Geometrical Condition for Lowering Side Glass and Body Side Assembly in Automobiles, Monografías, Ensayos de Ciencia de materiales

¡Descarga Geometrical Condition for Lowering Side Glass and Body Side Assembly in Automobiles y más Monografías, Ensayos en PDF de Ciencia de materiales solo en Docsity! 4.1 Body in White 1] Cx Cz 40 .35 38 31 .36 27 .34 23 .32 e .19 30 2 15 af | 5 25 5 10. 15 20 25 10. 15 20 A X= 20 mm AR a mn O E — X= 220 mm Fig. 4.33. Example of the influence on c, and c of position and heigth of the front bumper spoiler. c,o is the result without spoiler. NX ¡x 0 Y Vete Fig. 4.34. A) Center line section of shape study by Prof Morelli with Pininfarina, to suppress overall lift. In the lower figure, the distribution of computed (a) and ex- perimental (b) lift along center line are reported. B) Nissan model 350 Z, performing c ,= 0.28 and zero lift. 2 4 Body Work Fig. 4.35. Examples of vibrating mode for a 3 Box and a 5 Box body, for which reso- nance frequency are lower than 100 Hz. The dynamic deformation scale is magnified, to help interpretation. A and C: global torsion; E: front body torsion; B: side bending; D and E: front bending. The main heat sources are the environment, mainly for solar radiation, hot powertrain components (heat exchangers and exhaust pipes) and the air condi- tioning system. Noise and heat transmission are similar effects and are dynamic phenomena, therefore the previously listed factors should be considered as dynamic events: noise transmission through solids, for instance, is manifest as vibration, the max- imum amplitude occurring at the resonance frequency of whole body or detailed body area (Fig. 4.35). In a similar way, noise transmitted by air has the highest amplification factor in accordance with the cavity resonance frequencies, that in our case are the cavity frequencies of passenger compartment (Fig. 4.36). In Volume II, the acoustic comfort chapter, those phenomena will be explained in more detail. With reference to the body, it is important to bear in mind that the outer body sheets represent an effective barrier against environment noise transmis- sion, but produce a high level of heat and noise reflection or diffusion, without any absorption. It must be kept in mind that, according to mass law, noise attenuation through a steel wall is increased 1 dB by every gauge increase of 0.1 mm and that, for the same sheet thickness, attenuation of aluminum is approximately 9 dB less than for steel. Moreover, in order to reduce disturbance through these sheets, when possible insulating sheets of low density materials (polyurethane foams, ultralight fibers En thinsúlate , textile fibers), are associated, being very effective in reduct- ing high frequency noise and thermal fluctuations. The same materials are also 4.1 Body in White 5 Fig. 4.38. Examples of twin pillar on some recent sedans: the triangular opening in between is stiffened with a bonded glass. and therefore: L=0,5(H— R=— $). (4.12) It is interesting to note that the mass per unit area ofa 4 millimeter thick glass is 10 kg/m?, 3.5 mm thick glass is 8.75 kg/m?, a steel sheet of 0.8 mm gauge is 6.24 kg/m”, two steel sheets one of which 0.8 mm thick and the other 0.7 are together 11.7 kg/m?; correspondingly, the door mass can be lowered by maximizing the glass surface and reducing the area of the double (inner and outer) steel panels. Instead, the opposite would be true, if aluminum or plastic door panels were adopted. In a similar way, in quarter panel area, a glass window between two small steel pillars could be lighter than one single steel pillar with the same surface area, made by inner frame and outer panel. In terms of visibility also, particu- larly in the case of parking manoeuvres, a wide steel quarter panel has a visual disadvantage in comparison to two small pillars with glass in between. However, from a cost and stiffness perspective, the opposite is true. Regarding the overall body torsional stiffness, in principle body side with two side windows and rear quarter with wide steel pillar is the best solution, followed 6 4 Body Work Si Fig. 4.40. Schematic drawing to analyze complete opening of side window down to the belt line. by the alternative of three side windows with two small quarter panel pillars; the solution with two side windows and one small quarter panel pillar offers the best all around visibility but is less rigid. Rear vision is often reduced by quarter panels tapered to the rear, with the purpose of drag improvement, by height of back glass lower side and by back glass slope to horizon, which can amplify glass defects for angles less than 30”. The other body components that can influence visibility are: mirrors, wind- shield wiper and lighting devices, the properties of which will be explained in their respective chapters. Reliability In the paragraph on Structural function some indications have already been given regarding fatigue breaking and fatigue location. In the paragraph on Insulation, squeaks and rattles, usually resulting from contact among different components, have been specified. Another issue acting to worsen body reliability is oxidation and sheet corrosion. 4.1 Body in White 7 Fig. 4.41. Corrosion levels: A) blistering; B) perforation; C) structural. Body driving environment presents various extreme physical and chemical conditions, depending on the geographic location and ambiental conditions. These situations include humidity, acids, deicing salt, hydrocarbons and so on. Resistance of materials to environmental attack is highly specific: body steel sheets, for example, could not be used for the body without special treatment. An untreated steel plate, when exposed to air, is completely covered by rust after just a few days. Corrosion can be classified into three different steps (Fig. 4.41): + cosmetic corrosion, the consequences of which only really affect appearance, being referred to as paint abrasion, cutting or blistering; corrosion perforation, when holes in sheets not involved in safety tasks, as fenders or doors, are manifest; structural corrosion, when holes and cracks appear which affect the body frames involved in safety tasks, as rails, crossmembers or reinforcements where suspensions are fitted. For many years, this considerable problem was a major concern, which had to be addressed using a complex treatment of preliminary detergent cleaning, phosphating, electrophoretic coating, application of primer, curing of primer and top coating with multiple layers, cured in bake oven and able to supply corrosion prevention fora limited number of years, under specified conditions. Using traditional protection processes, after one year some cosmetic stain could be expected to appear on the coating, but without damage to the steel sheet. After 2-3 years, blisters began to arise in some places due to rain, dirt and moisture entrapment. This was caused by humidity and electrochemical reaction with steel. When this starts, blisters break and cause corrosion perforation, often appearing first in the wheel house, door panels and underbody boxed frame without ventilation. Even repeated treatment of the box frame with injected wax to cover the unprotected steel (in some areas not reached even by dipping electrophoresis) was unable to provide sufficient prevention. 10 4 Body Work + Assembly of tubular or simple metal stamped elements (space frame), with bonded panels made from stratified fiberglass or resin transfer molding pro- cess (RTM) with injected resin or by sheet molding compound (SMC) of glass or carbon fiber prepreg, cured and hardened under pressure and heating. This process is mainly manual and used for low production rate and very high structural performance, particularly in terms of weight to stiffness ratio. Although investment levels may be limited, the cost of materials and labour are high. Assembly of macro elements injected in thermoplastic resin, locally reinforced with metal insert or reinforced thermosetting moldings, connected by rivets or screws and/or adhesives. Being a partially automated process, it is adopted by few models of low production rate, mainly with the purpose of demonstrating a new technology. The most critical aspect is achieving the required structural stiffness, due to low Young's modulus of thermoplastic components and to the medium modulus of reinforced thermoset components. Assembly of stamped aluminum sheets and/or die cast and/or extruded alu- minum pieces, welded and glued or riveted with additional die cast nodes. This process offers an average or high level of automation, developed as an improvement or evolution of traditional sheet stamping and assembling inher- ited from processing steel. The main difference from the steel process is part integration by the specific property of aluminum manufacturing process. For instance, a die cast aluminum piece can integrate ribs and punched bosses and have variable thickness, that in a steel part would require additional brackets and reinforcements. Despite innovation effort and many applications, costs of bodies produced in this way still remain higher than traditional steel bodies, while total weight is much lower (up to 40% less than steel) and stiffness sometimes higher (Fig. 4.43). Assembly of hydroformed sheets and/or tubes by welding or riveting. This process can be quite automated; it has been tested on some model of low or medium production, in order to reduce investments in dies (sheet hydroform- ing) and assembling fixtures (tubular hydroforming). Performance and weight are similar to traditional steel bodies, but sometimes sheet deep drawing in a hydroforming press (that uses only a matrix or a punch - Fig. 4.44) can assist forming very difficult parts, even with undercuts. Assembly process The assembling process includes welding (resistance spot welding, arc welding, braze welding, brazing, laser welding), permanent cold fastening (riveting, clinch- ing), chemical linking (bonding). Spot welding creates a melted nugget common to both sheets due to the heat obtained from resistance to electric current flow through sheets held together under pressure by electrodes. 4.1 Body in White 11 m!kgl — A 4004 . B m=17.5VC ] A C =D! 3004 20074 1004 m=14.6VC-67.1 | A (R?=0.88) 0 r r — r 0 5 10 15 20 25 VC [m?] Fig. 4.43. Parametric comparison of body mass m in aluminum and steel, referred to body dimension VC (product of length times width square). Line A: interpolation of steel body masses; B: ULSAB prototype; C: aluminum bodies; line D: interpolation of aluminum bodies. The spot can be direct, when generated by two opposite electrodes, operated by welding guns of various sizes, configurations and clamping modes (Fig. 4.45). Instead, an indirect spot can be generated by an electrode pushed against only one side of sheet assembly, while a copper plate supports the opposite side, with the purpose of keeping the surface smooth, avoiding marks and puncture wounds, that could be present on the electrode nose side. Roll welding is operated with two rollers or rotating electrodes, rotating as the seam welding is in progress, while the current flow frequency is controlled. Arc welding uses an electric arc between one wire electrode and the surfaces to be welded, instead of current flow between electrodes, in order to melta consumable wire in the joint; in the case of thin sheets, this process must be used in blind areas and only when no other joining process is possible, because this kind of welding is uneven and thermal impact can bring metal stresses. This kind of welding requires inert gas (helium or argon) envelope to be shielded from atmosphere, particularly in the case of aluminum. Braze welding use as filler material a wire of non ferrous alloy with a melting point which is lower than plates to be joined; the result is a groove or fillet or plug weld. Laser welding makes use of a laser beam, concentrated by a lens directly on the Plates to be joined or on a braze wire, in order to generate the melting energy. Cold assembling processes are increasingly sought after due to potential energy savings, investment reductions, reduced frequency of servicing and, specifically, 12 4 Body Work Fig. 4.44. Sheet hydroforming: blank positioning (1); blank holder closing (2); fluid pressurization (3); punch stroke (4). the possibility of join non weldable materials with relative ease, for instance het- erogeneous materials. Among the cold processes, clinching is the most efficient in terms of the materials required, since it does not require additional pieces needed, for instance, by blind riveting (Fig. 4.46). Both riveting and clinching cannot be used on aesthetic surfaces; moreover, they should normally be operated by standing fixtures and only therefore on subassemblies. Bonding is generally used as a complementary function to welding, for some sheet assembling, mainly in aluminum and in hem operations (as will be ex- Plained in the specific components chapters). Bonding is compulsory in struc- tural plastic assembly (as composites) and in heterogeneous fittings that cannot be riveted, as with the windshield and the fitting of the back glass to the body. Adhesives are usually in the form of semi-liquid paste or semi-solid tapes; they are extruded and their most common chemical families are polyurethane or epoxy resins. Adhesive curing requires time and accelerating factors such as humidity or temperature. When assembled parts must be moved and therefore there is a risk of relative sliding before paint baking, local heating and baking of joint is needed, for instance by induction heating. When designing glued parts, 4.1 Body in White 15 e2 -0.4 -0.3 -0.2 -01 0 1 2 3 4 FeP04/0.8mm e DP 600 /Imm BH180/0.8mm — = —-—.— FEESGO/mm — TRIP800/1mm — AA6016/1mm Fig. 4.48. A) example of forming limit curve; B) comparison among experimental FLC of steel and aluminum grades and thicknesses. In this figure, the regions below CLF are risk free (for example, point x s), respectively in drawing region (quadrant 1) and stretching region (quadrant 2). Instead, regions above CLF are risky (quadrant 3) as in the case of point Xe. In Fig 4.48, table B compares examples of limit forming curves obtained by testing different materials. Another property that makes a difference among draw sheets is spring back, which relates to the capacity of fibers not yet collapsed in drawing to return to their original shape following release of the forming load. This effect is more relevant as material yield strength increases, but can be tackled, within limits, by increasing fiber stretching through blank holder loading in a double crank press (a press in which punch and blank holder are both movable). The test that enables to rate the spring back property of a material consists of forming a number of the same 2 profiles with increasing blank holder loads, measuring the final configuration of profile wings or permanent deflection of wing edges after forming (Fig. 4.49). At this stage it is appropriate to review the most commonly used materials. Steel sheets Traditional iron sheet alloy has an extremely low carbon content (from 0.02 to 0.08 %), very low controlled quantity of phosphorus, sulphur and manganese and potentially up to 0.3% of titanium, defining the deep drawing capacity, elongation at break above 40% and limited thinning (r index > 1.8). 16 4 Body Work DEF. [mm] e 60 AAA mu a BHF BHF $0 —+- FeP04 ON Ñ + HSS 40 SS — Al 0 T T T T T 0. 20 40 60 80 100 120 BHF [kN] Fig. 4.49. Influence of blank holder force BHF on spring back DEF of specimen made with two steel grades and one aluminum, U shaped by drawing. On left side, a section explaining how the specimen appears out of the die. No other material exhibits similar deep drawing properties. After forming, parts appear perfectly smooth and do not require manual finishing. Bending at 180 degrees and welding, both spot- and wire-welding, can be applied without major problems. Gauges used in body start from 0.60 mm and rise up to 2.0 mm and more, for structural non visible parts. The main drawback is weight (mass is 7.8 Kg per square meter of sheet gauge 1.0 mm) and, secondly, dent strength when the thickness is less than 0.80 mm. Furthermore, the problem of easy corrosion can be considered to have been overcome, as steel automotive sheets are generally zinc coated or galvanized on both sides during the rolling mill process. Over recent years, body designers have been forced to face two additional problems, influencing steel makers research and innovation toward new steel grades. The first was safety strength, that could be resolved with traditional steel only by increasing sheet thickness and therefore weight. The second is fuel consumption and hence air pollution, directly influenced by the weight. No steel sheet gauge can be thinner in practice than 0.6:0.7 mm, for reasons of both thinning and breaking during forming and as concerns elastic instability and dent weakness (referring to outer panels). Therefore, body weight could be reduced only by selecting the thinnest sheet possible, consistent with dent and safety strength. In practice, designers needed to be provided with a choice of increased yield steel, without losing traditional drawing capacity. In this context, considerable innovation by steel makers over recent years must be acknowledged, as a wide choice of steel grades with a large range of yield strength and break 4.1 Body in White 17 A[%] 50 BÁKE HARDENING 40 = Lo( ¿ 30 7 20 . MICROALLIÉD 10 'ARTENS 0 - Y [MPa] 0 200 400 600 800 1000 1200 Fig. 4.50. Properties of major steel grade families used for bodies over recent years. Y : yield strength; A: elongation at break. elongation has been made available, which are capable of satisfy a range of different requirements (Fig. 4.50). Some of these steel grades (Dual Phase and Multiphase) have the property of changing their ductility as a consequence of plasticity during the forming stage and following hot curing in the painting oven, meaning they are sufficiently drawable in the stamping stage but then increase hardness and yield strength when the body is painted. In order to illustrate the application of different steel grades available so as to save body weight, an international consortium of steel makers has promoted a study and the application of new technologies, resulting in a series of prototypes, named ULSAB (Fig. 4.51). Table 4.1 summarizes the mechanical and forming properties of some steel grades used on ULSAB-AVC prototypes. Aluminum sheets The main advantage of aluminum over zinc coated steel is the lower specific mass to reduce parts weight, since the yield strength of aluminum alloys is not so much lower than most steel grade today in use. Traditionally, the performance equivalence between steel sheets and aluminum sheets was rated in terms of the bending stiffness of a plate and therefore was established by elastic modulus ratio, in favour of steel with a factor of approx. 3. Since the plate bending stiffness is related to the third power of gauge, in order to obtain a stiffness equivalence, the required increase of aluminum thickness should be cubic root of 3 or approximately 1.43. Usually in practice, to replace 20 4 Body Work Fig. 4.52. Examples of Mercedes E aluminum panels and frames. investments, because the only specific tool needed for each section is the ex- trusion matrix. Structural plastics Only composites suitable to replace steel or aluminum parts for structural goals (stiffness, strength, crash absorption) can be considered structural plastics. These composites should therefore have Young's modulus and ultimate strength not lower than aluminum. In practice today only carbon fiber composites can reach the required values and in this case they can be used for panels as well as for frames. The average mass density of these components is between 1.4 and 1.8 kg/dm*, meaning the potential for weight saving is very high when compared to steel and still of interest when compared to aluminum, even though the minimum thickness recommended for this kind of application is 1.2 mm. The Young's modulus can rise to 150 GPa, falling in between steel and aluminum. Assembling of composite parts, after steam curing, is usually made by bonding with structural adhesives such as epoxy resins. The cost of such components is still only acceptable for niche, extreme performance vehicles. A family of composites adopted for mean production vehicles is that of SMC (Sheet Molding Compound ), mostly for outer panels or movable parts (hood, door, fender, decklid, liftgate). Such composites incorporate impregnated patches in vinylester or polyester, reinforced by random glass fiber of small/medium 4.1 Body in White 21 length and sometimes by long continuous glass fiber, cut and laid down in the open die, then stamped under pressure and temperature close to 120 "C (the curing time in the die about 30 seconds per millimeter thickness). Minimum thickness is 2.0 mm for non visible parts, while usually it is difficult to accept less than 4.0 mm for outer panels. The average density is between 1.5 and 2.0 kg/dm3. The Young's modulus, depending on glass fiber percentage, is between 10 and 20 GPa. Usually, a SMC part weighs at least 90% of the corresponding steel part, although sometimes it can replace more than a single steel piece, integrating their functions. The main problem is that of surface appearance quality, which is the primary aim for all outer panels, since these composites, if reheated after stamping at a temperature higher than that used during curing, tend to eject gases through very small holes, that remain open and visible after painting. In order to minimize scraps, itis first necessary to coat the piece in the die with an in-mold- coating and ensure that the bake temperature during all painting stages is maintained lower than the die curing temperature. The cost of a SMC component can be kept at the level of steel only for very low production volumes, thanks to the die investment advantage, because material and manufacturing process are generally more expensive than for steel. Another composite family for small production is obtained by the RTM (Resin Transfer Molding) process, which uses a preformed light fiber cloth, inserted in the die as a core and then injected at medium pressure with thermosetting resin (for instance, glass reinforced polyester) while the die is closed (Fig. 4.53). The resulting piece is light and stiff, thanks to its sandwich structure and can be used for outer panels of movable parts. As regards painting, the same problems as with SMC arise, while the overall costs are comparable. As regards reinforced thermoplastics, their Young's modulus is much lower than 10 GPa and therefore they can be recommended for deformable compo- nents, such as bumpers and wings, but not for structural body parts. Some technological constraints affecting aesthetics During style modeling and pre-engineering, body is divided conceptually into macro elements, each being not only paired with the preferred material but also designed with regard to boundary configurations, mainly angles and curved connection fillets: at this stage, technological constraints and manufacturing op- portunities are sometimes in conflict with the styling targets. Some general rules, relating to the manufacturing process constraints are listed here. + All outer panels, stamped by sheets, should have a main curvature radius not larger than 2,000 mm, in order to avoid sheet pumping and visible surface waves. The risk of flat regions and pumping increases with the yield strength of the sheet. 22 4 Body Work Fig. 4.53. Resin Transfer Molding: 1) insert layers feeding; 2) insert preforming; 3) die closing; 4) low pressure injection and reaction process; 5) stamped piece. A B 2 10T y as uy $ 3T 2 7 048 YE ( Fig. 4.54. Dimensional rules for aluminum 2000 and 6000 hem tool design, to avoid cracks while hemming: preform hem operation (A); final hem operation (B). 4,2 Body Side — 15 Fig. 4.56. The body side of a 5 door car can be analyzed as a structure with 8 hinges connected by 9 beams (1); some hinges can be replaced by extremely stiff nodes and only 5 hinges (2); or only 3 hinges in case of pillarless body (3). + Door opening and closing loads, locks noise. + Corrosion resistance in humidostatic chamber and special tracks. + Aging and environmental resistance of plastic and rubber components. + Hail and stone denting resistance of outer panels. + Roof crushing under distributed load (snow) and local load (roof rack). 4.2 Body Side The body passenger compartment can usually be conceived to be a box sur- rounded by six main walls with frame in wall intersections and more precisely a floor, a roof, two side walls, named body sides, a firewall or dash panel, a rear bulkhead. The latter is always missing in SW and SUV, and is occasionally lacking or limited in sedan cars. The body side is a lean ring frame, empty in the middle and therefore subject to shear stresses in the wall plane, as a consequence of roof to floor or firewall to bulkhead sliding. The body side is loaded by static and dynamic loads concentrated on doors hinges and latches, by distributed dynamic inertia forces while driving and by Pulse loads on certain areas in the event of impact. Body side perimeter elements, as individual components of a unitized structure, should therefore be sized in bending and torsion, especially close to nodes. In a typical side frame, eight main nodes can be identified (Fig. 4.56). The most relevant stresses on side frame assembly arise from body torsion (Fig. 4.57 - A) and from crashes, mainly frontal (Fig. 4.57 - B) and side crash. At least four of the eight nodes include roof and floor crossmembers connections between body sides: the stiffness of these nodes is a priority in order to achieve adequate torsional stiffness and impact protection (Fig. 4.57 - C). Moreover, at latches and hinges fitting positions, adequate reinforcing brackets are welded. 15 4 Body Work 400 600 XxX x C T 400 Z 200 Z J Hg, S Z ==> Fig. 4.57. A) Main loads on body side in body torsion; B) loads applied to body side by front crash; C) example of side frame reinforcement for D node. Access to the passenger compartment is consistent with a size increase of nodes F-C-I (Fig. 4.56) and related beams only. As a consequence, the weaker part of body side is the upper one, particularly between nodes H and D. These members are usually reinforced by an inner diaphragm of adequate thickness and high strength plate. A similar weakness condition is present in the pillarless body side (Fig. 4.56 - 3), a countermeasure being to stiffen as much as possible the whole rear pillar or quarter to the D node. As concerns material selection, since they have to be strong and stiff as well, the main candidates are steel plates with adequate properties or aluminum as- semblies (made by different alloys and processes such as stamping, extrusion, rolling or die casting) or in few cases carbon fiber reinforced moldings. 4.2.1 Body Side Setting In order to understand how design relates to the above listed problems, we can start by considering a typical saloon body side setting. The main parts included in most body side assembly, with reference to Fig. 4.58, are: 1. body side outer (from 0.7 to 0.9 mm thick - steel grade FEPO4 - FEPO5), 2. front pillar reinforcement (from 1.0 mm FEE220BH to 1.6 FEP04), 3. central pillar reinforcement (from 1.2 mm FE600DP to 2.0 FEP04), 4. central pillar boxing (from 0.7 to 1.5 mm FEE220BH), 5. quarter panel assy (from 0.75 to 0.9 mm FEP04), 4,2 Body Side — 15 Fig. 4.58. Split view of main body side elements. 6. windshield pillar box (from 1 mm FEE220BH to 2.0 FEP04), 7. windshield pillar reinforcement (from 0.8 mm FEPO4 to 1.2 mm FEE355), 8. front pillar rail (from 0.8 to 1.2 mm FEP04), 9, roof rail box (from 0.8 to 1.5 mm FEP0O4 or 1.0 mm FE600DP), 10. roof rail (from 0.8 to 1.2 mm FEP04), 11. rear pillar rail (from 0.8 to1.2 mm FEP04), 12. rear side brace (from 0.8 to 1.5 mm FEP04), 13. reinforcement plate (from 1.5 mm FEE355 to 2.5 mm FEP02). (Where: FEP are low carbon deep drawing steel parts, FEE high yield strength steel, BH means bake hardening, DP dual phase). Sometimes a quarter boxing frame is present, the inner wheel house (when not included in underbody assembly) and some local reinforcements for safety belt anchorages, hinges and latches attachment. In summary, the body side assembly is made of an outer panel (better visible with the doors open) defined by body side styling and door shapes; an inner panel (box or rail) completely covered by internal trim, defined mainly by structural analysis and designed to static and dynamic stress, complete with holes, slots, insert, brackets; intermediate members (diaphragm or reinforcement), coupled 15 4 Body Work Fig. 4.61. A) Example of metal housing welded and bonded to quarter panel; F: housing bottom; S: spot welds; AS: structural adhesive. B) Example of thermoplastic (polyammide) filler flap, both free and fitted to body; SNI: hinge holes; AR: reaction spring clamp; MA: integral hook. In some aluminum bodies, a lower number of spaced weld spots has been combined with continuous bonding of flanges with epoxy resins, resulting in a significant increase of joint stiffness. Nevertheless, in some fitting positions of different parts with structural fune- tion, there is no way to spot weld sheet lips due to lack of flanges; therefore arc or laser welding seams or braze welding is needed. 4.2.2 Fuel Filler The fuel filler housing is usually located in the body side, most commonly in the quarter panel. Due to its depth, the housing is normally drawn and trimmed, with insertion of a metal or composite plastic stamped bottom and covered by a door or flap, made by various materials and process (Fig. 4.61). Tf the filler flap and the quarter panel are made of the same metal, they are spot welded in the quarter subassembly and sealed by a structural adhesive, cured seccessively in the painting bake oven. In the case of heterogeneous materi- als, spot welds are replaced by rivets or adhesives cured by a specific subassembly baking. The filler door can be stamped out of steel or aluminum sheet, with perimeter downflange and trim or hem, or molded with thermoplastic or thermosetting 4,2 Body Side — 15 Fig. 4.62. The layout of dimensional measurement of door opening deformation in body torsion test. resin: in this case, the design of hinges is critical and therefore the selected material should be sufficiently tough. Thermoplastics do not allow the molding of thick ribs due to risk of sink marks on the door surface. Thermosetting flaps, on the other hand, lack toughness and are therefore relatively easy to break, especially with misuse. 4.2.3 Body Side Specifications The critical design properties of the body side are: + dimensional precision of door housing, + stiffness of nodes and beams, * resistance to concentrated loads on hinges, latches and belt anchorages, + fatigue strength, * impact resistance, * air and water tightness. The precision and stiffness of the door housing are major targets in order to ensure effectiveness of the body side weather strips and door operation for any vehicle attitude. A typical dimensional door opening tolerance is + 1 mm, whilst the allowed diagonal length change in the torsion test is less than 1% (Fig. 4.62). Node stiffness, obtained by appropriate parts matching and local reinforce- ments, is a primary objective in the first stage of body analysis: design data are available to preliminary select the archetypes required to achieve the body stiffness target. Among the most critical nodes affecting stiffness is node D (Fig. 4.56) particularly in two box cars with liftgate. 15 4 Body Work Fig. 4.63. Node A is the most conditioned by body openings and styling, E: roof panel; A: body side outer; B: roof rail; C: pillar boxing; D: front roof crossmember. The usual way to deal with the concentrated loads of latches, hinges and safety belts is not only provide sheets of adequate thickness and yield strength, but also additional reinforcements welded to box parts with thickness often exceeding 2 mm. During side impact, nodes B and C are the most stressed, whereas in frontal impact it is nodes F and A. The latter is the node most affected by dimensional constraints and therefore certain advantages can be obtained through contribu- tory connection with the front roof crossmember (Fig. 4.63). The most critical body side features as concerns fatigue behavior are the flange lips unions between the roof rail and central pillar and the door opening flange at node D. Occasionally fatigue cracks appear on the quarter panel near the belt line, due to the sharp section change from the lower quarter panel to the rear pillar, the small union radius and the high degree of panel stretching in drawing stage (Fig 4.64). Computer analysis of fatigue stress cycles indicates where the main stresses are most likely and therefore enables the selection of materials, thickness and design to reduce shape overstress. Furthermore production variations influencing local gauge reduction, or increased notches or wrinkles can deviate to some extent from computed results. It is recommended, therefore, that a set of full body endurance test on fatigue bench be performed, using as input is a paved road time history; usually endurance tests are not performed on the free body side, because the test would give limited and less reliable results in the same amount of time. Regarding the properties of individual body side parts, the primary contri- bution to the body stiffness is provided by sills, and the basic resistance to side impact by the central pillar. The sill section can be designed with some discre- tion, principally in Y direction, while central pillar is heavily affected by the 4,2 Body Side 16 100 l Fig. 4.67. Sealing by a thermo-expandable buffer positioned in node C: 1) start posi- tion of expandable diaphragm; 2) volume after expansion. Regarding air and water ingress, the main area of risk is the roof to body side junction: one specific design solution shall be explained in roof and weather strip chapters (Fig. 4.66). Flange lips, where weather strips are fitted, should be free of waving, making hard weather strip insertion and toe sealing (therefore laser welded or bonded lips are better than spot welded) and without gaps or steps (as in split body side, not in unitized one and moreover between two boxing parts, mostly when the thickness is different). In the case of the split body side, it is therefore recommended to fill with braze and finish by grinding, gaps between contiguous sheets, in order to provide a continuous surface for the weather strips. Moreover, the flange of door openings should always be designed with a radius of 60 mm or more, to avoid wrinkles in weather strip bulbs. If this is not feasible, instead of having a continuous closed section, the weather strip should be molded in angle areas and therefore with an open side. An often critical region for the transmission of rolling noise is the base of the central pillar, because the sections usually designed to provide embedding of the pillar in the sill and the belt retractor housing result in a hard to seal node. One solution consists of positioning a thermo-expandable buffer in white body side that later expands (while passing in the bake painting oven) to fill the contiguous cavity (Fig. 4.67). 16 4 Body Work sec.C-C sec.S-S a 210.5 Fig. 4.68. Examples of matching zones between the front fenders and body parts. 4.3 Fenders Today the main function of the fender is related to aesthetics and aerodynamics, however the first reason for their introduction was to shield wheels and protect passengers compartment from projection of water, mud and stones. In comparison to the body frame, the structural contribution of fenders is zero with the exception of a few cases (mainly spiders) where the front fenders are welded to the body frame in order to increase the overall torsion stiffness. Usually front fenders are only screwed on, while on the rear (quarter panels) they are welded to the body side. Metal front fenders commonly comprise few parts: a main drawn panel, a back completion flanged to main panel, the purpose of which is to seal the space between fender panel and lower front pillar, and some brackets to fit the fender to body and hold accessories. Fig. 4.68 shows some typical fender to body fitting sections. Some fender properties are specified by manufacturers as standard such as: the fastening characteristics, the masked gap section, the fitting of interior mudguards (Fig. 4.69). In this figure, the free distance available for fender crushing under pedes- trian impact should be, according to standard approximation, at least 80 mm in order to limit pedestrian head acceleration. The main advantages of detachable front fenders are : the fitting process for movable parts can follow a longitudinal path, starting from quarter panel and ending with front fender; in this way, a degree of lack of precision in body side or doors can be recovered; + fenders, mainly front, are frequently exposed to impact and damages: conse- quently the cost of repair is lower for fittings which are detachable; + fenders can be made from a different material than the body. 4,3 Fenders 16 Fig. 4.69. An example of fender and wheel opening section. A: fastening with spacer, serew and riveted nut; B: lokari** fitting; C: masked gap; b: amount of deflection designed for pedestrian impact absorption. As a result of the ease of denting fenders due to normal use conditions and the fact that the absence of structural requirements tends to promote the use of thinner gauges (even less than 0.7 mm in the case of steel sheets), these parts represent the most appropriate candidates for materials which are more flexible than steel, such as aluminum or plastics. Moreover, plastic moldings offer the advantage of integrating in a single shot, brackets and other ancillary parts, which would otherwise need to be welded, and thus compete with steel in terms of cost even for mass production. In principle, the benefits of thermoplastic molded fenders with respect to steel are as follows: + lower die cost, + lower weight, + higher flexibility, mainly an advantage with respect to pedestrian impact, + removal of mudguards or at least lower area of wheel housing which needs to be protected against stone, sand, etc.; * straightforward integration of brackets and completion panels, 16 4 Body Work a E POS N Pr A 3-5 l a, B 0-1 Ñ loc : Ñ Cc 0-2 D 1-3 E 1.2 > 100 Fig. 4.71. Fiat specifications for position and number of front fenders fittings. P: fender; L: body surface; RE: rivet nut. moreover should be verified during full body endurance testing, with regard to fenders and underbody protection from abrasion and noise absorption. 4.4 Roof Assembly The conventional task of the roof panel is that of compartment shielding and connection of body sides, front and rear roof cross members. In the case of roll over, compartment integrity is provided by the compartment frames, whilst the roof target is only to avoid the ejection of passengers, providing protection against hard local contact (Fig. 4.72). Correspondingly a wide selection of suitable materials is available: steel, alu- minum, thermosetting plastic sheet, glass. In the case of steel grade FEPO4, the usual thickness is in the range 0.70.:0.85 mm. In all cases, any roof panel ma- terial requires braces to prevent panel flexing and vertical waviness, commonly known as oil canning: for this purpose light steel bows are introduced, which are welded or riveted or screwed to the body side and bonded to roof panel by 4,4 Roof Assembly 16 Fig. 4.72. Typical set of roof assembly parts for a metal closed roof (A) and in case of partly sliding top (B). TA: front roof cross member; TP: rear cross member; C: bows; SR: brace for roof top opening. FeP04 sp 0.8 SE sp 0.8 sp 0.7 C sp 0.8 Fig. 4.73. Some examples of roof bows: A) open section, uniform thickness; SE: thermo-expandable sealant strips; B) open section, tailored blank; S: extruded sealant; C) boxed bow; S: extruded sealant. smooth flexible adhesives such as butyl mastic (in order to avoid sink marks on outer surface), mainly providing a damping effect (Fig. 4.73). These bows can moreover provide a connection between the two body sides and the roof in a side impact. For that reason, simple bows are made from steel sheet at least 1.0 mm thick or from steel tailored blanks 0.70 mm thick in the central area, 1.4 mm at both sides. Side bow edges, preferably screwed to the body side for tolerance adjustment, must in any case include deep drawn ribs, to avoid local buckling under compression and bending loads. In the case of higher class vehicles, preferred are boxed bows made from FEPO4 steel and thickness 0.7 4.8 mm for both welded elements. It is always recommended to verify that roof panel and bows assembly do not exhibit any resonance frequency coincident with compartment cavity frequencies. Another task of roof bows is to support the interior roof liner (see roof liners chapter ) which requires a number of fittings both in the central and boundary 16 4 Body Work Fig. 4.74. Examples of open (A) and boxed (B) sections of front roof cross member assembly. P: roof panel; O: inner frame; SC: boxing member; S: sealant. areas to avoid deflections and vibrations. Fittings are usually plastic clips or velero SR strips. Some roof liners, mostly on less expensive vehicles, are bonded directly to roof panel. Continuous roof panel connection to the body sides, windshield and back glass, mainly horizontal, help to avoid compartment frame lozenging under shear stresses, therefore contributing to torsional body stiffness. In order to achieve this goal, it is not necessary to weld the roof panel to the body side; instead it is sufficient to bond it with a medium stiffness adhesive, as used for the windshield and back glass. Regarding the front roof crossmember (Fig. 4.74), lighter cars have open sec- tions, thickness 0.7 - 0.8 mm, material steel FEPO2, while heavier cars adopt more robust sections, often boxed, with thickness between 0.65 mm in steel FEE355 and 0.80 mm in steel FEPO2 or FEE275. Rear roof crossmembers have completely different sections depending on the body type (with or without lift- gate — Fig. 4.75). Three-box cars without tailgate do not require a cross member designed for torsion; a simple open section beam, with thickness in the range 0.70 to 1.2 mm, above the back glass being adequate. In the case of the liftgate, the cross member contribution to the body and opening stiffness is usually relevant; therefore the cross member has wide sections and is boxed, with thickness up to 1.2 mm, both for the cross and boxing members, while a higher yield strength steel is recommended, for instance bake hardening FEE220BH. The roof contribution to body stiffness reduces if a top is mounted in a large opening (e.g. panorama top). Even the resonance frequencies and body vibration modes can be significantly influenced, in case of a large opening to accomodate a movable top (Fig. 4.76). 4,4 Roof Assembly 17 Fig. 4.78. Examples of roof sections designed for laser braze welding. PB, 1 3 5 7 EOS. Ef P.sc L P L XA F PLE, E PEE P + PO sc 2 4 6 8 Fig. 4.79. Examples of Y-Z sections of roof to body side junction. From 1 to 5: with drip mold housing; 1) B= 16 mm; 2) B=37 mm; 3) B=22 mm; 4) B=17 mm; 5) B= 13 mm; 6: folded and roll welded flanges; 7 and 8: side welded drip and body side boxing. 17 4 Body Work and rolled flanges presents sealing problems. Side junction design is consistent with doors that have a window frame wrap around body side frame if body side weakening is considered acceptable. For each design, sealing ease must be granted. 4.4.1 Roof Specifications The delivery tests specified for roof assembly are static, dynamic and impact. The purpose of static testing is to verify that the roof assembly exhibits no permanent deflection due to a distributed load representing snow load: the dis- tributed pressure applied is in the range 10:15 N/dm?. Dynamic bench and paved road tests are intended to verify the absence of roof contribution to compartment noise, due to panel drum-like vibration and the lack of bow damping effect. As concerns impacts, the roof contribution to both side crash and roll over is mainly related to the roof cross-members and their connection nodes to the body sides. Roof bows, as already mentioned, must have strong edge fittings. Impact testing is performed on the assembled body, in accordance with regulations. For these properties it is not easy to provide standard design specifications: finite element analysis represent the best approach to reach an appropriate design solution. 4.5 Front Frame The front frame is the assembly between the firewall and front bumpers. For most cars, this frame surrounds and supports the power train and its auxiliaries, and is therefore relatively complex and specialized. Moreover, fitted to the front frame are the front suspension links, steering box, part of the air conditioning system and front lamps. Last but not least, the front frame is in responsible for absorbing front crash energy, impact loads together with the compartment frames (body side and floor) and for attenuating potential injury to vulnerable road users as pedestrians in the event of an impact. If the boundary cabin frame has been conceived to be a strong cage, these functions could be satisfied, for instance, using a cantilever embedded in the cage, with an increasingly strong section from bumpers to firewall, equipped with brackets to support the different various mechanical sub-systems, and sufficiently distant from the critical area to avoid direct contact with vulnerable road users during impact (Fig. 4.80). However, this simple scheme must comply with other requirements such as volume and obstruction restrictions, and enable operation of the different sub- systems, specifically the power train itself (which may be longitudinal, trans- verse, inclined, horizontal mounted), exhaust and intake systems and the wheel motion envelope. Moreover, such a cantilever should be connected to a resilient 4.5 FrontFrame 17 Fig. 4.80. Main load position on front frame and their cause. MP: power train; SP: suspension and steering; FE: front end; CR: crash. compartment boundary frame (for example, the body side or floor members) and not directly to a wall such as the firewall for example. This is not only due to the extremely high loads (hundreds of thousand N) to be faced during front impact, but also to avoid the fact that a cantilever bending could excite vibrations of the firewall and floor, giving raise to air pumping and noise within the compartment cavity. Many configurations of front rails embedded in the compartment cage can be found (Fig. 4.81). In practice, all known configurations are a combination of basic members, shown in view (1) of Fig. 4.81, with additional members that could be longitudinal, as in case (3) of same figure, cross members as in case (2), inclined as in case (4) and (5) or parallel to tunnel as in case (6). Front rails P, mainly responsible in front crash handling, are connected to upper shorter rails R, by vertical strut towers D, where the spring and shock absorber housings are located. The assembly of these three members which are always present is the structural block supporting the wheels vertical, longitudinal and side loads. The front rails P, that in Fig. 4.81 appear to be straight and with constant section, often have a twisted axis and variable section both in the planar and lat- eral view, caused by the space restrictions due to the mechanical parts and their operation, in particular the suspensions, steering system and transmission links displacement. Therefore respective impact tuning is very difficult and the differ- ente allowable section configurations (circular, squared, rectangular, hexagonal) do not each exhibit the same energy absorption effectiveness. In order to evaluate the crash performance of front frames, in addition to computational tools, drop tower or pendulum real frame testing (Fig. 4.82) is often used, in which a front car prototype is struck by a stiff mass. 17 4 Body Work 3 MINA O 10 20 30 40 50 EA[%] T— | TA | O 10 20 30 40 50 60 — EA[%] Fig. 4.83. Contributions of front frame single members in terms of front crash energy absorption EA. A) impact at 56 km/h against offset rigid barrier - Auto Motor und Sport; B) full front impact at 56 km/h against rigid barrier - U.S.A. NCAP. Contrib- utors: TI) lower frame; PS) upper rail; PP) main front rail; CB) crash box; TA) front cross member. 15 km/h is the impact test speed for all vehicles compared in terms of repair cost rating and consequently car manufacturers” design target. Overall impact resistance in insurance testing is provided by bumpers, cross member and crash box; therefore the preferred approach is to assign to a sin- gle supplier (usually the bumper supplier) the task of meeting with the target through the design of all three parts included in the system. Correspondingly this aspect is discussed further in the chapter on bumpers. As concerns manufacturing the front frame, usually the firewall and each wall dividing the front volume from passengers cabin are included. These walls enable a number of interface functions, through the openings for mechanical control devices (as steering system, gear operation, pedals, hood release cable), multiple connectors for electrical harness and for electronic systems as air bags, housings for components as air conditioning. Moreover, the firewall (also called lower dash panel) supports the cowl top or top dash panel, fulfilling the important function of front pillars connection and 4.5 FrontFrame 17 ---- 1 60 40 ñ ! 20 ¿ rm 0 7 7 r r 0 100 200 q im e00 400 500 Fig. 4.84. Effect of minor front frame changes on energy absorption: 1) load F - crush d recorded in pendulum test for original frame; 2) after black marked changes in frame figure. windshield support. To these panels, an instrument panel assembly, supported by a frame, is usually fitted. The most common arrangement of lower and upper dash panel are shown in Fig. 4.86. In practice, the upper dash panel is used as the housing for the air conditioning and windshield wiping systems. The lower dash panel, where openings for operations and controls are cut, can be designed with a twin or single wall with a reinforcement cross member, or sandwich panels with a plastic damping sheet in between, included to attenuate dash panel vibrations. 4.5.1 Front Frame Specifications 1. Progressive crush load in front impact (X direction), capable of generating a mean body acceleration between 10 G to 30 G. 2. Resistance to maximum vertical shock absorber load. 17 4 Body Work Fig. 4.85. Example of body installed crash box (A), view and section of absorbing device (B), bumper beam and crash box assembled (C) and deformed after offset impact test (D). Fig. 4.86. Vertical simplified sections of different dash panel. 1) dash upper; 2) twin fire wall; 3) rear fitting of front suspension arm; 4) reinforce cross member, sometimes used to fit steering housing; 5) sanwich assembly with plastic sheet in between; 6) cow] top; 7) front floor connection to dash lower panel. 4.7 Compartment Floor 18 Fig. 4.88. Conceptual cost comparison C between two floor types, referred to produc- tion volume VOL. ST) traditional steel floor (20 kg); CP) fiber reinforced plastic floor (1 piece, 10 kg). also been developed of sandwich composites floors, made of two plastic (usually thermosetting) stamped parts with a closed cell stiffening core. In theory also thermoplastic floors offer a feasible solution when reinforced by metal braces to meet structural targets (in terms of impact and stiffness) and should result in lower vibration levels. Fig. 4.88 shows a qualitative cost comparison between two different floor tech- nologies, illustrating the potential advantage of a polymeric floor for low pro- duction volumes. Today typical floor parts are (Fig. 4.89): two longitudinal rails (LA) below front floor and two (LP) below rear floor; two side sills (LL); a front seats cross member (TA) and a rear seats cross member (TS); a cross member between rear wheel houses (T) and a rear cross member (TP); a front floor (PA); a rear floor (PP), sometimes split in two pieces for stamping requirements. Only in a few cases is a single piece floor feasible. The front floor with central tunnel (needed in rear-wheel and four-wheel drive cars) is more articulated; in this case, if the floor cannot be drawn as a single piece including the tunnel, it is usual to divide the floor into three longitudinal spot-welded parts: the tunnel and two side floors. Longitudinal front rails are commonly straight, whilst rear rails have variable sections with twisted axis, extending from the side sills toward the inside, then turning around the inner wheel house before straightening towards the rear. Cross members are usually straight, with the exception of the tunnel wrap- ping, where they are arched; in this case, a smaller section is designed above 18 4 Body Work Fig. 4.89. Frame elements and panels of a typical floor for 2 or 3 box car. tunnel. It is useful to note that cross members can better resist heavier loads (as in belt anchorage testing) if fitted below the floor, because in this case the reac- tion to cross member loading is distributed across the entire surface of contact between the cross member flanges and the floor, instead of being concentrated at the weld spots. On the other hand, as concerns corrosion resistance, a floor with inner cross members is preferred since joint surfaces are not exposed to dust and salt pro- jection. However, if the floor sheets are not zinc coated, the risk of humidostatic corrosion could be higher inside boxing inner cross members. In the case of vehicles with higher ground clearance and higher floor, such as Vans and SUVs, the floor frame may be simplified to be a grid of linear beams, folded or rolled, as in case of Fig. 4.90. Floor specifications are referred to two main goals: absence of resonances coincident with compartment cavity modes (meaning for most cars no resonances in the range 50 to 70 Hz and 120 to 140 Hz) and resistance to dynamic seat and belt loads. To face the first problem, floor panel and frames are designed by computer analysis so as to avoid resonances coinciding with the body frame; in any case, the amplitude of panels vibrations should be attenuated using heavy damping patches (made by viscoelastic bituminous or polymeric materials, bonded or melted to the floor, with a mass of at least 3 kg/m?). Fig. 4.91-A are shows floor and tunnel regions usually covered by thermofused elastomers; Fig. 4.91-B shows areas and dimensions of damping patches opti- mized by computer analysis to reduce area covered and hence weight/cost while maintaining effective vibration attenuation. Fig 4.91-C compares the damping performance of the conventional with the computer- optimized solution for dash lower panel and floor panels. For safety belt and seat fittings resistance tests, related to front crash be- havior, it is recommended to bolt all anchorages through local plates to cross members and avoid traction stresses on plate spot welds. 4.7 Compartment Floor 18 Fig. 4.90. Floor of Fiat Multipla: PA) front floor; PP) rear floor; LL) sills; LC) logitudinal floor rails; LP) rear floor rails; R) floor rails reinforcements; T) main seats cross members; t) ancillary cross members; TP) rear cross member. 0 100 [2] 200 300 Fig. 4.91. In A, traditional damping patches on floor panels (a,c) and tunnel (b,d) are shown. In B, an example of computer optimized design (patches ef) resulting in weight and vibration reduction. In C picture, comparison of panels mean square speed V2,¡in case of traditional (Base) and optimized (Opt) patches on floor and dash panel. 18 4 Body Work Fig. 4.93. Examples of off-road (A) vehicle with chassis and SUV (B) with integrated underbody frame. Fig. 4.94. Examples of off-road chassis (A) and SUV underbody frame (B). cars, with ground clearance enough to tackle small sterams and every kind of track, while using the simplest manufacturing technologies possible in order to maintain production costs low and facilitate repair even without official service. For example, an Off-road vehicle can be manufactured with aluminum riveted sheets, because aluminum can be easily embossed, cut, part replaced and hand riveted without the risk of corrosion. 49 Spider, Coupe and Cabrio The bodies of cars in this class are the most sophisticated in terms of styling while also requiring a characteristic design for the following reasons. Coupes can be classified as sports car with high speed and road holding tar- gets; therefore their static and dynamic stiffness, principally torsional stiffness, represent the primary goal for body design. Usually this target is easier to achieve 4.9 Spider, Coupe and Cabrio 18 than with a 5 door sedan since coupes have only two side doors, a relatively small liftgate and above all a rear seat pan. Today, a coupe body torsional stiffness is usually higher than 1,000,000 Nm/rad, reaching or even exceeding 1,500,000 Nm/rad. However the static stiff- ness is often not enough: ideally, the design should result in first global torsion mode of the complete car over 40 or better 45 Hz, in order to avoid coupling with suspension main resonance, which usually falls in the range 1520 Hz. Also, coupe aerodynamics must be accurately tuned, primarily in terms of guaranteeing road holding or road contact and therefore lift properties. In most cases coupes are provided with spoilers or wings, resulting in a zero lift rear axle condition or sometimes down force. The front bumper and underbody are designed to minimize front lift, providing an advantage also in terms of induced drag. Spiders (2 places) and Cabrios (4-5 places) are usually considered to be free- time, more environment friendly products and therefore they do not aim to provide the same levels of high speed performance as coupes. Due to the fact that the upper frame is missing, some typical problems of these cars are: + lack of torsional stiffness, +* lack of protection in the case of roll-over, + high structural unevenness and risk of fatigue local weakness, + lack of sealing against water and air ingress (both with soft and hard top). The lack of stiffness is a natural consequence of the spider concept, more similar to a platform than to a sedan or coupe. It can be observed that the torsional stiffness of a vehicle underbody, without upper body frame, could reach 200,000 Nm/rad, just 20 30 % of the stiffness of current production mid sized sedans. Some conventional measures to increase the platform torsional stiffness are, for instance, to increase the sills section and/or rocker panels and sills thickness, if not detrimental to the ergonomics of passengers access. However in this way itis not possible usually to increase the stiffness by more than 20.25 %. In most cases it is possible to connect the sills rear end with a very stiff cross member or a boxed rear seat pan: the aim is to unite five subassemblies (two sills, a rear cross structure, underbody and firewall) to result in a stiffened box missing just one wall. Further stiffening can be obtained using a large cowl top with small open- ings connected to the front suspension strut towers and by a tunnel strongly embedded in the firewall and rear seat pan. Finally, boxed frames can be added between the sills and rear suspension strut towers. All these features or at least most of them, shown in Fig. 4.95, have resulted in levels of torsional stiffness which are close to sedans (400,000 to 600,000 Nm/rad) and a first torsional vibration mode sufficiently far from the suspension main 18 4 Body Work Fig. 4.95. Examples of recent spider frames. resonance. However such results have not been achieved by all spiders in the market and never yet by cabrios (due to the cabrio wheelbase and side door opening which is much longer than in spiders). Regarding roll over protection, which is always more difficult to guarantee for cabrios than for spiders, additional features are commonly added in two regions: windshield and seat back. The windshield frame is reinforced by strengthening A pillars, both with a stronger embedding of the lower pillar end in the body side and by inserting a tubular or hydroformed reinforcement of high strength steel between the body side outer and inner (Fig. 4.96-A). The roll bar (Fig. 4.96-B) is an additional frame, usually fitted to the body frame, in steel, aluminum or carbon fiber, specified according to design invention and engineering analysis. Regarding structural unevenness, one precaution is to specify all rather than just a limited number of the features suggested in order to enable the higher stiffness to be shared across different nodes to avoid local overstressing, bear- ing in mind that F.E.M. analysis represents the only reliable tool for a valid performance forecast. Regarding water and air tightness, related issues shall be examined in weather strip chapter. 4.9.1 Spider and Cabrio Soft Top Traditionally spiders and cabrios feature with soft tops which are foldable and stowable behind passengers seats, according to the following two solutions: 4.9 Spider, Coupe and Cabrio 19 Fig. 4.98. Main soft roof cloth parts. A, B, C, D: nodes; A1) first bow; AN) last but one bow; EL) side elastic strip; CTL) tensioning side wire; MT) pretensioning spring; AP) side rear run fitting; AB) loop ring; RL) back glass radius (> 3500 mm); GF) side front weather strip; GC) side central weather strip; GP) side rear weather strip; GU) last bow gasket. Through the summation of soft top and body tolerances, it may be possible to find a fitting difference of +5 mm at the top mounting station. Regarding obstructions of kinematic devices, a fewer number of rods is gen- erally preferable. Gasket installation shall be examined in the specific chapter. Soft top specifications With reference to the conventional body tests in open top conditions, additional delivery tests with closed top for spider and convertibles are: + bench durability test; * squeak and rattles bench test after thermal cycling; + noise road test; * aerodynamic noise and rustle in wind tunnel; 19 4 Body Work TE 25 Fig. 4.99. Schematic view of detachable back glass - soft top junction. LU) back glass; TL) top cloth with female velero %* strip; TE) outer layer with male velero %F strip; CI) sew and bond seam; C) sewing stitch. + dynamic top cloth deformation in wind tunnel; less than 60 mm is generally acceptable; + water leakage in high pressure spray water chamber; * dust or powder ingress; + door closing and opening durability with side window glass operation; * system misuse test; unusual or incorrect operation by customers. Subassembly delivery tests on soft top and its components are: + salt fog corrosion resistance of coated top frame; + kinematic system durability through raising and lowering tests for at least 8,000 cycles; * static traction top cloth test; physical and chemical tests on cloth layers and weather strips, to verify re- sistance to hydrocarbons, chemicals, abrasion, UV radiation and thermal cy- cling; * current absorption, in the case of electromechanical top; + durability test of motoring system, both electromechanical or hydraulic. 4.9 Spider, Coupe and Cabrio 19 Fig. 4.100. Example of locks for manual operated soft top with rear deck. A) front hook; B) front centering pin; C): rear deck to top latch; D) rear deck to body latch; RE) adjusting stroke: 248m; SE) latch; SC): striker; PE) overmoulded (e.g by Hytrel Janti-noise pivot. 4.9.2 Convertible Top For many years winter hard-tops in fiberglass or metal sheet with back glass were provided as optionals to replace the soft top when needed. However, in recent years, many new models have been put in production with a retractable hard top (comprising a number of retractable segments) operated by motorized kinematics; in this way, a spider can be effectively changed into a coupe in just a few seconds (Fig. 4.101). Although these solutions are much more expensive than traditional soft top, when the top is closed, acoustic insulation and water tightening are much more effective; moreover full or partial glass tops are commonly available which enable a panoramic view of the surroundings. However one issue with these solutions relates to the volume of the folded parts, commonly obstructing 80% of already limited luggage compartment. An interesting solution has been designed for Ferrari Superamerica (2006) (Fig. 4.101-B) which features a glass roof with carbon fiber frame rotating while opening and stopping when laid down above the decklid; in this way, a much lower obstruction of luggage compartment is achieved. The contribution of hard tops to the body stiffness is relevant and therefore their stress condition is specified. On the other hand, the same car, in spider configuration, should offer ride and road-holding performance not too different from the coupe configuration. 19 4 Body Work Fig. 4.103. Stamped sheet main sub-assemblies of a truck cabin: A) floor assembly; B) back panel assembly; C) windshield frame; D) RH and LH side frame, + The engine compartment cover (central floor panel, as in Fig. 4.103) is unique as well as floor rails, + The windshield frame has standard pillar outer panels, while cross members relate to cabin width. + Cross elements, back panel assy and front cabin assy, are related to width. Different cabin lengths are obtained by adding cross floor panels relating to width and additional rail extension. Moreover, the rear side pillar is replaced by side stiffened panels, and assembled to the door opening and back panel. Chassis The chassis configuration is nominally ladder shaped, with two main longitudinal rails (usually constant cross section for small trucks and variable section for heavy duty semitrailers) and a number of cross members (Fig. 4.108). The chassis rails and cross member can be steel cold or hot rolled or aluminum extruded profiles, welded in the case of tapered section. Rails and cross members can be joined using arc or spot welding, fasteners, bolts, screws. The assembling technology used is often not related just to engineering or design analysis but to the available plant facilities and common practice of the manufacturer. It should 4.10 Commercial Vehicles and Trucks 19 Fig. 4.104. Example of stamped elements for a truck windshield frame: 1) pillar outer; 2) top cowl; 3) windshield header; 4) header reinforcement; 5) header boxing. be borne in mind that arc welding seams, mainly those between aluminum pro- files, must be certified with x-ray images and 100% process parameters control. Trucks specifications The following list summarizes the most common specifications for these vehicles, according to commercial vehicles targets. + Modal analysis of complete body: identification of resonance frequencies and associated torsion, bending and mixed vibration modes. + Chassis acceleration measurement in vehicle mission targeted tracks and fol- lowing frame fatigue test by bench three-axial loading: during durability test- ing, the cabin and suspended masses are missing. + Chassis vertical acceleration in road targeted driving and following excitation in a climatic chamber, together with temperature cycling between -30” C and + 80” C, to verify cabin and trimming durability. + Cabin frame resonances; resulting seats and steering wheel vibrations. Strength and deflection of roof composite panel under snow and concentrated mass load. Stiffness and strength of frame extensions, insert, reinforcement and rear pillar panel under concentrated load. + Durability test of step sides, front fender and fender extension: verification of resistance to concentrated load and insert strength. 19 4 Body Work Fig. 4.105. Sheet stamped elements of a truck cabin side frame: 1) RH outer side frame; 2) RH inner side frame; 3) outer rear pillar; 4) rear pillar boxing; 5) rear pillar reinforcement; 6) RH upper side outer panel. Fig. 4.106. Sheet stamped elements of a cabin RH floor: 1) RH side floor; 2) RH floor reinforcement; 3) RH rail; 4) RH rail extension; 5) RH rail front end brace; 6) RH rail rear end bracket. 4,10 Commercial Vehicles and Trucks 20 400 1340+2 view B Fig. 4.109. Details of a pick-up or commercial cabin shielding frame, to protect the back cabin panel from forward freight motion. most common cause of fatigue cracks on paved tracks; it is therefore necessary to avoid small radii and sharp stiffness change from boxed to open sections. In the other case, due to lack in synergism of cabin and bed, the body stiffness in the interface region is conferred to the chassis frame, usually consisting in longitudinal rails and cross members and therefore the local stress is higher in this region. As concerns the specifications and design criteria, the cabin has the same target as the reference vehicles, while the beds are similar to commercial vehicles and therefore have the following characteristic specifications: + safety and stability of freight: for this purpose, the cabin back includes ad- equate trusses and shielding frames (Fig. 4.109) whereas the bed includes a number of hooks for goods clamping (Fig. 4.110); + safety for other road users: side and rear protection bars are provided for this purpose (Fig. 4.111); + warping and bending resistance of bed side walls and tailgate; + absence of road noise, squeaks and rattles; * resistance to environment-induced corrosion; 20 4 Body Work Fig. 4.110. Details of freight clamping devices in a pick-up bed: G) hooks; P) bed floor; B) frame; SP) bed wall; CA) bed. Fig. 4.111. Examples of side (A) and rear (B) protection for other road users, CA) pick- up bed; AU) chassis; BA) side protection bar; BP) rear protection bar; LB) un- derbody rail; G) ground. 4,10 Commercial Vehicles and Trucks 20 TA (1 CE LO MP ST Pc — 100 Fig. 4.112. Map of bed walls supporting devices and typical section of rail and cross member intersection. SP) side wall; CS) wall lock; CE) wall hinge; LO) platform rail; TD) intermediate cross member; PC) frame; TA) wall stop bott; ST) outer brace for platform fitting; PI) cantilever brace, one side zinc coated (cantilever side); ME) can- tilever, one side zinc coated (interface with rail and brace); LS) upper rail, inner side zinc coated; SE) diaphragm between rails, two sides zinc coated. + resistance to electrochemical or galvanic corrosion, mainly depending on the materials used for underbody, bed and fittings (Fig. 4.112); * resistance to abrasion; + fatigue resistance of rear bed to underbody fittings. 4.10.3 Commercial Vehicles, Vans Vehicles belonging to this family are mid-sized vehicles in the range between cars and trucks, usually featuring a unibody offering performance closer to cars than to trucks. The speed of these vehicles is similar to cars, while the cargo capacity and large side/rear opening dimensions cause an overall stress condition which is much more severe than in the automobile body. Underbody design is usually ladder shaped, with longitudinal rails and cross members welded to the floor; these parts are usually bent or rolled or stamped when required. The upper frame includes a cabin (usually featuring a line of three seats) and a cargo volume, separated from the cabin by a protection panel. The body side is made from a drawn outer panel and inner members stamped or curved as rings in the vertical plane, made from rolled or cut and bent steel sheets. The most critical part of this assembly is the rear end frame, ring shaped and strongly boxed. The roof, commonly stamped in steel, is welded to the body sides with con- ventional automotive tools and stiffened by bows similar to cars. For some high 5 Body Components 5.1 Outer Body Components 5.1.1 Bumpers Before the 1970's, bumpers were usually chrome plated or rolled and formed stainless steel leafs, the main function being aesthetic enrichment and protecting the car body against small impacts (Fig. 5.1). Thanks to the Experimental Safety Vehicles (E.S.V.) (and later Research Safety Vehicle) Program , many studies and considerable research led to the definition of some basic design concepts of relevance such as: + Frontand rear end of vehicles should be able to absorb energy. + The stiffness of body parts committed to energy absorption should increase as the passengers cabin is neared. The properties of traditional bumper leafs are completely opposite to those required, as they collapse in bending, with only low levels of energy absorbed. As a consequence, the soft nose (Fig. 5.2) was born, consisting in: a) an outer flexible plastic shell (thermoset as polyurethane molded element by RIM. - Reaction Injection Molding process or thermoplastic injection molded as polyolephine or polycarbonate or blended thermoplastics); b) a metal support cross-member fitted to body frame through energy absorption devices; c) some polyure-thane or polyolephine foam insert in the space between. L. Morello et al.: The Automotive Body, Vol. 1: Components Design, MES, pp. 207-437. springerlink.com Ye Springer Science + Business Media B.V. 2011 20 5 Body Components AAA Fig. 5.2. Schematic section of high absorption front end: A) flexible skin; B) supporting bar; C) foam insert; D) absorbing/damping device. 5.1 Outer Body Components 21 Fig. 5.4. In most recent front end, as in Porsche Cayenne, the bumper skin is extended to bonnet contour, with aesthetic continuity. It can be concluded that, following a rather complex and on-going process of evolution (Tab. 5.1 - Fig. 5.5), the design of bumpers is once more determined by its aesthetic properties, while the protection function in case of impact, originally less important, is now achieved by specific devices hidden under the bumper itself. Consequently, the bumper of today is really an integrated body part (even made of a different material, despite being body colored), while before the 1970's it was essentially a component added for cosmetic purposes. The mission of the bumper The main bumper tasks include the following: * aesthetics; + overall body protection in parking impact (up to a speed of 4 km/h) or ac- cording to individual State safety rules; + energy absorption and controlled transfer of stress to body frame, when im- pacted at 15 km/h (insurance impact test ); * aerodynamics; + friendly contact (or absence of injury) in case of pedestrian's impact; + support of winches or tow hooks for off-road vehicles (fig. 5.6). Today most production vehicles have plastic bumpers painted in the same color as the body; in the case of special mission (e.g. use of bumper to tow or lift heavy masses as in case of off-road vehicles), metal painted or plastic coated 21 5 Body Components Fig. 5.5. Examples showing the evolution of bumper appearance (see A,B,C,D in table 5.1). 5.1 Outer Body Components 21 Fig. 5.6. Example of off-road steel bumper with integrated winch housing. bars are in use, strongly connected to body; cheap vehicles, to reduce cost and chromatic body matching troubles, feature a different color for body and bumper (in this case bumpers are mass colored, usually grey or black). Regarding crash tests required for the mandatory homologation, different reg- ulations are applied in different countries; for instance, for years Canada and U.S. have required car components protection at a higher speed than in Europe, resulting in different designs of bars and absorption devices. Fig. 5.12 shows a view of most common solutions in Europe, Japan and Korea. Aesthetics (Style) Bumper shape, gaps with respect to adjacent parts (lamps, fenders, radiator grille, bonnet), color, roughness (skin grain) are properties relevant to the aes- thetics of the vehicle and are therefore modeled and specified by the styling centre. Skin radius must comply with the regulation limit of > 2.5 mm (in some areas > 5 mm) for all surface points that can be contacted by a 100 mm sphere. Even the plastic blend used to mold the bumper should have a mass color not so different from the final bumper painted color in order to keep any abrasion or surface marking less evident. In some cases, only zones less exposed to damage are painted, while most zones with high risk of contact such as bumper fascia are left grey or black. Protection in low speed crash (parking) International regulations are explained in depth in Volume IL Here it is appropriate to recall that, for European and Arabian State Rules (ECE 42), bumpers, both front and rear, must enable permanent functional damage to the vehicle to be avoided when impacted by a pendulum of mass 21 5 Body Components Elk] 16 ) 144 dm] 12 - 107 84 6, 4) 2), 0 7 7 7 7 7 7 r 1.4 15 16 17 18 1.9 20 21 S [mm] Fig. 5.7. Parametric properties of a typical crash-box. E: absorbed energy; d: total crush; s: wall thickness. already mentioned in the chapter on Bodywork. The properties of these absorbers are controlled deformation and high efficiency, limited maximum load (so rails do not deform permanently) and minimized body crush, in order to prevent damage to vital components such as the radiator and engine pulleys. Their actual goal is to reduce repair cost rather than avoiding damage altogether mostly by avoiding removal and remounting of the power train. In these solutions, bumpers share their function with a subsystem which is not crushable at impact speeds slightly exceeding 4 km/h which becomes a filter capable of absorbing energy in the speed range 5:15 km/h. This subsystem is usually integrated in a front assembly module or a rear assembly module called respectively front end and rear end (Fig. 5.8). Aerodynamics Bumpers perform two main aerodynamic tasks: the first, as a body shape part influencing both drag and lift, the second, as flow conveyors or extractors both for the engine compartment and underbody. In detail, the front bumper usually features a spoiler or dam, extending to the underbody (Fig. 5.9), having the purpose of accelerating the underbody air flow. In this way, negative pressure variation is generated, thereby decreasing front lift and, at the same time, the air suction from the engine compartment is facilitated. This process causes a change in the lift contribution of the different 5.1 Outer Body Components 21 6 ES Fig. 5.8. Split view of the components usually assembled in a front-end module: 1) frame; 2) upper cross member; 3) radiators; 4) bonnet lock; 5) air flow channel; 6) fans; 7) central brace; 8) bumper bar; 9) projectors; 10) bumper; 11) foam energy absorbers; 12) dam; 13) fascia. Fig. 5.9. Examples of front dam and fastenings. 21 5 Body Components 0.4 ns A Cz e Cx 2. Cx A 0.3 0.2 + 7 7 r 1 0.05 0.1 0.15 0.2 0.25 — Czant Fig. 5.10. Influence of front lift coefficient Czane On overall lift coefficient C, and on drag coefficient C, in relation to different spoilers tested on the same car. body zones, as it usually increases rear lift, resulting in a downward pitching moment . Nevertheless, the induced rear lift is usually less than the reduction in front lift, the combination being lower overall lift. The additional result is lower induced drag; therefore the advantage of an optimized front spoiler is reduced lift as well as drag coefficient. Fig. 5.10 provides an example of the explained effect: the reported values have been measured with different front spoilers mounted on the same vehicle without other modifications. Moreover, the bumper hump at the pendulum impact position, together with the front spoiler, determines the effectiveness of the radiator air intake. Accord- ing to the aerodynamics of today's cars, the air intake is usually positioned in the lower bumper area, because here the peak aerodynamic pressure is exhibited, while over the hump and below the spoiler a negative pressure can often occur. The dimensions and position of the air intake should be investigated in detail for a new model during the pre-engineering stage; later in the process itis much more difficult to change the style model while more powerful engine and air con- ditioning systems could require larger radiators that may not be compatible with the engine compartment space available and the car weight target. Therefore, an adequate virtual and experimental analysis is needed, both for air intake and ejection. 5.1 Outer Body Components 22 50 m% ante 1995 m% post 1995 A B € D Fig. 5.12. Market analysis of different bumper types. A: self supporting shell; B: with metal supporting leaf, C: with plastic boxing; D: with foam insert and metal supporting bar. 22 5 Body Components 20 al] HA A) 40 80 120 160 200 1D [um] Fig. 5.13. Influence of embossed grain depth 1D on bumper extraction requirements: A is the draft angle required. temperature (<-30” C), a property which is clearly a priority for the bumpers of cars sold in very cold countries (Fig. 5.14). With some exceptions, today polyolefin are used for most applications, even of large size, due to lower cost and substantially adequate properties in each of the salient tests (impact, chemical and aging resistance, moldability, paintability). A persistant problem for all thermoplastics is the presence of unevenness or waviness on flat surfaces, due to cooling after moulding and aging under load (creep). These waves are unacceptable for class A parts and sometimes can be reduced by using ribs which must be very thin (otherwise sink marks can arise); otherwise support metal blades can be snapped on to the bumper. High pressure injection is the most effective process, both in terms of the material properties and production cost (when production volume is consistent with higher die and press investments). For small production volumes, low pressure injection of thermoplastics can be used or Reaction Injection Molding of thermoset (mostly polyurethane) plas- tics; in the latter case, the blend can be stiffened by adding short glass fibers (R.R.LM. process). Polyester and vinylester thermoset resins are not recommended due to their higher stiffness and lower resiliency. Moreover all thermoset materials face the problem of recycling. Regarding energy absorbers for low speed crash, foam inserts (polyurethane or polypropylene or polystyrene) are used as well as plastic honeycomb injected 5.1 Outer Body Components 22 FLEXURAL| ELONG. |BRITTLE |THERMAL | oc yyy, | SOFTEN. DENSITY | MODULUS |AT BREAK | TEMPER. |EXP. COEF. TEMPER. (Mpa) 05 (e) xto.spC | HARDNESS eo) TPO 0,97 1100 35 -40 11 65 80 PC/PBT| 1,22 1700 120 -40 8 120 145/180 PA/PPO| 1,07 2000 80 >-40 9,5 >100 | 130-175 RIM 1,05 200 250 -45 18 20 - RRIM | 1,13 500 100 -30 5 20 pa Fig. 5.14. Typical bumper materials properties in the 19905. polypropylene or polystyrene blocks, full of thin ribs; the selection is made ac- cording to cost and weight (Fig. 5.15). Design criteria, materials and technologies As explained above, the design specifications are related to: + Impact, for contact with pendulum or barrier and fitting areas. + Dynamic stress, where openings and notches can weaken the bumper. Thermal and mechanical stress in proximity of hot parts (eg. engine, exhaust system). * Abrasion or break for ground contact, against ramps or platforms. Size is related to material choice (in Fig. 5.16, materials used recently are shown): for polyolefin (polypropylene, polypropylene with EPDM) the recom- mended outer skin is 33.5 mm, if a support metal blade exists; 3.5 mm, ifa plastic boxing is welded to the outer skin; 4.0 mm if the bumper is self support- ing. Plastic boxing requires at least 3 mm thickness. The highest thickness is needed with low flexural modulus polypropylene (550-800 Mpa), the lowest if high modulus is used (up to 1800 Mpa). In Fig. 5.14 the mechanical and thermal properties of frequently used materials are compared. For energy absorbers, polyurethane or polypropylene foam with a density of 4050 km/m' are commonly used.