Fundamentals of Fracture Mechanics: Understanding Ductile and Brittle Fracture, Schemes and Mind Maps of Materials science

Download Fundamentals of Fracture Mechanics: Understanding Ductile and Brittle Fracture and more Schemes and Mind Maps Materials science in PDF only on Docsity! Material Engineering I! (MEng 2091) Failure Fundamentals Of OE 4 Simple fracture is the separation of a body into two or more pieces in response to an imposed stress that is static (i.e., constant or slowly changing with time) and at temperatures that are low relative to the melting temperature of the material. + The applied stress may be tensile, compressive, shear, or torsional. Steps in fracture: * crack formation * crack propagation Failure 10/09/2021 Brittle vs. Ductile 4 Ductile materials - extensive plastic deformation and energy absorption (“toughness”) before fracture 4 Brittle materials - little plastic deformation and low ene Brittle acture Stress | | | | | | | | | | C Strain Failure 10/09/2021 Brittle vs. Ductile es —H---- — “Vor ty & (a) (b) (c) a) Very ductile, soft metals (e.g. Pb, Au) at room temperature, other metals, polymers, glasses at high temperature. 100% reduction in area. b) Moderately ductile fracture, typical for ductile metals. Failure 10/09/2021 c) Rrittle fractiira cold meatalec caramicc. Ductile Fracture (a) () Fibrous: (d) (e) 45° maximum Shear ettkracc (c) ‘Shear Crack Grows 90° to Applied stress Stages in the cup-and-cone fracture. (a) nitial necking. (b) Small cavity formation. (c) Coalescence of cavities to form a crack. (d) Crack propagation. (e) Final shear fracture at a 452,Aan9|¢oelabve to the tensile direction. Brittle fracture Brittle Fracture cont’d ee A. Transgranular fracture: Fracture cracks pass through grains. Fracture surface have faceted texture because of different orientation of cleavage planes in grains. SEM Micrograph + Grains Path of crack propagation (a) Brittle Fracture cont’d B. Intergranular fracture: Fracture crack propagation is along grain boundaries (grain boundaries are weakened or embrittled by impurities segregation etc.) SEM Micrograph 4 Grain boundaries Path of crack propagation ™—. Engineering Fracture - Avoidsharpcorners. _o, [(a\ — © | K alm a0| 2 : Oo P, So © max M's, 2.5 A r, fillet 2.0 increasing wih radius 0 0.5 1.0 ih <— sharper fillet radius (b) Impact Fracture Testing eee + Is employed to ascertain the fracture characteristics of materials. 4 Two standardized tests, the Charpy and Izod, were designed and are still used to measure the impact energy, sometimes also termed notch toughness. + For both Charpy and Izod, the specimen is in the srzn7 anf a har anf CANTIArN Ane “action 8mm (0.32 in.) Specimen used for Charpy and Izod impact tests. 10 mm (0.39 in.) 10 mm (0.39 in.) .ailure 10/09/2021 Impact Fracture Testing ‘10mm’ (0.39 in.) re The hammer is released from fixed height h and strikes the specimen; the energy expended in_ fracture’ is reflected in the difference between h and the’ swing height h’. Izod Pointer ‘Starting position Hammer Failure 10/09/2021 Ductile-to-Brittle 0 RS (e.g., Cu, Ni) Low-strength (FCC and HCP) metals a iron at T < 914°C) Low-strength steels (BCC) Britt! More Ductile e Impact energy I I I ! ! High-strength materials I I 1 Ductile-to-brittle transition temperature ' Temperature Failure 10/09/2021 iS) rR Ductile-to-Brittle Transition Fatigue 22 4 Fatigue is a form of failure that occurs in structures subjected to dynamic and fluctuating stresses (e.g., bridges, aircraft, and machine components). 4 Under dynamic and fluctuating stresses , it is possible for failure to occur at a stress level considerably lower than the tensile or yield strength for a static load. 4 Estimated to causes 90% of all failures of metallicstructures (bridges, aircraft, machine components, etc.) 4 Fatigue failure is brittle-like (relatively little plastic deformation) - even in normally ductile materials. Thus sudden and catastrophic. Failure 10/09/2021 Fatigue: S-N curves (1) (stress-number of cycles to failure) Fatigue properties of a material (S-N curves) are tested in rotating-bending tests in fatigue testing a i Flexible coupling Counter | Specimen Load Load > Result is commonly plotted as S (stress) vs. N (number of cycles to failure) » Low cycle fatigue: high loads, plastic and elastic deformation. » High cycle fatigue: low loads, elastic deformation (N > 105) Fallure 2009/2024 Fatigue: S—N curves (Il) 5 Fatigue limit (endurance limit) occurs for some materials (Some Fe and Ti allows). In this case, the S—N a Maximum stress amplitude below which the material never fails, no matter how large the number of cycles is. Fatigue limit Stress amplitude, S 103 104 10° 10® 107 108 102 = 1010 Cycles to failure, N (logarithmic scale) 21 Fatigue: S—N curves (Il) In most nonferrous alloys (e.g., aluminum, copper, magnesium), S decreases continuously with N. In this cases the fatigue specified number of cycles (e.g. 107) 4 Fatigue life: Number of cycles to fail at specified stress level Stress amplitude, S Sj--------- Fatigue strength at Ny cycles | | 10° ~—104 Fatigue life 107 , 108 109 ~—100 at stress S$} Cycles to failure, NV (logarithmic scale) Factors that affect fatigue life: environmental effects Eee o Thermal Fatigue. Thermal cycling causes expansion and contraction, hence thermal stress, if component is restrained. Solutions: o eliminate restraint by design a use materials with low thermal expansion coefficients Corrosion fatigue. Chemical reactions induce pits which act as stress raisers. Corrosion also enhances crack propagation. Solutions: 4 decrease corrosiveness of medium, if possible 4 add protective surface coating Failure 10/09/2021 Creep 4 Creep is a time-dependent and permanent constant 10ad ata Nig emperature (> VU. Examples ' Creep testing: Constant load Stages of creep Rupture x 2] | Po < © | Primary at Tertiary mM le >| Ae 3 | 5 |< Secondary | | | oh | Instantaneous deformation ! ! Time, t t 1. Instantaneous deformation, mainly elastic. 2. Primary/transient creep. Slope of strain vs. time decreases with time: work-hardening 3. Secondary/steady-state creep. Rate of straining is constant: balance of work-hardening and recovery. 4. Tertiary. Rapidly accelerating strain rate up to failure: formation of internal cracks, voids, grain boundary