Materials Selection-Concepts of Structure of Materials-Lecture Handout, Exercises of Structures and Materials

Download Materials Selection-Concepts of Structure of Materials-Lecture Handout and more Exercises Structures and Materials in PDF only on Docsity! Unified Engineering Lecture M21 12/2/2003 Materials Selection docsity.com Objective • Aim to provide coherent overview of material selection – Materials (and structural configurations and processes) should be selected for applications based on measurable criteria docsity.com References • Material Selection in Mechanical Design, M.F Ashby, Pergamon Press, Oxford, 1992 • Ashby and Jones, Engineering Materials I, Chapter 6 docsity.com Materials for Mechanical Elements - performance indices • Design of a structural element is specified by three parameters, or groups of parameters (performance indices): – Functional requirements (F), Geometry (G) and Material Properties (M) • We can quantify the interdependence if we can specify performance, p, as a function of F, G and M: » p = f(F, G, M) • We can simplify further if the three groups of parameters are separable, i.e: » p = f1(F) . f2(G) . f3(M) docsity.com Ex: Lightweight stiff rod - tensile load P P AL Material, modulus E, density r - note these are a property of the material, and cannot be independently selected • Mass of rod given by m = rAL AE • Stiffness of rod, given by k = P = d L • Combining, by eliminating free variable, A: r r m = kL2 = k ⋅L2 ⋅ E E F G M Choose material with low r/E ratio!!! docsity.com MODULUS - DENSITY RATIOS OF SOME MEMS MATERIALS Material Density, r, Kg/m3 Modulus, E, GPa E/ r GN/kg-m Silicon 2330 165 72 Silicon Oxide 2200 73 36 Silicon Nitride 3300 304 92 Nickel 8900 207 23 Aluminum 2710 69 25 Aluminum Oxide 3970 393 99 Silicon Carbide 3300 430 130 Diamond 3510 1035 295 Silicon performs well, diamond, SiC and SiN significantly better docsity.com DEFLECTION OF CIRCULAR PLATE 0.67 mga2 2d = m = pa tr p Et 3 2a δ t mg The elastic deflection of a telescope mirror (shown as a flat disc), under its own weight. (Adapted from Ashby.) Ê0.67/ gˆ1 2 pa 4 Á Ê r 3 ̂ 1 2 r 3 m = Á ˜ M = Ë d ¯ ÁË E ˜̃ E¯ docsity.com Example 3 - Telescope Mirror 3 r • Choose materials with high M = E The distortion of the mirror under its own weight can be corrected by applying forces to the back surface. (Adapted from Ashby.) docsity.com STRENGTH-DENSITY MAP 10000; T 2. STRENGTH-DENSITY METAL AND POLYMERS: YIELD STRENGTH ENGINEERING ] CERAMICS AND GLASSES: COMPRESSIVE STRENGTH. ALLOYS 4 ELASTOMERS : TENSILE TEAR STRENGTH J COMPOSITES: TENSILE FAILURE MEA:88-3) 1000 ENGINEERING COMPOSITES 5s (Ct ao = 100b = f£ AR 6b PanaLLp = tf ffm Ef a Py roeres = “eS Ww 0 CERAMICS = NGINEERING “oF n POLYMERS ae oy { - GUIDE LINES oot FOR MINIMUM | 7 POLYMERS ee x WEIGHT DESIGN] J FOAMS rd 1 Z 3 4 2 - 7 ae j L at: J J | p ote” hy i a=C at=c B B 01 L cont bt 1 2 L Ol 03 1 3 10 30 DENSITY p_ (Mg/m3) Ashby docsity.com CTE-THERMAL CONDUCTIVITY oo yt at 4 10. EXPANSION- CONDUCTIVITY | 2, wm 10° 10° J CONTOURS © Ye (W/m) “No wv vy 7 MFa68-91] s , 4 “ re “ = | if Be “ | ce LARGE THERMAL “ Z ee 2 1000E—|stRAIN MISMATCH 7 E z / ce E 7 ¢ 10 E NGINE ERING c L “BOlYMERS f #: 4 POLYMERS s Fs Pol CERAMICS “ENGINEERING LINEAR EXPANSION COEFFICIENT & (10 0.1 poe iii fm _DIAMOND ii SMALL THERMAL | ] STRAIN MISMATCH 0.1 THERMAL CONDUCTIVITY A W/mK) 1000 Ashby docsity.com CTE-MODULUS MAP Determines thermal stress, thermal buckling limits for thin tethers, also Feasibility of thermal actuation Ashby docsity.com