CH560 Engineering Material Selection PDF

Engineering Material Selection CH 560 Prof. Yehia M. Youssef Copyright © YM Youssef, 24-Oct-24 CH560 - Engineering 1 Material Selection  Engin...

Engineering Material Selection CH 560 Prof. Yehia M. Youssef Copyright © YM Youssef, 24-Oct-24 CH560 - Engineering 1 Material Selection  Engineering Materials & Their Properties Metals Polymers Ceramics Composites Elastomers Glasses The menu of engineering materials. Copyright © YM Youssef, 24-Oct-24 CH560 - Engineering 2 Material Selection  Engineering Materials & Their Properties Engineering Materials fall into six broad classes as shown in the figure: –Metals –Polymers –Elastomers –Ceramics –Glasses –Composites which are combinations of two or more of the above Members of each class have similar properties, similar processing routes and often they can be be used for similar applications. Copyright © YM Youssef, 24-Oct-24 CH560 - Engineering 3 Material Selection  Engineering Materials & Their Properties Metals – have relatively high moduli – their good ductility allows them to be formed by deformation processes, e.g. rolling, forging and extrusion. – even high strength metals show some ductility (e.g. spring steel ~ 2%) and they generally fracture in a ductile manner. – they are prone to fatigue. – of all classes of materials, they are the least resistant to corrosion Copyright © YM Youssef, 24-Oct-24 CH560 - Engineering 4 Material Selection  Engineering Materials & Their Properties Ceramics and glasses – have high moduli i.e. stiffness. – they are brittle. – they are hard and abrasion resistant (hence they are used in bearings and cutting tools). – they retain their strength at high temperature. – they are corrosion resistant. – because they have no ductility, they have a low tolerance for stress concentration (such as holes or cracks) and for high contact stresses (e.g. at clamping points). Copyright © YM Youssef, 24-Oct-24 CH560 - Engineering 5 Material Selection  Engineering Materials & Their Properties – Plastic deformation can occur in more ductile materials allowing accommodation of the stress concentration by redistributing loads more evenly. – Ceramics, like other brittle materials, display a wide scatter in strength and, as a consequence, design with ceramics is quite difficult. Copyright © YM Youssef, 24-Oct-24 CH560 - Engineering 6 Material Selection  Engineering Materials & Their Properties Polymers and Elastomers – they have low moduli (50 times less than metals). – they are strong. – elastic deflections can be large. – they creep even at room temperature (a polymer under load can, in time, develop a permanent set). – their properties show a large temperature dependence e.g. At 20°C tough and flexible At 4 °C brittle At 100°C can creep rapidly – their strength is inadequate above 200°C. Copyright © YM Youssef, 24-Oct-24 CH560 - Engineering 7 Material Selection  Engineering Materials & Their Properties Their advantages – combinations of their properties can be good e.g. strength per unit weight – they are easy to shape – complicated parts performing several functions can be moulded from a polymer in a single operation – their large elastic deflections allow the design of polymer components which snap together, making assembly fast and cheap – by accurately sizing the mould, no finishing is required – polymers are corrosion resistant – they have low friction coefficient – they are being increasingly exploited Copyright © YM Youssef, 24-Oct-24 CH560 - Engineering 8 Material Selection  Engineering Materials & Their Properties Composites – combine the attractive properties of the other classes of materials while avoiding their drawbacks. – widely used types include polymer matrix composites, e.g. epoxy or polyester reinforced with fibres of glass, carbon or Kevlar. – at room temperature, their performance can be outstanding. – for polymer matrix composites, they cannot be used above 250°C because the polymer matrix softens. – components made from composites are expensive and they are difficult to form and join. – the engineer will use composites when the increased performance justifies the higher cost. Copyright © YM Youssef, 24-Oct-24 CH560 - Engineering 9 Material Selection  Engineering Materials & Their Properties Metals Fe, Al, Ni, Cu, Ti, Pb, etc Strong, Stiff, Dense, Tough, High Energy to Produce Polymers Ceramics PE, PAN, PP, etc Co/WC C, Al2O3, SiC, B4C Easily Formed, Soft Kevlar/Al Cutting Tool Hard, High Melting But Fibres are Stiff and Arall Stiff Strong Epoxy/Carbon Intermediate Density Low Density Can be Low Energy Composites Low Energy to Produce to Produce Polyester Al/SiO2 Car Tyre Glass Glass Rubber GRP Inorganic Elastomers Polymeric Toughened High Extensions Metallic Polystyrene The five generic classes of engineering materials and their combinations in composite materials Copyright © YM Youssef, 24-Oct-24 CH560 - Engineering 10 Material Selection  Definitions of Materials Properties Density (usual units Mg/m3) is the weight per unit volume. We measure density by weighing in air and in a fluid of known density. Elastic Modulus (units GPa or GN/m2) is given by the elastic part of the stress-strain curve. – Young’s modulus, E, describes tension or compression – Shear modulus, G, describes shear loading – Bulk modulus, K, describes the effect of hydrostatic pressure – Poisson’s ratio, n, is dimensionless and is the negative of the ratio of the lateral strain to the axial strain  2 in axial 1 loading. – The slopes of stress-strain curves give the moduli but more accurate values are measured dynamically. Copyright © YM Youssef, 24-Oct-24 CH560 - Engineering 11 Material Selection  Definitions of Materials Properties Commonly 1 3 n= G E KE 3 8 3G E E E= G= K= 1+ G 2 (1 + n ) 3 (1 − 2n ) 3K Copyright © YM Youssef, 24-Oct-24 CH560 - Engineering 12 Material Selection  Definitions of Materials Properties Strength, sf (units MPa or MN/m2) For metals, sf is taken as the 0.2% offset yield strength sy. For polymers, sf is defined as the stress sy at which the stress-strain curve becomes markedly non linear (typically at 1% strain). Polymers are slightly stronger (by ~20% ) in compression than tension. For ceramics and glasses, strength strongly depends upon the mode of loading. In tension, strength is the fracture strength sft whereas in compression, it means the crushing strength sf which is much larger, such that c s  15s c f t f Copyright © YM Youssef, 24-Oct-24 CH560 - Engineering 13 Material Selection  Definitions of Materials Properties For composites, the strength is often taken as the 0.5% offset linear elastic behaviour. Composites containing fibres are weaker by up to 30% in compression than in tension because the fibres buckle. Here, the strength of composites is taken as the tensile strength. Copyright © YM Youssef, 24-Oct-24 CH560 - Engineering 14 Material Selection  Definitions of Materials Properties Design Limiting Material Properties Class Property Symbol and Units General Relative Cost CR (--) Density  (Mg/m3) Mechanical Elastic moduli E, G, K (GPa) Strength (yield, ultimate, fracture) s (MPa) Toughness Gc (kJ/m2) Fracture toughness Kc (MPam1/2) Damping capacity  (--) Fatigue ratio f (--) Thermal Thermal conductivity  (W/m K) Thermal diffusivity a (m2/s) Specific heat Cp (J/kg K) Melting point Tm (K) Copyright © YM Youssef, 24-Oct-24 CH560 - Engineering 15 Material Selection  Definitions of Materials Properties Glass temperature Tg (K) Thermal expansion coefficient  (K-1) Thermal shock resistance T (K) Creep resistance --- (--) Wear Archard wear constant KA (MPa-1) Corrosion/ Corrosion rate -- (--) Oxidation (parabolic rate constant) Kp (m2/s) Copyright © YM Youssef, 24-Oct-24 CH560 - Engineering 16 Material Selection  Definitions of Materials Properties For multiaxial loading: Metals – Von Mises yield function works well ( s1 − s2 ) + ( s2 − s3 ) + ( s3 − s1 ) 2 2 2 = 2s 2 f Polymers – the yield function includes the effect of pressure P 2   P  ( s1 − s2 ) + ( s2 − s3 ) + ( s3 − s1 ) 2 2 2 = 2s 1 − 2 f   sf  where  is a numerical coefficient which characterises the pressure dependence of flow strength, and 1 P = − ( s1 + s2 + s3 ) where s1, s2, s3, are the 3 principal stresses, positive when tensile. Copyright © YM Youssef, 24-Oct-24 CH560 - Engineering 17 Material Selection  Definitions of Materials Properties When a material is difficult to grip (e.g. ceramics), then the strength is often measured in bending. The modulus of rupture (MOR), (units MPa or MN/m2) is the maximum surface stress in a bent beam at the instant of failure. b F F, d Ff t X l 3 Ffl σ max = 2 = MOR 2 bt d Copyright © YM Youssef, 24-Oct-24 CH560 - Engineering 18 Material Selection  Definitions of Materials Properties Ultimate tensile strength su (units MPa) For brittle solids e.g. ceramics, glasses su is equal to the tensile failure stress. For metals, ductile polymers and composites su is larger than strength sf by a factor of 1.1 → 3 due to work hardening (or in composites) load transfer to the reinforcement. Hardness H in MPa is measured by pressing a pointed diamond or hardened steel ball into the surface of a material. Hardness is defined as the indenter force divided by the projected area of the indent – and it is related to strength by H ~ 3 sf. Copyright © YM Youssef, 24-Oct-24 CH560 - Engineering 19 Material Selection  Definitions of Materials Properties Hardness H in MPa is measured by pressing a pointed diamond or hardened steel ball into the surface of a material. Hardness is defined as the indenter force divided by the projected area of the indent – and it is related to strength by H ~ 3 sf. Large hardness means: - resistance to plastic deformation or cracking in compression. - better wear properties. apply known force measure size e.g., of indent after 10 mm sphere removing load Smaller indents D d mean larger hardness. most brasses easy to machine cutting nitrided plastics Al alloys steels file hard tools steels diamond increasing hardness Copyright © YM Youssef, 24-Oct-24 CH560 - Engineering 20 Material Selection  Definitions of Materials Properties Copyright © YM Youssef, 24-Oct-24 CH560 - Engineering 21 Material Selection  Definitions of Materials Properties Toughness Gc (in kJ/m2) and Fracture Toughness K (in MPam1/2 or MN/m3/2) measure the resistance of the material to the propagation of a crack. The fracture toughness is measured by loading a specimen containing a crack of length 2c. The tensile stress sc is measured at which the crack propagates and Kc is then calculated from: K c = Yσc πc and the toughness: 2 K Gc = c E (1 + ν ) where Y is the compliance which depends on specimen and crack geometries, E is Young’s modulus and n is Poisson’s ratio Copyright © YM Youssef, 24-Oct-24 CH560 - Engineering 22 Material Selection  Definitions of Materials Properties Although K and G are adequate for brittle solids, excessive plasticity can occur at the crack tip in more ductile materials, requiring more sophisticated analysis (based on J or COD) s sc 2c X K Ic = σc πc  Copyright © YM Youssef, 24-Oct-24 CH560 - Engineering 23 Material Selection  Definitions of Materials Properties The loss coefficient  (dimensionless) measures the degree to which a material dissipates vibrational energy. When a material is loaded elastically to a stress s, it stores elastic energy per unit volume: σ max 2 σ U=  0 1 σ  dε = 2  E If it is loaded and then unloaded, it dissipates energy: The loss coefficient is: ΔU =  σ  dε ΔU η= 2 π U A measure of damping is given by the specific damping capacity ΔU D= U Copyright © YM Youssef, 24-Oct-24 CH560 - Engineering 24 Material Selection  Definitions of Materials Properties The loss coefficient  measures the fractional energy dissipated in a stress-strain cycle. F F A0 σ= A0 U U l δl ε= l ΔU η= 2πU Copyright © YM Youssef, 24-Oct-24 CH560 - Engineering 25 Material Selection  Definitions of Materials Properties F σ= A0 su METALS X F A0 sy l σ Slope, E = ε 0.2% δl ε= l Copyright © YM Youssef, 24-Oct-24 CH560 - Engineering 26 Material Selection  Definitions of Materials Properties F σ= A0 σcf CERAMICS F A0 l σft Tension X δl ε= l Copyright © YM Youssef, 24-Oct-24 CH560 - Engineering 27 Material Selection  Definitions of Materials Properties POLYMERS Copyright © YM Youssef, 24-Oct-24 CH560 - Engineering 28 Material Selection  Definitions of Materials Properties Fatigue – cyclic loading not only dissipated energy but it also causes crack initiation and growth and eventually fatigue failure. Some materials e.g. mild steel, have a fatigue limit, a stress amplitude below which fatigue failure does not occur. Here, we use the fatigue ratio, f (dimensionless) which is the ratio of the fatigue limit to the yield strength sy. Copyright © YM Youssef, 24-Oct-24 CH560 - Engineering 29 Material Selection  Definitions of Materials Properties Fatigue = failure under cyclic stress. specimen compression on top Adapted from Fig. 8.18, Callister 7e. (Fig. 8.18 is motor from Materials Science in bearing bearing counter Engineering, 4/E by Carl. A. Keyser, Pearson Education, flex coupling Inc., Upper Saddle River, NJ.) tension on bottom Stress varies with time. s s max -- key parameters are S, sm, and S frequency sm s min time Key points: Fatigue... --can cause part failure, even though smax < sc. --causes ~ 90% of mechanical engineering failures. Copyright © YM Youssef, 24-Oct-24 CH560 - Engineering 30 Material Selection  Definitions of Materials Properties Design for Fatigue S = stress amplitude case for unsafe steel (typ.) Fatigue limit, Sfat: --no fatigue if S < Sfat Sfat safe Adapted from Fig. 8.19(a), Callister 7e. 10 3 10 5 10 7 10 9 N = Cycles to failure Sometimes, the fatigue limit is zero! S = stress amplitude case for unsafe Al (typ.) safe Adapted from Fig. 8.19(b), Callister 7e. 10 3 10 5 10 7 10 9 N = Cycles to failure Copyright © YM Youssef, 24-Oct-24 CH560 - Engineering 31 Material Selection  Definitions of Materials Properties Design for Creep Sample deformation at a constant stress (s) vs. time s s,  0 t Primary Creep: slope (creep rate) decreases with time. Secondary Creep: steady-state i.e., constant slope. Adapted from Fig. 8.28, Callister 7e. Tertiary Creep: slope (creep rate) increases with time, i.e. acceleration of rate. Copyright © YM Youssef, 24-Oct-24 CH560 - Engineering 32 Material Selection