Lecture 26: Mechanical Properties I: Metals & Ceramics Reading assignment: Callister 6.1-6.3, 6.5-6.9, 13.8-13.9 Learning objectives: Understand the difference between elastic and plastic deformation Know how to determine mechanical properties from the results of a tensile test Elastic modulus • Yield strength Tensile strength • Strain to failure Understand how the mechanical properties of ceramics differ from those of ductile metals Lecture 26, summer 2007 Mechanical Properties I: Metals & Ceramics ENGR 145, Chemistry of Materials Case Western Reserve University
Engineering Stress & Strain [Callister 6.2] tension compression from Callister Lecture 26, summer 2007 Mechanical Properties I: Metals & Ceramics ENGR 145, Chemistry of Materials Case Western Reserve University
Elasticity vs. Plasticity [Callister 6.2] Elastic behavior 0 when 0 Reversible deformation — no permanent shape change after load is removed Plastic behavior ≠0 when 0 Some strain remains after load has been removed Lecture 26, summer 2007 Mechanical Properties I: Metals & Ceramics ENGR 145, Chemistry of Materials Case Western Reserve University
Poisson’s Ratio [Callister 6.5] Elastic dimensional change will occur transverse to applied uniaxial load: Poisson’s ratio Lecture 26, summer 2007 Mechanical Properties I: Metals & Ceramics ENGR 145, Chemistry of Materials Case Western Reserve University
Elastic Behavior [Callister 6.3] Stress-strain curve for loading … … is retraced on unloading Linear elastic behavior: Hooke’s law modulus of elasticity (a.k.a. Young’s modulus) Note: not all elastic behavior is linear (see e.g. Callister Figure 6.6) … … but all elastic behavior is reversible Lecture 26, summer 2007 Mechanical Properties I: Metals & Ceramics ENGR 145, Chemistry of Materials Case Western Reserve University Callister Fig. 6.5
Modulus of Elasticity and the Interatomic Potential [Callister 6.3] Recall that energy between atoms depends on their separation Recall also that Minimum in energy zero net force Applying tension or compression raises energy of material compression tension Callister Fig. 2.8b Lecture 26, summer 2007 Mechanical Properties I: Metals & Ceramics ENGR 145, Chemistry of Materials Case Western Reserve University
Modulus of Elasticity and the Interatomic Potential [Callister 6.3] F = d(energy)/dr Modulus of elasticity dF/dr E d2(energy)/dr2 — curvature of interatomic potential near ro Lecture 26, summer 2007 Mechanical Properties I: Metals & Ceramics ENGR 145, Chemistry of Materials Case Western Reserve University Callister Fig. 2.8 a
Modulus of Elasticity and the Interatomic Potential [Callister 6.3] Modulus of elasticity dF/dr high modulus low modulus Lecture 26, summer 2007 Mechanical Properties I: Metals & Ceramics ENGR 145, Chemistry of Materials Case Western Reserve University Callister Fig. 6.7
Mechanical Properties of Metals — Elastic Behavior High modulus strong bonding (high curvature of interatomic potential near ro) (from Callister) Lecture 26, summer 2007 Mechanical Properties I: Metals & Ceramics ENGR 145, Chemistry of Materials Case Western Reserve University
Elastic Modulus: Temperature Dependence [Callister 6.3] E gradually as T Callister Fig. 2.8 Callister Fig 6.8 Lecture 26, summer 2007 Mechanical Properties I: Metals & Ceramics ENGR 145, Chemistry of Materials Case Western Reserve University
Mechanical Properties of Metals [Callister 6.5] Plastic behavior: some strain remains after removal of load Yield strength: stress that will result in a specified residual strain 0.2% yield strength 0.002=0.2%) Lecture 26, summer 2007 Mechanical Properties I: Metals & Ceramics ENGR 145, Chemistry of Materials Case Western Reserve University
Mechanical Properties of Metals [Callister 6.5] Plastic behavior in a tensile test Yielding Tensile strength (M) (a.k.a. ultimate tensile strength) Necking Fracture (F) Callister Fig. 6.11 Lecture 26, summer 2007 Mechanical Properties I: Metals & Ceramics ENGR 145, Chemistry of Materials Case Western Reserve University
Mechanical Properties of Metals [Callister 6.5] Brittle vs. ductile Two measures of ductility: % elongation % area reduction Callister Fig. 6.13 Lecture 26, summer 2007 Mechanical Properties I: Metals & Ceramics ENGR 145, Chemistry of Materials Case Western Reserve University
Mechanical Properties of Metals — Plastic Behavior Lecture 26, summer 2007 Mechanical Properties I: Metals & Ceramics ENGR 145, Chemistry of Materials Case Western Reserve University
Plastic Behavior: Effects of Temperature Callister Fig. 6.14 Lecture 26, summer 2007 Mechanical Properties I: Metals & Ceramics ENGR 145, Chemistry of Materials Case Western Reserve University
Modulus of Resilience [Callister 6.6] Measure of a material’s capacity to absorb mechanical energy elastically Area under stress-strain curve has units of energy per unit volume Approximate this integral with: Callister Fig. 6.15 Lecture 26, summer 2007 Mechanical Properties I: Metals & Ceramics ENGR 145, Chemistry of Materials Case Western Reserve University
Modulus of Toughness [Callister 6.6] A measure of energy absorbed during fracture Area under stress-strain curve has units of energy per unit volume Approximate this area as or Lecture 26, summer 2007 Mechanical Properties I: Metals & Ceramics ENGR 145, Chemistry of Materials Case Western Reserve University Callister Fig. 6.13
True Stress and Strain [Callister 6.7] Engineering stress and strain: based on initial dimensions True stress and strain: based on instantaneous dimensions (i) Callister Fig. 6.1 Lecture 26, summer 2007 Mechanical Properties I: Metals & Ceramics ENGR 145, Chemistry of Materials Case Western Reserve University
True Stress and Strain [Callister 6.7] Engineering stress and strain: based on initial dimensions True stress and true strain: based on instantaneous dimensions Callister Fig. 6.16 Start of necking Lecture 26, summer 2007 Mechanical Properties I: Metals & Ceramics ENGR 145, Chemistry of Materials Case Western Reserve University
Mechanical Properties of Ceramics [Callister 13.8] Virtually no plasticity at room T Strain to failure typically < 0.2% Linear to fracture Lecture 26, summer 2007 Mechanical Properties I: Metals & Ceramics ENGR 145, Chemistry of Materials Case Western Reserve University Callister Fig. 13.29
Mechanical Properties of Ceramics [Callister 13.8] Flexural strength: measured in 3-point bending a.k.a. modulus of rupture, 3-point bend strength, fracture strength Compression Tension rectangular cross-section circular cross-section Lecture 26, summer 2007 Mechanical Properties I: Metals & Ceramics ENGR 145, Chemistry of Materials Case Western Reserve University Callister Fig. 13.28
Mechanical Properties of Ceramics [Callister 13.8] Moduli usu. higher than for metals • Wide spread in strengths Lecture 26, summer 2007 Mechanical Properties I: Metals & Ceramics ENGR 145, Chemistry of Materials Case Western Reserve University