2 Mechanical Properties Stiffness - Elastic Modulus or Young’s Modulus (MPa)Strength - Yield, Ultimate, Fracture, Proof, Offset Yield. Measured as stress (MPa)Ductility - Measure of ability to deform plastically without fracture - Elongation, Area Reduction, Fracture Strain - (no units or mm/mm)Toughness, Resilience - Measure of ability to absorb energy (J/m3).Hardness - Resistance to indentation/abrasion (Various scales, e.g.; Rockwell, Brinell, Vickers.)
3 Stress and StrainIn a simplistic sense, stress may be thought of as Load/Area.Similarly, strain is the deformation of the component/original length.A stress may be direct, shear, or torsional - leading to corresponding deformations.Stress cannot be measured directly, but deformation can be.
10 Duke’s Quick Tip!Express Load in Newtons (N) and Area in mm2 to get Stress in MPa.Mechanical properties of metals are almost always given in MPa or ksi.Imperial units: Load in kips (1000 lbf) & Area as in2 gives Stress in ksi (kips/in2)1000 psi = 1 ksi = 6.89 MPa
11 Hooke’s Law Elastic Deformation Elastic deformation is not permanent; it means that when the load is removed, the part returns to its original shape and dimensions.For most metals, the elastic region is linear. For some materials, including metals such as cast iron, polymers, and concrete, the elastic region is non-linear.If the behavior is linear elastic, or nearly linear-elastic, Hooke’s Law may be applied:Where E is the modulus of elasticity (MPa)
14 Shear Stress and Strain Shear Strainshear stress, t = Shear Load / Areashear strain, g = angle of deformation (radians)shear modulus, G = t /g (elastic region)
15 Elastic Properties of Materials Poisson’s ratio: When a metal is strained in one direction, there are corresponding strains in all other directions.For a uniaxial tension strain, the lateral strains are constrictive.Conversely, for a uniaxial compressive strain, the lateral strains are expansive.i.e.; the lateral strains are opposite in sign to the axial strain.The ratio of lateral to axial strains is known as Poisson’s ratio, n.
16 Poisson’s Ratio, n For most metals, 0.25 < n < 0.35 in the elastic rangeFurthermore:
17 Plastic Deformation Sy Sy Sy Stress Strain Most Metals - Al, Cu Elastic PlasticElastic PlasticElastic PlasticSySySyStress0.0020.002Strain0.002Most Metals - Al, CuClad Al-AlloysLow carbon Steel
18 Microstructural Origins of Plasticity Slip, Climb and Slide of atoms in the crystal structure.Slip and Climb occur at Dislocations and Slide occurs at Grain Boundaries.tt
19 Elastic and Plastic Strain (e,S)StressTotal StrainThe 0.2% offset yield stressis the stress that gives a plastic(permanent) strain ofStrainPlasticElasticepee
22 Ductile Vs Brittle Materials Only Ductile materials will exhibit necking.Ductile if EL%>8% (approximately)Brittle if EL% < 5% (approximately)Engineering StressEngineering Strain
23 Toughness & Resilience Toughness: A measure of the ability of a material to absorb energy without fracture. (J/m3 or N.mm/mm3= MPa)Resilience: A measure of the ability of a material to absorb energy without plastic or permanent deformation.(J/m3 or N.mm/mm3= MPa)Note: Both are determined asenergy/unit volume
24 Toughness, Ut Su Sy Engineering Stress, S=P/Ao Engineering Strain, e = DL/Lo)
25 Resilience, Ur Su Sy ey Engineering Stress, S=P/Ao E Engineering Strain, e = DL/Lo)
26 Typical Mechanical Properties Metals in annealed (soft) condition