 # Mechanical Properties of Metals

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Mechanical Properties of Metals

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.)

Stress and Strain In 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.

Direct Stress Examples
Engineering Stress Engineering Strain Direct Stress - Tension Direct Stress - Compression

Tension Test Typical Universal Testing Machine Measures P Extensometer
Measures DL Typical Universal Testing Machine

Modern Materials Testing System
Hydraulic Wedge Grips Specimen Extensometer

ASTM Tension Test Specimen
Ao=0.20 in2 2” Gauge Length Lo

Raw Data Obtained Load, P (kN) Elongation, DL (mm) Total Elongation

Engineering Stress-Strain Curve
Elongation Sy 0.2% offset yield stress Engineering Stress, S=P/Ao (Ultimate) E Su E Proportional Limit Engineering Strain, e = DL/Lo)

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

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)

Modulus of Elasticity - Stiffness

Atomic Origin of Stiffness

Shear Stress and Strain
Shear Strain shear stress, t = Shear Load / Area shear strain, g = angle of deformation (radians) shear modulus, G = t /g (elastic region)

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.

Poisson’s Ratio, n For most metals, 0.25 < n < 0.35
in the elastic range Furthermore:

Plastic Deformation Sy Sy Sy Stress Strain Most Metals - Al, Cu
Elastic Plastic Elastic Plastic Elastic Plastic Sy Sy Sy Stress 0.002 0.002 Strain 0.002 Most Metals - Al, Cu Clad Al-Alloys Low carbon Steel

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. t t

Elastic and Plastic Strain
(e,S) Stress Total Strain The 0.2% offset yield stress is the stress that gives a plastic (permanent) strain of Strain Plastic Elastic ep ee

Ductility - EL% & AR% Elongation Area Reduction Lo Ao Lf Af

Ductile Vs Brittle Materials
Only Ductile materials will exhibit necking. Ductile if EL%>8% (approximately) Brittle if EL% < 5% (approximately) Engineering Stress Engineering Strain

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 as energy/unit volume

Toughness, Ut Su Sy Engineering Stress, S=P/Ao
Engineering Strain, e = DL/Lo)

Resilience, Ur Su Sy ey Engineering Stress, S=P/Ao E
Engineering Strain, e = DL/Lo)

Typical Mechanical Properties
Metals in annealed (soft) condition