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Mechanical Properties of Metals

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Mechanical Properties Stiffness - Elastic Modulus or Youngs 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/m 3 ). Hardness - Resistance to indentation/abrasion (Various scales, e.g.; Rockwell, Brinell, Vickers.)

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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.

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Direct Stress Examples Direct Stress - TensionDirect Stress - Compression Engineering Stress Engineering Strain

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Tension Test Typical Universal Testing Machine Extensometer Measures L Measures P

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Modern Materials Testing System Hydraulic Wedge Grips Specimen Extensometer

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ASTM Tension Test Specimen 2 Gauge Length A o =0.20 in 2 LoLo

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Raw Data Obtained Load, P (kN) Elongation, L (mm) Uniform Deformation Total Elongation Elastic Deformation X Maximum Load, P max Load, P f

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

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Dukes Quick Tip! Express Load in Newtons (N) and Area in mm 2 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 in 2 gives Stress in ksi (kips/in 2 ) 1000 psi = 1 ksi = 6.89 MPa

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Hookes 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, Hookes Law may be applied: Where E is the modulus of elasticity (MPa)

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Modulus of Elasticity - Stiffness

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Atomic Origin of Stiffness

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

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Elastic Properties of Materials Poissons 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 Poissons ratio,.

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Poissons Ratio, For most metals, 0.25 < < 0.35 in the elastic range Furthermore :

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Plastic Deformation Stress Strain SySy SySy SySy Elastic Plastic Most Metals - Al, CuClad Al-AlloysLow carbon Steel Elastic Plastic

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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.

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

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Elastic Recovery Strain Stress Loading Unloading Loading Unloading Reloading elastic strain Strain

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Ductility - EL% & AR% Elongation Area Reduction LoAoLoAo LfAfLfAf

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

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Toughness & Resilience Toughness: A measure of the ability of a material to absorb energy without fracture. (J/m 3 or N.mm/mm 3 = MPa) Resilience: A measure of the ability of a material to absorb energy without plastic or permanent deformation. (J/m 3 or N.mm/mm 3 = MPa) Note: Both are determined as energy/unit volume

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Toughness, U t Engineering Strain, e = L/Lo) Engineering Stress, S=P/Ao SuSu SySy

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Resilience, U r Engineering Strain, e = L/Lo) Engineering Stress, S=P/Ao SuSu SySy E eyey

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Typical Mechanical Properties Metals in annealed (soft) condition

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