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Mechanical Behavior

# Mechanical Behavior - Mechanical Behavior Chapter 6 8...

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Mechanical Behavior Chapter 6 & 8

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Material Properties Chemical Physical Mechanical Tensile test – elastic modulus (stiffness), tensile strength, ductility Impact test - toughness Hardness test – hardness (wear resistance or durability) Thermal (thermal conductivity, heat capacity, thermal expansion coefficient) Electrical (electric conductivity, dielectric constant) • Magnetic Optical (reflectivity, index of refraction) Corrosive (electrochemical reactivity) Needed for material selection and manufacturing processes
Mechanical Properties The material’s ability to carry or resist mechanical forces or stresses These properties affect how the material can be worked Low hardness leads to poor machinability High ductility leads to poor surface finish Typical properties: Elastic modulus (stiffness) Hardness Ductility Rigidity Strength Toughness Viscoelasticity

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Stress Stress is the resistance offered by a material to external forces or loads per unit area. Normal stress: σ = F/A in which F A Shear Stress: τ = F/A in which F // A Stress is measured in pounds per square inch (psi or ksi) or Newton per square meter (N/m 2 or Pa, MPa, GPa) Normal stress is perpendicular to the surface on which the forces act, such as tension or compression; whereas shear stress is parallel to the surface. F F A F F F F A A
Strains Normal Strain , ε , also called unit elongation (in/in, mm/mm) – Total elongation divided by the original length of the material as a result of force on the materials – Two forms of normal strain • Longitudinal ε longitudinal (= Δ L/L original ) • Lateral ε lateral – The ratio of these two forms of strain is known as the Poisson’s Ratio, ν =- ε lat / ε long Shear Strain (shape deformation): γ (angle in radian) Strain is the material’s response to stress, thus there exists a relationship.

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Finding Stress-Strain Relationship Tensile Testing Extensometer Specimen Rubber Polycarbonate Aluminum
Universal Testing Machines at ETID Lab #3 Lab #9

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Hook’s Law: σ = E· ε Where E is the elastic modulus or Young’s modulus. Hook’s law governs the linear elastic behavior of a mat’l. Stress-Strain Relationship Slope of this section = Elastic Modulus or Young’s Modulus, E Ultimate Strength