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
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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
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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
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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
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Stress-Strain Diagrams • These diagrams are used to determine how material will react under different loads Elastic Range: material will resume its original dimension after load is removed – Linear elastic: straight line section from which E is defined – Nonlinear elastic: material behaviors nonlinearly and ends at a point called elastic limit Plastic Range: there is permanent deformation after
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This note was uploaded on 04/06/2008 for the course ENTC 206 taught by Professor Fang during the Spring '08 term at Texas A&M.

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Mechanical Behavior - Mechanical Behavior Chapter 6 & 8...

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