Tensile test apparatus 5 stress and strain to

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Unformatted text preview: 5 Stress and Strain To eliminate effect of sample area, define stress & strain as follows engineering stress Ao - original area Units - psi or Mpa = 1E6 N/m2 145 psi = 1 MPa F = A0 li - l0 l = = l0 l0 6 engineering strain lo - original length li - instantaneous length Unitless ELASTIC DEFORMATION At small levels of stress reversible little shape, volume change = E w/temp) E = modulus of elasticity /stiffness ( 50 - 400 Gpa metals 70 - 500 Gpa ceramics 1E-3 - few GPa polymers tangent modulus, secant modulus if nonlinear NO effect from microstructure, composition, prior history Determining E - Linear Materials Determining E - Nonlinear Materials Microscopic Behavior Atomic forces Repulsive Attractive F = 0 @ equilibrium Strain = bond str-e-e-tching Moving atoms apart Strain related to bond forces E dF/dr So E & atomic bonding related Ceramics Stress-strain behavior similar to metals Linear region May fracture before plastic deformation Smaller strains e.g., 1E-3 Testing m...
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This note was uploaded on 03/26/2008 for the course ENGR 213 taught by Professor Dr.stubbs during the Fall '04 term at Texas A&M.

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