Section 3B Stress_Strain

Section 3B Stress_Strain - MATE 210 Section 3 STRESS...

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Unformatted text preview: MATE 210 Section 3 STRESS & STRAIN (Smith Chapter 6.2-6.6) How do we measure the strength of materials? Materials in load-bearing applications are structural materials How does the structure of a material affect the mechanical properties ? MATE 210 MATE 210 Mechanical Properties Example Stiff Strong Tough Light Not stiff enough (need bigger E) modulus of elasticity Not strong enough (need bigger y ) yield strength Not tough enough (need bigger K ic ) fracture toughness Too heavy (need lower ) density All OK ! 2002, M.F. Ashby and D. Cebon MATE 210 elastic : returns to original size and shape upon removal of load plastic : permanent change or deformation fracture : specimen separates into pieces (i.e., breaks) failure : inability of a component to perform as intended Deformation response to mechanical force MATE 210 Plastic deformation:- permanent-atoms move around- bonds break & reform- energy adsorbed- slip planes Brittle fracture:- little energy adsorbed- fast or catastrophic- atoms pulled apart- fracture plane MATE 210 l l l l l = = [unitless] Strain : change in length/length engineering strain: Stress : force/area engineering stress: A F = [MPa] 1 MPa = 10 6 Newtons/m 2 = 145 psi mega pascals MATE 210 linear relationship E = elastic modulus stiffness Youngs modulus [GPa] Elastic Regime :- no change upon release of load Hookes Law: F = kx- stretching of atomic bonds Poissons ratio : ratio of transverse to axial strain (elastic regime only!) a t = [0.25-0.35] / Cu = 110 GPa Mega - 10 6 Giga - 10 9 lateral axial MATE 210 Example Problem A piece of copper originally 50 mm long is pulled in tension with a stress of 276 MPa. 1. What is the stress in psi? 2. If the deformation is entirely elastic, how much will the rod change in length(mm)? 3. What would the strain be on the rod if the tensile force was removed? Cu Rod E cu = 110 GPa MATE 210 The Tension Test sample specimen geometries: 2 long & 0.5 Instron Tensile Test Machine test procedures by ASTM MATE 215 MATE 210 y p Plastic Regime : permanent deformation movement of dislocations (slip planes) n p K = K- strength coefficient n- strain hardening exponent plastic elastic nonlinear behavior: linear relationship E = Proportional limit Yield strength if C = .002 MATE 210...
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This note was uploaded on 04/02/2008 for the course MATE 210 taught by Professor Niebuhr during the Spring '05 term at Cal Poly.

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Section 3B Stress_Strain - MATE 210 Section 3 STRESS...

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