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Unformatted text preview: Chapter 7 Mechanical Properties Introduction • Mechanical behavior of a material reflects the relationship between its response or deformation to an applied load or force • Important mechanical properties are strength, hardness, ductility, and stiffness • Factors to be considered: the nature of the applied load and its duration • Load can be tensile, compressive, or shear and its magnitude can be constant or time dependent Concepts of Stress and Strain • Most loads are torsional • Engineering stress, σ=F/A o A =area before loading • Engineering strain, ε=(l il )/l i = Δl/l o l i = instantaneous length o l = original length • above equations used for tensile an compression • For shear and torsional test the following equation is used o τ=F/A, shear stress o γ=tan(θ), shear stain • stress equations for pp’ plane at angle θ = complex stress state o σ’= σ*cos 2 (θ)=σ((1+cos(2 θ))/2) o τ’=σ*sin(θ)*cos(θ)=σ(sin(2 θ)/2) Stress Strain Behavior • Hooke’s Law, σ=Eε o E= Young’s Modulus= Modulus of Elasticity; GPa or psi • Elastic deformation = deformation in which stress and strain is proportional o Nonpermanent, meaning that when the applied load is released the piece returns to its original shape • Tangent Modulus= stress/strain at certain stress level • Secant Modulus= chg in stress/chg in strain from origin to certain stress level • Τ=Gγ o G=shear modulus Anelasticity • Time dependent elastic (nonpermanent) deformation Elastic Properties of Materials • Poisson’s ratio = ratio of lateral and axial strains o ν= ε X /ε Z =ε Y /ε Z • max value of Poisson’s is .5 • Relation between shear and elastic moduli o E=2G(1+ ν) • Plastic deformation= deformation that is permanent and nonrecoveable o Deformed beyond approx. ε=.005 results no more proportionality between stress and strain Tensile Properties •...
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This note was uploaded on 01/02/2012 for the course ENME enme382 taught by Professor Bruck during the Spring '10 term at Maryland.
 Spring '10
 Bruck

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