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Chapter 7 Mechanical Properties

Chapter 7 Mechanical Properties - Chapter 7 Mechanical...

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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 0 o A 0 =area before loading Engineering strain, ε=(l i -l 0 )/l i = Δl/l 0 o l i = instantaneous length o l 0 = 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 p-p’ 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 Z =-ε Y 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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