Chapter 6 Quiz Review Elastic deformation Plastic deformation o Occurs in elastomer o Occurs bc of coiling and uncoiling of spring like molecules o Elastic strain and stress in non linear o Most structures are designed for elastic deformation when stress is applied o From the graphs Can use hooke s law to find the elastic region when looking at the curve of metals slope young s elastic modulus point where are will return to original shape after load is gone yield strength where permanent deformation occurs won t go back to original shape o Material doesn t go back to original shape o Occurs bc breaking of bonds and those bonds get new neighbors o Yield strength measure of resistance to plastic deformation o Occurs by a slip in the material o A structure that has plastically deformed or experienced a permanent change in shape may not be capable of functioning as intended o From the curve can get Ultimate tensile strength the largest value of stress on the curve Is the max stress which the material can support without breaking When the necking occurs Strain hardening when the material is loaded again after the elastic region the curve will follow back to the UTS w the same elastic modulus Raising yield strength by permanently straining the material is this strain hardening process Fracture where stress decreases as necking and non uniform deformation occur The break finally occurs The stress strain is determined by the tensile test Engineering stress o The force over the unit area o Original diameter is used to measure changes in dimensions o Most common units are PSI and MPa Engineering strain o The elongation of the material length o Most common units are in in cm cm it often True stress o The load divided by the actual cross sectional area of the specimen at that load o Instantaneous changes in dimensions diameter as a function of force Chapter 6 Quiz Review True strain o The strain calculated using actual not original dimensions NOTE the engr and true curves are identical to the yield point After that the true curve goes higher Stress strain curve REVIEW Stress and strain These are size independent measures of load and displacement respectively Elastic behavior This reversible behavior often shows a linear relation between stress and strain To minimize deformation select a material with a large elastic modulus E or G Plastic behavior This permanent deformation behavior occurs when the tensile or compressive uniaxial stress reaches y Toughness The energy needed to break a unit volume of material Ductility The plastic strain at failure