A.E., Cer.E., Chem.E, Met.E., And Phys. 377 Fall 2001 Principles of Engineering Materials

Mechanical PropertyTestingPart 1Introduction:Properties of Engineering MaterialsDesign considerations relate needs to propertiesWhat materials meet our design requirements?Can you assure the quality and properties of a given piece or batch of material?Mechanical Property Testing:Static testsTensileBendCompressionCreepHardness

Dynamic testsImpactFatigueWearTensile Test:Conditions: static loadingroom temperatureStandard specimenCylindrical shapeDiameter = 0.505 inches; Ao = 0.2 sq in. Gauge length = 2.0 inchesCrude testLoad vs. Elongation (stretch)

Tensile Test Schematic

Load-stretch axes

Load-stretch elastic

Problem: - data depends on geometry!Same load will be more intense on smaller cross-section specimensLonger gage length will have more stretch more than shorter gage lengthWant data that describes the material, not the material and its geometryTherefore, we need to remove geometric effectsSimple Solution: Engineering Stress vs Engineering StrainDefine Engineering Stress as:Load / Original AreaLoad/ Ao

Define Engineering Strain as:Stretch / Original Gage Length(L Lo) / Lo

Engineering Stress - Strain

Elastic regionSlope StiffnessYoungs Modulus E = / (Also called Modulus of Elasticity)Stretching of atomic bondsSum of forces = 0 with added applied loadRecoverable i.e. elasticCompare materialsExtent where does plastic deformation begin? (i.e. dislocations begin to move)Yield pointOffset yield strength

A Stiffness Comparison

Yield point diagram

Typical Stress - Strain Curve

Offset Yield Strength

Plastic region:

Why a positive slope?What is this telling us?Chain analogy & strain hardeningPlastic deformation strengthens the metal!Ultimate tensile strength maximum attained stressNecking begins at ultimate tensile strengthBreaking strengthLoad = Strength x Area

d(Load) = Str (dA) + A (dStr)

Since dA is negative, a positive d(Load)is only possible if the (dStr) term isan over-riding positive!

Ductility

Elongation at Fracture(Lf-Lo) / Lo x 100%Reduction in area(Ao-Af) / Ao x 100%Glass and bubble gum examples

True stress and strain:

Points in plastic region are not correct (area is changing during the test!)True Stress = Load/ Aactual True Strain = ln (Lf / Lo)Strain hardening coefficient (n) true= K truen Determined from true stress true strain curve

True stress-strain vs Engineering stress-strain

Effect of Elevated Temperature:Decrease in strength and stiffnessModulus of ElasticityYield strengthTensile StrengthIncrease in ductilityElongationReduction in areaOther Tests:Bend tests3 point bendingFlexural strengthFlexural modulusCompression testAccount for friction at contacts