Transcription of Chapter 6: Mechanical properties of metals
1 1 Chapter 6: Mechanical properties of metals Outline Introduction Concepts of stress and strain Elastic deformation Stress-strain behavior Elastic properties of materials Plastic deformation Yield and yield strength Ductility Resilience ToughnessConcepts of stress and strain Tension, compression, shear, and torsion2 Stress-strain testing Tension tests engineering stress engineering strain Compression tests0AF= 000llllli == specimenextensometerGauge lengthShear stress Shear and torsional tests Shear stress Shear strain Geometric considerations of the stress state GAF==090 90 - y x = x/y= tan 3 Elastic means reversible!
2 Elastic deformation1. Initial2. Small load3. UnloadF bonds stretchreturn to initialF Linear-elasticNon-Linear-elastic6 Linear elastic properties Modulus of Elasticity, E:(also known as Young's modulus) Hooke's Law: = E Linear-elasticE FFsimple tension test4 Stress-strain behavior Stress-strain for linear elastic deformation Stress-strain for non-linear elastic deformationStress-strain behavior(continue) Modulus of elasticity E is proportional to (dF/dr)r Slope of stress strain plot (which is proportional to the elastic modulus) depends on bond strength of metal5 Elastic Shearmodulus, G: G = G Other elastic propertiessimpletorsiontestMM Special relations for isotropic materials:2(1 + )EG=3(1 2 )EK= Elastic Bulkmodulus, K:pressuretest.
3 =Vo. Vol VPPPP= -K VVoP VKVoComparison of Young s ,AluminumPlatinumSilver, GoldTantalumZinc, TiSteel, NiMolybdenumGraphiteSi crystalGlass -sodaConcreteSi nitrideAl oxidePCWood( grain)AFRE( fibers) *CFRE*GFRE*Glass fibers onlyCarbon fibers onlyAramid fibers onlyEpoxy 020060080010 001200400 TinCu alloysTungsten<100> <111>Si carbideDiamondPTFEHDP ELDPEPPP olyesterPSPETCFRE( fibers) *GFRE( fibers)*GFRE(|| fibers)*A FRE(|| fibers)*CFRE(|| fibers)*E(GPa)6 Comparison of yield strengthGraphite/ Ceramics/ SemicondMetals/ AlloysComposites/ fibersPolymersYield strength, y(MPa)PVCHard to measure, since in tension, fracture usually occurs before 6,6 LDPE702040605010010302003004005006007001 0002000 Tin (pure)Al(6061)aAl(6061)agCu(71500)hrTa (pure)Ti (pure)aSteel(1020)hrSteel(1020)cdSteel(4 140)aSteel(4140)qtTi ( )aW(pure)Mo (pure)Cu(71500)
4 CwHard to measure, in ceramic matrix and epoxy matrix composites, sincein tension, fracture usually occurs before Elastic properties of materials Poisson s ratiozyzx = = metals : ~ : ~ : ~ :E: [GPa] or [psi] : dimensionless7 Plastic means permanent!Plastic Deformation ( metals )F linear elasticlinear elastic plastic1. Initial2. Small load3. Unloadplanes still shearedF elastic + plasticbonds stretch & planes shear plasticPlastic deformation: yield and yield strength Yielding Proportional limit Yield strength8 Tensile strength Tensile strength: the stress at the maximum on the engineering stress-strain curve metals : occurs when noticeable necking starts Ceramics: occurs when crack propagation starts Polymers.
5 Occurs when polymer backbones are aligned and about to break yTypical response of a metalengineering TSstressengineering strain Plastic (permanent) deformation(At lower temperature: T< Tmelt/3) Simple tension test:engineering stress, engineering strain, Elastic+Plasticat larger stresspermanent (plastic) after load is removed pplastic strainElastic initially9 Elastic and plastic deformations Stress-strain relations under uniaxial loadingCeramicsPolymeric materialDuctility Ductility: a measure of the degree of plastic deformation that has been sustained at fracture10 Mechanic properties of typical metals ResilienceResilience Resilience: is the capacity of a material to absorb energy when it is deformed elastically and then, upon unloading, to have this energy Energy to break a unit volume of material Approximate by the area under the stress-strain curveBrittle fracture: elastic energyDuctile fracture.
6 Elastic + plastic energyvery small toughness (unreinforced polymers) Engineering tensile strain, Engineeringtensile stress, small toughness (ceramics)large toughness ( metals ) Adapted from Fig. , Callister 7e. Stressand strain: These are size-independentmeasures of load and displacement, respectively. Elasticbehavior: This reversible behavior oftenshows a linear relation between stress and minimize deformation, select a material with alarge elastic modulus (Eor G). Toughness: The energy needed to break a unitvolume of material.
7 Ductility: The plastic strain at Plasticbehavior: This permanent deformationbehavior occurs when the tensile (or compressive)uniaxial stress reaches y.
