Chapter8:MechanicalPropertiesofMetals ISSUESTOADDRESS 1 LECTURER: PRO. SEUNGTAE CHOI 8.1Introduction Mechanicalbehaviorofamaterialreflectsitsresponseordeformationin relationtoanappliedloadorforce. Keymechanicaldesignproperties:stiffness,strength,hardness,ductilityand toughness ASTM(AmericanSocietyforTestingandMaterials)Standards (http://www.astm.org) Roleofengineers 2 LECTURER: PRO. SEUNGTAE CHOI
8.2ConceptsofStressandStrain TensionTest(TensileTest) ShearandTorsionalTests A 0 l l 0 CompressionTest GeometricConsiderationsofthe StressState 3 LECTURER: PRO. SEUNGTAE CHOI ElasticDeformation 4 LECTURER: PRO. SEUNGTAE CHOI
PlasticDeformation(Metals) 5 LECTURER: PRO. SEUNGTAE CHOI 8.3ElasticDeformation Elasticdeformation(nonpermanent):Whentheappliedloadisreleased,the piecereturnstoitsoriginalshape. Linearelasticmaterials: =E Hooke'sLaw) E 6 LECTURER: PRO. SEUNGTAE CHOI
MechanicalProperties Theelasticmodulusisproportionaltotheslopeoftheinteratomicforce separationcurveatequilibriumspacing: d E dr rr 0 Callister&Rethwisch 9e. Callister&Rethwisch 9e. 7 LECTURER: PRO. SEUNGTAE CHOI Young smoduli:comparison E Callister&Rethwisch 9e. 8 LECTURER: PRO. SEUNGTAE CHOI
Poisson'sratio, Theaxialelongationisaccompaniedbylateralcontraction. Lateralstrainisproportionaltotheaxialstrain. Poisson sratio:theratioofthelateralstraintotheaxialstrain lateral strain x y axial strain Mostofmaterials: =0.25~0.35 Cork: =0(e.g.winecap) Rubber: =0.5(incompressiblematerial) LimitationonPoisson sratio Units: TypicalvaluesofPoisson sratio z z Callister&Rethwisch 9e. 9 LECTURER: PRO. SEUNGTAE CHOI OtherElasticProperties Elasticshearmodulus,G: G G M M Elasticbulkmodulus,K: P P V PK V 0 K V V 0 P P Specialrelationsforisotropicmaterials: E E G K 2(1 ) 3(1 2 ) V 0 V 10 LECTURER: PRO. SEUNGTAE CHOI
BasicConceptsinMechanicsofMaterials Threebasicequationsoflinearelasticity Terminologies 11 LECTURER: PRO. SEUNGTAE CHOI 8.4PlasticDeformation Plasticdeformation:permanentornonrecoverable deformation Simpletensiontest: p T T melt Callister&Rethwisch 9e. 12 LECTURER: PRO. SEUNGTAE CHOI
Tensile Test Tensile test to understand the mechanical behavior of materials Standard: ASTM (American Society for Testing and Materials) Typical tensile specimen Ǥ ͺǤʹǡ Callister & Rethwisch 9e 13 LECTURER: PRO. SEUNGTAE CHOI StressǦStrain Curve Typical stressǧstrain curve of mild steel 14 LECTURER: PRO. SEUNGTAE CHOI ǣ ǣ ȋ䚡 Ȍǡ Î ȋ ͳͳ ͳͷ Ȍ ǣ ȋ 䝉 䞈Ȍ ȋᴴḩ 䞈Ȍ Î ǣ ȋἥ䚐㢅 Ȍ ȋἥ䚐ᵉ⓸ȍ ǣ Ǣ ǡ ǣ ǯǣ Ǧ Î ǣ
Yield Strength and Yield Criteria Yield strength: stress at which noticeable plastic deformation has occurred. von Mises yield criterion: the yielding of materials begins when the second deviatoric stress invariant J2 reaches a critical value. ǡ ɐ ɐy Vy α ǡ H Hp α ͲǤͲͲʹ Ǥ ͺǤͳͲ ȋ Ȍǡ Callister & Rethwisch 9e. Vv 3 3 3 V ijcv ijc 2 i j 1 ª (V 1 V 2 ) 2 (V 2 V 3 ) 2 (V 3 V 1 ) 2 º¼ 2 15 LECTURER: PRO. SEUNGTAE CHOI Yield Strength: Comparison ʹͲͲͲ Ȁ Ȁ Ȁ Ȁ ͲͲ ǡ ʹͲͲ ȋͷ ǦʹǤͷ Ȍ ȋ Ȍ ȋ ͳͷͳͳȍ ȋ Ȍ ȋͷͳͷͳȍ ȋͳͳʹͳȍ ȋ Ͳ ͳȍ ȋͳͳʹͳȍ ȋ Ȍ ȋ Ȍ ȋ ͳͷͳͳȍ ͳͳͳ Ͳ Ͳ ͷͳ ͶͲ ȋ Ͳ ͳȍ Ͳ ʹͲ ͳͳ ȋ Ȍ 16 LECTURER: PRO. SEUNGTAE CHOI 㼳 ǡ ǡ ͲͲ ͲͲ ͷͳͳ ͶͲͲ ǡ ǡ Ǥ ɐ y ȋ Ȍ ͳͳͳͳ ǡ ǡ Ǥ ȋͷͳͷͳȍ ǤͶǡ Callister & Rethwisch 9e. α α α α α α Ƭ
WorkHardeningorStrainHardening y y1 y0 T A K n n n o 17 LECTURER: PRO. SEUNGTAE CHOI TensileStrength(UltimateStrength) TS y Callister&Rethwisch 9e. 18 LECTURER: PRO. SEUNGTAE CHOI
TensileStrength:Comparison TS 19 LECTURER: PRO. SEUNGTAE CHOI Callister&Rethwisch9e. Ductility EL L f L 0 L 0 EL EL L 0 A 0 A f L f Callister& Rethwisch 9e. e A 0 A RA f A 0 20 LECTURER: PRO. SEUNGTAE CHOI
Toughness Energytobreakaunitvolumeofmaterial Approximatebytheareaunderthestressstraincurve. Callister&Rethwisch 9e. e 21 LECTURER: PRO. SEUNGTAE CHOI ElasticEnergyDensity&TrueStress&Strain ElasticEnergyDensity(Resilience) U r y d 0 U r 1 y y 2 TrueStress&Strain T 1 ln 1 T T A i T ln i o Callister &Rethwisch 9e. Callister&Rethwisch 9e. 22 LECTURER: PRO. SEUNGTAE CHOI
Hardness Resistancetopermanentlyindentingthesurface. Hardnessmeasurement:simple,inexpensive,nondestructive Largehardnessmeans: D d 23 LECTURER: PRO. SEUNGTAE CHOI HardnessMeasurement 24 LECTURER: PRO. SEUNGTAE CHOI
DesignorSafetyactors Designuncertaintiesmeanwedonotpushthelimit. actorofsafety,n y w N N Example:Calculateadiameter,d,toensurethatyielddoesnotoccurinthe 1045carbonsteelrodbelow.Useafactorofsafetyof5. d y working N Lo 220 kn y 2 TS d /4 d 25 LECTURER: PRO. SEUNGTAE CHOI