CHAPTER 4 1/1/2016. Mechanical Properties Of Metals - II. Fracture of Metals Ductile Fracture. Ductile and Brittle Fractures

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1 //06 Fracture o Metals Ductile Fracture CHAPTER Mechanical Properties O Metals - II Fracture results in separation o stressed solid into two or ore parts. Ductile racture : High plastic deoration & slow crack propagation. Three steps : Specien ors neck and cavities within neck. Cavities or crack and crack propagates towards surace, perpendicular to stress. Direction o crack changes to 5 0 resulting in cup-cone racture. Fracture o Metals Brittle Fracture Ductile and Brittle Fractures No signiicant plastic deoration beore racture. Coon at high strain rates and low teperature. Three stages: Plastic deoration concentrates dislocation along slip planes. Microcracks nucleate due to shear stress where dislocations are blocked. Crack propagates to racture. SEM o ductile racture Exaple: HCP Zinc ingle crystal under high stress along {000} plane undergoes brittle racture. Ductile racture Brittle Fracture SEM o brittle racture 3 Brittle Fractures (cont..) Toughness and Ipact Testing Brittle ractures are due to deects like Folds Undesirable grain low Porosity Tears and Cracks Corrosion age Ebrittleent due to atoic hydrogen At low operating teperature, ductile to brittle transition takes place Toughness is a easure o energy absorbed beore ailure. Ipact test easures the ability o etal to absorb ipact. Toughness is easured using ipact testing achine 5 6

2 //06 Ipact testing (Cont ) Also used to ind the teperature range or ductile to brittle transition. Fracture Toughness Cracks and laws cause stress concentration. K Y a K = Stress intensity actor. σ = Applied stress. a = edge crack length Y = geoetric constant. 7 Sinking o Titanic: Titanic was ade up o steel which has ductile - brittle transition teperature 3 0 C. On the y o sinking, sea teperature was 0 C which ade the the structure highly brittle and susceptible to ore age. K Ic = critical value o stress intensity actor.(fracture toughness) Y a 8 Exaple: Al 0 T85 6.MPa / 30 alloy steel 60.MPa / Measuring Fracture Toughness A notch is achined in a specien o suicient thickness B. B >> a plain strain condition. B =.5(K Ic /Yield strength) Specien is tensile tested. Higher the K Ic value, ore ductile the etal is. Used in design to ind allowable law size. Fatigue o Metals Metals oten ail at uch lower stress at cyclic loading copared to static loading. Crack nucleates at region o stress concentration and propagates due to cyclic loading. Failure occurs when cross sectional area o the etal too sall to withstand applied Fracture started here load. 9 0 Fatigue ractured surace o keyed shat Final rupture Fatigues Testing Cyclic Stresses Alternating copression and tension load is applied on etal piece tapered towards center. Dierent types o stress cycles are possible (axial, torsional and lexural). Mean stress = Stress range = ax in r ax in Stress to cause ailure S and nuber o cycles required N are plotted to or SN curve. ax in Stress aplitude = a Stress range = R in ax

3 //06 Structural Changes in Fatigue Process Factors Aecting Fatigue Strength Crack initiation irst occurs. Reversed directions o crack initiation caused surace ridges and groves called slipband extrusion and intrusion. This is stage I and is very slow (0-0 /cycle). Crack growth changes direction to be perpendicular to axiu tensile stress rate (icrons/sec). Saple ruptures by ductile ailure when reaining cross-sectional area is sall to withstand the stress. Persistent slip bands In copper crystal Stress concentration: Fatigue strength is reduced by stress concentration. Surace roughness: Soother surace increases the atigue strength. Surace condition: Surace treatents like carburizing and nitriding increases atigue lie. Environent: Cheically reactive environent, which ight result in corrosion, decreases atigue lie. 3 Fatigue Crack Propagation Rate Stress & Crack Length Fatigue Crack Propagation. Notched specien used. Cyclic atigue action is generated. Crack length is easured by change in potential produced by crack opening. σ σ Δa ΔN Δa ΔN AK α (σ,a) When a is sall, / is also sall. / increases with increasing crack length. Increase in σ increases crack growth rate. = atigue crack growth rate. ΔK = K ax -K in = stress intensity actor range. 5 6 A, = Constants depending on aterial, environent, requency teperature and stress ratio. 7 Fatigue Crack Growth rate ΔK Log Log( A K ). Log( K) Log( A) Straight line with slope Liiting value o ΔK below Which there is no easurable Crack growth is called stress intensity actor range threshold ΔK th Fatigue Lie Calculation AK K A K Y a ( y a y a Integrating ro initial crack size a 0 to inal crack size a at nuber o atigue cycles N a N But Thereore Thereore Ay a ( ) ( ) a0 0 a a0 Integrating and solving or N N (Assuing Y is independent o crack length) Ay ( ) 8 ) 3

4 //06 Creep in Metals Creep is progressive deoration under constant stress. Iportant in high teperature applications. Priary creep: creep rate decreases with tie due to strain hardening. Seconry creep: Creep rate is constant due to siultaneous strain hardening and recovery process. Tertiary creep: Creep rate increases with tie leading to necking and racture. Creep Test Creep test deterines the eect o teperature and stress on creep rate. Metals are tested at constant stress at dierent teperature & constant teperature with dierent stress. High teperature Low teperature Mediu teperature Creep strength: Stress to produce iniu creep rate o 0-5 %/h at a given teperature. 9 0 Creep Test (Cont..) Larsen Miller Paraeter Creep rupture test is sae as creep test but aied at ailing the specien. Plotted as log stress versus log rupture tie. Tie rupture decreases with increased stress and teperature. Larsen Miller paraeter is used to represent creep-stress rupture ta. P(Larsen-Miller) = T[log t r + C] T = teperature(k), tr = stress-rupture tie h C = Constant (order o 0) Also, P(Larsen-Miller) = [T( 0 C) + 73(0+log t r ) or P(Larsen-Miller) = [T( 0 F) + 60(0+log t r ) At a given stress level, the log tie to stress rupture plus constant ultiplied by teperature reains constant or a given aterial. Larsen Miller Paraeter L.M. Diagra o several alloys I two variables o tie to rupture, teperature and stress are known, 3 rd paraeter that its L.M. paraeter can be deterined. Exaple: For alloy CM, at 07 MPa, LM paraeter is 7.8 x 0 3 K Then i teperature is known, tie to rupture can be ound. Exaple: Calculate tie to cause 0.% creep strain in gaa Titaniu aluinide at 0 KSI and 00 0 F Fro ig, P = = ( ) (log t 0.% + 0) t=776 h 3

5 //06 Case Study Analysis o Failed Fan Shat Requireents Function Fan drive support Material 05 cold drawn steel Yield strength 586 Mpa Expected lie 60 K (ailed at 3600 k) Visual exaination (avoid additional age) Failure initiated at two points near illet Characteristic o reverse bending racture Failed Shat Further Analysis Tensile test proved yield strength to be 369 MPa (lower than speciied 586 MPa). Metallographic exaination revealed grain structure to be equiaxed ( cold drawn etal has elongated grains). Conclusion: Material is not cold drawn it is hot rolled!. Lower atigue strength and stress raiser caused the ailure o the shat. 5 6 Recent Advances: Strength + Ductility Fatigue Behavior o Nanoaterials Coarse grained low strength, high ductility Nanocrystalline High strength, low ductility (because o ailure due to shear bands). Ductile nanocrystalline copper : Can be produced by Cold rolling at liquid nitrogen teperature Additional cooling ater each pass Controlled annealing Cold rolling creates dislocations and cooling stops recovery 5 % icrocrystalline grains in a atrix o nanograins. Nanoaterials and Ultraine Ni are ound to have higher endurance liit than icrocrystalline Ni. Fatigue crack growth is increased in the interediate regie with decreasing grain size. Lower atigue crack growth threshold K th observed or nanocrystalline etal