PowerPoint PresentationSlide 2Slide 3Slide 4Slide 5Slide 6Slide 7Slide 8Slide 9Slide 10Slide 11Slide 12Slide 13Slide 14Slide 15Slide 16Slide 17Slide 18Slide 19Slide 20Slide 21Slide 22Slide 23Slide 24Slide 25Slide 26Slide 27Slide 28Slide 29Slide 30Slide 31Introduction to Materials Science, Chapter 8, FailureUniversity of Virginia, Dept. of Materials Science and Engineering1How do Materials Break?Chapter Outline: Failure Ductile vs. brittle fracture Principles of fracture mechanics Stress concentration Impact fracture testing Fatigue (cyclic stresses) Cyclic stresses, the S—N curve Crack initiation and propagation Factors that affect fatigue behavior Creep (time dependent deformation) Stress and temperature effectsAlloys for high-temperature useIntroduction to Materials Science, Chapter 8, FailureUniversity of Virginia, Dept. of Materials Science and Engineering2Brittle vs. Ductile Fracture•Ductile materials - extensive plastic deformation and energy absorption (“toughness”) before fracture•Brittle materials - little plastic deformation and low energy absorption before fractureIntroduction to Materials Science, Chapter 8, FailureUniversity of Virginia, Dept. of Materials Science and Engineering3Brittle vs. Ductile FractureA. Very ductile: soft metals (e.g. Pb, Au) at room T, polymers, glasses at high TB. Moderately ductile fracture typical for metalsA. Brittle fracture: ceramics, cold metals, A B CIntroduction to Materials Science, Chapter 8, FailureUniversity of Virginia, Dept. of Materials Science and Engineering4 Steps : crack formation crack propagation FractureDuctile vs. brittle fracture•Ductile - most metals (not too cold): Extensive plastic deformation before crackCrack resists extension unless applied stress is increased•Brittle fracture - ceramics, ice, cold metals: Little plastic deformationCrack propagates rapidly without increase in applied stressDuctile fracture is preferred in most applicationsIntroduction to Materials Science, Chapter 8, FailureUniversity of Virginia, Dept. of Materials Science and Engineering5Ductile Fracture (Dislocation Mediated) (a) Necking, (b) Cavity Formation, (c) Cavities coalesce  form crack (d) Crack propagation, (e) FractureCrack grows 90o to applied stress 45O - maximum shear stressIntroduction to Materials Science, Chapter 8, FailureUniversity of Virginia, Dept. of Materials Science and Engineering6Ductile Fracture(Cup-and-cone fracture in Al)Scanning Electron Microscopy. Spherical “dimples”  micro-cavities that initiate crack formation.Introduction to Materials Science, Chapter 8, FailureUniversity of Virginia, Dept. of Materials Science and Engineering7Crack propagation is fastPropagates nearly perpendicular to direction of applied stressOften propagates by cleavage - breaking of atomic bonds along specific crystallographic planesNo appreciable plastic deformationBrittle Fracture (Low Dislocation Mobility)Brittle fracture in a mild steelIntroduction to Materials Science, Chapter 8, FailureUniversity of Virginia, Dept. of Materials Science and Engineering8A. Transgranular fracture: Cracks pass through grains. Fracture surface: faceted texture because of different orientation of cleavage planes in grains.B. Intergranular fracture: Crack propagation is along grain boundaries (grain boundaries are weakened/ embrittled by impurity segregation etc.)A BBrittle FractureIntroduction to Materials Science, Chapter 8, FailureUniversity of Virginia, Dept. of Materials Science and Engineering9Fracture strength of a brittle solid: related to cohesive forces between atoms. Theoretical strength: ~E/10 Experimental strength ~ E/100 - E/10,000Difference due to:Stress concentration at microscopic flaws Stress amplified at tips of micro-cracks etc., called stress raisersStress ConcentrationFigure by N. Bernstein &D. Hess, NRLIntroduction to Materials Science, Chapter 8, FailureUniversity of Virginia, Dept. of Materials Science and Engineering10Stress Concentration0 = applied stress; a = half-length of crack; t = radius of curvature of crack tip. Stress concentration factor 2/1t0ma2Crack perpendicular to applied stress: maximum stress near crack tip  2/1t0mta2KIntroduction to Materials Science, Chapter 8, FailureUniversity of Virginia, Dept. of Materials Science and Engineering11Two standard tests: Charpy and Izod. Measure the impact energy (energy required to fracture a test piece under an impact load), also called the notch toughness.Impact Fracture Testing CharpyIzodh’hEnergy ~ h - h’Introduction to Materials Science, Chapter 8, FailureUniversity of Virginia, Dept. of Materials Science and Engineering12As temperature decreases a ductile material can become brittleDuctile-to-Brittle TransitionIntroduction to Materials Science, Chapter 8, FailureUniversity of Virginia, Dept. of Materials Science and Engineering13Low temperatures can severely embrittle steels. The Liberty ships, produced in great numbers during the WWII were the first all-welded ships. A significant number of ships failed by catastrophic fracture. Fatigue cracks nucleated at the corners of square hatches and propagated rapidly by brittle fracture. Ductile-to-brittle transitionIntroduction to Materials Science, Chapter 8, FailureUniversity of Virginia, Dept. of Materials Science and Engineering14V. Bulatov et al., Nature 391, #6668, 669 (1998)“Dynamic" Brittle-to-Ductile Transition (not tested)(molecular dynamics simulation )DuctileBrittleIntroduction to Materials Science, Chapter 8, FailureUniversity of Virginia, Dept. of Materials Science and Engineering15Under fluctuating / cyclic stresses, failure can occur at lower loads than under a static load.90% of all failures of metallic structures (bridges, aircraft, machine components, etc.)Fatigue failure is brittle-like – even in normally ductile materials. Thus sudden and catastrophic!FatigueFailure under fluctuating stressIntroduction to Materials Science, Chapter 8, FailureUniversity of Virginia, Dept. of Materials Science and Engineering16Fatigue: Cyclic StressesCharacterized by maximum, minimum and mean Range of stress, stress amplitude, and stress ratio Mean stress m = (max + min) / 2Range of stress r =