Mechanical FailureOverviewFracture ModesFracture MechanismCrack PropagationDuctile vs Brittle FailureModerately Ductile FailureExample: Pipe Failuresc09f03abc09f04Brittle Fracturec09f06abc09f07abFracture MechanicsStress ConcentrationFracture ToughnessDuctile vs Brittlec09tf01stress-intensity factor (K)Critical StressCompact tension (CT) specimenFlaws are Stress ConcentratorsDESIGN AGAINST CRACK GROWTHDesign Example: Aircraft WingSensors made to mesh with planeSlide 26c09tf02c09f13Brittle Fracture of CeramicsFracture of PolymersImpact TestingDuctile to Brittle Transition Temperature (DBTT)c09f19c09f20Temperaturec09f24c09f23c09f30c09f29Slide 40Fatigue MechanismCrack growth ratec09f25Slide 44c09f27c09f33Improving Fatigue Lifec09f34High Temperature - CreepCreepc09f35Slide 52Creep Failurec09f37Secondary CreepSUMMARY1Mechanical FailureChapter 92Overview•Failure Modes –Fracture, Fatigue, Creep•Fracture Modes –Ductile, Brittle, Intergranular, Transgranular•Fracture Toughness•Stress Concentrators (Flaws)•Crack Propagation33Fracture Modes•Simple fracture is the separation of a body into 2 or more pieces in response to an applied stress that is static (constant) and at temperatures that are low relative to the Tm of the material.•Classification is based on the ability of a material to experience plastic deformation.•Ductile fracture–Accompanied by significant plastic deformation•Brittle fracture–Little or no plastic deformation–Sudden, catastrophicFracture MechanismImposed stress Crack Formation Propagation•Ductile failure has extensive plastic deformation in the vicinity of the advancing crack. The process proceeds relatively slow (stable). The crack resists any further extension unless there is an increase in the applied stress.•In brittle failure, cracks may spread very rapidly, with little deformation. These cracks are more unstable and crack propagation will continue without an increase in the applied stress. 455Crack PropagationCracks propagate due to sharpness of crack tip •A plastic material deforms at the tip, “blunting” the crack. deformed regionbrittle Energy balance on the crack•Elastic strain energy- •energy stored in material as it is elastically deformed•this energy is released when the crack propagates•creation of new surfaces requires energyplastic6Ductile vs Brittle FailureVery DuctileModeratelyDuctileBrittleFracturebehavior:Large Moderate%AR or %ELSmall• Ductile fracture is usually more desirable than brittle fracture.Ductile: Warning before fractureBrittle: No warning77• Evolution to failure:Moderately Ductile Failureneckingvoid nucleation• Resulting fracture surfaces (steel)50 mmparticlesserve as voidnucleationsites.50 mmFrom V.J. Colangelo and F.A. Heiser, Analysis of Metallurgical Failures (2nd ed.), Fig. 11.28, p. 294, John Wiley and Sons, Inc., 1987. (Orig. source: P. Thornton, J. Mater. Sci., Vol. 6, 1971, pp. 347-56.)100 mmFracture surface of tire cord wire loaded in tension. Courtesy of F. Roehrig, CC Technologies, Dublin, OH. Used with permission.fractureCrack propagationCoalescence of cavities88• Ductile failure: -- one piece -- large deformationFigures from V.J. Colangelo and F.A. Heiser, Analysis of Metallurgical Failures (2nd ed.), Fig. 4.1(a) and (b), p. 66 John Wiley and Sons, Inc., 1987. Used with permission.Example: Pipe Failures• Brittle failure: -- many pieces -- small deformations9Ductile vs. Brittle Failurecup-and-cone fracturebrittle fracture(a) SEM image showing spherical dimples resulting from a uniaxial tensile load. (b) SEM image of parabolic dimples from shear loading.10Ductile Failure1111Brittle FractureArrows indicate point at failure originationDistinctive pattern on the fracture surface: V-shaped “chevron” markings point to the failure origin.12Transgranular Fracture•Cleavage - in most brittle crystalline materials, crack propagation that results from the repeated breaking of atomic bonds along specific planes.•This leads to transgranular fracture where the crack splits (cleaves) through the grains.13Intergranular Fracture•Intergranular failure is typically due to elemental depletion (chromium) at the grain boundaries or some type of weakening of the grain boundary due to chemical attack, oxidation, embrittlement.Fracture MechanicsStudies the relationships between: material properties stress levelcrack producing flawscrack propagation mechanisms14Stress Concentration•The measured fracture strengths for most brittle materials are significantly lower than those predicted by theoretical calculations based on atomic bond energies.•This discrepancy is explained by the presence of very small, microscopic flaws or cracks that are inherent to the material.•The flaws act as stress concentrators or stress raisers, amplifying the stress at a given point.•This localized stress diminishes with distance away from the crack tip.Fracture Toughness•Fracture toughness measures a material’s resistance to brittle fracture when a crack is present.•It is an indication of the amount of stress required to propagate a preexisting flaw. •Flaws may appear as cracks, voids, metallurgical inclusions, weld defects, design discontinuities, or some combination thereof. •It is common practice to assume that flaws are present and use the linear elastic fracture mechanics (LEFM) approach to design critical components. •This approach uses the flaw size and features, component geometry, loading conditions and the fracture toughness to evaluate the ability of a component containing a flaw to resist fracture.Ductile vs Brittle•The effect of a stress raiser is more significant in brittle than in ductile materials. •For a ductile material, plastic deformation results when the maximum stress exceeds the yield strength.•This leads to a more uniform distribution of stress in the vicinity of the stress raiser; the maximum stress concentration factor will be less than the theoretical value. •In brittle materials, there is no redistribution or yielding.18Fracture Toughnessstress-intensity factor (K)•The stress-intensity factor (K) is used to determine the fracture toughness of most materials. •A Roman numeral subscript indicates the mode of fracture and the three modes of fracture are illustrated in the image to the right. •Mode I fracture is the condition where the crack plane is normal to the direction of largest tensile loading. 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