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FailureModule-081) Fracture, ductile and brittle fracture2) Fracture mechanics3) Impact fracture, ductile-to-brittle transition4) Fatigue, crack initiation and propagation, crack propagation rate5) Creep, generalized creep behavior, stress and temperature effectsContentsFailure – Classification Failure of a material component is the loss of ability tofunction normally or to perform the intended job! Three general ways failure:Excessive elastic deformation, E.g.: buckling. Controlled bydesign and elastic modulus of the material.Excessive plastic deformation, Controlled by yield strengthof the material. E.g.: loss of shape, creep and/ or stress-rupture at elevated temperatures.Fracture, involves complete disruption of continuity of acomponent – under static load: brittle or ductile, underfluctuating/cyclic load: fatigue, mode in which mostmachine parts fail in service.Fracture Fracture defined as the separation or fragmentation of asolid body into two or more parts under the action of stress. Fracture is classified based on several characteristicfeatures:characteristic terms usedStrain to fracture Ductile BrittleCrystallographic mode Shear CleavageAppearance Fibrous and grayGranular and brightCrack propagationAlong grain boundariesThrough grainsFracture modes Ductile and Brittle are relative terms. Most of the fractures belong to one of the following modes:(a) rupture, (b) cup-&-cone and (c) brittle.Parameter Ductile fracture Brittle fractureStrain energy requiredHigher LowerStress, during crackingIncreasing ConstantCrack propagation Slow FastWarning sign Plastic deformation NoneDeformation Extensive LittleNecking Yes NoFractured surface Rough and dull Smooth and brightType of materialsMost metals (not too cold)Ceramics, Glasses, IceDuctile fracture Vs Brittle fractureDuctile fracture Ductile fracture in tension occurs after appreciable plasticdeformation. It is usually preceded by necking. It exhibits three stages - (1) formation of cavities (2) growthof cavities (3) final failure involving rapid crackpropagation at about 45 to the tensile axis. Fractography of ductile fracture reveals numerous sphericaldimples separated by thin walls on the fractured surface. McClintock’s strain to ductile fracture, εf,)32()()1(sinh)2ln()1(00bafnblnDuctile fracture (contd….) Stages of void nucleation, void growth, crack initiation andeventual fracture under ductile fracture mode:Brittle fracture Brittle fracture intakes place with little or no precedingplastic deformation. It occurs, often at unpredictable levels of stress, by rapidcrack propagation. Crack propagates nearly perpendicular to the direction ofapplied tensile stress, and hence called cleavage fracture. Most often brittle fracture occurs through grains i.e.transgranular. Three stages of brittle fracture - (1) plastic deformation thatcauses dislocation pile-ups at obstacles, (2) micro-cracknucleation as a result of build-up of shear stresses, (3)eventual crack propagation under applied stress aided bystored elastic energy.Brittle fracture – Griffith Theory Nominal fracture stress that causes brittle fracture inpresence of cracks (length of interior crack=2c), the stressraisers, Griffith’s criteria: a crack will propagate when the decreasein elastic energy is at least equal to the energy required tocreate the new crack surface. Thus for thin plates: For thick plates: When plastic energy is also taken into account (Orowan’smodification):212cE212)1(2cE2121)(2cEpcpE214cEfFracture mechanics Relatively new field of mechanics, that deals withpossibility whether a crack of given length in a materialwith known toughness is dangerous at a given stress level ornot! Fracture resistance of a material in the presence of cracks,known as fracture toughness, is expressed in two forms.(1) Strain-energy release rate, G:(2) Stress concentration factor, K: Both parameters are related as:For plane stress conditions i.e. thin plates:For plane strain conditions i.e. thick plates:EcG2cKGEK2)1(22GEKFracture mechanics (contd….) K depends on many factors, the most influential of whichare temperature, strain rate, microstructure and orientationof fracture. The value of K decreases with increasing strainrate, grain size and/or decreasing temperature. Depending on the orientation of fracture, three modes offracture are identified as shown in the figure:Notch-impact testing Ductile and Brittle are terms used to distinguish twoextremes of fractures modes based on plastic deformationinvolved before fracture occurs. Three factors that aid transition from ductile to brittle-cleavage type of fracture are: (1) tri-axial state of stress (2)low temperature, and (3) rapid rate of loading. Since brittle fracture is most unpredictable, its been extendat a greater extent. Usually a notch will be introduced tosimulate the conditions. A notch increases the tendency for brittle fracture by fourmeans: (a) by producing high local stresses, (b) byintroducing a tri-axial state of stress, (c) by producing highlocal strain hardening and cracking, and (d) by producing alocal magnification to the strain rate.Notch-impact testing (contd….) A material’s susceptibility to different kinds of fracture ismeasured using notched specimen subjected to impact load.Further study involves examining the fracture surfaces, andcalculation of ductility. Two kind of specimen configurations & loading directions:Ductile-to-Brittle transition Energy absorbed during the notch-impact is plotted as afunction of temperature to know at what temperature range(DBTT) material fracture in a particular mode. In metals DBTT is around 0.1-0.2 Tmwhile in ceramics it isabout 0.5-0.7 Tm, where Tmrepresents absolute meltingtemperature.Fatigue failure Failure that occurs under fluctuating/cyclic loads – Fatigue. Fatigue occurs at stresses that considerable smaller thanyield/tensile stress of the material. These failures are dangerous because they occur withoutany warning. Typical machine components subjected tofatigue are automobile crank-shaft, bridges, aircraft landinggear, etc. Fatigue failures occur in both metallic and non-metallicmaterials, and are responsible for a large number fraction ofidentifiable service failures of metals. Fatigue fracture surface is perpendicular to the direction ofan applied stress.Fatigue failure (contd….) Fatigue failure can be recognized from the


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NIT-T MECH 786 - Failure

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