Module 08 Failure Contents 1 Fracture ductile and brittle fracture 2 Fracture mechanics 3 Impact fracture ductile to brittle transition 4 Fatigue crack initiation and propagation crack propagation rate 5 Creep generalized creep behavior stress and temperature effects Failure Classification Failure of a material component is the loss of ability to function normally or to perform the intended job Three general ways failure Excessive elastic deformation E g buckling Controlled by design and elastic modulus of the material Excessive plastic deformation Controlled by yield strength of the material E g loss of shape creep and or stress rupture at elevated temperatures Fracture involves complete disruption of continuity of a component under static load brittle or ductile under fluctuating cyclic load fatigue mode in which most machine parts fail in service Fracture Fracture defined as the separation or fragmentation of a solid body into two or more parts under the action of stress Fracture is classified based on several characteristic features characteristic Strain to fracture Crystallographic mode Ductile Shear terms used Appearance Fibrous and gray Brittle Cleavage Granular and bright Crack propagation Through grains Along grain boundaries Fracture 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 Ductile fracture Vs Brittle fracture Parameter Ductile fracture Brittle fracture Strain energy required Stress during cracking Higher Lower Increasing Constant Crack propagation Slow Warning sign Plastic deformation Deformation Extensive Necking Yes Fast None Little No Fractured surface Rough and dull Smooth and bright Type of materials Most metals not too cold Ceramics Glasses Ice Ductile fracture Ductile fracture in tension occurs after appreciable plastic deformation It is usually preceded by necking It exhibits three stages 1 formation of cavities 2 growth involving rapid crack failure final of propagation at about 45 to the tensile axis cavities 3 Fractography of ductile fracture reveals numerous spherical dimples separated by thin walls on the fractured surface McClintock s strain to ductile fracture f 32 1 sinh 2ln 1 00bafnbln Ductile fracture contd Stages of void nucleation void growth crack initiation and eventual fracture under ductile fracture mode Brittle fracture Brittle fracture intakes place with little or no preceding It occurs often at unpredictable levels of stress by rapid plastic deformation crack propagation Crack propagates nearly perpendicular to the direction of applied 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 that causes dislocation pile ups at obstacles 2 micro crack nucleation as a result of build up of shear stresses 3 eventual crack propagation under applied stress aided by stored elastic energy Brittle fracture Griffith Theory Nominal fracture stress that causes brittle fracture in presence of cracks length of interior crack 2c the stress raisers Griffith s criteria a crack will propagate when the decrease in elastic energy is at least equal to the energy required to create the new crack surface Thus for thin plates For thick plates modification When plastic energy is also taken into account Orowan s 212cE212 1 2cE2121 2cEpcpE214cEf Fracture mechanics Relatively new field of mechanics that deals with possibility whether a crack of given length in a material with known toughness is dangerous at a given stress level or not 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 22GEK Fracture mechanics contd K depends on many factors the most influential of which are temperature strain rate microstructure and orientation of fracture The value of K decreases with increasing strain rate grain size and or decreasing temperature Depending on the orientation of fracture three modes of fracture are identified as shown in the figure Notch impact testing Ductile and Brittle are terms used to distinguish two extremes of fractures modes based on plastic deformation involved 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 extend at a greater extent Usually a notch will be introduced to simulate the conditions A notch increases the tendency for brittle fracture by four b by a by producing high local stresses means introducing a tri axial state of stress c by producing high local strain hardening and cracking and d by producing a local magnification to the strain rate Notch impact testing contd A material s susceptibility to different kinds of fracture is measured using notched specimen subjected to impact load Further study involves examining the fracture surfaces and calculation of ductility Two kind of specimen configurations loading directions Ductile to Brittle transition Energy absorbed during the notch impact is plotted as a function of temperature to know at what temperature range DBTT material fracture in a particular mode In metals DBTT is around 0 1 0 2 Tm while in ceramics it is about 0 5 0 7 Tm where Tm represents absolute melting temperature Fatigue failure Failure that occurs under fluctuating cyclic loads Fatigue Fatigue occurs at stresses that considerable smaller than yield tensile stress of the material These failures are dangerous because they occur without any warning Typical machine components subjected to fatigue are automobile crank shaft bridges aircraft landing gear etc Fatigue failures occur in both metallic and non metallic materials and are responsible for a large number fraction of identifiable service failures of metals Fatigue fracture surface is perpendicular to the direction of an applied stress Fatigue failure contd Fatigue failure can be recognized from the appearance of the fracture surface Any point with stress concentration such as sharp corner or notch or metallurgical inclusion can act as point of initiation of fatigue crack Fatigue failure contd Three basic requisites for