2 Ductile or Brittle Failure Following elastic deformation, two differentprocesses can occur – plastic deformationleading to ductile failure or movement tobrittle fracture with little or no plasticity.The stress strain curve is shown for ductilefailure. After the UTS the ductility is shownby the necking of the material. This wouldonly be for round bar.The stress strain curve for brittle materialsis indicated, which would be for glass andceramics.Brittle FailureSvS stops here
3 Fracture Processes For ductile materials which have plasticity Ligament of materialStretches between voidsFor ductile materials which have plasticityonce the neck starts, a sequence ofstructural changes occurs as the metalproceeds to failure. Internally voids areinitiated . Both the true stress and strainare both still increasing. With furtherincrease in strain the voids becomelarger, as they become circular orellipsoidal in shape. The small ligamentsof metal between the voids eventuallytears and an internal crack is initiated.With further true stress and strain increasethe last areas to fail are those connectingthe internal crack to the surface. A “shear lip”at 45o to the surface characterizes this regionleading to “cup and cone” fracture in rod.
4 Micro Failure Processes. In single phase materials, small voids appearat the cell walls as each dislocation effectivelycarries a vacancy with it. These voids thengrow with further stressing
5 Micro Failure Processes In two phase material, several differentmicro processes can lead to void initiationgrowth and failure.If the second phase is either large or continuous,then a dislocation pile up in one phase will leadto a void formation. This will then grow inphase. The second phase will have strengthenedthe material but will also have eventuallyinitiated the failure process.Examples include pearlitic steels andother platelet structures such as thosein titanium alloysSecond Phase45oVoid from dislocationpile up on slip planes
6 Micro Failure ExampleExample of toughened material?
7 Micro Failure Processes When the second phase is small andnon discontinuous, other processes inaddition to dislocation pile up and a voidon the slip plane are possible. The controllingfactors are the strength of the second phaseparticle and interface between the secondphase and the matrix material.If the second phase is weaker or morebrittle than the matrix, then the second phaseparticle may crack.Age hardened aluminum alloys behave inthis manner
11 Fracture Surface Features Ductile failure in a titanium alloy which is two phase showing dimples from void formation
12 Final Failure in Shear Lip Shear dimples in area of shear lip.
13 Brittle Failure Fracture Surface A faceted type of fracture surface is often seen with a brittle failure as cracks are on a singlecrystal plane, no shear lip would be found..
14 Fracture Mechanics.Originally to explain why glass has low toughness where toughness is the ability to resist crackgrowth in a material. Simple energy balance used – energy applied is the stress and it is used up bycreating fresh surface. Works well for brittle materials where no plastic deformation. For ductilematerials energy absorbed is complicated by plastic deformation. Fracture mechanics relates the sizeof a defect in the material to the stress is can withstand before failure.Kc = Ysc (pa)0.5ModeIIIIII
16 Ductile to Brittle Transition In some materials, mainly steels, ductility can decrease very sharply with temperature, so a ductilematerials becomes brittle – know as the ductile brittle transition. The standard test is to use an impacttester – a pendulum type hammer and the energy absorbed in failure is measured by how far thehammer swings through after impact – the further the less energy.Shear ratio to impact energyCarbon content effect on DBTT for Steels
17 HomeworkFor a material with a single crystal yield strength of 300 MPa, calculate the yield strength for a grain size of 50 micron and for a grain size of 2.5 microns. Assume k =1Decide the crack initiation process in a tensile test for the following conditions:-Matrix shear strength Matrix Normal Strength Interface Strength Particle StrengthabcdData units MPa.
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