Chapter 7. Mechanical Properties of Metals II Fracture and ...

1 1 Chapter 7 Chapter 7. Mechanical Properties of Metals II Fracture and Failure1. Fracture - ductile Fracture - brittle Fracture - ductile to brittle transition T2. Fatigue: - fatigue failure- fatigue crack growth rate3. Creep (time-dependent deformation)- creep rate- Larsen-Miller parameterHow to improve Mechanical characteristics?2 Chapter 7 Ductile and Brittle FracturesDuctile fractureBrittle Fracture after extensive plastic deformation slow defect/crack propagation along crystallographic (cleavage) planes rapid crack propagationFracture results in separation of stressed solid into two or more parts3 Chapter Fracture of Metals Ductile FractureDuctile Fracture : high plastic deformation & slow crack propagationThree steps.

2 - Specimen forms neck and cavities within neck- Cavities form crack and crack propagatestowards surface, perpendicular to stress- Direction of crack changes to 450resulting in cup-cone Fracture Scanning electron micrograph showing conical equaxialfeatures produced during the Fracture of a steel sample4 Chapter 7 Brittle FractureNo significant plastic deformationbefore fractureCommon at high strain rates and low T Three stages1. Plastic deformation concentrates dislocations along slip planes2. Microcracks nucleate due to shear stress where dislocations are blocked3. Crack propagates to fractureEx.: hcp Zn single crystal under high stress along {0001} plane Due to defects like:- porosity- tears and cracks- corrosion damage- embrittlement due to atomic hydrogenMost brittle fractures are transgranularScanning electron micrograph showing brittle Fracture in ferritic ductile iron5 Chapter 7 Ductile to Brittle Transition (DBT) TemperatureAt low T, high stress levels or fast loading rates ductile to brittle transition takes placeOperation at low temperaturesSinking of Titanic: Titanic was made up of steel which has low DBT temperature.

3 On the day of sinking, sea temperature was 20C which made the structure highly brittle and susceptible to more damage6 Chapter 7 Fracture ToughnessFigure and flaws cause stress concentrationaYK =1K1 - Stress intensity factor - Applied stressa- edge crack lengthY- geometric constantKIc- critical value of stress intensity factor ( Fracture toughness)aYf =Measuring Fracture Toughness:notchis machined in a specimen of thickness BB >> a plain = (KIc/Yield strength)2 Specimen is tensile testedHigher the KIcvalue, more ductile the metal is7 Chapter Fatigue of Metals Metals often fail at much lower stress at cyclic loadingcompared to static loading Crack nucleatesat region of stress concentration and propagates due to cyclic loading Failure occurs when cross sectional area of the metal too small to withstand loadFatigue: the phenomenon leading to Fracture under repeated stresses having the maximum value less than the ultimate strength of the materialDifferent types of stress cycles are possible.

4 Axial, torsional and flexuralFatique-fractures surface of steel shaft8 Chapter 7 Structural Changes in Fatigue Process Crack initiation first occurs Reversed directionsof crack initiation caused surface ridges and groves extrusion and intrusion: first stage, very slow ( ) Crack growth changes direction to be perpendicular to maximum tensile stress (rate few microns/sec) Sample rapture by ductile failure9 Chapter Creep in MetalsCreep is progressive plastic deformationunder constant stress with timeImportant in high temperature applicationsPrimary creep:creep rate decreases with time due to strain hardeningSecondary creep:Creep rate is constant due to simultaneous strain hardening andrecovery processTertiary creep:Creep rate increases with time leading to necking and fractureCreep test: constant load (stress) different temperaturesCreep rate / t10 Chapter Larsen-Miller Parameters Larsen Miller parameteris used to represent creep-stress rupture data P (Larsen-Miller) = T[log tr+ C]T - T(K), tr= stress-rupture time, h.

5 C - Constant (order of 20) or P (Larsen-Miller) = [T(0C) + 273(20+log tr)At a given stress level, the log time to stress rupture plus constant multiplied by temperature remains constantfor a given materialQ:Using the parameter plot at a stress of 207 MPa (30ksi), determine the time to stress-rupture at 980oC for directionally solidified alloy CM 247 (upper curve)11 Chapter Ductility and Strength Fatigue crack growth is increased in the intermediate regime with decreasing grain sizeCoarse grained low strength, high ductilityNanocrystalline high strength, low ductility (because of failure due to shear bands)Ductile nanocrystalline copper : Can be produced by old rolling at liquid nitrogen temperature Additional cooling after each pass Controlled annealing12 Chapter 7 Strengthening in Metals The ability of a metal to plastically deform depends on the ability of the dislocation to move Mechanical strength can be increased by hindering dislocation motion Methods for strengthening:1.]

6 Make the grains smaller: misalignment between grains at the boundaries acts as a barrier2. Make a solid state solution (alloy): impurity atoms introduce strain, minimum strain energy, if located at dislocation3. Strain hardening: cycle stress many times, increase dislocation density, dislocations interfere with each other