2 Dynamic FatigueFatigue is failure due to a stress being repeatedly appliedto a metal. After a number of cycles the component fails.Examples include aircraft, axles, and shafts.StressesCyclic – tension compression fully reversedtension – zero – tensiontension – non zero – tensionsine wave, saw tooth,trapezoidal with delay.2. Stress Rato R= smax/smin
3 Stress V Log Nf CurvesFerrous, that is BCC iron based have a “knee” in the stress, number of cycles to failuredata, while non-ferrous does not. Safety ranges can be determined for ferrous, while theStress range for 107 cycles is used for non-ferrous metals.
4 Micro ProcessStage I – Slip lines intersect with surface of material at 45o to stress axis. Single slip system operating.Form “Persistent Slip Band” – can’t be removed by annealing processes.Crack initiates in slip band and grows at 45o to stress – single slip system % NfStage II – At crack tip, as tensile stress increases, multiples slip systems, crack grows along them bothproducing “ Striations”, identifying fatigue., 74 – 49% NfStage III – Crack length such that it is approached K1c value, so overload failure as stress increases.
5 Fatigue Fracture Surface .FractureSurfaceExposed SurfaceShaft broken by fatigue. Initiation site isat the top. Note the change in appearancewhen going from stage II to stage III.Tube fractured by fatigue. Note river linesrunning from initiation sites on surface as well asmultiple stage I initiation sites on surface. Riverlines connect cracks.
6 Stage II Crack Growth Striations Fine striations on surface. River line indicatescrack growth direction while striations indicatecrack front.Coarse striations on surfaceFine
7 Mixed Mode Crack Growth Transition where some striations and some overload failure. Anisotropic effect?Striations indicate fatigue process lead to failure
8 Fatigue Crack Growth Testing Cyclic stress on a pre cracked sample and measure crack length “a” as a function ofnumber of cycles to produce a da/dN curve. Know “a” so can calculate DK, alsofrom da/dN curve, for given value of N slope of curve is da/dN, growth rate per cycle.DK= YDs (pa)0.5
9 Crack Growth Rate DataTypical crack growth curve shown with three regions. The first is a threshold regionwhich indicates a safe crack size below which no crack growth occurs. Then a steadystate region of crack growth, and the third stage of rapid failure when plane strainfracture toughness is met. Lifing of components can be done using stage II. If the cracklength and service stress is known, DK can be calculated and the crack growth rateobtained from graphs. The number of cycles before the crack length becomes criticalcan be determined and so the remaining life of the cracked component can be estimated
10 Factors Affecting Fatigue. Microstructure – plate structure of Ti-6Al-4V better fatigue life than particle structure.Compressive stresses in the surface stop cracks opening – shot peenSurface roughness – rougher surface initiates cracks easier than smooth surface.Corrosion decreases fatigue life by initiating cracks easily so modifying stage I.Cyclic frequency – increase frequency decreases fatigue life.Cyclic waveform – trapezoidal detrimental for some titanium alloysExamination – ultrasonic inspection, penetration X ray, die penetrant,
11 Creep MechanismsCreep in metals is extension to failure under combined stress and temperature. The temperature isusually above 0.4 the melting point in oKelvin. The process occurring during creep involves grainboundaries at 45o to the stress axis sliding relative to each other. Diffusion of atoms is involved.sGrain Boundaryslides due to atomsmoving. Voids openup and failure occursgrain boundary controlled
12 CreepCreep can be split into three sections – primary, decreasing creep rate, secondary witha steady state creep rate and tertiary creep with an increasing creep rate.Factors affecting creep are grain size, temperature,Small grain size detrimental for creep – single crystal turbine blades used these days.Don’t want blades elongating as they would rub against containment. Other areainclude nuclear reactors.
13 Creep PolymersCreep in polymers is measured by applying a load and measuring the extension after a setperiod of time. The creep modulus is then the stress divided by the strain. The higher thecreep modulus the better. Note the low temperatures compared to metals and reinforcement effect.