CHE 333 Class 20 Fracture continued

Dynamic Fatigue Fatigue is failure due to a stress being repeatedly applied to a metal. After a number of cycles the component fails. Examples include aircraft, axles, and shafts. Stresses Cyclic – tension compression fully reversed tension – zero – tension tension – non zero – tension sine wave, saw tooth, trapezoidal with delay. 2. Stress Rato R= smax/smin

Stress V Log Nf Curves Ferrous, that is BCC iron based have a “knee” in the stress, number of cycles to failure data, while non-ferrous does not. Safety ranges can be determined for ferrous, while the Stress range for 107 cycles is used for non-ferrous metals.

Micro Process Stage 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. 25 -50 % Nf Stage II – At crack tip, as tensile stress increases, multiples slip systems, crack grows along them both producing “ Striations”, identifying fatigue., 74 – 49% Nf Stage III – Crack length such that it is approached K1c value, so overload failure as stress increases.

Fatigue Fracture Surface . Fracture Surface Exposed Surface Shaft broken by fatigue. Initiation site is at the top. Note the change in appearance when going from stage II to stage III. Tube fractured by fatigue. Note river lines running from initiation sites on surface as well as multiple stage I initiation sites on surface. River lines connect cracks.

Stage II Crack Growth Striations Fine striations on surface. River line indicates crack growth direction while striations indicate crack front. Coarse striations on surface Fine

Mixed Mode Crack Growth Transition where some striations and some overload failure. Anisotropic effect? Striations indicate fatigue process lead to failure

Fatigue Crack Growth Testing Cyclic stress on a pre cracked sample and measure crack length “a” as a function of number of cycles to produce a da/dN curve. Know “a” so can calculate DK, also from da/dN curve, for given value of N slope of curve is da/dN, growth rate per cycle. DK= YDs (pa)0.5

Crack Growth Rate Data Typical crack growth curve shown with three regions. The first is a threshold region which indicates a safe crack size below which no crack growth occurs. Then a steady state region of crack growth, and the third stage of rapid failure when plane strain fracture toughness is met. Lifing of components can be done using stage II. If the crack length and service stress is known, DK can be calculated and the crack growth rate obtained from graphs. The number of cycles before the crack length becomes critical can be determined and so the remaining life of the cracked component can be estimated

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 peen Surface 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 alloys Examination – ultrasonic inspection, penetration X ray, die penetrant,

Creep Mechanisms Creep in metals is extension to failure under combined stress and temperature. The temperature is usually above 0.4 the melting point in oKelvin. The process occurring during creep involves grain boundaries at 45o to the stress axis sliding relative to each other. Diffusion of atoms is involved. s Grain Boundary slides due to atoms moving. Voids open up and failure occurs grain boundary controlled

Creep Creep can be split into three sections – primary, decreasing creep rate, secondary with a 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 area include nuclear reactors.

Creep Polymers Creep in polymers is measured by applying a load and measuring the extension after a set period of time. The creep modulus is then the stress divided by the strain. The higher the creep modulus the better. Note the low temperatures compared to metals and reinforcement effect.