Fatigue

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Design factors

Any noth or geometrical discontinunity can act as a stress raiser and fatigue crack insitiation site, these design features include grooves, holes, keyways, threads and so on. The sharper the radius of curvature the more stress concentration.

Surface Treatments

Small scratches and grooves can limit fatigue life. Improving the surface finish by polishing will enhance fatigue life. One way to increase fatigue performance is by imposing residual compressive stresses within a thin outer surface layer. Thus a tensile stress of external orgin will be partiall nullified. The net effect is that the likelihood of crack formation and therefore fatigue failure is reduced. Case hardening is a technique by which both surface hardness and fatigue life are enchanced for steel alloys. It is done by carburizing or nitriding process by which a componenet is exposed to a carbonaceous or nitrogenous atmosphere at elecated temperature. The improvement of fatigue properties from increased hardness tihin the case as well as desired residual compressive stresses.

Thermal fatigue is normally induced at elevated temperatures by fluctuating thermal stresses; mechanical stresses from an external source need not be present. The orgin is the restraint to the dimensional expansion and/or contraction that would normally occur in a structural member with variations in temperature.

Thermal stress depends on coefficient of thermal expansion, modulus of elasticity and temp change.

Sigma = alpha *E*deltaT

Corrosion fatigue. Corrosive environments have deleterious influence and produce shorter fatigue lives. Small pits may form and serve as stress coenctration. Crack propogataion rate enhanced by corrosive environment.

Materials are often placed in service at elevated temperatures and exposed to static mechanical stresses. Deformation under such circumstances is termed creep. It is time dependant and permenant deformation. Observed in all material types, for metals it is important for temp greater than 0.4Tm, where Tm is absolute melting temperature.

Creep tests are conducted to understand material properties at constant stress while maintaining temperature constant.

Creep curve consitsts of primary occurs first, it is decreasing creep rate and curve is decreasing. It is being strain hardened and difficult for material to be strained. Secondary creep(steady state) is longest. Material retains its ability to experience deformation but is also strain hardening. Third step is ultime failure also called rupture and results form microstructural changes such as grain boundary speration. Most important property of creep test is the slope. It is the parameter for long life applications.

- Instantaneous strain at the time of stress application increases

- Steady-state creep rate increases

- Rupture lifetime decreases

Strain(steady state) = k1 sigma^n , n and k are constnats

Strain(steady) = k2 sigma^n exp(-qc/RT) , q=activation energy for creep.

Several mecahnicsms have bee proposed to explain the creep behavior materials, these mechanisms involve stress induced vacancy diffusion, grain boundary diffusion, dislocation motion and graind boundary sliding. ef