Fatigue Flashcards
Define fatigue failure
Failure of a component after repeated loading at levels lower than the static failure load. Failure occurs suddenly when the crack length is above the critical crack length and in a brittle manner.
fatigue/endurance limit
The largest value of repeated stress that will not cause failure for an infinite number of stress cycles.
Fatigue strength
The stress level at which failure will occur for some specified No. of cycles
failure Life
No. of cycles to cause failure at a specific stress level
Paris Law stage 1
K is less then K threshold, the crack growth rate is insignificant. when k does become larger than k threshold, the crack growth rate increases rapidly.
Paris Law Stage 2
Stable crack propagation. Follows the paris law, da/dn = CK1^m
Paris Law stage 3
As k1 approaches Kc, crack growth rate increases rapidly until final failure occurs.
Crack formation stage 1
There is back and forth slip of crystallographic planes, leading to the formation of slip bands within the material.within the slip bands, pores will nucleate and coalesce together. The slip of crystallographic planes can also form extrusions and intrusions. Due to the stress concentrations, there is much faster micro crack formation.
Crack formation Stage 2
The micro cracks in the material start to join and form macro cracks. The cracks escape the shearing stress control and propagate perpendicular to the loading direction. Continuous crack growth occurs each cycle, with plastic deformation occurring in the area in front of the crack tip. the crack propagates along a plane normal to the loading and in a linear manner.
Crack formation Stage 3
The final catastrophic failure occurs as there is separation of the intrusions, as well as mass pore formation.
Extrusions and intrusions
The sticking out and digging in of slip planes into and out of the surface of a material. The plastic deformation occurs in regions of high stress concentration. Due to the cyclic stress patterns, the continuous plastic oscillation of metal layers in and pout of the surface occurs, producing ridges.
Striations
When a load is applied, the crack opens under the load and high shear stresses develop at +- 69 degrees. Thus high stress concentration leads to plastic deformation at the lobes in front of the crack tip.
the material in front of the last striation naturally rounds out in order to reduce the stress concentration and avoid plastic deformation.
The crack length increases such that an equilibrium is reached between the applied load and stresses ahead of the crack tip.
During the compressive loading phase, plastic deformation is not reversed.the bulge at the nose collapses due to the opposite 69 degree shear stresses. A new striation has now been formed.
Residual Stresses
Residual stresses come from material processing methods. the induced residual stresses are in effect ‘locked into; the component in the absence of external loading. If opposite in sign to the applied load, it increases the yield stress. If the same sign as the applied load, then weakens the material. Since fatigue cracks originate on the surface of the material, inducing surface residual stresses improves fatigue properties.
Shot peening
Firing small steel balls at the surface of a material to form dents into the surface and induce compressive residual stresses.
Carburizing
Carbon is diffused into the surface of the steel at elevated temperatures and then quenched rapidly. This forms a had layer of Martensite on the surface. The quenching causes a structure change to BCT and due to the volume expansion, there is residual compressive stress induced.
