Design Of Machine Elements
- 1. Gandhinagar institute of technology Subject:Design of Machine Elements(2151907) Topic:Fatigue Failure
- 2. Fatigue Failure • The premature failure of strength of a material due to repetitive stresses which may be above or below its yield strength. • Fatigue failure is defined as time delayed fracture under cyclic loading. • Magnitude of fatigue failure decreases as the number of stress cycles increase.
- 3. Fatigue Failure • As shown in figure,there is a wire of diameter 2 to 3 mm. • If we want to cut it into two pieces, one method is to shear the wire by two equal and opposite forces P1 and P2 as shown in fig.(a). • Practically it is very difficult to cut the wire by this method.
- 4. Fatigue Failure • Other two figures (b) and (c) indicates bending and unbending of the wire. • As shown in fig.(b) when the wire is bent, A is subjected to tensile stress while B to compressive stress. • While in fig.(c), there is compressive stress at A abd tensile at B. • Hence,the cycle is made by compressive and tensile stresses. • Therefore,after few cycles wire can be cut very easily.
- 5. Fatigue Failure • Fatigue failure begins with a crack at some point in the material. • The crack is more likely to occur in following regions: – Regions of discontinuity like oil holes,screw threads etc. – Region of irregularities like scratches,stamp and inspection mark. – Internal cracks due to defects in material like blow holes.
- 6. Fatigue Failure • Fatigue failures are common in transmission shafts, connecting rods,gears,vehicle suspension springs and ball bearings.
- 7. Fatigue Failure • Fatigue cracks are not visible till they reach the surface of the component and by that time, failure has already taken place. • The fatigue failure is sudden and total. • It is relatively easy to design a component for a static load, because fatigue failure depends upon number of cycles, mean stress, stress amplitude, stress concentration, residual stresses, corrosion and creep. • Therefore,design of components by fatigue loading are very complex.