A mechanical component may fail i.e. it may be unable to perform its function satisfactorily, as a result of any one of the following three modes of failure:

1. Failure by elastic deflection.
2. Failure by Yielding
3. failure by Fracture

1. Failure by elastic deflection

• In applications like transmission shaft supporting the gears, the maximum force acting on the shaft, without effecting it performance is limited by the permissible elastic deflection. Sometimes the elastic deflection results in unstable conditions, such as buckling of columns or vibrations. The design of mechanical component, in all these cases, is based on the permissible lateral or torisional deflection. The stresses induced in the component are not significant and properties of the material are not of primary importance. The moduli of elasticity and rigidity are the important properties and dimension of component are determined by the load deflection equation.
• In short, in a components like : columns, beams, shafts etc.,the torsional deflection in an elastic region is termed as failure of the component

2. Failure by Yielding

• For ductile material deformation occurs after the yield point, resulting in permanent deformation of the machine element which ultimately breaks at breaking point. Hence for ductile materials, failure is usually considered to have occurred when yielding i.e. plastic deformation reach a limit, when engineering usefulness of the part is destroyed, even through there is no rupture or fracture of machine part. Thus, the yield point is criterion of failure of ductile materials subjected to static loading.
• In short, when a mechanical component, made of ductile material, undergoes yielding or plastic deformation, its functional utility comes to an end and it is termed as failure of the component. Such failure is known as elastic failure.

3. Failure by fracture

• In case of brittle materials the yield point and ultimate strain is very nearly equal to unity. So brittle materials are considered to have failed by fracture with little or no permanent deformation.
• Sudden separation or a breakage of a material along the cross-section normal to the direction of stress is known as fracture. Fracture is a sudden failure without plastic deformation. The failure of components made of brittle material is due to fracture.