Crank shaft is one of the vital components of the engine which is greatly stressed and subjected to bending and torsional loads. It works with fourlink mechanism converting reciprocal displacement of piston to rotatory motion. The design and operational efficiency of crankshaft determines the life span for an engine. Let us consider a single cylinder four stroke cycle engine which is an Internal Combustion Engine (IC Engine).

                     An IC engine works on slider-crank mechanism its constituents being a crank shaft, connecting rod and piston. The combustion of gas in the combustion chamber gives rise to linear displacement of the piston. The piston acts as a slider in the mechanism whose sliding motion is converted to rotatory motion. The displacement of piston is accompanied with sudden shocks and may cause damage to the devices taking input from the engine. A crank shaft functions in converting the sudden shocks to smooth rotatory output. Some devices which use this mechanism are generators, compressors, pumps.

                      The crank shaft experiences a lot of force on its entire geometry by the thrust given to piston, connecting rod, crank shaft sequentially one on another. The parameters determining the magnitude of force on crank shaft are piston, piston rings, pin, connecting rod dimensions and weight, crank radius. Two types of loading impact are created on the crank structure by forces of combustion and Inertia, namely torsional load and bending load. Failure in the engine occurs due to operational, mechanical and wear-out factors. Crank-shaft ICE has its failure mainly due to fatigue at fillet areas due to bending load caused by torsion. The stress range is large during the life span of the crank fillet, because of the design and geometry of the crank and engine. At the root of the fillet areas large stress is subjugated with high stress range. These locations with high stress range are the points where the cyclic loads could cause fatigue crack initiation leading to fracture. Cranks free from internal flaws are also expected to have bending and torsional fatigue resulting in cracks at pin surface, oil hole surface or crank radius