A~Inves~tions have been carried out to study the relaxation of the surface residual stress in 0.23% C steei due to the application of fatigue loading. The residual stress was induced in the specimen by pre-straining and was measured by X-ray back reflection method using Cr-Ku radiation. The surface residual stress induced, depends on the plastic strain and appears to bear a relation of the type uRr = u&P)o.=. The decay of the residual stress appears to depend on log N, given by the relation uRal = am0 -K log N, where N is the number of fatigue cycles. The constant K depends on the initial value of the residual stress.
RESIDUAL stresses have been found to affect the fatigue life of materials. In a recent paper[lj it has been shown that introduction of pre-straining increases the crack nucleation period and decreases the crack propagation, as a result of which the fracture life is increased. Tensile pre-straining is reported121 to introduce a compressive residual stress on the surface layer of the material. A model of the introduction of the compressive residual stress is shown in Fii. 1, where it could be seen that the surface layers being free on one side can easily flow whereas the core, being restricted from free elongation follows a different stress-strain relation. On relieving the load, the surface layer is compressed by the core, resulting in a compressive residual stress. Application of an alternating stress is reported[3] to decrease the residual stress, the decrease being very sharp in the first cycle itself.
The aim of the present work is to establish q~nti~tively the surface compressive residual stress for different values of pre-strain and to study how the relaxation of the residual stress takes place on the application of a fatigue type of loading.