TRANSIENT RETARDATIONS IN FATIGUE CRACK-GROWTH FOLLOWING A SINGLE PEAK OVERLOAD

Retardation in the fatigue crack growth rate following the application of a single peak overload in a fatigue loading sequence has been studied for a low carbon structural steel. Tests have been performed at load ratios of R = 0.2 and R = 0.6 at a baseline stress intensity range, DELTA-K(b), corresponding to fatigue crack growth rates in the Paris regime. Single peak overloads were applied at a crack-length to specimen-width ratio of a/W = 0.5. At the load ratio of R = 0.6 monotonic or "static" fracture modes were observed upon application of the overload, and these produced an immediate increase in growth rate. A subsequent retardation is attributed to the presence of a residual compressive stress field ahead of the crack tip. A similar retardation was observed at a load ratio of 0.2. The importance of residual stress was established by performing stress relieving experiments. In addition, removal of the surface deformation after an overload by machining "T" sidegrooves resulted in an extended transient, which could not be explained by residual machining stresses.