This paper presents an analytical and experimental study of crack shape evolution in steel plate specimens under cyclic loading. It is widely known that the introduction of compressive residual stresses by cold working the surface can be highly beneficial in improving the fatigue performance of structural components. Although it is recognized that relaxation of surface compressive residual stress can reduce the potential benefits, the effects of residual stress on crack shape evolution are often overlooked. Previous studies have shown that the intensity of the surface compressive residual stress has a pronounced effect on fatigue crack initiation. A recently developed technique termed controlled stitch cold working, which applies differing intensities of compressive residual stress at specific regions in a structure, is shown in the paper to influence fatigue crack propagation life considerably. The approach can be used to prevent crack growth in particular directions, for example, encouraging a controlled or fail-safe scenario. The paper describes fatigue crack growth tests on shot peened specimens tested under uniaxial tension. The specimens were peened to varying intensities and of differing coverage areas. Cracks were initiated from starter notches after peening, meaning the approach can be used for fatigue performance enhancement at the construction stage or later as part of a maintenance and repair strategy. Crack growth retardation is apparent in all tests, and this is attributed to the constrained crack shape. Further work will consider the use of laser and ultrasonic peening to contain cracks from rapidly propagating from stress concentrations. This approach used with careful design of stiffness can significantly extend the fatigue crack propagation resistance of ship and offshore structures.
Numerical modelling of the fatigue crack shape evolution in a shot-peened steam turbine material
