Residual stresses are those which are locked into a component, independent of any external, or applied loads. Residual tensile stresses, often introduced during machining or as a result of uneven cooling of welds or castings are detrimental as they facilitate crack propagation.
Shot peening adds a layer of residual compressive stress beneath the surface of a component, which can counter-act tensile residual stresses, and applied (in service) stresses, significantly increasing the life of a component before failure.
Understanding the residual stress within a component can prevent cracks and failures by helping to identify which areas benefit most from surface treatment and tuning the process to suit. The Electronic Speckle Pattern Interferometry (ESPI) method is one of the ways to measure the effect of residual stress.
ESPI, sometimes referred to as TV Holography, is a technique which uses laser light, together with digital imaging and processing to visualise minute displacements on the surface of components. Combined with the hole drilling technique this can provide accurate measurement of the residual stress within a component.
The hole drilling technique involves drilling a small blind hole into the surface of a component. When a small amount of material is removed, the remaining material finds a new stress equilibrium, resulting in a slight distortion on the surface surrounding the hole. The measurement of this distortion allows the stresses which were previously present to be measured. Repeated measurements through incremental hole drilling gives a stress profile through the near surface.
Traditionally the displacement has been measured using a rosette strain gauge applied to the surface. Electronic Speckle Pattern Interferometry (ESPI) is an optical measurement technique whereby laser light is split and projected onto the surface of the component and images of the illuminated surface are captured by digital processors. By capturing images before and after material removal, sub-micron displacements can be measured across the entire image. From this displacement, together with information about the material, the residual stress can be calculated.
With accurate information of the residual stresses, it is possible to optimise shot peening parameters to enhance the components life and/or performance. The introduction of ESPI testing is a significant investment for Sandwell. It will aid the understanding of the effects of shot peening on components and support many of the latest research projects taking place at Sandwell.