Fatigue life extension programs use a range of residual stress treatments, and the most widely used processes are shot peening, laser shock peening, and hole cold expansion. For a given objective and application, these processes should be considered.
Laser Shock Peening
Laser shock peening, often called LSP or laser peening, is another process that produces a layer of compressive residual stress at the surface of a part. The layer of compressive stress from LSP is typically much deeper than the layer from shot peening, as shown for high strength aluminum in the figure above. The large depth of the compressive stress gives LSP great potential for fatigue life extension.
LSP allows variation in process intensity. A low-intensity process is used in softer materials, and a high-intensity process in harder materials. For a given material, varying the intensity varies the depth of the compressive stress. The figure below shows the effect of five variations in process intensity, measured by laser irradiance (power per area), on the residual stress versus depth profile produced in Ti-6Al-4V. The depth of residual stress produced by LSP can be tightly controlled and can be optimized for a given objective.
LSP and shot peening deform the part surface. LSP creates a wavy surface, as shown below, which is not detrimental to fatigue performance. Shot peening, in contrast, produces a rough surface that degrades fatigue performance, and process design for shot peening must ensure a larger benefit from compressive residual stress than the detriment from surface roughness.
Because LSP imparts deeper stress, is a more controlled process, and leaves a good surface, laser peening typically outperforms other processes for fatigue life extension. However, LSP has a relatively high cost that can be prohibitive for many applications.
Hole Cold Expansion
These technologies allow us to comprehensively support customer needs in fatigue life extension programs. Fatigue crack growth forecasts for various process options, like those shown at right, are a typical deliverable.