Fatigue Life Extension

Compressive residual stresses are known to improve fatigue performance and enhance damage tolerance. To provide fatigue life extension of existing parts, compressive residual stress treatment processes have been developed that create a region of compressive stress, either at the part surface (e.g., shot peening or laser shock peening) or in a high stress area (e.g., cold expansion of holes or coining). The compressive stress acts to slow the growth of fatigue cracks, providing an extended fatigue life and economic value (e.g., longer part lifetimes, longer inspection intervals, reduced sustainment costs, increased system availability, and higher safety margins).

Stress-life fatigue data illustrating life or strength improvements from shot peening and laser shock peening of a titanium alloy

Hill Engineering uses residual stress engineering to enable efficient development of compressive residual stress treatments for fatigue life extension. Our systematic approach establishes the potential value of competing processes by weighing estimated process benefits against production costs and schedules. Modern engineering tools and well-designed tests are then applied to address process qualification and meet certification requirements. Our work is backed by extensive experience in fatigue analysis and design, engineering software tools that support fast and efficient process design, and a proven record of quick response. Because Hill Engineering is not a residual stress treatment provider, our customers receive advice that is independent and objective, and engineering work that is portable if changes in process are needed down the line.

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.

Shot Peening

Shot peening produces a layer of compressive residual stress on a treated surface. It entails directing a stream of shot at the workpiece surface with sufficient velocity to create plastic deformation and a layer of residual stress. Shot can be metallic (often steel), glass, or ceramic particles. Shot peening is inexpensive and widely available. The compressive stress layer is typically shallow (0.01 to 0.02 inch (0.25 to 0.5 mm)), which leaves it susceptible to degradation by mechanical impact or thermal exposure. In the right application, shot peening is a very effective means for fatigue life extension, and shot peening is commonly applied to springs, gears, and shafts, as well as turbine blades, landing gear, and airframe elements.

Residual stress produced by shot peening (SP) and laser shock peening (LSP) in 7050-T7451 aluminum (measured by slitting)

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.

Residual stress from LSP for five different intensities (measured by slitting)

Surface deformation from LSP

Hole Cold Expansion

Hole cold expansion is a cost effective way to produce a ring of compressive residual stress around a through-hole in a part. The typical process pulls an oversized tapered mandrel through a hole, which causes plastic deformation around the hole and a resulting compressive residual stress. Typical expansion produces near yield-strength level compressive stress at the edge of the hole, which decays with distance from the hole edge, crossing zero at a distance of 0.5 to 1.0 times the hole diameter. Hole cold expansion has been applied extensively in aircraft applications in parts made from aluminum, titanium, and steel.

Compressive residual stress near a cold expanded fastener hole (measured by the contour method).

Assessing Options

Residual stress engineering enables an assessment of options for fatigue life extension. Key technologies Hill Engineering brings to fatigue life extension are

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.

Forecast of crack growth in a part without residual stress and with three options for residual stress treatment

Further information on residual stress engineering
Fatigue Life Extension
Quality Control
Distortion Engineering