in Vol. 9 - January Issue - Year 2008
Fatigue Testing Used To Verify Benefits Of LaserPeen® Processing
Figure 1: Three point bend fatigue test results for LaserPeen
Figure 2: View of LSPT
The benefit provided by LaserPeen® processing is significant; how does LSPT assist its customer with the application of this process?
Surface enhancement processes such as shot peening and laser peening are applied to increase the fatigue life of metallic components. While conventional metal shot peening is used as a broad area coverage process to extend the fatigue life over the surface of the components, laser peening is used to provide deep compressive stress at critical high-stress locations on the component. For these applications, there is a requirement to quantify the specific fatigue benefit achieved at the location laser peening is applied.
LSPT works with its customers to quantify the benefit of the process on their components. This initial step starts by evaluating the process on coupons to determine the residual stress profile that will provide the desired benefit needed for the component. Next, fatigue tests are generally conducted on coupons or on components to measure the actual fatigue benefit achieved for the intended application. In the past, these tests have typically been conducted by the customer in their facilities. However, LSPT has recently made a large investment and installed the capability to conduct fatigue tests on components at its facility.
What test facilities have been installed at LSPT to accomplish this testing?
LSPT has developed the capability to conduct tests on both coupons and actual components. Coupons with specific geometrical configurations can be tested on our servo-hydraulic fatigue stand to generate stress-life curves to evaluate the effect of deep compressive surface stress. Geometrical configurations may include specific types of notches to evaluate the effect of stress concentrations versus surface residual stresses on the fatigue life. This information is useful to initially evaluate the benefits of the surface residual stresses and to generate engineering data for component design. For example, the effects of laser peening on three point bend fatigue results for Ti-6Al-4V alloy are shown in Figure 1.
Once basic fatigue data is generated on coupons, the actual benefits of a surface enhancement process for a component are verified by component fatigue tests. Component fatigue tests are an important method to confirm that laser peening achieved the desired benefit and is generally required for engineering substantiation prior to starting production.
Component fatigue tests can be conducted on one of three fatigue systems at LSPT. Two of these involved a forced fatigue response where the test can be controlled to excite a specific fatigue mode in the component. One of these is shown in Figure 2 where a blade component is being fatigue tested on a shaker table. This system can achieve high frequencies and provides the ability to excite a variety of higher order fatigue modes in a component. Generally, component tests are conducted a lower order modes such as in first or second bend where the test frequency is typically under 1,000Hz. In these cases, the siren tests (forced response) and air horn tests (natural response) provide sufficient capability to conduct component fatigue tests. The test setup for these two systems is relatively easy and the test can be easily reproduced without a lot of difficulties.