Date of Award
Master of Science (MS)
Oliver J. Myers
Laser shock peening is a cold working process which is used to improve material properties like surface hardness, fatigue life, wear and corrosion resistance, etc. It is widely used to treat turbines, fans, compressor blades, aircraft and automotive parts. When the material is irradiated by high power density laser beams, shock waves are generated, which plastically deforms the material surface and induces high compressive stresses within subsurface area. The amount of residual compressive stress and plastically affected depth depend on laser parameters (laser power density, pulse duration, wavelength, repetition rate, spot size and shape), materials, ambient environment, etc. To improve the application of laser shock peening, it is of critical importance to optimize the process by fully understanding the effects of different parameters. Extensive studies have been devoted to this area. Recently, thanks to the advance of laser technology, high repetition rate lasers could significantly improve this technique by increasing compressive residual stress and plastically affected depth. This research studies the effect of laser repetition rate at different spot sizes and different scanning patterns of shot application on the final shock peening results by finite element modeling.
A two-dimensional finite element model is developed to simulate the interaction between metal samples and laser-induced shock waves. Multiple laser impacts are applied at each location to increase plastically affected depth and compressive stress. The in-depth and surface residual stress profiles are analyzed at various repetition rates and spot sizes. It is found that the residual stress is not sensitive to repetition rate until it reaches a very high level. At extremely high repetition rate (100 MHz), the delay between two shock waves is even shorter than their duration, and there will be shock wave superposition. It is revealed that the interaction of metal with shock wave is significantly different, leading to a different residual stress profiles. Stronger residual stress with deeper distribution will be obtained comparing with lower repetition rate cases. The effect of repetition rate at different spot sizes is also studied. It is found that with larger laser spot, the peak compressive residual stress decreases but the distribution is deeper at extremely high repetition rates.
A three-dimensional finite element model is developed to study the effect of scanning pattern and repetition rate. The final residual stress distributions are studied at repetition rates of 0.1 MHz, 1 MHz, 10 MHz and 20 MHz for 5 different patterns. It is found that there are no major differences in residual stress profiles due to variation of scanning patterns except for circular pattern. It is also revealed that the minimum residual stress decreases and non-uniformity increases with increase in repetition rate due to interaction of relaxation waves with incoming pressure pulses. To minimize the effect of relaxation waves, two zig-zag patterns are studied. The overlap between successive spots is less in zig-zag pattern-1, and it is completely absent in zig-zag pattern-2. It is found that by applying the newly proposed zig-zag pattern-2, the residual stress uniformity can be significantly improved at repetition rates higher than 0.1 MHz.
Kosaraju, Sai Hamsini, "Optimization of Laser Shock Peening Process Using Finite Element Modeling" (2018). All Theses. 3252.