13th International Conference on Fracture June 16–21, 2013, Beijing, China -1- Evaluation of Optimal Condition in Laser Shock Peening Process by AE Method Tomoki Takata1,*, Manabu Enoki1, Akinori Matsui2, Yuji Kobayashi2 1 Department of Materials Engineering, School of Engineering, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japan 2 Shintokogio, Ltd., 1-1-1 Nishiki, Naka-ku, Nagoya city, Aichi 460-0003, Japan * Corresponding author’s e-mail address: takata@rme.mm.t.u-tokyo.ac.jp Abstract Laser shock peening (LSP) is now recognized as an efficient surface treatment to improve the fatigue life of metal components. To evaluate the process conditions, Almen strip is used as well as shot peening. As this is limited to specific material and size, we cannot apply to a wide variety of substrates. Acoustic emission (AE) method is expected to be useful for understanding LSP process because an elastic wave generates when laser is radiated on a target. The water layer on a target confines the shock wave due to laser irradiations during LSP process. In this study, laser irradiations during LSP process were monitored by AE method with varying the temperature and the thickness of the water layer on a target, respectively. Impact forces during LSP process were obtained from detected AE waveforms by deconvolution technique. In addition, laser irradiations were observed by high speed camera, and the image was compared with the detected AE signal. This result shows that shock wave was emitted by not only laser ablation but also collapse of a bubble which was generated after laser irradiation and a bubble was affected by the water layer. Keywords Laser shock peening, Acoustic emission, Cavitation bubble 1. Introduction Peening is one of surface treatment method to improve the fatigue strength of metals by inducing compressive stresses generated from mechanical means such as hammer blows or by blasting with shot (shot peening). Laser shock peening technique, which use laser to generate the reaction force near material surface by laser ablation, can impact a layer of compressive stress four times deeper than that attainable from conventional shot peening. However, the effect of laser irradiation on materials and phenomenon near radiation point are not well understood yet. The residual stress in depth direction is generally used to evaluate the degree of laser peening. A sample for residual stress measurement has to be cut and electrochemically polished, which is a destructive method. Although there are several in-situ methods to evaluate residual stress during laser peening process such as probe beam polarization and VISAR method, these are some limitations such as an exact adjustment required for fiber position and applicability for only thin samples for the first and latter methods respectively. Acoustic emission (AE) method is one of the nondestructive evaluation methods to evaluate the size, location and generation time of deformation and damage in real-time. AE method has been successfully applied to shot peening method to analyze impact behavior of particles and evaluate the impact force by inverse analysis [1]. In this paper, AE method was applied to laser peening process. A high speed camera was also used to observe the laser radiation behavior. The impact pressure due to laser ablation was evaluated from the detected AE signals and simulated waveforms. The laser radiation phenomenon was discussed with results of AE and high speed camera observations. Cavitation is phenomenon that a bubble generates by pressure difference in stream and collapses with emitting shock wave after the short time. In our present group, a cavitation bubble was observed during laser shock peening process [2].
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