13th International Conference on Fracture June 16–21, 2013, Beijing, China -5- 0 200 400 600 800 0 200 400 600 800 y = -1.2372 + 0.99631x R= 0.99962 Figure 4. Relationship between bubble collapse time, tc and AE interval time, Δ t in room temperature 3. Results and discussion 3.1 Detected AE waveform compared with photos by high speed camera During laser shock peening process, a bubble was generated after laser irradiation. Fig. 3 shows typical AE waveforms detected during laser peening process coupling with photos recorded by the high speed camera and a graph demonstrated a change of radius of a bubble on sample surface. Two AE events were generated with time duration of several hundred microseconds for each laser irradiation. The results of high speed camera showed that after laser irradiation a bubble generated and expanded its size up to the maximum radius. Then it became smaller and finally collapsed. According to this observation, the first and latter AE events were considered to correspond to laser irradiation and bubble collapse, respectively. Let t1 and t2 are times for the first and second AE events, respectively while tc and Rmax are time for bubble collapse and maximum bubble radius obtained from the high speed camera results, respectively. A time interval between two events, Δt, is defined as t2 – t1. A plot of Δt vs. tc and Rmax vs. tc for experiment condition (a)-1 is shown in Fig. 3 and 4, respectively. A linear relationship of these parameters supported that the first and latter AE events were generated from impacts due to laser irradiation and bubble collapse, respectively. Fig. 4 shows a plot of in room temperature. It is clear that larger bubble required longer collapse time at constant temperature. 3.2 A cavitation bubble A cavitation bubble was generated by laser irradiation, expanded, shrank and collapsed. A bubble was almost hemisphere, but after bubble collapse a bubble became ellipsoid, repeated expanding and shrinking, and finally became some smaller bubbles (Fig. 3). In the case of laser irradiation from horizontal direction (Fig. 1(a)), a bubble comes off the sample after second collapse, so AE event was not detected. In case of laser irradiation from vertical direction (Fig. 1(b)), they are sometimes detected more AE event than three when a bubble does not come off. Time detected third and fourth AE event is equal to second and third bubble collapse time respectively (Fig. 5). By using HyperVision HPV-2A, collapse of a cavitation bubble was observed in detail with the sampling rate of 1,000,000 fps and pixels of 312 × 256 (Fig. 6). Immediately after bubble collapse,
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