Optical in-situ analysis of the cavitation damage on technical alloys under repeated single bubbles

Single cavitation bubbles were used to investigate the effect of cavitation collapses onto a solid surface in a spatially and temporally controlled way. The focus of this work was on damage to Cu- and Fe-base alloys, as well as pure aluminum to make the connection to existing research. The single bubbles were induced by a focused laser pulse in distilled water. The expansion and the collapse of the bubbles in the vicinity of solid samples were recorded with up to two high-speed cameras using shadowgraphy. Also, a microscope observed damage on the solid surface in situ. This method provides insights into both small surface changes caused by the collapse of one single bubble as well as the cumulative damage caused by up to hundred-thousands of single-bubbles. Damage from many single bubbles formed patterns that showed to be mainly dependent on the stand-off distance g (the ratio of the distance from the bubble center to the surface and the maximum bubble radius). Somewhat surprisingly, even on the hardest material tested the first collapsing bubble could induce damage in the form of pits. The number of pits caused by one bubble varied stochastically from bubble to bubble. Across all materials, some bubbles caused no damage, while the maximum number of pits caused by one bubble decreased as the hardness of the tested material increased. As a parameter for the rate of early damage formation – the pitting – rate was defined as the slope of a linear fit to the number of pits per bubble. The pitting rate was dependent on both the stand-off distance and the bubble radius. The latter relation indicated a non-zero limit of the bubble radius that can cause pitting. For g ranging from 0.3 to 2, a correlation between pitting and the presence of stronger, non-axisymmetric regions during bubble collapse (for most g values predominantly observed during the second collapse) was identified. These non-axisymmetric regions were the part of the bubble that collapses last, and thus were found to be associated with non-simultaneous nature of the collapse. For g > 2, no damage could be observed even for many single-bubble collapses. For g = 1.4 the stronger collapse regions were also the regions where a strong shock wave was emitted during the second collapse.