The Effect of Surface Flaws on Nanosecond Shock Wave Induced Brittle Fracture
Nano pulse lithotripsy (NPL) is a novel medical technology to fragment urinary calculi through electrical discharge. Compared with traditional shock wave lithotripters utilizing focused shock waves, NPL is ideal for investigating the stress field induced by surface acoustic waves (SAWs), such as the leaky Raleigh waves (LRWs), on the fluid-solid boundary. A pioneering study has recently demonstrated the generation of LRWs induced by the spherically divergent shock wave at the borosilicate glass-water boundary in NPL treatment. The resultant tensile stress field was found to play an important role in the initiation of cracks and the formation of ring-like fracture on the glass surfaces. This prior work motivates us to investigate the effect of SAWs on surface flaws that can be artificially and controllably created on the glass surface by microindentations to mimic the surface erosion induced by cavitation. In this study, we used a microhardness tester with a Vickers indenter and applied a load of 1.0 kg with a dwell time of 10 s to produce indentations at various radial distances (1.0 mm, 1.5 mm, 2.0 mm and 2.5 mm) on the borosilicate glass samples (50 x 50 x 3.3 mm in LxWxH). Each indentation creates a pyramid shaped impression with an average diagonal length of 55 m and penetration depth of 7.8 m. At a standoff distance of 1.5 mm, the shock waves generated by the NPL probe were applied to the glass surface until the glass was broken. After each shock impact, the crack initiation and extension around each indentation site were recorded, from which the speed of crack development represented by the arc length per shock was calculated. Through these experiments, we have made the following important observations. First, the artificially induced surface flaws made by the microindentation can well control the location of the crack initiation since the presence of surface flaws will significantly weaken the glass surface. Under the effect of the maximum tensile stress (σ_(T,max)) generated by NPL shock wave impact, the cracks extending from the indentation impression site are predominantly aligned perpendicular to the direction of LRWs (which is also the direction of σ_(T,max)). Second, compared with the number of shocks required to initiate the ring-like fracture on the original (untreated) glass surface, fewer shock waves are needed to initiate the ring-like crack formation and extension from the impression site presumably due to the higher stress concentration built up at the tip of the surface flaws during NPL, which can greatly reduce the tensile stress or stress integral required to initiate a crack. The average speed of ring-like fracture formation at a radial distance about 1.5 mm is 0.26 mm per shock on the glass without microindentations. By contrast, the average speed of crack extension at the same radial distance with microindentations can increase to 0.34 mm per shock. Furthermore, the speed of crack extension varies with the radial distance of the microindentation from the NPL probe axis, largely following the variation of the local tensile stress integral generated by the LRWs. Altogether, these findings suggest potential synergy between shock wave-induced LRWs and surface flaws (e.g., produced by cavitation erosion pitting during shock wave lithotripsy) that may lead to improved stone comminution, which warrants future investigations.
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