Quality Waterjet Newsletter – 11/29/2005 Material Behaviors under the Impact of Waterjets A fundamental study of material behaviors under the impact of waterjets was done by Beutin et al.* in 1974. Today it is still a very useful source of information for this subject. This article will highlight its main points. An intensifier pump was used to generate pressure up to 5000 bar (72.5 psi) in their study. As a jet is discharging from the nozzle, it starts as a single column of liquid and then gradually turns into droplets because of friction of the surrounding air. The impact of a high-speed droplet on a solid body produces initial impact pressure (water-hammer pressure), followed by stagnation pressure at the impact center and shear stress caused by the radial outflow of the liquid. The typical duration of the initial impact pressure is only several microseconds. The loading history of the solid body under repetitive impacts of water droplets is a quasi-static load superimposed with high frequency cyclical loads. The load generated by a continuous single-column jet is steady stagnation pressure except at the initial moment of impact. Mass losses under the impact of a waterjet for three different metals (pure aluminum, zinc, and armco iron) were measured as a function of stand-off distance. The results agree with the theory – as the jet travels across the space, the number of droplets increases and thus the destruction potential (up to a certain point). However, in the case of artificial sandstone that was tested, the trend was different. Due to its low compressive strength, mass-loss occurred even at the jet’s stagnation pressure and therefore damage declined as the stand-off distance increased. Another series of tests with lead and aluminum as the target materials were done to further confirm these observations. Because lead is so soft (low in resistance again plastic flow), the stagnation pressure is enough to cause macroscopic plastic deformation. In fact when the stand-off distance was short (2.5 and 5 cm), the jet pushed the material aside. When the velocity of jet was below a certain point, no damage was observed, indicating that the stagnation pressure was below the threshold of plastic flow. In the case of aluminum it only responded to the impact pressure because its compressive strength was above the stagnation pressure. The repetitive impact of water droplets is similar to the loading in a fatigue test.  By using 100 mg mass loss as a criterion for material destruction under the jet impact, a pressure-time curve was plotted to compare with the standard fatigue-test curve for pure aluminum and AlMg5 aluminum alloy. The similarity is remarkable. A scanning electron microscope (SEM) was used to examine the damages caused by the jet on the three metals: pure aluminum, zinc, and armco iron.  The study revealed cracking and plastic deformation to different degrees for each of these metals. High deformation speed was believed to play an important role. Pure aluminum showed some strengthening effect under low impact pressure because of its ductility. * Beutin, E. F., Erdmann, F., and Louis, H., “Material behaviour in the case of high-speed liquid jet attacks,” Proceedings of the 2^nd International Symposium on Jet Cutting Technology, Cambridge, England, April 2– 4, 1974.

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