Unsteady penetration of a target by a liquid jet.

It is widely acknowledged that ceramic armor experiences an unsteady penetration response: an impacting projectile may erode on the surface of a ceramic target without substantial penetration for a significant amount of time and then suddenly start to penetrate the target. Although known for more than four decades, this phenomenon, commonly referred to as dwell, remains largely unexplained. Here, we use scaled analog experiments with a low-speed water jet and a soft, translucent target material to investigate dwell. The transient target response, in terms of depth of penetration and impact force, is captured using a high-speed camera in combination with a piezoelectric force sensor. We observe the phenomenon of dwell using a soft (noncracking) target material. The results show that the penetration rate increases when the flow of the impacting water jet is reversed due to the deformation of the jet-target interface--this reversal is also associated with an increase in the force exerted by the jet on the target. Creep penetration experiments with a constant indentation force did not show an increase in the penetration rate, confirming that flow reversal is the cause of the unsteady penetration rate. Our results suggest that dwell can occur in a ductile noncracking target due to flow reversal. This phenomenon of flow reversal is rather widespread and present in a wide range of impact situations, including water-jet cutting, needleless injection, and deposit removal via a fluid jet.

Determination of the temperature rise within UHMWPE tibial components during tribological loading.

The wear of ultrahigh molecular weight polyethylene (UHMWPE) is considered as one of the major reasons for revision of artificial joints. While in vivo measurements have shown a significant temperature increase in knee implants, the amount of heat dissipated within the UHMWPE tibial component and its influence on the friction behavior when paired with a cobalt-chromium (CoCrMo) femoral component is unknown. Our goal was to address these questions by measuring the temperature rise over a wide range of tribological loading conditions that mimic certain spots on artificial knee joints. The temperature rise as a function of lubricant, sliding velocity, coefficient of friction and maximum load was determined and analyzed. Additionally, the heat gradient during constant loading was investigated that allows the calculation of heat flow. The test setup consists of a wheel-on-flat laboratory testing device. Tests were performed in ambient air and different lubricants. During the tests, the temperature rise in the polyethylene was recorded with embedded thermocouples. The temperature rise was high and shown to be directly linked to load, coefficient of friction and relative velocity. Because it is generally assumed that the applied energy is an indicator for the development of wear in particles, some considerations for the design of knee joints are proposed based on our observations. The amount of heat dissipated in the polyethylene under cyclic loading was measured and is discussed in comparison with the theoretical model of temperature in friction pairs.