Viscoplastic Effects Occurring in Impacts of Aluminum and Steel Bodies and Their Influence on the Coefficient of Restitution

摘要

Generally speaking, impacts are events of very short duration and a common problem innmachine dynamics. During impact, kinetic energy is lost due to plastic deformation nearnthe contact area and excitation of waves. Macromechanically, these kinetic energy lossesnare often summarized and expressed by a coefficient of restitution, which is then used fornimpact treatment in the analysis of the overall motion of machines. Traditionally, thencoefficient of restitution has to be roughly estimated or measured by experiments. How-never, more recently finite element (FE) simulations have been used for its evaluation.nThereby, the micromechanical plastic effects and wave propagation effects must be un-nderstood in detail and included in the simulations. The plastic flow, and thus the yieldnstress of a material, might be independent or dependent of the strain-rate. The firstnmaterial type is called elastic-plastic and the second type is called elastic-viscoplastic. Innthis paper, the influence of viscoplasticity of aluminum and steel on the impact processnand the consequences for the coefficient of restitution is analyzed. Therefore, longitudinalnimpacts of an elastic, hardened steel sphere on aluminum AL6060 rods and steel S235nrods are investigated numerically and experimentally. The dynamic material behavior ofnthe specimens is evaluated by split Hopkinson pressure bar tests and a Perzyna-likenmaterial model is identified. Then, FE impact simulations and impact experiments withnlaser-doppler-vibrometers are performed. From these investigations it is shown thatnstrain-rate effects of the yield stress are extremely small for impacts on aluminum but arensignificant in impacts on steel. In addition, it is demonstrated that it is possible tonevaluate for both impact systems the coefficient of restitution numerically, whereas for thenaluminum body a simple elastic-plastic material model is sufficient. However, for the steelnbody an elastic-viscoplastic material model must be included. u0001DOI: 10.1115/1.4000912