An analytical and experimental study of the behavior of semi-infinite metal targets under hypervelocity impact

In this study the material strength and strain rate effects associated with the hypervelocity impact problem were considered. A yield criterion involving the second and their invariants of the stress deviator and a strain rate sensitive constitutive equation were developed. The part of total deformation which represents change in shape is attributable to the stress deviator. Constitutive equation is a means for analytically describing the mechanical response of a continuum under study. The accuracy of the yield criterion was verified utilizing the published two and three dimensional experimental data. The constants associated with the constitutive equation were determined from one dimensional quasi-static and dynamic experiments. Hypervelocity impact experiments were conducted on semi-infinite targets of 1100 aluminum, 6061 aluminum alloy, mild steel, and commercially pure lead using spherically shaped and normally incident pyrex projectiles. The strain rates encountered in such impacts were determined in the vicinity of the axis of symmetry taking into account the initial hydrodynamic and the subsequent material strength affected regimes. The determination of strain rates was based on the attenuation of the peak pressure with the advancing shock front in the target. Using the developed constitutive equation and the strain rates in the material strength affect regime, the stress-strained curve and the dynamic yield strength were obtained. This dynamic yield strength was adapted into an existing penetration equation and the crater depths were predicted. Good agreement was found to exist between the theory and experiment indicating that the dynamic yield strength of the target material is an important parameter. The error in predicting the crater depths was within 5% except in the case of lead targets. The discrepancy with reference to lead targets may be attributed to the complex behavior of lead in quasi-static as well as dynamic testing because of its soft nature and probable thermal effects