Validation of the Preston–Tonks–Wallace strength model at strain rates approaching ?1011 s?1 for Al-1100, tantalum and copper using hypervelocity impact crater morphologies

Data from hypervelocity impact experiments shows that the size of an impact crater is a non-linear function of the projectile diameter for micrometre-scale and smaller sized impactors. This non-linearity is thought to arise due to the strain rate hardening of the target materials at the ultra-high strain rates experienced during the impact event (approaching 1010 s?1). Here we investigate this ultra-high strain regime using a combination of experimental and simulated results. The experimental work involved samples of monodisperse silica and sodalime glass spheres with diameters between 500 nm and 22 ?m which were fired onto aluminium 1100 alloy, high purity (99.5+%) tantalum and copper targets at a velocity of ca. 6 km s?1 using a light gas gun. Precise measurements of the resulting crater diameters were made using scanning electron microscopy (with a resolution of a few tens of nanometers). We also ran hydrocode simulations of the impact events using ANSYS’ AUTODYN to compare the modelled results against the experimental data. The Preston–Tonks–Wallace (PTW) constitutive model was used in the hydrocode as it specifically deals with loading events at very high strain rates. Comparison is also made with results obtained using the Steinberg-Guinan strength model which is also applicable at high strain rates.

The results demonstrate that current literature values for the PTW model need refining in order to accurately model micrometre-scale (and below), hypervelocity impact events. We present updated model parameters for Al, Cu and Ta and thus show that impact craters give an indirect measurement of the yield strength of a material at strain rates approaching 1011 s?1.