High strain-rate shear and friction characterization of fully-dense polyurethane and epoxy

Abstract

A series of oblique plate impact experiments were conducted in an effort to observe the high strain-rate mechanical behaviors of two thermosetting polymers; fully dense, high durometer – low viscosity formulations of polyurethane (PUR) and epoxy. Two configurations of oblique impact experiments were utilized. For both, an angled projectile was accelerated towards an angled target via slotted-barrel light-gas gun, facilitating the oblique-planar impact of the projectile/target faces. This produced high strain-rate (~105 s−1) combined normal (pressure) and shear stress loading in specimens with magnitudes up to 1.0 and 0.2 GPa, respectively. To observe friction characteristics, a polymer flyer impacted an aluminum anvil and a coefficient of friction was calculated from the ratio of shear to normal stress measured at the interface. Alternatively, an aluminum flyer impacted a thin-polymer specimen confined between two aluminum anvils. Polymer strength was then calculated from measured shear stresses, observed over a range of confining pressures by increasing impact velocity. For both configurations, stresses were inferred from free surface particle velocity histories acquired through transverse photon Doppler velocimetry techniques.Coefficients of friction (COFs) of 0.11 and 0.33 were found for PUR, while values of 0.26 and 0.48 were found for epoxy. Both saw an increase in COF with decreasing pressure, a trend consistent with literature in polymer tribology. Shear strengths were found to increase with pressure for both tested polymers. For PUR only, a pressure threshold was observed after which strength decreased. The mechanism for this decrease is not known but has been previously observed in shock compression experiments conducted on polymethylmethacrylate. The applicability of results in demonstrated by implementing data into constitutive and equation-of-state models in CTH and successfully reproducing experimental results.