Investigation of Sample Geometry and Strain-Rate Dependence in Shear Characterization of a UHMWPE Unidirectional Cross-ply for Finite Element Simulation of Composite Processing

Abstract

In-plane shearing is the dominant mode of deformation in many composite forming processes, including thermoforming. The shear-frame, or picture-frame, test is a widely accepted method to characterize the shear behavior of a material system to find the properties to use in forming simulations, and this test has been shown to be applicable for providing shear stiffness as a function of the state of shear for a variety of woven-fabrics based material systems. The current research explores the use of shear-frame testing of a non-woven material system made of Ultra High Molecular Weight Polyethylene (UHMWPE), specifically DSM Dyneema® HB210 unidirectional fiber cross-ply. The material system was characterized at an elevated temperature for processing applications. The effects of sample size and sample geometry were investigated. The load contribution from the sample arms is of particular interest for these types of materials, so an investigation of appropriate gage area and normalization methods was performed. Finite element simulations of the shear-frame test were completed to validate the characterization methodology. Also, during the processing of UHMWPE material systems into complex shapes, the rate of shear in different sections of the part will differ. Therefore, the effects of strain rate on shear characterization were explored by performing constant strain-rate shear frame testing over a range of shear rates. In a typical shear-frame test, the crosshead rate remains constant which leads to an increasing shear strain rate. The decreasing crosshead rate required to maintain a constant shear rate was obtained and implemented into the testing software. Representative shear curves were examined to determine if rate-dependence is required in a finite element simulation of the thermoforming process.