Challenges associated with shear characterization of a cross-ply thermoplastic lamina using picture frame tests

The current research investigates characterization of soft composite materials used in the manufacture of combat helmets. The material system considered is a cross-ply unidirectional (UD) sheet comprised of ultrahigh molecular weight polyethylene (UHMWPE) fibers consolidated with a polyurethane (PUR) based matrix. This paper presents efforts toward characterization of the material’s shear behavior using the picture frame (a.k.a. trellis frame, shear frame, rhombus) test method and systematically investigates parameters influencing measurement of shear stiffness including sample arm geometry, forming temperature, strain rate, and mechanical conditioning. A specific emphasis is placed on the importance of arm geometry in the shear characterization. Overestimation of the force curve is found to result from interference of supporting arm material outside the central region of the test specimen. Removal of the excess arm material results in more accurate measurements of shear stiffness, which are subsequently available for input into FE models that simulate forming processes.