An assessment of experimental techniques for measuring the mode I fracture toughness of UHMW-PE composites

Ultra-high molecular weight polyethylene (UHMW-PE) composites belong to the material class for protection systems with one of the best weight-specific performance against most common threats. Typically, ballistic composites show material specific deformation and failure phenomena during ballistic impact. Besides elastic-plastic deformation, shear plugging and failure of matrice and/or fibers, delamination of layers are on of the most important energy absorption mechanisms. The degree of delamination defines the amount of energy absorbed and the shape of back face bulge of the target. The back face bulge must be predicted throroughly, particularly for components close to the body, such as vests and helmets. The most common experimental technique for determining the fracture toughness is the Double Cantilever Beam test (DCB). Typically, tests are carried out using standardized DCB specimens. However, as will be shown in this paper, the standard test method is not suitable for measuring the mode I fracture toughness of UHMW-PE composites. If the standard test procedure is followed, the cantilever portions of the DCB specimen break due to the relatively low bending stiffness of the material. For this reason, several modifications to the standard test setup were tested and evaluated for their potential to produce reliable mode I fracture toughness data. This paper presents a discussion on the advantages and limitations of the different test setups. Finally, an alternative specimen geometry and test setup, which are particularly suitable for flexible composites, are presented.