Shear strength of homopolymer and copolymer aramid fibers

Abstract

Novel shear experiments with homopolymer and copolymer high performance aramid fibers were conducted via a custom designed MEMS device to examine the correlation between shear and tensile strength. A finite element analysis was utilized to design the shear test fiber specimen geometry in order to eliminate the stress singularity at the notch tip and ensure uniform shear stress in the majority of the test shear zone. The experimental results from individual aramid fibers pointed to the significantly higher shear strength of the copolymer fibers: the average shear strength values of the homopolymer and the copolymer fibers were 85 ± 7.6 MPa and 169 ± 17 MPa, respectively. In comparison, the tensile strength of the homopolymer fibers was 3.97 ± 0.3 GPa, and of the copolymer fibers 5.1 ± 1.2 GPa, namely a 100% increase in shear strength resulted in 25% higher tensile strength. This drastic difference in fiber shear strength is attributed to stronger intermolecular interactions stemming from improved crystallite order in copolymer aramid fibers with 2–4° misorientation angles, as opposed to an average 16.7° misorientation angle in the homopolymer fibers. The shear failure surfaces of copolymer fibers were smooth, comprised of highly oriented fibrils, compared to the wavy failure surfaces of homopolymer aramid fibers, which were characterized by interfibrillar splitting.