Dynamic behaviour of UHMWPE yarns and addressing impedance mismatch effects of specimen clamps

The dynamic stress–strain behaviour of Spectra® 900 yarns, made of highly oriented ultra-high molecular weight polyethylene (UHMWPE), is determined by means of a tensile split Hopkinson bar arrangement incorporating special grips to clamp the specimens. Test results show that a five-element viscoelastic constitutive model captures the behaviour of Spectra® 900 yarns with good fidelity. Not uncommonly, as in this instance, grips used to clamp specimens introduce an impedance mismatch with the input/output bars, and the design of the clamps may not be amenable to modification to yield an impedance match. Consequently, an algorithm is developed to relate the strain signals from the input/output bars to what is experienced by the grips, and thus what is imposed on the specimen. This enables the extraction of corrected quantities for insertion into the Hopkinson bar equations. The proposed method is validated by finite element analysis.