Constitutive behavior of a Twaron fabric/natural rubber composite: Experiments and modeling

Ballistic fabrics made from high performance polymeric fibers such as KevlarRTM and TwaronRTM fibers and composites utilizing these fabrics are among the leading materials for modern body armor systems. Polymeric fibers used to produce ballistic fabrics often behave viscoelastically and exhibit time- and rate-dependent stress-strain relations. This necessitates the study of the constitutive behavior of composites filled by ballistic fabrics using viscoelasticity models.;In the present work, the constitutive behavior of Twaron CT709 RTM fabric/natural rubber (TwaronRTM/NR) composite is studied using three viscoelasticity models (i.e., a three-parameter generalized Maxwell (GMn=1) model, a four-parameter Burgers model, a five-parameter generalized Maxwell (GMn =2) model) and a newly developed para-rheological model. The new model utilizes a three-parameter element to represent the TwaronRTM fabric and the affine network based molecular theory of rubber elasticity to account for the deformation mechanisms of the NR constituent. The uniaxial stress-strain relation of the TwaronRTM/NR composite at two constant strain rates is experimentally determined. The values of the parameters involved in all the models are extracted from the experimental data obtained in this study. The stress-relaxation response (under a uniaxial constant strain) and the creep deformation (under a uniaxial constant stress) of the composite are also experimentally measured.;The stress-strain relation at each strain rate predicted by the newly developed para-rheological model is seen to be in good agreement with the measured stress-strain curve over the entire strain range studied. It is shown that the new model also predicts the elastic moduli and ultimate stress of the TwaronRTM/NR composite well. An implicit solution provided by the para-rheological model is shown to predict the creep response of the composite more accurately than all the other models at both the primary and secondary stages. The mathematical complexity that arises from including an additional Maxwell element to the GMn =2 model to obtain the GMn =2 model with enhanced predictability is traded with the use of simple characteristic time functions in the para-rheological model. The relaxation and creep trends predicted by the para-rheological model indicate that the long time viscoelastic response of the composite lies between that of a crosslinked polymer and a semi-crystalline thermoplastic.