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Recently, a hyperelastic-viscoplastic constitutive model was developed for PET and the noncrystallizing copolymer PETG (R. B. Dupaix, Ph.D. thesis, MIT, 2003). The materials were found to behave very similarly under monotonic loading conditions and the single constitutive model was able to capture both materials' behavior. However, differences were observed upon unloading, and it is expected that additional differences would be observed under more complex loading conditions. Here their behavior is investigated under nonmonotonic loading conditions, specifically under load-hold conditions. The model of Dupaix and Boyce (R. B. Dupaix, Ph.D. thesis, MIT, 2003) is modified to include Ahzi's upper-bound model for strain-induced crystallization [Ahzi et al., Mech. Mater., 35(12), pp. 1139–1148 (2003)1. The crystallization model is adapted to include criteria for the onset of strain-induced crystallization which depend on strain rate and level of deformation. The strain-rate condition prevents crystallization from beginning prior to the deformation process slowing significantly. The level-of-deformation condition delays crystallization until the material has deformed beyond a critical level. The combined model demonstrates differences in behavior between PET and PETG during complex loading situations, indicating its ability to capture the fundamental criteria for the onset of strain-induced crystallization. [ABSTRACT FROM AUTHOR]

An investigation of two tests commonly used to determine resistance to slow crack growth in PE pipes and materials is detailed, in order to gain a greater understanding of the mechanisms involved and to resolve differences in results observed. The full notch creep test (FNCT) is carried out on small notched bars machined from sheet or pipe loaded to create high constraint at the notch tip. The notched pipe test (NPT) is a pressure test on pipe containing external machined notches. In this test, it has been observed that the use of more flexible materials allows deformation in the crack tip region and contributes to slow crack growth resistance via crack tip blunting. Good pipe performance can be achieved by selecting materials with high inherent slow crack growth resistance or by combining inherent resistance with blunting mechanisms promoted by a lower density material. It is concluded that the FNCT test, while useful for an indication of inherent slow crack growth resistance, cannot be used to predict pipe performance for a range of materials, and therefore is unsuitable as a reference test for a pipe product specification. The NPT test remains the benchmark test for pipe performance and is referenced by many specifications. [ABSTRACT FROM AUTHOR]

The mechanical properties of four-step three-dimensional braided composite at high strain rates are important in evaluating this kind of composite under dynamic and impulsive loading. The in-plane and out-of-plane compressive properties of E-glass/epoxy four-step three-dimensional braided composite at strain rates from 300 to 3500 s-1 were tested with split Hopkinson pressure bar. The compressive properties at quasi-static strain rate (10-3 s-1) were also tested and compared with those at high strain rates. From the experimental data, the stress-strain curves, compressive stiffness, and compressive deformation of the composite are rate-sensitive in in-plane and out-of-plane compressive directions. In the out-of-plane direction, the compressive deformation mode was observed to have a deformation transition from axial compression to shear as the increase of strain rate, whereas only compressive deformation was found for in-plane direction compression. The failure stress and strain under compression at various strain rates have no regular tendency as the strain rate increased from 10-3 to 3500 s-1. [ABSTRACT FROM AUTHOR]