This paper summarizes our extensive investigation on the low cycle (up to Nf1/4 5 10 4, where Nf is the number of cycles to failure) fatigue behavior of short glass-fiber-reinforced poly(ethylene terephthalate), or PET, thermoplastic [PET - Thermo plastic polyester for injection molding]. The modes of fatigue test include tension-tension, compression-compression, four-point bending (flexural) - all at frequency f 1/4 1-3 Hz, and flexural fatigue at f 1/4 30 Hz (ASTM D-671). All tests were stress-controlled with stress ratio R 1/4 Smin/Smax1/4 0.1, except for flexural fatigue at f 1/4 30 Hz where stress ratio R 1/4 1. The fracture surfaces of tested specimens were analyzed using scanning electronic microscopy (SEM). The results from this investigation provide comprehensive, up-to-date information and recommendations concerning methods for fatigue testing of injection-molded specimens and models, prediction and optimization of low cycle fatigue properties that play a key role in determining a highly stressed plastic parts life and end-use performance, preselection of PET-based plastic for various industrial applications.
High-impact polystyrene (HIPS) has been processed into a specific microstructure which exhibited the hard-elastic behavior previously restricted to crystalline polymers. Unidirectionally stacked profuse crazes formed in uniaxially elongated HIPS resembled the row nucleated structures of stacked lamellar aggregates bridged by extended fibrils. Films in this form possessed repeated high recovery from large extension without rupture. Stress-strain and stress-relaxation tests, performed in air and in a variety of liquids, and thermoelastic experiments indicated that this material has similar characteristics to those of hard-elastic crystalline polymers. Our results confirmed that the recovery process of elastic HIPS, like crystalline systems, is composed of an instantaneous component and a time-dependent healing mechanism. The first was found to be energetic in nature and the second conformational (entropic). A mechanism, based on the reversible extensive formation of fresh surface of the craze (or interlamellar) fibrils giving rise to the elastic restorative force, is proposed. Stress-induced sorption and release of liquid, a newly discovered property, is also discussed.
Models are proposed for initiation and completion of failure in unidirectionally reinforced composites under longitudinal shear. The models allow characterization of the effect of fiber volume content on the average shear strength of composites considered as homogeneous material. Results are compared with experiments.
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