Yield and transient effects during the plastic deformation of solid polymers

Abstract

Tensile tests were performed on seven commercial polymers at 22° C and at constant true strain rates of 10⁻⁴ to 10⁻¹ sec⁻¹. The constant strain rates were imposed on the minimum section of each sample with the aid of a diametral transducer, an exponential function generator and a closed-loop hydraulic testing machine. The polymers investigated were: high and low density polyethylene, polytetrafluoroethylene, polypropylene. polyvinylchloride and polyamide 6 and 66. True yield drops were observed in the rigid glassy polymers, whereas yielding was more gradual in the semi-crystalline or plasticized polymers. Strain rate change tests were also performed, during which one order of magnitude increases and decreases were imposed on the specimens. “Normal” transients were observed at small strains in the samples containing a rubbery phase, while the transients were of an “inverse” nature in the samples containing a glassy phase. With an increase in the strain at which the change was initiated, the “normal” transients changed in character to “inverse”. Transient tests were also performed in which straining was interrupted to permit a period of stress relaxation or of holding in the unloaded condition prior to the resumption of straining. A quantitative model is proposed, based on the dynamics of plastic waves which accounts for the transition from “normal” to “inverse” transient behaviour with increasing strain, and also explains the opposite effects of stress relaxation and of specimen unloading on the restraining transients.