Pre-Yield Strain Hardening in Thermoplastics
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PRE-YIELD STRAIN HARDENING IN THERMOPLASTICS DAVID C. MARTIN The University of Michigan, Macromolecular Research Center, 1600 Bonisteel Blvd., Ann Arbor, MI 48109 Current Address: The University of Massachusetts at Amherst, Polymer Science and Engineering, 701 Graduate Research Center, Amherst, MA 01003 ABSTRACT Successive stress relaxation testing was used to investigate the strain hardening of polypropylene and polystyrene in the stage of deformation before yielding. By combining this information with that of a stress relaxation test it was possible to measure the change in flow stress with plastic strain or "workhardening" parameter K. K has been associated with the nucleation of "defects" of some sort which slow down the kinetics of the deformation process. Both polymers were found to strain harden in this deformation region. In polystyrene, the amount of time need to relax through a fixed stress increment reached a plateau at a point corresponding with visible crazing in the gage section of the sample. The workhardening parameter K was determined and found to decrease with stress. By plotting the rate of change of flow stress with total strain plots were obtained which avoided the use of strain, an ill-defined parameter in materials which change state during deformation. From these plots it was seen that polystyrene exhibits a well-defined linear region at small strain whereas polypropylene deviates from linearity immediately. Hardening of polystyrene was observed even in the linear response regime. INTRODUCTION The molecular origins of plastic deformation in completely or partially amorphous systems like conventional thermoplastics is often compared with the defect-based dislocation mechanism known to operate in crystalline materials. Several molecular models have been invoked to explain the origins of small strain plastic behavior in polymers including "strophons" E1], "rotons" and "tubons" (2), "dislocations" [3), "disclinations" [4), and "kinks" (5). Recently, Escaig et al. [3,63 have described a method of characterizing the plastic deformation of polymers in the stage before general yield. Their technique defines and measures a "workhardening" parameter K which is proposed to serve as a measure of defect generation in this regime. Their scheme involves both conventional stress relaxation testing and successive step relaxations through a small stress increment. This K parameter has been shown to be sensitive to the extent of cross-linking in polyaminobismaleimide (PADM) [3) and physical aging in atactic poly(methyl methacrylate) (PMMA) (6). In this study, the macroscopic "hardening" behavior of polypropylene and polystyrene in the region of deformation before general yield were studied by mechanical testing. The stress dependence of the hardening response was also investigated.
Mat. Res. Soc. Symp. Proc. Vol. 79. -1987 Materials Research Society
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METHODS
of an Cp(t).
in the polymer E(t) may be considered as the sum The strain strain Ee(t) and a plastic recoverable strain elastic or a gives: with r
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