A Reduced Variable Approach to Relating Creep and Creep Rupture in PMMA
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A REDUCED VARIABLE APPROACH TO RELATING CREEP AND CREEP RUPTURE IN PMMA G. B. MCKENNA AND J. M. CRISSMAN Polymers Division, National Bureau of Standards,
Gaithersburg, Md.
20899
ABSTRACT Creep and creep rupture of PMMA at high stresses have been characterized and found to be relatable by use of reduced variables, it is shown that when the creep compliances can be correlated by a superposition principle for which the vertical shift is the ratio of the applied stress to a reference stress and when strain at failure is a constant, a commonly used failure criterion (that the product of the strain rate at failure and the time to failure is constant) becomes valid. The reduced variables approach is found to apply to two greatly different thermal histories. Consistent with the concept of physical aging, the response of a quenched sample is simply shifted along the log time axis to shorter times relative to the response of the aged sample. I.
Introduction
The description of the viscoelastic and creep rupture behaviors of polymers subjected to large stresses has received considerable attention Kinetic and reaction rate models [1-3,10], which have successfully [1-10]. described the failure behavior of polymers subjected to monotonically increasing loading histories, appear to be unsuitable when the loading Similarly, there have been several history is more complicated [9,10]. successful attempts to correlate the nonlinear viscoelastic response of polymer glasses under monotonic loading histories using the concept that the applied stress, or strain, simply shifts the viscoelastic spectrum That is, the creep, or stress relaxation, along the time axis [4,6,7]. curves at different stress (strain) levels can be superposed to form a master curve by a shift, ac, along the log time axis. The idea of a "stress clock" in nonlinear viscoelasticity has also been formalized [IIJ, although its suitability for describing the behavior of polymeric glasses subjected to complicated loading histories is at this point unknown. In addition to the consideration of the time dependent behavior of polymers, it is known that the mechanical response of polymeric glasses is also dependent upon the thermal history to which they have been subjected Struik [6,7] has shown that, to a first approximation, the [6,7,12]. effect of the changes in the volume of the glass after a quench from above the glass transition temperature, Tg, is to shift the viscoelastic spectrum along the time axis. This phenomenon is generally referred to as physical aging. While much work has been done to describe either the failure behavior of polymers or the effects of stress, strain, or volume on their viscoelastic response, little work has been directed toward studying both aspects simultaneously. In this work we present the initial results of a program designed to address both the issue of physical aging in polymeric glasses and their failure behavior under conditions of tensile creep. The creep behavior at relatively large stresses has been examined for specimens of polyme
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