Quantitative Prediction of Long-Term Failure of Polycarbonate

Abstract

Time-to-failure of polymers, and the actual failure mode, are influenced by stress, temperature, processing history, and molecular weight. We show that long-term ductile failure under constant load is governed by the same process as short term ductile failure at constant rate of deformation. Failure proves to originate from the polymer's intrinsic deformation behavior, more particularly the true strain softening after yield, which inherently leads to the initiation of localized deformation zones. In a previous study, we developed a constitutive model that includes physical aging and is capable of numerically predicting plastic instabilities. Using this model the ductile failure of polycarbonates with different thermal histories, subjected to constant loads, is accurately predicted also for different loading geometries. Even the endurance limit, observed for quenched materials, is predicted and it is shown that it originates from the structural evolution due to physical aging that occurs during loading. For low molecular weight materials this same process causes a ductile-to-brittle transition. A quantitative prediction thereof is, however, outside the scope of this paper and requires a more detailed study.

Quantitative assessment and prediction of contact area development during spherical tip indentation of glassy polymers

“…At large deformation, namely, the effect (of the thermal-history) has been erased by the plastic deformation, i.e. the 21 [Insert figure 2 about here]…”

“…In previous work we developed an 3D elasto-viscoplastic constitutive equation that accurately captures the deformation characteristics of polymer glasses [15,16,20,21]. The basis of this constitutive model is the division of the total stress into two contributions, first proposed by Howard and Thackray [22]:…”

“…The so-called driving stress s in equation (4) accounts for the rate-dependent plastic flow response, attributed to intermolecular interactions on a segmental scale [15,21] and is represented by a compressible Leonov model [20,24]:…”

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“…For polycarbonate (PC) the parameters in the model (see table 1 [21]) proved to be independent of the molecular weight distribution and the key parameter, needed to adjust for differences in thermal history (illustrated in figure 2b), is the initial value of the state parameter S a .…”