Role of microstructural homogeneity on failure variability in environmental stress cracking test methods

Environmental Stress Crack Resistance (ESCR) is a highly valued assessment of polymer materials for estimating their service life in often vital infrastructures; however, its test methods, like the well-known Bell Test, often display a relatively low degree of repeatability in their results, leaving gaps in technical data for design engineers to grapple with alone. It was hypothesized in the present study that this undesirable variability is associated with the spatial uniformity in microstructure present in prepared plaques used for testing. An investigation followed to validate this theory by comparing ESCR results of compression-molded high-density polyethylene (HDPE) plaques made with a standard cooling plate design versus one with improved temperature uniformity. A bulk descriptor for microstructural uniformity in these plaques was devised based on Dynamic Mechanical Analysis (DMA). Infrared thermal imaging coupled with computational fluid dynamics (CFD) simulations in ANSYS Fluent was used to characterize the cooling conditions in both plate designs. The new plate with its improved cooling uniformity was found to lower the statistical variance in Bell Tests by more than 95 % without causing major changes in the average failure time (30–40 h originally vs. ∼30 h for the new design). Comparing the storage moduli determined by DMA of specimens cut from the plaques proved effective in characterizing the microstructural variability in specimens used in the Bell Test, ultimately confirming the hypothesis of this study by finding more consistency in the elastic modulus throughout molded plaques corresponded with decreased ESCR variability.