Understanding environmental stress cracking in the Bell Test at the notch region

Abstract

The Bell Test (ASTM‐D1693) is commonly used to accelerate part failure in order to evaluate resin selection for applications like piping and tanks where environmental stress cracking is possible, even though the method tends to offer less than desirable repeatability. The working hypothesis for this new study with polyethylenes was that the variability seen in failure times may be attributed to a higher sensitivity of the phase morphology primarily at or near the surfaces of a notch than more commonly referenced bulk descriptors of a polymer, affecting stresses and the localized absorption of the preferred stress cracking agent. Using an organic dye tracer to monitor penetration of stress cracking agent into the polymer along with image analysis for the notch dimensions and a load cell, the local aspects of the mechanism were evaluated for up to four grades of polyethylene of varying crystallinity. By this detailed analysis, it was revealed that for failure to occur by ESC, influences in the polymer by absorption of the stress cracking agent preceded the changes in stresses in the specimen, contrary to original expectation. There was an imminent rate increase in both stresses and notch expansion once absorption of the surfactant reached a critical concentration. This critical nature was shown to be dependent on the crystallinity of the polymer under observation, considering the size/arrangement of crystals present and entanglements formed through the amorphous contents to dictate the rate by which IGEPAL can be absorbed into a polyethylene specimen. The results of this study validate the need for greater focus on polymer properties at the notch rather than the bulk of the specimen, though more techniques for providing analysis of these properties at the notch surfaces are needed for future improvements to the mechanism.Highlights Bromocresol dye acts as a novel visualization aid in tracking IGEPAL CO‐630 sorption Crystallinity was related to a plasticization threshold for absorption of IGEPAL Accelerated increase in internal stresses follow absorption. Variability of the Bell Test was related to crystalline properties at the notch

Effect of hexyl‐branched backbone size on the size distribution and coalescence of free volume around an amphiphilic molecule

“…The cracks reduce the strength and durability of polyethylene 4 . ESC is believed to happen in three steps: (i) amphiphilic liquid absorption, (ii) crack propagation, and (iii) brittle fracture 5,6 . It is important to point out that the rate of each stage varies depending on the microstructure of the PE sample and on the conditions to which it is exposed such as temperature, stress level, and type and concentration of the amphiphilic liquid.…”