Linear elastic fracture mechanics approach to evaluate environmental stress cracking resistance of HDPE

In this work the Environmental Stress Cracking Resistance (ESCR) of two grades of High-Density Polyethylene (HDPE), widely used for the production of bleach bottles, was evaluated by using a linear fracture elastic mechanics (LEFM) approach. Two solutions with similar composition to commercial bleach (with and without sodium hypochlorite) were chosen as active environment. Tests were conducted on notched samples in three and four-point bending configurations, by applying two different loading histories: constant displacement rate and constant load, respectively. The fracture behavior of the two materials was studied in terms of both the initiation and propagation phases of fracture, and the influence of the active environment was evaluated. Tests were performed at different temperatures and a time-temperature superposition scheme was applied to fracture data both in air and in the active environment. This study was carried out considering firstly the stress intensity factor K and then the strain energy release rate G as fracture parameters and the results of the two approaches were compared. According to LEFM, K and G are related through the Young Modulus E, whose evolution over time was derived from the fracture data. These values were compared with the relaxation modulus determined from previous tensile tests on the same materials, to assess the validity of the K-G relation for a non-linear viscoelastic material such as HDPE.

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