A comparison between $K$ and $G$ approaches for a viscoelastic material: the case of environmental stress cracking of HDPE

Abstract

According to Linear Elastic Fracture Mechanics the stress intensity factor and the energy release rate are two fracture parameters linked by the elastic modulus and Poisson's ratio of the considered material. This concept has been extended to the analysis of linear viscoelastic materials, by introducing time-dependent quantities; it is also used for nonlinear viscoelastic polymers, even if its accuracy in this case is still an open question. In this work the Slow Crack Growth and the Environmental Stress Cracking resistance of two highdensity polyethylene grades were investigated, differing for their molecular weight distribution and fracture resistance. The description of the fracture behaviour of the two materials provided by the stress intensity factor or the energy release rate turned out to be equivalent, despite the nonlinear mechanical behaviour exhibited by the two polymers. Moreover, a time-dependent effective modulus, related to the two fracture parameters, was evaluated: its value was in good agreement with the modulus experimentally determined from tensile tests. A constant relevant effective strain was found despite the different testing conditions (i.e. applied mechanical loading, temperature and presence of an active environment), its value being equal for the two considered polyethylenes.

Environmental stress cracking of high-density polyethylene under plane stress conditions

“…A blow moulding HDPE grade having a monomodal molecular weight distribution, previously studied in [32][33][34] and hereby indicated as HDPE-MONO, was considered.…”

“…To determine the ESC resistance of HDPE-MONO, the employed active environment was derived from a commercial bleach, usually supplied in the bottles considered in this work. This environment was an aqueous solution containing sodium hydroxide, sodium carbonate, perfume and surfactants (Sol.B in [32][33][34]); the latter are a well known [1,3, ESC agent for polyethylene. At variance with commercial detergents, the solution used in the tests did not contain sodium hypochlorite (the actual bleaching agent) since, as proved in [33,34], this chemical does not affect the fracture resistance of the considered HDPE.…”

“…To cover this gap the resistance to SCG and ESC of a blow moulding High-Density Polyethylene (HDPE) grade, used for the manufacturing of bleach bottles, was studied on compression moulded thin specimens subjected to tests under plane stress conditions. The obtained results were then compared with the relevant plane strain behaviour of the same compression moulded polymer, previously studied in [32][33][34]. Finally, thin specimens, directly obtained from blow moulded bleach bottles, were also tested, thus allowing an investigation of the effect of the material production process on its resistance to SCG and ESC.…”

Environmental Stress Cracking of High-Density Polyethylene Applying Linear Elastic Fracture Mechanics

“…Polymers, and thus also PE-HD, are visco-elastic, time-dependent materials which cannot be described by a static Young's modulus only (Equation ( 4)). This visco-elastic behavior requires minor modifications and changes to the analysis, as demonstrated by Rink et al [43] as well as Andena et al [44] and approved recently for SCG and ESC by Contino et al [45][46][47][48] and Kamaludin et al [49][50][51] applying LEFM. This is especially relevant in the case of indirect crack length determination using compliance analyses, representing the relationship between the monitored ∆COD and the resulting crack length.…”

Multidisciplinary Optimization for Weight Saving in a Variable Tapered Span-Morphing Wing Using Composite Materials—Application to the UAS-S4

“…The values acquired from various sources are averaged for this work [60][61][62][63]. In terms of Poisson's coefficient, the value of 0.4 is considered as shown in Table 5, the same as the value used by Contino et al in [64]. In aircraft wings, the spar is one of the most critical components, as it must be able to withstand the most severe aerodynamic loads and to provide support for other wing components.…”