• Thuy, Maximilian

Abstract

The phenomenon of stress cracking due to environmental influences remains an important topic both in materials engineering and from a scientific point of view. Particularly in the case of packaging materials made of high-density polyethylene, estimating the potential for premature damage due to stress cracking is complex in view of the large variety of potential liquid filling goods. The known craze-crack mechanism of environmental stress cracking is accelerated by detergents and emulsifiers usually used to address the environmental stress cracking resistance of a material, but the interaction with other components, such as organic solvents, is often unknown. A particular challenge in this context is presented by crop protection products that consist of several different admixtures in addition to the biologically active ingredients. Using four typical container materials made of high-density polyethylene, the damage or stress cracking behavior in contact with model liquids for crop protection agents was investigated. These model liquids are specified in the German regulations for the approval of dangerous goods containers and consist of typical admixtures for crop protection agents, but without biological active ingredients. The two model liquids differ in a water-based solution and a composition based on a range of organic solvents, which are absorbed by the high-density polyethylene to a considerable extent. Therefore, extensive sorption measurements have additionally been carried out. The main tests are based on the standardized method of the full-notch creep test, in which a test specimen bar with a four-sided notch is simultaneously subjected to static load far below the yield stress and an environmental medium. First-time use of model liquids for crop protection products in the full-notch creep test demanded a detailed investigation, hence with optical monitoring the notch was observed during testing. Digital image correlation analysis thus reveals the notch opening under tensile load. The individual components of the model liquid were additionally used in their pure form but also in binary and modified mixing ratios to further improve the understanding of the interaction of solvent and detergents on stress cracking. Different degrees of plasticization in the material have also been examined within solvent environments. In addition, relevant parameters of the full-notch creep test have been applied to static loaded compact tension specimens to characterize crack propagation rates by applying linear elastic fracture mechanics. The crack propagation rate under static loading as a function of stress intensity factor follows the Paris-Erdogan law in all environments. As in full-notch creep test, crack propagation in compact tension specimens exposed to surface-active media is strongly accelerated. Since solvents cause plasticization, followed by intense blunting of the sharp notch, significantly retarding both crack initiation and propagation, the crack growth rate in solvents is greatly reduced. For strongly plasticize full-notch creep test specimens eventually no crack initiation occurs and similar to a creep test, ductile failure under shear deformation develops. Supplementing both methods, full-notch creep test and compact tension specimen, the fracture surfaces after failure are analyzed by laser scanning microscopy and scanning electron microscopy. Based on the results, the effect of different surfactants as well as the influence of organic solvents on stress cracking is discussed. Scanning electron micrographs of the fracture surface indicate more pronounced structures with a high degree of plasticity and a overall lower crack propagation rate, indicating the distinct creep behavior of fibrils. Also, the fracture surface of a slow and continuous crack propagation during stress cracking exhibits self-affine isotropic roughness exponents on the local scale, but resolved over the macroscopic fracture surface, a clear position dependence is observed. This varies according to the stress state distribution in the specimen, suggesting that the exponents are closely related to the locally dominant dissipation processes during craze-cracking.

Topics

• density
• impedance spectroscopy
• surface
• scanning electron microscopy
• crack
• plasticity
• isotropic
• creep
• creep test
• surfactant
• sorption measurement