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04-11-2019

Fast Evaluation of the Long-Term Stress Crack Resistance

A Test Method to Characterize Thermoplastics

The strain hardening test allows for an evaluation of the material's strain hardening behaviour via uniaxial short-term tensile tests (© SKZ)

In Würzburg, Germany, SKZ scientists are intensively researching the usability of the Strain Hardening Test (SHT, ISO 18488:2015) as a method for a fast and cost-effective characterization of the stress crack resistance.

Up to now this test method is mainly used for Polyethylene (PE). The transferability from PE to different other thermoplastics was basically shown in a successfully completed research project. The applicability of SHT for testing PE component's stress crack resistance is currently being investigated in another research project.

Quick Statements on the Material's Behaviour

Established test methods used to characterize the stress crack resistance of thermoplastics and components made thereof are often time consuming. Additionally, quantitative results are no longer achieved in justifiable testing times, especially when it comes to highly stress crack resistant materials for long-term applications. However the SHT enables the determination of stress crack resistance by evaluating the strain hardening behaviour in uniaxial short-term tensile tests. Instead of generating crazes, which turn into stress cracks, the method mimics the microscopic material behaviour on macroscopic scale.

Preparation and pre-treatment of samples as well as the influence of test parameters were systematically investigated for Polypropylene (PP), Polybutene 1 (PB-1), Polyamide 12 (PA12) und Polyvinylchloride (PVC-U) materials. The evaluation algorithm was modified and a validation on the basis of reference data as well as on molecular and morphologic characteristics was carried out.

Focus on Practical Suitability and Significance

"Currently implementation for different durable PE products like geomembranes, sheets for container engineering and pipes is being investigated. Thereby a special attention is paid to a systematic consideration of production-related influences such as orientations and residual stresses. Finally, a workable test specimen removal from components shall be defined and the informative value of the results shall be evaluated," explains SKZ researcher Britta Gerets. Interested companies are invited to attend the project meetings free of charge.

The final report can be ordered at www.skz.de .

Publically Funded Research Projects

The research projects 18649 N and 20225 N of Fördergemeinschaft für das Süddeutsche Kunststoff-Zentrum e. V. (FSKZ) were/are supported through Arbeitsgemeinschaft industrieller Forschung e. V (AiF) within the framework of industrielle Gemeinschaftsforschung (IGF) by Bundesministerium für Wirtschaft und Energie (BMWi) on the basis of a decision of the German Bundestag.

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