Slow crack growth resistance of modern PA-U12 grades measured by cyclic cracked round bar tests and strain hardening tests

Abstract

A critical failure mechanism in long-term applications of plastic structures, such as piping systems, is considered to be crack initiation and subsequent Slow Crack Growth (SCG). Thus, safe installation and operation during service lifetime of such structures are not conceivable without the knowledge of the resistance against SCG for any new material, like Unplasticized PolyAmide 12 (PA-U12). Unfortunately, long-term static tests at elevated temperatures lead to unreasonably long test times and measurements may also be affected by thermal aging and hydrolysis. Against this backdrop, the current study examines two accelerated test methods, namely the cyclic Cracked Round Bar (CRB) test as well as the Strain Hardening (SH) test, in order to characterize the SCG behavior. Both were originally developed for PolyEthylene (PE) and successfully implemented in recent years. While the cyclic CRB test measures SCG directly, accelerated through cyclic loading, the SH test quantifies the molecular disentanglement resistance, which determines craze formation and breakdown. The focus of this work was initially put on the extension of the CRB test towards PA-U12 grades, checking the occurrence of actual crack initiation and propagation. Afterwards a correlation to SH test results was done in terms of SCG resistance. Thereby, adequate test conditions were developed and the influence of the Molecular Weight (MW), expressed by the Viscosity Number (VN) of the PA-U12 grades, was considered. Results confirm the suitability of each method to rank SCG resistance and show a good correlation of the abovementioned SMall scale Accelerated Reliable Test (SMART) methods, highlighting their sensitivity to long-term relevant molecular parameters.