To increase the effectiveness and durability of pipe systems, there are rigorous regulations with regard to expected lifetimes. Pipe materials have to withstand at least 50, or sometimes 100 years in application. Seeing that testing under real conditions cannot be done within feasible amounts of time and due to vast improvements in quality of polymer materials over the last years, the necessity for new, faster and more selective test methods has arisen. One of these faster methods is the cyclic cracked round bar test, which is currently under ISO standardization for polyethylene pipe grade materials used in gas and water supply pipes. However, other materials, such as polypropylene or polyamide are also in dire need of faster and more selective tests, in order to remain competitive.
The newly developed cyclic method, which is based on fracture mechanics and was developed for PE only, has been applied to different polymer pipe materials, to examine the feasibility as a tool for long-term property screening. It was found, that the test can be applied to pipe materials such as polypropylene, polyvinylchloride, polyamide and polybutylene. Especially for polypropylene, which has been the focus of this thesis and is very diversely used both in pressurized and unpressurized applications, at ambient and elevated temperatures, the test showed promising results. Experiments conducted under various load and temperature conditions showed the enormous importance of the crack initiation phase during fatigue testing for PP. Initiation accounted for up to 80% of total fatigue lifetime. Therefore, influence of notching procedures has been investigated. Additionally, the influences of reinforcement and morphology have been investigated in depth during this study. Specifically, the differences in properties of polypropylene block-, random-, and homopolymer were investigated.
It showed that the cyclic cracked round bar test possesses vast potential to rank materials in the different failure modes. For example, in the short term failure region materials can be compared within hours to days of testing. Even long-term failure, which is referred to as “quasi-brittle”, could be achieved for most materials within days or weeks of testing. Compared to more than 10,000 hours of testing using classical methods, this is an enormous improvement. Fracture surfaces, hysteresis analysis and compliance development have also been used to further investigate the specific damage mechanisms of individual materials.
Besides mechanical failure mechanisms, also ageing of the material can play a major role with regard to the lifetime of pipe systems made from polymers. To characterize the influence of physical and chemical ageing, one of the examined polypropylene materials, has been investigated after severe artificially accelerated ageing at elevated temperatures. It was found, that the used combination of primary and secondary antioxidants could delay chemical ageing over a period of more than 18 months at 80°C. Results of the ageing study also clearly showed that additional physical processes which only occur due to the accelerated ageing itself can significantly influence material performance. In this case, annealing lead to a significant decrease in residual stress in the material. Due to this decrease, fracture mechanical properties seemingly improved with ageing time. However, this physical ageing process does not occur in real application, and could lead to non-conservative results in lifetime estimations if ignored.