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Fatigue crack growth testing in chlorinated water at elevated temperatures - Test equipment and test procedure

 

Papers # 2018 Las-Vegas

In the present paper, a novel test system for conducting fatigue crack growth (FCG) experiments under superimposed mechanical-environmental conditions is presented. FCG experiments were performed on a polypropylene (PP) grade and a short-glass fiber reinforced polyamide 66 (PA-GF) grade as model materials. The environmental conditions were air, non-chlorinated water and chlorinated water at elevated temperatures (80°C and 95°C). In terms of ranking, the FCG resistance of PP in the various environments decreased in the order non-chlorinated water, air and chlorinated water. For PA-GF, the FCG resistance in chlorinated water was reduced with increasing chlorine content at least in the range from 1 to 10 mg/l.

To prevent the spread of waterborne diseases, a sufficient water disinfection is essential for meeting hygiene standards for potable and other tap water worldwide. Chlorine is the most widely utilized and the most affordable water disinfectant. Due to the increasing application of polymeric materials in water supply systems, there is a great need for adequate test methods that are capable of determining the crack growth resistance of polymers under superimposed mechanical loading and exposure to chlorine. Hence, a novel test arrangement was designed and implemented on a conventional electrodynamic test machine that permits for such superimposed mechanical-environmental loading conditions in a cyclic manner. Simultaneously, it allows for the determination of the quasi-automatic determination of the fatigue crack growth kinetics via an optical crack length measurement system. The test arrangement was developed to ensure tests with cracked round bar and compact type specimens. By using sodium hypochlorite in a controlled system under continuous flow, preventing also the faster decrease in reactive chlorine content at elevated temperatures, constant chlorine contents in the range between 0.1 and 10 mg/l over the complete testing time are assured. Fatigue crack growth results for a polypropylene pipe fitting grade and a polyamide pipe push-fitting grade used as model materials and tested at different temperatures and chlorine concentrations are compared and contrasted to highlight the potential of this novel test procedure. As expected, for both materials, the more critical test environment with the lowest crack growth resistances was found to be the liquid environment with the highest chlorine content.

Related keywords : Polypropylene fittings, polyamide push-fittings, chlorinated water, fatigue crack growth resistance, superimposed mechanical-environmental loading.
Please note that the whole article content is available on PPCA website only


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