Characterisation of quasi-brittle fatigue crack growth in pipe grade polypropylene block copolymer

The aim of this work was to characterise quasi-brittle fatigue crack growth in a pipe grade polypropylene block copolymer. Cyclic cracked round bar specimen tests have been identified as a possible way to induce cyclic quasi-brittle crack growth within a reasonable amount of time. To ensure quasi-brittle failures, fracture surfaces have been examined using scanning electron microscopy. For further acceleration of the tests, higher temperatures with adapted load ranges have been applied. For 50 °C and 80 °C, a decrease in testing times of approx. 60% and 80% was achieved without significant changes in failure behaviour (e.g. cyclic crack opening displacement (Δδ) curves or fracture surfaces) of the tested specimens. Relationships between applied load and testing times, as well as between crack initiation and failure have been examined. A change in ratio of crack initiation to total failure time was observed at higher temperatures. This has to be considered in future tests, especially when comparing different types of polypropylene at temperatures, differing from the real application.