This master- thesis deals with the analysis of crack- propagation behaviour of the pipe materials Polybutene (PB) and crosslinked Polyethylene (PEX- a). The materials were investigated by cyclic Cracked Round Bar (CRB)– Test, which was implemented in resent years in cooperation of the chair of Materials Science and Testing of Plastics at the University of Leoben (Leoben, A) and the Polymer Competence Center Leoben GmbH (Leoben, A). In real applications polyolefin pipes are exposed long term stress by low inner pressure, whereat slow crack growth (SCG) can be respected as generally accepted critical failure mechanism. For assessment of life expectancy it is necessary to test so close to reality as possible. In this respect the CRB- Test provides an advantage, because the fatigue stress on CRB- specimens increases tendency of slow crack growth. Based on the ISO- standard 18489 for PE, the CRB- Test was applied on the pipe materials above- mentioned, that came to important conclusions to this or similar materials for scientific and industrial characterisations in the future. Hence, we can benefit of the estimation of testing parameters, such as testing temperature, testing time and stress load. The paramount goal of this thesis was to evaluate the behaviour of time-to-rupture curves of both materials at different temperatures using cyclic CRB- Test and involving pictures of fracture surfaces as well as crack opening progressions. For better understanding it was necessary to do extensive material characterisations followed by a direct comparison of properties of uncrosslinked Polyethylene (PE) and crosslinked Polyethylene (PEX-a). Furthermore the determination of crack kinetics based on a compliance calibration curve has been carried out with additional tests of optical crack growth at different temperatures. Primarily, it was possible to determine the quasi brittle transition of PB at 80°C. Therefore a testing temperature of 23°C seems to be inappropriate. For PEX-a it was neither for a testing temperature of 23°C nor for 80°C possible to determine the quasi brittle transition, whereat the time-to-rupture curves illustrate high crack growth- and temperature- resistance. For that reason crosslinked thermoplastic materials, such as PEX-a should be preferred for stress over a long time period. Otherwise PB stands out for better mechanical behaviour on the ductile failure section. Because of long testing time on PEX-a, the suitability of CRB- Test, as rapid quality test, can be recommended only for PB.