The lifetime of pre-insulated district heating pipes is commonly evaluated by thermal ageing at elevated temperatures and is calculated using the Arrhenius equation. In this investigation, the effects of a repetitive shear stress during thermal ageing of pipes were studied. The degradation of polyurethane foam, especially at the interface with a steel pipe was evaluated from measurements of the adhesion strength and of alterations in the chemical structure of polyurethane by Fourier transform infrared spectroscopy. The main conclusion was that the thermal degradation of mechanically stressed district heating pipes was significantly faster than that of non-loaded pipes aged at the same temperature. It was also shown that the faster degradation of the mechanically loaded pipes is mainly due not to fatigue but to accelerated chemical degradation of the polyurethane foam. The results suggest that this methodology should be considered as an accelerated test method in order to avoid overestimation of the lifetime of district heating pipes and to show better ageing characteristics of mechanically stressed pipes, especially those intended for use in the fourth generation district heating networks