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

预绝缘的区域供热管的使用寿命通常通过高温下的热老化来评估，并使用Arrhenius公式进行计算。在这项研究中，研究了管道热老化过程中反复剪切应力的影响。通过用傅立叶变换红外光谱法测量粘合强度和聚氨酯化学结构的变化来评估聚氨酯泡沫的降解，特别是在与钢管的界面处。主要结论是，受机械应力的区域供热管的热降解速度明显快于在相同温度下老化的无负荷管道。还显示出，机械加载的管的更快降解主要不是由于疲劳，而是由于聚氨酯泡沫的加速化学降解。结果表明，应将这种方法视为一种加速测试方法，以避免过高估计区域供热管道的寿命，并显示出机械应力管道的更好的老化特性，尤其是打算用于第四代区域供热网络的管道