To increase performance of modern pipe systems functionalized multi-layer build ups become increasingly interesting to answer market demands. Whereas single-layer pipe systems made from polyolefins have been the focus of research for many years, the influence of multi-layer build ups has not been investigated in detail yet. Often multilayer pipe consist of three different layers. For example a pipe may consist of an inner layer which is resistant against abrasion, chemicals and slow crack growth, a middle layer with high stiffness for mechanical strength and an outer layer which offers resistance against external scratches and point loads. To describe the long-term behaviour of a system as defined above, detailed knowledge about the single materials, as well as the influence of the interfaces in between is required.
This work focuses on the impact of multi-layer build ups on long-term performance of polymer pipes. Long-term resistance against crack growth of the single materials has been investigated using accelerated fatigue methods. To estimate changes in mechanical response compared to homogenous pipes, finite element analysis has been used. Additionally, local interface properties have been investigated using X-ray scattering and Raman-microscopy. Due to long service times of pipes, ageing processes in the materials have to be regarded as well. To induce artificially accelerated ageing, materials were stored at elevated temperatures for up to 18 months. Afterwards, changes due to physical and chemical ageing were investigated using thermic, mechanical and fracture mechanical methods.
With regard to crack propagation through a pipe wall, it was found, that the presence of interfaces influences the resulting crack driving force significantly when compared to a homogenous pipe. Depending on the miss-match of elastic properties of the materials, cracks may be accelerated or decelerated in the vicinity of an interface. Spectroscopy and scattering techniques showed promising results in regard to interdiffusion and mechanical stability of the interfaces. The ageing study revealed, that even after 18 months of accelerated ageing, used stabilizer combinations were sufficient to inhibit chemical degradation of materials. Accelerated ageing procedure led to effects which do not occur under actual application conditions. These have to be considered to avoid non-conservative results.