Investigating the time- and space- dependent mechanical, physical and chemical properties of aged polyethylene gas pipes using nanoindentation tests

Macroscopic tensile tests are conventionally used to obtain mechanical properties of aged polyethylene (PE) gas pipes, but cannot characterize the inhomogeneous aging behavior of the specimens caused by diffusion-limited oxidation (DLO). Nanoindentation test procedures have, therefore, been adopted to obtain time- and space-dependent mechanical properties of small-sized PE pipe samples, especially, creep properties directly from two types of the PE pipes in accelerated aging and natural conditions. The results indicate that the energy-based method to calculate the initial contact stiffness is not only suitable for cementitious materials and rocks, but also for PE materials. The combination of nanoindentation creep test and generalized Kelvin model can extract detailed parameters that are more representative of the differences in material deformation. The change of crystallinity with aging times probably leading to the difference of mechanical properties was revealed by physical and chemical properties obtained by the DSC and FITR technique. In addition, the correlation between nanoindentation results and structural information also indicates crystallinity plays a dominant role on mechanical performances of the PE pipe. This method using nanoindentation can significantly promote the accuracy of mechanical properties characterization and possess well fitness for PE pipes in accelerated aging conditions or in-service.