The increasing use of polymers in engineering applications requires investigation new and efficient methodologies to assess the ability of materials to withstand long-term service loads. Impact tests are widely used to characterize breaking strength because they simulate the most severe loading form to which a material can be subjected. Although impact is not the most probable of operational risks, it is considered as the most critical loading conditions in service life given the fact that failure usually occurs at high deformation rates and in the presence of notches or stress concentrators. The impact strength analysis of extruded polyethylene tubes presents an additional difficulty, due to the interaction between orientation and crystallization of the structure, leading to a complex anisotropic morphology. This study is based on the extension of the methodology for the determination of the radial resilience of polyethylene pipe using arc-shaped samples as numerically formulated by Niglia et al. [1] and in accordance with ISO/DIS-17281. The study investigates the evolution of the critical fracture energy (GIc) through the pipe wall using a specific numerical function (Y) relating crack length to specimen width (a/w). The samples are cut and machined under appropriate conditions from a HDPE-100 tube with a 200 mm nominal diameter and a specific dimension ratio (SDR) of 16. The first batch of impact specimens is prepared to establish impact fracture energy (U; kJ/m²) measurements at 6 different positions (rings) along the pipe (z-direction) and each position (or ring) is represented by 5 machined C-specimen specimens covering the entire pipe circumference (q-direction). The second lot of specimens is also C-shaped with an inner central increasing notch used to measure the impact fracture energy when changing positions from the outer layer to the inner layer of the pipe (r-direction). It is found that average experimental GIc values vary between 7.86 and 8.18 kJ/m² for the z-direction and between 7.49 and 8.24 kJ/m² for the q-direction. The low standard deviations indicate that GIc can be considered as a rather constant property for a given pipe wall depth and at any position along z-direction. This is a verification that the manufacturing process is delivering similar products with homogeneous properties following melt extrusion and associated water cooling. However, in the radial direction (i.e. for different depths), average GIc values grew about 5 times; explicitly from 5.28 kJ/m² up to 25.90 kJ/m² when going from the outer side to the inner side of the pipe. Consequently, as the position varied across the pipe wall; it was possible to establish a trend for GIc values. The impact toughness is found to be lower at the outer pipe layers compared to the inner ones. Such conclusion is in accordance with morphological and structural data of literature.

[1] J. Niglia, A. Cisilino, R. Seltzer, P. Frontini, Determination of impact fracture toughness of polyethylene using arc-shaped specimens, Engineering Fracture Mechanics 69 (2002) 1391–1399.