Fatigue Crack Growth and Process Zone Development in a PE‐HD Pipe Grade in Through‐Thickness Direction

Abstract

Improvements in the polymerisation process of polyethylene (PE) over the last two decades have led to commercial PE materials (PE 80, PE 100), which continuously gain importance as pipe materials in gas and water supply systems. It is generally acknowledged that the resistance of thermoplastics pipes against quasi-brittle failure determines their long-term performance. This failure phenomenon consisting of crack initiation and crack propagation can be realized in principle under laboratory conditions by various mechanical approaches. Despite the basic difference in loading conditions, some research groups favour fatigue test approaches in evaluating new pipe grades and in evaluating the long-term performance under constant amplitude loading conditions for the following reasons: Firstly, fatigue loading principally leads to accelerated crack growth; secondly, similar characteristics have been found in creep crack growth and fatigue crack growth (FCG) in various PE materials. [1±5] Many fatigue investigations on PE, based on cyclical pressurized pipe tests [2,6,7] and fracture mechanics methods [4,8±10] have shown that FCG in PE occurs either by a quasi-continuous mechanism or by a discontinuous (stepwise) mechanism. As different work groups have shown, [10,11] the crack growth mode depends on various test parameters such as the R-ratio (minimum to maximum load), test temperature and the stress level.The objectives of the research reported here were (1) to characterize the fatigue crack growth behaviour of a high density polyethylene (PE-HD) pipe grade using a method based on a linear elastic fracture mechanics (LEFM) approach and (2) to examine the development of the crack front and the front of the process zone ahead of the crack in the thickness direction of the specimen during the fatigue tests. In contrast to other research reports in the literature, [10±14] the novelty of this work was the determination of a very dense array of data points in the through-thickness direction. The effects of the stress intensity factor and frequency were investigated systematically. The experimental data were determined in order to calculate the effective crack length based on compliance relationships. Results and Discussion FCG KineticsA typical fracture surface of a specimen tested at 5 Hz is shown in Figure 1. Striations on the fracture surface document the stepwise crack growth mechanism. The fracture surface is characterised by the typical attributes of craze formation and breakdown (voids and fibrils). No clear differences in the details of fracture surfaces could be found as a result of the variation of frequency.When examining the FCG data in Figure 2, where FCG rates, da/dN, are shown as function of the stress intensity factor range, DK I , it becomes apparent that with increasing test frequency there is a noticeable increase in crack growth resistance in the low crack-growth regime. At higher crackgrowth rates, the curves coincide. This frequency sensitivity of FCG in polymers can be explained in terms of the eff...

Simulation Study of Fatigue Lifetime in PE100 Pipes

“…Furthermore, the Figure 7 shows the evolution of standard dimension ratio (SDR) as a function of number of cycles, we remarked that when the thickness (t) decreases then SDR increases, the crack growth to direction of thickness was propagated with a very speed which as the fatigue lifetime decreases. The analysis of the effect of the standard dimension ratio SDR are show by the authors [1,8,24,26].…”

Direct comparison of the compliance method with optical tracking of fatigue crack propagation in polymers

“…[6,17] Inspired by these excellent results, the method was adopted to determine fracture characteristics for other materials that are not linear elastic and isotropic, such as bone [18][19][20][21] and polymers, including reinforced [22,23] and nonreinforced thermoplastics, [24], which includes ultra-high molecular weight polyethylene [25,26] and polyethylene pipe grades. [27][28][29][30] It is not trivial to apply a method developed for linear elastic behaviour on non-linear, time dependent materials where the (apparent) modulus strongly depends on loading conditions and loading time. Modifications of the method were, therefore, proposed; e.g.…”