Micromechanisms of slow crack growth in polyethylene under constant tensile loading

doi:10.1016/S0032-3861(01)00476-1

Polymer

Volume 42, Issue 23, November 2001, Pages 9551-9564

https://doi.org/10.1016/S0032-3861(01)00476-1 Get rights and content

Abstract

Circumferentially notched specimens of a first generation and a third generation pipe-grade of high density polyethylene with similar weight average molar masses have been subjected to constant tensile loads at 80°C. A transition from full ligament yielding to failure by stable sub-critical crack growth was observed as the applied load was decreased. The specimen lifetimes in this latter regime were dependent on the initial stress intensity factor, K_i, and failure was associated with slow crack propagation preceded by formation of a wedge-shaped cavitational deformation zone at the notch tip. The fibril diameters in the deformation zones decreased with stress intensity factor near the transition, the limiting behaviour of a relatively slow crack growth resistant third generation grade at the lowest K_i being inferred from testing in Igepal™ to be the breakdown of diffuse zones of interlamellar voiding. This regime was not directly accessible to testing in air within the allotted experimental times. However, comparison with the results of accelerated testing in cyclic fatigue has indicated stable interlamellar voiding in the third generation grade not to necessitate the presence of Igepal. Moreover, in both grades, very similar modes of deformation were observed in air and in Igepal at relatively high K_i. Igepal was therefore inferred not to lead to qualitative changes in the range of mechanisms that are characteristic of slow crack growth in polyethylene.

Introduction

Current interest in slow crack growth (SCG) in polyethylene (PE) is motivated by practical concerns over its long term performance in service, where it may be subject to significant pressures and contact with water or other industrial fluids. Hydrostatic pressure testing of pipes is still widely used to assess their performance. However, for many modern PE grades, failure times may be of the order of years [1]. Pre-screening of different grades therefore generally involves accelerated testing, usually of notched specimens either taken from a pipe or moulded ad hoc. Accelerated SCG can be achieved by testing at high temperature and/or in the presence of a surfactant and/or under cyclic loading conditions. Non-ionic surfactants such as Igepal™ (nonyl phenol ether glycol) [2] are particularly effective for reducing the failure times of the tougher materials without affecting their ranking with respect to other grades tested under the same conditions. Indeed, according to Fleissner, “surfactant-assisted stress cracking is a means to markedly reduce testing time without changing the failure mechanism” [3]. This is important if such tests are to be of direct relevance to a wide range of service conditions, although the fundamental reasons for such behaviour remain unclear [4].

In the present study, we have used various microscopical techniques to examine the microdeformation mechanisms associated with SCG under tensile loading in two grades of high density PE (HDPE) at 80°C in air and in Igepal. The aim was to gain insight into the micromechanisms of long-term failure in these materials and the differences in the behaviour of the different grades. The work was also motivated by our interest in the extent to which Igepal results in qualitative modifications in microdeformation behaviour, and hence the extent to which accelerated testing is representative of failure in air under conditions not accessible to experiment within convenient times.

Section snippets

Materials

The HDPE grades used in this study were two pipe-grade ethylene–butene copolymers (PEA and PEB) from Solvay, with similar melt flow indices and densities. Data for the two materials are given in Table 1.

Mechanical testing

All the mechanical tests to be described in what follows were performed on notched specimens, under creep loading conditions and at a temperature of 80°C. As discussed in the introduction, the resistance to SCG may be investigated using a range of techniques. However, since part of our aim was

CNB testing in air (series PE1 and PE2)

Fig. 3 shows creep curves from PEA specimens loaded at different ligament stresses, illustrating the three distinct regimes typical of this type of test. The initially large deformation rate corresponded to the application of the load, the period of transient loading lasting about 30 s in this case. The subsequent evolution of the global strain was dominated by the deformation of the ligament defined by the notch, associated with stable necking and/or SCG at the notch tip, depending on the

Discussion

The deformation induced structures at the crack tip described in the previous section varied considerably in scale, ranging from diffuse zones of interlamellar deformation with fibril dimensions roughly commensurate with the lamellar spacing (or thickness) of about 10 nm, to craze-like deformation zones with fibril diameters of 0.1 mm or more. Indeed the central macroscopic necks observed in all the specimens may be considered to represent the limiting case of the trend towards coarser

Conclusions

In notched HDPE specimens subject to static loading, high stress, short-term failure is initiated by yielding across the whole of the load-bearing ligament, accompanied by coarse cavitation in the specimen interior. As K decreased, there was a transition to SCG, characterised by the formation of a fibrillar crack tip damage zone. The fibrils became progressively finer as K decreased further, and in notched specimens of a relatively tough, 3rd generation pipe grade tested for about a month at

Acknowledgements

We acknowledge the financial support of Solvay Polyolefins Europe, Belgium throughout this work and the technical support of the Interdepartmental Centre of Electron Microscopy of the EPFL.

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Micromechanisms of slow crack growth in polyethylene under constant tensile loading

Abstract

Introduction

Section snippets

Materials

Mechanical testing

CNB testing in air (series PE1 and PE2)

Discussion

Conclusions

Acknowledgements

Polymer

Polymer

Polymer

Polymer

Trends Polym Sci

Polym Engng Sci

Trends Polym Sci

AST STP

Proc 9th Plastic Fuel Gas Pipe Symp

J Mater Sci

Proc 11th Plastic Fuel Gas Pipe Symp

Polym Engng Sci

Int Plastic Pipe Symp

Int Plastic Pipe Symp