Damage evolution and failure mechanisms in polyethylene


Fracture - RCP # 2004 Milan

Recent experimental data (Ting et al (1-2) and Ting (3)) has shown that the onset and subsequent evolution of the damage (craze) was found to be sensitive to constraint effects, in addition to rates. In particular, the effect of notch depth was found to exert a greater influence on the material response than through the thickness effects. The results showed that the deeper the notch (crack), the greater was the notch tip (plastic) constraint to promote constrained brittle failure where cavitation was preferred. As the notch tip constraint was relieved, the crack tip ductility was increased to invoke the blunting process, which in turn increased the separation (or displacement) and energy values. This dependency (on constraint) therefore can lead to contrasting damage-failure behaviours between different grades of polyethylene. Different macroscopic deformation mechanisms such as, voiding, cavitation, lamellar fibrillation, large-scale fibrillation, multiple crazing and shear deformation, were observed leading to distinct failure behaviour. The pipe grade polyethylene exhibited a craze-shear transition at low constraint and rate conditions. Clearly, there is a competition between craze and shear formation at this condition. On the other hand, the damage once initiated in a high-density polyethylene often led to the growth of radial cracks. Consequently, the nature and the form of the damage affect the overall energy dissipation, Gc. The implication on the fracture toughness and hence the long-term fracture properties is significant. This paper attempts to establish the linkage between the structural and mechanical properties. Examination of the influence of the microstructural properties and their interrelated nature on the crack growth resistance forms a basis of discussion. The study performed here should provide pipe manufacturers a valuable understanding of the crack growth behaviour of polyethylene.

Please note that the whole article content is available on PPCA website only

Related papers

2001 Munich : Fully Predictive Model of RCP in Plastics Pipes

Author(s) : Ivankovic, Tropsa, Jasak, Leevers

This work presents a 3D coupled solid-fluid model for predicting fast failures in pressurised plastic pipes. It is developed within a unified computational procedure where both solid pipe and pressurising media are discretised using the Finite Volume method. The coupling is achieved across the pipe-fluid interface...

1998 Gothenburg : Fast brittle fracture of gas pressurised plastic pipes

Author(s) : Venizelos, Greenshields, Ivankovic

Over 30 years ago, fast brittle fracture was identified as an inherently catastrophic failure mode in pressurised plastic gas pipelines. Although fast cracks seemed difficult to initiate, subsequent rapid crack propagation (RCP) was thought to be possible at pressures lower than the rated pressure, imposed by design...

1995 Edinburgh : Characterising Toughness for Pipe Grad Materials

Author(s) : Williams

A review of the Fracture Mechanics methodology for characterising toughness is given. It is shown that the fracture resistance R is derived by computing an energy difference. It is noted that R is dependent on the constraint in the crack tip region and that this leads to the very complex behaviour observed in Charpy...

1995 Edinburgh : Material, structural and pressurising fluid models for RCP tests on

Author(s) : Leevers, Greenshields, Venizelos, Ivankovic

The development of pipe-grade materials to resist slow crack growth and ductile rupture has been spectacularly successful. It is a measure of this success that concern with Rapid Crack Propagation (RCP) has increased to the level evident at this conference. Stronger materials encourage the use of higher...

1992 Eindhoven : Rapid Crack Propagation along Pressurized PE Pipe - Small Scale Testing and Numerical Modelling

Author(s) : Leevers, Venizelos, Ivankovic

The small-scale 'S4' test for rapid crack propagation seems set to acquire the status of a specifying standard for polyethylene gas distribution pipe. This paper describes parallel research projects on pressureinstrumented S 4 t e s t s a n d on Finite-Volume numerical analysis of S4 and full-scale tests....

1974 Southampton : The Analysis of practical failures of plastic pipelines

Author(s) : Marshall, Williams

During the last ten years, it has often been predicted that plastics will be used in increasing quantities as true engineering materials, Whilst there has been an increased acceptance of GRP as a positive alternative to metals in many applications, the use of thermoplastics for major load bearing structures has been...

Members of the Association

BOREALISBOROUGEFormosa Plastics CorporationINEOS O&PIRPCKazanorgsintez PJSCKorea Petrochemical IND. Co., LTD (KPIC)LyondellBasellPetroChina Dushanzi Petrochemical CompanyPRIME POLYMERSABICSCG Chemicals & Thai PolyethyleneSinopecTASNEE