Ductile deformation in Pipe Grade PE [PE]

 

# 1998 Gothenburg

Rapid Crack Propagation in PE-100 gas pipes is governed primarily by a brittle-tough transition temperature TBT,above which ductile deformation will absorb energy and promptly arrest a fast-running brittle crack. Even at low temperatures, a layer of plastically deformed material is formed at the pipe bore during fracture, but only becomes effective when the pipe transition temperature is exceeded. To relate this post-yield deformation behaviour to the microscopic propenies at the bore of a PE-I00 pipe, it is important to isolate the 'true' or local macroscopic behaviour of the material under high-rate tension. A simple tensile test at constant displacement rate will certainly characterise yielding of PE-100, but post-yield deformation is generally inhomogeneous, and local changes in sample geometry and strain rare can lead to erroneous impressions of material response. To overcome this, true stressltrue strain data for pipe grade PE has been captured at a constant local strain rate using a video controlled tensile machine. Localised post-yield deformation in PE-I00 is characterised ar low strain rates over a range of temperatures, giving a family of isothermal true stressltrue strain curves. However, at the extremely high strain rates produced in rapid crack propagation, adiabatic heat generation increases material temperature and deformation becomes thermally coupled. A Finite Element model incorporating the isothermal true stressltrue strain data is therefore used to explore the strain rate and associated temperature dependence of rapid deformation in this material.

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