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Determining the residual quality of PE pipes using the strain hardening test

 

Papers # 2016 Berlin

Samples machined directly from pipe show that orientation has a large influence on the strain hardening modulus. No effects arising as a result of residual stresses were found. Samples machined in the axial direction from a hydrostatic pressurised pipe showed no effect as a result of this mechanical degradation. For now, also no clear effect on the is observable for samples machined in tangential direction, but more research is needed to confirm this.

The strain hardening test (SHT), carried out in accordance with ISO 18488, is a very quick and reliable test to determine the disentanglement capacity of the tie molecules in polyethylene (PE). The resulting strain hardening modulus () is an intrinsic property related to the slow crack growth (SCG) resistance. The power of this simple tensile test is especially obvious when testing PE 100RC material, which has an extremely high SCG resistance and makes laying pipes without a sand bed possible.

Previous studies have shown that the SHT on re-melted samples is a resin test and gives no information about the residual quality of the pipe. In this study, test samples were machined directly from PE pipe to circumvent re-melting. Two effects were taken into account, namely orientation and residual stresses. The PE molecules in a pipe are oriented as a result of the extrusion process. The machining direction of the test bars may therefore influence the results. Samples were machined in the axial, tangential (parallel to the pipe circumference), and radial (from the inner wall to the outer wall) pipe direction. Uneven cooling of the extruded PE pipe results in residual stresses. Test bars machined axially from the outer wall, the wall centre and the inner wall were also tested.

Orientation appears to be of great influence. The axial and tangential samples had a higher , while the radial samples had a lower than re-melted samples. In the axial and tangential machining directions, there was almost no difference in between the locations in the wall described above. This indicates that residual stresses do not influence the .

This knowledge was used to evaluate the residual quality of old first-generation PE pipes, which had been additionally mechanically degraded using hydrostatic pressure. The results for samples machined directly from pipe were compared with re-melted samples and with the results from the Pennsylvanian Notch Test (PENT).

Samples machined in the axial direction from the hydrostatic pressurised pipe showed no effect on the as a result of this mechanical degradation. More research using pipes with longer hydrostatic loading times is needed to obtain better trend with samples machined in tangential direction. For now, no clear effect on the is observable for the samples machined in this direction. More research is also needed to explain why the axial samples had a higher than the samples prepared from granulate, while the tangential samples sometimes had a higher and sometimes a lower .

Related keywords : Strain Hardening Test, SHT, polyethylene raised resistance to cracking, PE 100RC, Slow Crack Growth, SCG, orientation, residual stresses.
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