PSI- Issue 9

231

Fatima Majid et al. / Procedia Structural Integrity 9 (2018) 229–234 Fatima MAJID et al / Structural Integrity Procedia 9 (2018) 229 – 234

231

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In fact, the energy damage modeling based on residual ultimate energies is represented by the variation and fluctuation of the surfaces under the ( σ , ɛ ) tensile curves. The energy loss is very well over these curves. It decreases with the growth of the loading level. It depends on the represented parameters. Thus, the tensile curves that lead directly to the estimation of the released energy have used in order to calculate the surfaces under the curve (σ, ɛ ) for naturally damaged old pipes’ specimens . 3. Results To consider the energy model for the old pipes, an efficient approach has been adopted to quantify the damage of thermoplastic pipes through the dissipated energy allowing the rupture of the specimens. This energy is determined graphically through iterative or numerical calculations of the areas under the tensile curves of the aged specimens as shown in Figures 1 and 2. The tensile tests are carried out according to ASTM D 638.

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106,5 Elongation (mm) 124,5 142,5

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Fig 1. Calculation of the equivalent energy under the surface of the tensile curves of neat HDPE specimens.

A 3D representation of the tensile curves of aged HDPE specimens in comparison to neat ones has shown that the studied specimens undergoes a significant decrease in their energy proportionally natural degradation level. Indeed, this decrease is manifested by a significant reduction in the plastic strain surface until its disappearance for some specimens. In addition to maintaining the level of the elastic limit, which has remained more or less on the same level, the HD PE specimen’s behavior has shifted for some samples of the aged pipe from a perfectly ductile behavior to a very brittle one. This phenomenon, showing an irregular degradation, can be explained by several physical phenomena related to the direction of wear, the laminar or turbulent flow of the transported product and the amount of abrasives elements contained in the product. Besides, creep and fatigue phenomena, related to the over-pressurization or cyclic fluctuation of the service pressure, can be expected along with other natural degradation phenomena of thermoplastic materials.