PSI - Issue 25

Fatima Majid et al. / Procedia Structural Integrity 25 (2020) 430–437 Author name / Structural Integrity Procedia 00 (2019) 000–000

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Fig. 5. Limits of damage and reliability evolution for an old HDPE Pipe

6. Conclusion We proved through the damage models that we could represent the damage of old HDPE pipes through three main parameters, which are stresses, strains and energies. These models are satisfactory and represent the damage differently. By using them, we can predict the behavior of the HDPE pipe and determine the critical life fraction. We determined the three stages of damage for a better maintenance experience. So, the critical stage, stage II, is contained between βc1 and βc3. Then we noticed a fluctuation of the damage curves with a symmetrical axe at 52% of the life fraction, which is confirmed as a critical one by the burst pressure model. All the damage models progress within the limits of this loop, energy and strain. Thus, we consider 52% as the critical life fraction for the studied material. A crossing of the damage curves with the ones of reliability allowed us to confirm all the obtained results and confirm the damage stages. Furthermore, we calculated the number of cycles for each specimen. Then we established the Wöhler curve to predict the lifetime of an old HDPE pipe. From it, we compared the obtained lifetime of the studied material to the curves of the ISO code. It has been shown that the one of the old HDPE pipe have been reduced from a total number of cycles of 10 6 for a neat material to 10 5 cycles for an old HDPE pipe. According to the comparison to the lifetime given in the ISO code, we determined that the studied HDPE has been working for 15.4 years. The study shows that damage is variable from one specimen to another. It is proving a big variability of the ageing effect through the wall of the HDPE cylinder. A strategic maintenance decision have to be taken while we reach he critical life fraction. Besides, we showed that defects investigation is obligatory in the second stage of damage.

References

American, A. and Standard, N. (2010) ‘Standard Test Method for Resistance to Short-Time Hydraulic Pressure of Plastic’, Annual Book of ASTM Standards , 99(Reapproved 2005), pp. 1–3. doi: 10.1520/D1599-99R05.2. Bui-Quoc, T. et al. (1971) ‘Cumulative fatigue damage under stress-controlled conditions’, J. Basic Eng. Trans. ASME , 93, pp. 691–698. Devilliers, C. et al. (2010) ‘Characterisation of aged HDPE pipes from drinking water distribution : investigation of crack depth by Nol ring tests