PSI - Issue 3

Fatima Majid et al. / Procedia Structural Integrity 3 (2017) 387–394 Fatima Majid et al. / Procedia Structural Integrity 3 (2017) 387 – 394

393 393

allowed us to define precisely the critical life fraction for both of them. Indeed, the critical life fractions are 52% and 58% for both HDPE and PPR respectively. The damage stages of them are different. So, the first stage limit is 20% and 38%, the second one 75% and 78% and third over the last values for HDPE and PPR respectively. The performance of HDPE of 5.8 mm and PPR of 12 mm of thickness have a similar performance regarding the damage behavior and the criticism of thickness reduction. However, the two materials are very different considering the elastic and the rupture limits and the time to failure. In fact, the neat PPR reaches the burst pressure of 135 bars in 14 s. meanwhile, the HDPE reaches the burst pressure of 69.5 bars at 49 s. The discrepancies of these values can be explained by the range of each pipe, the PPR pipes are PN20 and those of HDPE are PN16, and the thickness differences. In fact, we used the existing pipes in the market. In this paper also, we were able to validate our research over HDPE, Majid (2017), by comparing the HDPE and PPR performances and developing the modified version of the static damage model based on the burst pressures instead of the stresses as published in the literature, Bui-Quoc (1971).

0,00 0,10 0,20 0,30 0,40 0,50 0,60 0,70 0,80 0,90 1,00

Miner Dommage statique basé sur les pressions d'éclatement HDPE Fiabilité basée sur les pressions d'éclatement HDPE Dommage statique basé sur les pressions d'éclatement PPR Fiabilité basé sur les pressions d'éclatement PPR

Damage

β c1

β c2

0,00

0,20

0,40

0,60

0,80

1,00

Fraction de vie( β )

Fig. 7. PPR and HDPE’s damage -reliability evolution in function of the life fraction.

5. Conclusion The comparison of PPR and HDPE materials allowed us to check the performance of these two material by exposing them to an increasing internal pressure until burst. The obtained pressure evolution was compared for neat and notched specimens. From then, we confirmed the ductile behavior of them and focus on the discrepancies of the obtained curves. The harmfulness of the notches has showed the critical impact of them over the burst pressure and the time to failure. Indeed, we noticed a drastic drop of the burst pressure and the time to failure. The crossing of damage-reliability curves allowed us to define precisely the critical life fraction of the studied polymers. Beyond the critical life fractions, 52% for HDPE and 58% for PPR, the materials become instable and a serious maintenance policy must be engaged. The non-linear static damage model of unified theory showed that the two materials has almost a similar damage evolution with different stages for the initiation, propagation and acceleration phases. The presented approach is a simplified method which can be used by industrials to do quality checks of these materials by using only static test instead of dynamic one. This approach is coast effective and permit assessing the damage and characterize the materials through burst tests.