PSI - Issue 23

Jan Poduška et al. / Procedia Structural Integrity 23 (2019) 293–298 Author name / Structural Integrity Procedia 00 (2019) 000 – 000

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middle layer thickness affects the lifetime significantly in the Case 3. In Case 3a. the middle virgin layer was 4.92 mm thick (60% of total wall thickness). In Case 3b, the inner virgin layer was 3.28 mm thick (40% of total wall thickness). The stress intensity factor dependencies K I (a) were calculated for all the cases using the FEM model described above. The investigated cases are pictured in Fig. 3.

Fig. 3. Representation of the cases of crack propagation in the multi-layer pipe: (a) Case 1; (b) Case 2; (c) Case 3 The lifetime estimations were calculated by integration of the power equation (1) between an initial crack length a ini and final crack length a fin . The integration provides only an estimation of the lifetime, as the calculated value accounts for the time of slow crack growth only, it does not include the time of initiation. The initiation can take up to 50 % of the total lifetime. The time of slow crack growth t SCG is calculated as: = ∫ ( ( )) (2) The calculated lifetimes are plotted against the hoop stress caused by the internal pressure in Fig. 4.

Fig. 4. The lifetime estimations t SCG plotted against the hoop stress σ hoop in the pipe wall for all the investigated cases

The single-layer pipes made of PE100 and PE100RC both meet the conditions for PE100 pipes – a lifetime of at least 50 years at the hoop stress of 10 MPa. The recycled PE pipe could not meet this condition at all, which was expected, as the recycled PE has much lower resistance against SCG. If the crack propagates in the inner layer (made of PE100 or PE100RC) of a multi-layer pipe that contains a middle layer of recycled PE (Case 2), the lifetimes are not much shorter in comparison to the homogenous pipe made of PE100 and PE100RC respectively.