PSI - Issue 2_B

T. Coppola et al. / Procedia Structural Integrity 2 (2016) 2936–2943 Author name / Structural Integrity Procedia 00 (2016) 000–000

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water filling volume. The material behavior is anisotropic with plastic damage accumulation according to model described in section 2. In the first case the model is formed by two load steps. In the first step the pipe is bent (four point bending) with internal pressure (p=108 bar) to form the wrinkle in the intrados, in the second step the pipe is pressurized up to burst. The maximum pressure at instability from simulation is 241 bar, to be compared with that reached during the test of 240.5 bar. The plastic strain at failure was measured by the thickness reduction on the necked section. The failure point has been located on the fracture locus reconstructing the stress history by FEM and determining the mean T and X values. The result is shown in Figure 2. It can be seen in Fig. 2 (a) that the measured failure point is placed on the FEM predicted limit curve ( X =0.55, T =0.42). To note that the pipe failure is not located in the point where the plastic strain is higher (the wrinkle) but at the bulging point near the extrados, as shown in Fig. 2 (b). In the second case a single pressurizing step up to burst is modelled. The experimental plastic strain to failure has been determined by the deformation of an electro-chemically etched square grid placed on the reduced thickness zone before the testing, as shown in Fig. 3 (a). The maximum strain calculated by the model in Fig. 3 (b) is very close to than measured in Fig. 3 (a). It can be seen in Fig. 4 (a) that the measured failure point is placed on the FEM predicted limit curve ( X =0.1, T =0.6), corresponding to a near plane strain condition, as also verified in Fig. 3 (a). The maximum pressure at instability from simulation is 156.8 bar, to be compared with the experimental burst pressure of 156.3 bar, as shown in Fig. 4 (b).

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(b) Fig. 2. (a) fracture locus for X80 grade with experimental test failure point; (b) plastic strain distribution in full scale test simulation

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(b) Fig. 3. (a) strain map measured on fracture region of X70 full scale test; (b) plastic strain distribution calculated by FEM.