PSI - Issue 37

Jan Kec et al. / Procedia Structural Integrity 37 (2022) 598–605 Jan Kec / Structural Integrity Procedia 00 (2019) 000 – 000

603

6

To compare the effectiveness of the two sleeves, the stress ratios of the bare pipe and the sleeves were converted to uniaxial stress using Von Mises criterion. This comparison can be seen in Fig. 7. It can be seen that the CL sleeve has a reduction in a strain of about 70% compared to the bare pipe, which can be considered as a good reinforcing effect of the sleeve. However, for the CS sleeve, higher strain can be observed on the sleeve than on the bare pipe, which is due to the bulging of the corrosion defect on the inside of the pipe. It can be seen that the ratio of strain on the sleeve and pipe decreases with increasing pressure, the sleeve suppresses the bulging of the corrosion defect. The pressure range was between 2 and 6,3 MPa with a total number Nf=10 000 cycles, which corresponds to about 27 years of operation at a frequency of one cycle per day. Each cycle had a 'saw tooth' shape. No strain increments were observed on the monitored strain gauges during the fatigue pressure test. No leakage of the pressure medium was observed for tested pipes. The burst test therefore proceeded. The DN500 with CL sleeve was failed at 16,91 MPa on the bare pipe. For circumferentially and also longitudinally oriented strain gauges located on the surface of DN500 with CL, the yielding appeared around 11 MPa. For the CL sleeve, a change in the orientation of the longitudinal deformations beyond the yield strength can be observed. Above the yield strength, the deformations start to turn towards lower values, until then a complete transition to negative values occurs, which is the result of material bulging at the edges of the sleeve and the induction of additional bending. The failure of DN700 with CS occurred at a pressure of 15,78 MPa at the internal defect, which was reinforced with CS sleeve. In Fig. 8, the arrow (pressure 14,6 MPa) indicates the step-change in pressure at which the reinforcing effect of the sleeve was lost and the sleeve failed. It is not possible to determine exactly whether full or partial breakage has occurred because the failure occurred at the internal defect which was reinforced by the sleeve and the sleeve was therefore fully broken at the end of the test. At a pressure of approximately 10,4 MPa, the stress-strain behaviour of the CS sleeve changed. Up to a pressure of 10,4 MPa, the strain response of strain gauges T5 to T8 is linear, but above this value, the character of the longitudinal strain gauges changes considerably and they started to take negative values. The surrounding material of pipe presses on the sleeve and thus induces compressive deformation and additional bending.

Fig. 7. Comparison of sleeve efficiency