PSI - Issue 48

Mersida Manjgo et al. / Procedia Structural Integrity 48 (2023) 155–160 Manjgo et al/ Structural Integrity Procedia 00 (2023) 000–000

159

5

Table 5. Remaining exploitation period - crack in the HAZ on the side of the exploited BM

Loadcase

Geometric term Y

Allowable stress range  ,MPa

Assumed crack length, m

Critical crack length a c ,mm

Number of cycles  N

5.58 6.01 6.21 6.39 5.58 6.01 6.21 6.39

0.01 0.02 0.025 0.027 0.01 0.02 0.025 0.027

17470 6554 4682 4005 607655 227949 162859

I

211

27

II

46

27

139303 * Applies in the event of the appearance of an initial crack with a length of 5 mm, for the case of a crack in the HAZ on the side of the exploited BM: C = 5.5٠10-9, and the exponent m = 2.33 4. Discussion At an assumed working load (stress) equal to the yield stress for the given material under working conditions ( R p0.2 = 211 MPa), the number of cycles required to reach the critical crack length is very small. Such a small number of cycles is expected, bearing in mind that this is low-cycle fatigue, i.e., the action of the load close to the yield stress. The greater the assumed length of the crack and the range of permissible stress, the lower the number of cycles until eventual failure and uncontrolled breakage. In the case when the assumed working load (stress) is realistic, i.e., the load determined using the tensometric method and is 46 MPa, the number of cycles required to reach the critical length of the crack is much higher, because the stress level is lower. However, although the stress level is lower, it is still higher than the stress level at the fatigue threshold, ΔK th , so that the growth of the assumed crack occurs in any case. Furthermore, the analysis also took into account the location of the possible crack, i.e. whether it occurs in BM, WM or HAZ. Also, the influence of the time of exploitation was analyzed, i.e. whether a possible crack occurs in new or exploited BM. From the tables for the case of the actual load of 46 MPa, it is visible:  In the case that an initial crack of length 5 mm in a new BM reaches a critical length of 41 mm, an exploitation period of 25.7 years is required, and an initial crack of the same length in an exploited BM reaches a critical length of 28 mm, an exploitation period of 4.6 years.  In the case that the initial crack of 5 mm length in the WM reaches a critical length of 63.5 mm, an exploitation period of 29.6 years is required.  In the case that an initial crack of length 5 mm in the HAZ on the side of the new BM reaches a critical length of 32 mm, an exploitation period of 21.4 years is required, and that an initial crack of the same length in the HAZ on the side of the exploited BM reaches a critical length of 27 mm, an exploitation period of 4.1 years is required. 5. Conclusion Based on the obtained results, it is evident that the remaining life of the vessel in exploitation, i.e., the number of cycles, ΔN until reaching the critical length of the crack, a c , depends on:  assumed crack lengths, a,  range of permissible stress, Δσ and  places of crack initiation Considering that the safety of the structure is evaluated according to its weakest component, it can be stated that for the case of a real load of 46 MPa, and for the case that the initial crack of 5 mm reaches the critical crack length of 27 mm, in the HAZ on the side of the exploited BM, an exploitation period of at least 4.1 years of work. This conclusion implies permanent control of the technological process of pressure vessel operation, NDT control of all critical places on the structure, including BM.