PSI - Issue 17

Juan Cruz Castro et al. / Procedia Structural Integrity 17 (2019) 115–122 Juan Cruz Castro et al./ Structural Integrity Procedia 00 (2019) 000 – 000

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5.2. Cumulative Usage Factor

The evaluation of the CUF was based on Equivalent completely reversed stress amplitude obtained in critical regions of the bridge girder. The results obtained with these theories have been compared in Fig. 3. Goodman ’s theory is conservative and Gerber ’s theory is inaccurate for tensile mean stresses in accordance with the experimental research made by Dowling in 2004 and 2009. He e stablished that the Morrow’s theory can be adjusted with the experimental data in steels, rather than Goodman’s equation , by using the true fracture strength. On the other hand, Dowling does not present experimental results that vali date that Gerber’s theory. Nevertheless, in the results discussed in this paper, Gerber’s theory values are highly non -conservative compare with Morrow, Walker and Goodman. As previously stated, Morrow and Walker’s theories can be considered adequate for t he equivalent stress estimation used in the calculation of the CUF.

Fig. 3. Cumulative Usage Factor with different theories of correction of mean stress in the bridge girder.

6. CONCLUSIONS

A Time-Limited Aging Analysis (TLAA) of mechanical fatigue was developed. It was related with a postulated case of the structural girders of a crane. This analysis must be presented in the application for the renewal of an operational license of a Nuclear Power Plant. This will be evaluated by the regulatory body on nuclear matters. The Aging Management Program (AMP) XI.M23 “Inspection of Overhead heavy load and light load (related to the refueling) handling systems” also must be evaluated to determine if the program is adequate during the extended period of license renewal. This document was designed for managing the effects of aging in the cranes used in Nuclear Power Plants. In most of the operational experience records in Nuclear Power Plants, the failures in cranes are related to human mistakes, for example, inappropriate maintenance that can result in excessive corrosion in bolted unions, loose bolts, pre-loaded bolts, wear in rails, etc. Therefore, a crane cannot fail for fatigue only as was shown in this paper but it can fail if other circumstances like those mentioned above. The Cumulative Usage Factor (CUF) has been determined in the structural girders of an overhead traveling bridge crane, which is located inside the reactor building of a Nuclear Power Plant type BWR. These analyses have shown several critical regions located near the contact between the trolley and the rail of bridge girder due to the cyclic loads applied. The critical cases depend on the theory of correction of mean stress. Some theories are more conservative (Goodman) than others; noting that Morrow and Walker ’s theories show good life estimates. Two critical cases were identified, the first is when the crane is tested at the end of an outage. In this case, its maximum capacity is exceeded by 25%. The second takes place when the crane handles the waste casks. The resulting values of the CUF for the structural girders of the crane comply with 10 CFR 54.21 (c) (1) (ii), showing that the CUF remains below one during the extended period of operation; obtaining a CUF of 0.003, 0.016, 0.021 and 0.044 with different theories of correction of mean stress.