Protection against local failure is one of the integral components in the design-by-analysis requirements in ASME BPVC Section VIII, Division 2. Of the methods offered by the ASME, the Local Strain Limit procedure outlined in is the typical calculation method. However, it has been found that relying on this procedure alone can lead to untenable utilization results if used on certain analyses with varied load paths.

The flange described in this study was calculated using “design by analysis” according to Part 5 of ASME BPVC Section VIII, Division 2. The elastic-plastic stress analysis method was used. The flange was loaded with an initial bolt pre-tension and then with internal pressure. During the local failure calculation, an abnormal condition was encountered in the form of a large spike in the history curve of the ratio between plastic strain and limiting triaxial strain.

An investigation found that despite being in a stress state below yield stress, some nodes had a non-zero plastic strain and high triaxiality factor. This was caused by the load sequence: first, the bolt pre-tension and then internal pressure. The flange was first bent due to the pre-tension load, and later experienced bending in the opposite direction after the internal pressure load was applied. This resulted in a relatively low stress state with a high triaxiality factor and non-zero plastic strain in certain areas, which then showed high utilization under the local failure strain limit criterion.

This paper will discuss how this issue can be avoided by using the strain limit damage calculation procedure outlined in ASME BPVC Section VIII, Division 2.

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