Local thinning is a common defect in elbows, but research on this type of defect mainly focuses on straight pipes at home and abroad, and there are few literature reports on local thinning of elbows. Through detailed finite element calculation and theoretical analysis, this paper studies the influence of local thinning on the ultimate bearing capacity of elbows under internal pressure and bending moment, the interaction effect of multiple local thinning under internal pressure, and the strengthening effect of straight pipes on the ultimate load of elbows under bending moment. Some experimental verification has been carried out and the following research results have been obtained:

1. The finite element method was used to systematically analyze and calculate the ultimate load of locally thinned elbows under internal pressure. It was found that the ultimate pressure of locally thinned elbows is different from that of locally thinned straight pipes. The ultimate pressure of elbows not only depends on the size of local thinning, but also on the location and bending radius of local thinning. If the calculation method of locally thinned straight pipes is used to evaluate elbows, it will result in unsafe or overly conservative results; The influence of thinning width on the ultimate load cannot be ignored. On the basis of the finite element analysis, the calculation formula of the limit pressure of the locally thinned elbow is given. The calculation result of the formula is quite consistent with the finite element calculation and the experimental results, and is on the safe side. The calculation formula can be actually applied to the safety assessment of the locally thinned elbow, which supplements the blank of this research.

2. Through finite element analysis, the mutual interference effect between multiple local thinning under internal pressure was studied. The study showed that the mutual influence of multiple local thinning is not only related to spacing, but also to the depth of thinning. When the thinning depth is shallow, the axial local thinning spacing is greater than 2 times the wall thickness, and the ultimate load of double local thinning is basically the same as that of a single local thinning; When the thinning depth is deep and the axial local thinning spacing is greater than 4 times the wall thickness, the ultimate load of double local thinning is basically the same as that of a single local thinning, which supplements the shortcomings of existing research.

3. Through the finite Metacomputing, the strengthening effect of the straight pipe connected to the elbow on the ultimate bending moment of the elbow is studied. It is pointed out that the straight pipe connected to the elbow will increase the ultimate bending moment of the elbow. When the bending radius is different, the increase of the ultimate load of the elbow is different. When the length of the connected straight pipe is greater than 3 times the pipe diameter, the strengthening effect of the straight pipe on the elbow no longer increases. This study supplements the lack of research on the strengthening effect of straight pipes on elbows.

4. Through finite element analysis, the influence of local thinning on the ultimate bending moment of the elbow was studied in detail. It was found that the magnitude of the ultimate bending moment of the locally thinning elbow under the action of in-plane bending moment is related to the thinning position, thinning size, and bending radius. Research has shown that the influence of geometric nonlinearity is significant under the action of bending moments. Based on the finite element analysis of the local thinning of the inner wall and large deformation, the calculation formula of the ultimate bending moment of the locally thinned elbow under the in-plane bending moment is given. The calculation results can accurately and conservatively reflect the results of the finite Metacomputing, and are consistent with the experimental results. This study fills the gap in this field.