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Local thinning is a common defect of elbow, but the domestic and foreign researches on this kind of defect mainly focus on straight pipe, and there are few literature reports on local thinning of elbow. Through detailed finite element calculation and theoretical analysis, this paper studies the internal pressure and bending moment under the action of the local thinning to bend the influence of ultimate bearing capacity, as well as the internal pressure under the action of many local thinning the mutual interference effect and under the action of bending moment of straight pipe to elbow limit load of strengthening effect, and some experimental verification, get the following results:
1. Using the finite element method under the action of internal pressure limit load of local thinned elbow has carried on the systematic analysis and calculation, it is concluded that the local thinned elbow ultimate pressure is different from local thinning of the straight pipe, elbow of extreme pressure not only depends on the size of the local thinning, also associated with local thinned position and bending radius, such as using the calculation method for the evaluation of local thinned straight pipe elbow, Results may be unsafe or too conservative; At the same time, the influence of thinning width on ultimate load cannot be ignored. Based on the finite element analysis, the formula for calculating the ultimate pressure of local thinning elbow is given. The results of the formula are in good agreement with the finite element calculation and experimental results, and are safe. The formula can be applied to the safety assessment of local thinning elbow, which complements the blank of this research.
2. Through finite element analysis, the mutual interference effect of multi-local thinning under internal pressure is studied. The study shows that the mutual influence of multi-local thinning is not only related to the spacing, but also to the thinning depth. It is pointed out that when the thinning depth is shallow, the axial local thinning spacing is greater than 2 times of wall thickness, and the ultimate load of double local thinning is basically the same as that of 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 single local thinning, which complements the deficiencies of existing studies.
3. Through finite element calculation, the strengthening effect of connected straight pipe on the ultimate bending moment of elbow is studied, and it is pointed out that the ultimate bending moment of elbow increases with the straight pipe connected to elbow, and the ultimate load increases with different bending radius. When the length of connected straight pipe is more than 3 times of pipe diameter, the strengthening effect of straight pipe on elbow is no longer increased. This study complements the deficiency of the research on strengthening effect of straight pipe on elbow.
4. The influence of local thinning on the ultimate bending moment of the elbow is studied in detail by finite element analysis. It can be concluded that the ultimate bending moment of the local thinning elbow under the action of internal bending moment is related to the thinning position, the thinning size and the bending radius. The results show that the influence of geometric nonlinearity is significant under bending moment. Based on the finite element analysis of the local thinning of the inner wall and large deformation, the formula for calculating the ultimate bending moment of the local thinning elbow under the action of the internal bending moment is given. The calculated results can reflect the finite element calculation results accurately and conservatively, and agree with the experimental results. This study fills in a gap in the field.