Comparison of experimental and finite element analytical results for the strength and the deformation of pipes with local wall thinning subjected to bending moment

摘要

Fracture behaviors of pipes with local wall thinning are very important for the integrity of power plant piping system. In this study, monotonic bending tests without internal pressure are conducted on 48.6 mm diameter Schedule 80 (thickness, 5.1 mm) STS370 full-scale carbon steel pipe specimens. Fracture strengths of locally wall-thinned pipes were calculated by elasto-plastic analysis using finite element method. The elasto-plastic analysis was performed by FE code ANSYS. We simulated various types of local wall thinning that can be occurred at pipe surface due to coolant flow. Locally wall thinned shapes were machined to be different in size along the circumferential or axial direction of straight pipes. We investigated fracture strengths and failure modes of locally wall thinned pipes by four point bending test. And, the allowable limit of pipes with local wall thinning was investigated. In addition, we compared the simulated results by finite element analysis with experimental data. The failure mode, fracture strength and fracture behavior obtained from FE analyses showed well agreement with experimental results. From the test results, we identified three types of failure modes into ovalization, local buckling and crack initiation. These failure modes could be classified according to thinned depth, thinned length and thinned angle of a pipe. For locally wall-thinned specimens, maximum moments (M_(max)) were estimated by using the net-section stress criterion. Pipes with local wall thinning can be estimated using σ_u instead of σ_f because of 1.19σ_f ≈ σ_u. Also, the axial strain affects failure modes occurred on local wall thinning, the allowable limit of local wall thinning for carbon steel pipe used can be given as follows; in the case of M_(max) > M_y, if 10 ≤ l < 25mm, d/t can be allowed to about 55%, and if 25 ≤ l < 100 mm, d/t can be allowed to about 50%. Also, if 100 ≤ l < 120 mm, d/t can be allowed to about 29%.