Plastic collapse and LBB behavior of statically indeterminate piping system subjected to a static load

The plastic collapse and LBB behavior of statically indeterminate piping system were investigated in this study, compared with those of the statically determinate piping system. Special attention was paid to evaluate the crack opening displacement after a crack penetrated wall thickness. The main results obtained were as follows:

1. The reduction of ultimate strength caused by a crack was relatively small in the statically indeterminate piping system. The main reason is thought to be that a sufficient redistribution of the bending moment occurs in this system.

2. A method to evaluate the crack opening displacement after crack penetration in a pipe with a non-penetrating crack was proposed. From this method, it was known that the crack opening displacement could be evaluated by using the incremental plastic rotation angle.

3. The acceptable defect size considering the deformation of a pipe was estimated by comparing the plastic moment at the defective part and the gross yielding moment at the non-defective part.

Article Outline

1. Introduction

2. Theory

2.1. Evaluation of plastic collapse load

2.2. Method for predicting COD

2.3. Net-section stress approach in pipe

3. Material and testing procedure

4. Test results and consideration

4.1. Plastic collapse and LBB behavior

4.2. Evaluation of COD

4.3. Gross yielding in pipe section

5. Conclusion

Appendix A. Nomenclature

References

1. Introduction

The structure integrity and reliability are required on nuclear piping systems, high-pressure vessels and LNG tanks and so on. Thus, in order to prove the structure integrity and reliability and to prevent a severe accident, attention is paid to the LBB design method on which various studies have been occurred. When the LBB concept is applied to such energy-related plants, it requires not only a piping fracture analysis but also a leakage analysis in crack parts of piping system. In particular, the leakage analysis is directly related to the evaluation of COD (Crack Opening Displacement). Studies on the piping fracture and the evaluation of COD due to cracks in structure have been mainly performed on statically determinate systems (Liu et al., 1996). As a result, many useful results were reflected on the standards to improve designs and inspections design or inspection. However, it is essential to investigate statically indeterminate systems, considering that most piping systems of energy-related plants consist of statically indeterminate ones ( Liu and Ando, 1996a). Liu et al. have made it clear that the statically indeterminate system had a higher safety margin in the viewpoint of the LBB concept than the statically determinate system from a series of studies on the plastic collapse behavior and LBB characteristic of a statically indeterminate system. However, proof from experiments has not been found for the LBB characteristics of the statically indeterminate system. Therefore, the LBB behavior in the statically indeterminate piping system was evaluated by comparing that of the statically determinate piping system from a series of experimental results.Furthermore, on the LBB evaluation, it is essential to estimate COD or COA (Crack Opening Area). The method of COD or COA evaluation has been established on the pipe, including a fully through-wall crack circumferentially. But if the LBB design method is considered, it is natural that a non-penetrating crack penetrates during a loading, then the contents leak than a fully through-wall crack is assumed initially. For this purpose, this study describes an approach to predict COD when a non-penetrating crack penetrates during a loading in pipe was proposed in this study.

2. Theory

2.1. Evaluation of plastic collapse load

The evaluation of plastic collapse load was based on the plastic design method (Liu and Ando, 1996b). The selected case in the present study was the system fixed at one end and simply supported at the other. The corresponding plastic collapse model obtained from this case is illustrated in Fig. 1. From Fig. 1, the evaluation value of plastic collapse load (P_C) can be drawn from the following relation, respectively.