M.K. Sahu’s research while affiliated with Bhabha Atomic Research Centre and other places

In service, the piping components (pipes/elbows) are subjected to different operational loadings such as thermal stress, internal pressure, self-weight, external loading from soil movement, flow induced vibration, vortex induced vibrations and third party damage. Structural integrity assessment of these piping components under postulated accidental conditions is essential for ensuring smooth operation of power plant piping system. In order to understand crack growth behaviour of piping components under operating conditions, fracture studies were carried out on two straight pipes and three elbows of 219 mm OD having circumferential through-wall notch with internal pressure. Fracture tests on pipes were carried out under four-point bending and elbows were tested under in-plane bending. A sealing arrangement consisting of stainless steel plate as backing material and silicon rubber sheet with two-component super strength epoxy based adhesive was used to seal the through-wall notch in the pipes and elbows. The pipe and elbows specimens were filled with water through threadolets and the desired internal pressure was achieved using an automatic high-pressure hydraulic pump. The internal pressure was varied from 10 to 25 MPa. The load carrying capacity of pipes decreased by a factor 1.95 with the increase of initial notch angle from 90.5° to 152.8° under an internal pressure of 10 MPa. In the case of elbows, with an increase of internal pressure from 10 to 25 MPa, load-line displacement corresponding to maximum load increased by 41.1%. Ovalization (intrados-extrados) in the elbows was found to be varying from 2.74% to 6.85% at the end of fracture tests. The pipe specimens have undergone initial stable crack growth followed by crack instability and crack growth deviated from the initial plane of the notch. The collapse load obtained for through-wall cracked elbows under internal pressure was found to be conservative compared to the values obtained using twice elastic slope method.

The fracture property, J-R curve is an essential material property for assessing the cracked structures for unstable ductile tearing. For obtaining this material property, fracture test is conducted and resultant test data namely load, load line displacement (LLD)and relevant crack extension data are post processed to obtain J-R curve. However, in this method, geometry functions η and γ are essential which are available for only limited geometries. To avoid this limitation, recently two approaches based on R6 failure assessment diagram were proposed. First was the load based R6 approach where load vs. crack growth data is converted to fracture property J-R curve using R6 Failure Assessment Diagram (FAD). This method was further modified to displacement based R6 approach where LLD vs. crack extension data is converted to J-R curve using R6 FAD. The predicted J-R curves were in good agreement with conventionally calculated values especially for displacement based approach. However, when these load and displacement based approaches are attempted to be used in case of throughwall cracked elbows, the predicted J-R curve deviated significantly with respect to those obtained using conventional method. On further investigation, it is found that the assumption involved in the displacement based approach is violated for cracked elbows because of severe ovalization of cross section. Hence, a modified approach is proposed in this work where both load and displacement parameter are used for calculation of J-R curves and named as hybrid approach. Further, this approach is adopted for the six through wall cracked pipes which are already investigated by load and displacement based R6 approaches for J-R curves. The predicted J-R curves are found to be in good agreement for all the investigated pipes and elbows. Hence, a simpler yet more accurate hybrid approach based on R6 method is proposed in this paper for post processing the test data to obtain, the material J-R curve.

Determination of fracture toughness, J-R curve from experimental test results requires certain geometry factors such as η and γ. These expressions of geometry factors are available for standard geometries and loading configurations. However, these expressions are not available in open literature for complex geometries and loading configurations. R6 Failure Assessment Diagram (FAD) methodology is used for assessment of failure in the failure regime of cleavage fracture, ductile crack initiation and plastic collapse. Recently, one load based approach was proposed for calculation of fracture toughness by R6 using experimental load vs. crack extension data. On comparison with already reported J-R curves calculated using conventional η -factor approach, predicted results were showing good match for some cases while for few cases it became significantly non-conservative. In this paper, the load based approach is modified to displacement based approach and instead of load vs. crack extension data, displacement vs. crack extension data is used for calculation. The calculated fracture properties using displacement based approach show a marked improvement over results using load based approach while comparing with the conventionally calculated J-R curves. The problem of non-conservatism of predicted fracture properties by load based approach is also resolved by the proposed displacement based approach.

Elbows are one type of components widely used in various industrial applications in conventional and nuclear power generating plants, chemical processing plants and petroleum refineries. Due to the complexities of geometric structure, elbows are one of the piping components that are prone to failure in the piping system of nuclear power plants (NPPs). In service, the piping components are subjected to internal pressure in addition to bending loads, and the internal pressure is known to have significant effect on the load carrying capacity of these components. Hence, fracture studies were carried out on 406 mm diameter elbows with and without internal pressure. The investigations have shown that the location of notch plays an important role in the load carrying capacity of the elbows and their failure behaviour. The internal pressure reduced the load carrying capacity of the elbow when the notch was located at the intrados in circumferential direction and the behaviour is vice-versa when the notch was located at the intrados in an axial direction, when subjected to opening moment, in both the cases.

This paper reports defect assessments of experiments on large-scale straight pipes and elbows using failure assessment diagram (FAD) methods. The pipes and elbows were of various pipe diameters and contained a range of sizes of cracks sharpened by fatigue loading. Solutions in the literature for stress intensity factor and limit load have been evaluated and up-to-date solutions used as inputs to the FAD assessments. The assessed loads for crack initiation are close to those observed experimentally for straight pipes under pure bending. For elbows, the assessed loads using the FAD approach are generally lower than the experimental loads for crack initiation demonstrating conservatism in the FAD approach. The FAD assessments give confidence in the use of the modern stress intensity factor and limit load solutions for practical defect assessments. The paper indicates how constraint modifications to the FAD may be used to improve accuracy.