I.A. Khan

A numerical prediction of flexural strength probability for NBG-18 nuclear grade graphite using strength pair model

In this work, a strength pair model has been proposed for the numerical prediction of flexural strength probability of NBG-18 nuclear grade graphite. The input to the proposed model is a random strength pair of tensile and compressive strengths whose value is based on its probability of occurrence in the experimental data. A finite element–based deterministic numerical approach has been implemented. To account for the large difference in tensile and compressive strengths, Drucker–Prager failure criteria has been implemented. The failure envelope of the Drucker–Prager failure criteria is assumed to have uniaxial fit with Mohr–Coulomb model in the principal stress space. A total of 292 simulations with random pairs of tensile and compressive strength are performed on a three-point bend specimen to obtain a set of flexural strength data. The flexural strength data obtained through numerical simulations are fitted using normal and Weibull distributions. The flexural strength probability obtained from the proposed model is found on conservative side. A goodness-of-fit test concludes that Weibull distribution fits the numerical data better than normal distribution.

Significance of Finite Compliance of a Cracked Piping System on Fracture Integrity Assessment

One of the thrust areas in the integrity analysis of cracked nuclear piping system is concern with the reduction in moment, at the crack section due to combined effects of local and global residual compliance. However an important consideration in the design of piping system, which is generally not considered, is the re-distribution of load that occurs due to finite compliance of the piping system. The load at the crack section reduces while it increases generally at support/anchor locations, which may be high stressed locations. In case of stiff-piping system this re-distribution of load may be quite significant. Hence for the complete integrity of the piping system these un-cracked locations should also be re-assessed. A generalized procedure is suggested to take care of the reduction in load at the cracked section and corresponding increase in reactions at the support/anchor locations in a 3-D cracked piping system. Thus the stability of cracked section as well as other highly stressed locations can be simultaneously assessed. Here it is assumed that the remaining piping system behaves in a linear elastic manner and the plasticity remains confined to the cracked section only. Detailed finite element analyses are performed on circuitous (3-D) cracked piping system to validate the developed approach. Results presented in this article clearly show that due to reduction in moment the crack driving force, for the same external load, reduces significantly.Copyright

Monotonic Fracture Studies on Bi-metallic Pipe Weld Joints Having Circumferential Through-Wall Crack

Bi-metallic welded joints are necessary in pressurized and boiling water reactors where heavy section low alloy carbon steel components are connected to stainless steel primary piping systems. The behaviour of such bi-metallic weld joints is very difficult to understand, and very limited experimental data are available in the literature on the fracture behaviour of such joints. It is essential to develop fracture assessment methodologies for these joints and validate them experimentally to ensure safe operation of power plants. In this background, fracture studies were carried out under monotonic bending on seven numbers of bi-metallic pipe weld joints of 324 mm nominal outer diameter having circumferential through-wall crack. The specimens were made of SA 508 Gr.3 Cl-1 low alloy steel (ferritic) pipe on one half and SA312 Type 304LN stainless steel (austenitic) pipe on the other half. The initial through thickness notch was located in the different regions of the weld joints such as weld centre, buttering, heat-affected zones and base metals (low alloys steel and stainless steel). Initial notch angle was either 60° or 90°. Prior to the fracture tests, fatigue pre-cracking was carried out to create sharp crack front. Subsequently, fracture tests were conducted under four-point bending and displacement control. The collapse load of the weld joint having crack in the buttering layer is 12% less compared to that having crack in the heat-affected zone. Increase in the crack angle present in the heat-affected zone from 60° to 90° reduced the collapse load of the weld joint by 19%.