Investigation of Fracture Behavior of Steam Generator Tubes of Indian PHWR using PLT Specimens

Steam generator (SG) tubes made of Ni-Cr-Fe based alloys such as Alloy-800 are used in nuclear power plants like Indian Pressurized Heavy Water Reactors (PHWRs) in the form of thin-walled tubular structural members as heat exchangers. These tubes are exposed to aggressive environments such as high temperature, internal pressure and flow-induced vibration etc. Corrosion aspects like intergranular-stress-corrosion-cracking (IGSCC) of such alloys received much attention in literature, whereas work on fracture behavior of these materials is limited, particularly for thin-walled tubes. For structural integrity assessment of SG tubes, a valid fracture mechanics parameter must be obtained. As their thickness is only about 1 mm, it is not feasible to machine standard fracture mechanics specimens from them. Moreover, the plane-strain condition of state of stress cannot be achieved at the crack-tip of these tubes which is a requisite of ASTM standard tests for fracture toughness. Thus, non-standard tests have been carried out on axially-cracked tubular specimens, directly machined from SG tubes in a test-setup known as Pin-Loading-Tension (PLT) setup for derivation of shape function from compliance for determining stress intensity factor and fracture toughness. Finite element analysis (FEM) has also been done to determine the shape function and the other required geometric functions for obtaining J from experimental data. A load-normalization technique was used for estimation of crack growth. The results obtained will be highly useful for residual life assessment of SG tubes of Indian PHWRs.