Experimental Investigations on Effects of Simulated Seismic Loading on LBB Assessment of High Energy Piping

Abstract

The current Leak Before Break (LBB) assessment is based primarily on the monotonic fracture tearing instability. In it the maximum design accident load is compared with the fracture-tearing resistance load. The effect of cyclic loading has generally not been considered in the fracture assessment of nuclear power plant piping. It is a well-known fact that the reversible cyclic loading decreases the fracture resistance of the material, which leads to increased crack growth. Indian nuclear power reactors consider Operational-Basis-Earthquake (OBE) and Safe-Shutdown-Earthquake (SSE) event in the design of various structures, systems and components. Keeping this in view a series of cyclic tearing test have been conducted on straight pipes, made of ASTM SA333 Gr.6 carbon steel. This is the material of primary heat transport (PHT) piping material of Indian Pressurised Heavy Water Reactor (PHWR). In this series 13 tests have been carried out on circumferentially through wall cracked seamless and circumferential seam welded straight pipes under reversible cyclic bending loading. All the tests have been conducted under quasi-static i.e. slow loading rates and the dynamic effect is not considered. The cyclic test results have been compared with the corresponding monotonic pipe fracture test results. These test results and its comparison with corresponding monotonic tearing clearly illustrates the need of addressing the reduction in apparent fracture toughness of material under reversible cyclic loading and safe number of load cycles in the LBB assessment.

Cyclic-tearing behaviour and J–R curves of Indian NPP pipes under displacement-controlled cyclic loading

“…The crack extension at both crack tips, and corresponding load, LLD and CMOD are obtained from image analysis technique as described in Refs. [28,29]. This image based technique unlike other conventional methods (e.g., ACPD or DCPD) has provided reliable measurement of both component (in-plane and out of plane) of crack growth on outer surface.…”

Comparing fatigue life prediction capability of critical plane models using multiaxial test database on 17 materials

“…16 Zhao-Jiang 17 model resulted in slight under-predictions for GH4169 alloy 51 and Ti-6Al-4 V alloy 54 and accurate fatigue life assessments for Al 7050-T7451 alloy. 52 Chu models 13,15 /Gupta et al 16 on TC4 alloy at room temperature and GH4169 alloy at 650 C, 55 SA 302B; SA 333 Gr. 6 steels; SS 347; S460N; and Al 5083 alloys, 16 SAE1045 steel.…”

“…In past, various CP models have been studied with regard to different multiaxial loading conditions and materials. The CP models have been classified into four different families, which are (i) stress-based CP models, [4][5][6][7] (ii) strain-based CP models, [8][9][10][11] (iii) strain energy density-based CP models, [12][13][14][15][16][17][18][19][20] and (iv) CP models based on mixed formulations. 21,22 These classifications have been done on the basis of the definition of fatigue damage function f FD ð Þ used in quantification of material damage.…”

“…16 • Strain energy density-based, Smith-Watson-Topper (SWT) 12 was used for fatigue life assessment of TC4; GH4169 alloys, 51 Al 7050-T7451 alloy, 52 titanium alloy (Ti-6Al-4V), 45 SS 347; and SA 302B steel. 16 Zhao-Jiang 17 model resulted in slight under-predictions for GH4169 alloy 51 and Ti-6Al-4 V alloy 54 and accurate fatigue life assessments for Al 7050-T7451 alloy. 52 Chu models 13,15 /Gupta et al 16 on TC4 alloy at room temperature and GH4169 alloy at 650 C, 55 SA 302B; SA 333 Gr.…”

“…52 Chu models 13,15 /Gupta et al 16 on TC4 alloy at room temperature and GH4169 alloy at 650 C, 55 SA 302B; SA 333 Gr. 6 steels; SS 347; S460N; and Al 5083 alloys, 16 SAE1045 steel. 15 As the critical plane models predict the planes of maximum fatigue damage, therefore, cracking plane orientation can also be predicted using such models.…”