The Huge Missing Factor in LBB Analysis - How A Circumferential Through-Wall-Crack in A Pipe System Changes the Flexibility and Reduces the Applied Moments

In typical leak-before-break (LBB) analyses in the nuclear industry, the uncracked piping normal operating forces and moments are applied in a cracked-pipe analytical procedure to determine normal leakage, and the combined forces and moments under normal operating condition and safe shutdown earthquake seismic loading are used in a fracture analysis to predict margins on "failure". The International Piping Integrity Research Program (IPIRG) performed in 1990 to 1998 provided some insights to typical LBB behaviors where pipe system tests were conducted with simulated seismic loadings. The test results showed a large margin on LBB which was also recognized in 2011 when the Argentinian Atucha II plant was analyzed using a robust full FE model. It was found that when circumferential through-wall cracks were put in the highest stressed locations, the applied moment dropped for both normal operating and N+SSE loading as the crack length increased. The through-wall crack size for causing a double ended guillotine break (DEGB) was greater than 90%-percent of the circumference. Similar results were also found for a petrochemical pipe system where thermal expansion stresses are much higher than the primary stresses. Even with very low toughness materials, the critical crack size leading to DEGB was greater than 80% of the circumference. The implication of this work is that pragmatically there is much higher margin for DEGB failure in nuclear plant operation, and efforts would be better focused on the potential for a small-break loss-of-coolant accident (SB-LOCA).

Probabilistic Analysis of Fatigue Behavior of Single Lap Riveted Joints

This research deals with the fatigue behavior of 200 small single lap multiple-riveted joint specimens, widely used for aeronautic structures. The tests were performed with three different levels of stress with stress ratio R = 0.05; three levels were set: 90 MPa, 120 MPa and 160 MPa. The fatigue life and critical crack size for all tested specimens were analyzed. According to the results’ analysis, two types of fracture, through-hole and in proximity of the hole, were observed, depending on the level of stress: the higher the applied stress, the more through-hole cracking. Indeed, under the fatigue load with a stress level of 90 MPa, less than 30% of specimens showed cracks propagating through the hole, while, at the stress level of 120 MPa, the percentage reaches 36.3%. At the stress level of 160 MPa, 100% of specimens failed through the hole. Moreover, aimed to use experimental data for probabilistic methods, a statistical analysis was performed according to the Anderson–Darling test. This method allowed the analysis of the datasets, in terms of both fatigue life and critical crack size, providing information about the best distribution function able to fit experimental results.

Case Studies of Aircraft Fuselage Cracking

Fatigue plays a significant role in the crack growth of the fuselage skin structures. In addition, the fuselage may suffer also from the corrosion damage, and the wear defects. The proper maintenance and scheduled test intervals can avoid the sudden skin failure. Therefore, the inspection interval has to be shortened. Nevertheless, the young machines may be also suffering from the unexpected skin rupture. The cracks are emanating from the rivets and the holes under cyclic loading. The stress concentration around the notch has an effective role under the effect of cyclic loading. The cracks propagate toward the high stressed area such as the notches or other crack locations. The propagation into a critical crack size is rather fast and causes a sudden aircraft fuselage cracking. Hence, the number of cycles to failure will be decreased dramatically. During the last decades, the fracture toughness, design, and the new alloying element have been enhanced. The previous fuselage failures show that the inspections against the cracking are recommended even after a few thousand of cycles. To prevent the crack extending, the crack arresting is recommended to use around the fuselage.

Comparison of Leak-Before-Break Assessment of Main Loop Piping Lines Fabricated of Different Materials

Leak-Before-Break (LBB) assessment is used for the design of nuclear reactor coolant system main loop piping to lower the cost of construction and operation in China. In these applications, the materials of main loop piping lines are cast austenitic stainless steel (CASS) or wrought stainless steel (WSS) due to the different type of reactor design. According to US.NRC SRP3.6.3, LBB assessment includes two major calculations, such as critical crack size calculation and leakage flaw size calculation. The elastic-plastic instability analysis or plastic instability analysis is chosen to perform critical size calculation depending on material properties, especially fracture toughness. In this paper, LBB assessment in the guidance of SRP 3.6.3 was performed to evaluate main loop piping lines of CASS and WSS. The JR curve tests and the adjustment due to thermal aging are performed to achieve reasonable material properties. J integral/tearing modulus approach is used to determine critical crack size of CASS pipe and net section collapse (NSC) approach is used to determine critical crack size of WSS pipe. Leakage flaw size under 1gpm leakage detection capability is determined based on Henry’s homogeneous nonequilibrium critical flow model. In order to demonstrate that fatigue crack growth is not a potential source of pipe rupture for the evaluated piping lines, the fatigue crack growth of a postulated circumferential part-through-wall crack under nuclear power plant full life time operating transients and the fatigue crack growth of a circumferential through-wall crack under one time safe shutdown seismic are analyzed. And the LBB assessment procedure and results of CASS pipe and WSS pipe are compared.

Improvement of the LBB.ENG2 Circumferential Through-Wall Crack J-Estimation Scheme

The LBB.ENG2[1] circumferential through-wall crack (TWC) J-estimation scheme forms the basis for the Extremely Low Probability of Rupture (xLPR)[2] probabilistic pipe fracture analysis for TWC elastic-plastic fracture mechanics (EPFM) stability assessment. The LBB.ENG2 methodology uses a reduced thickness pipe wall analogy to approximate the behavior of actual cracked pipe and sets the thickness of the reduced section by making the usual cracked pipe limit load assumption. Sometime during the original LBB.ENG2 development process, it was discovered that LBB.ENG2 was not as good as desired at predicting the maximum moment carrying capacity of pipe fracture experiments with longer cracks. Accordingly, the effective thickness equation was modified to be 1.0 at crack angles less than π/4, 4/π at angles greater than π/3, and linear between these values using a so-called ψ function. When LBB.ENG2 was coded for the TWC stability module for xLPR, TWC_fail, the behavior described above was implemented. Quite unexpectedly, with the new coding, exploration of TWC_fail’s bounds uncovered two discontinuities in the complete moment-pressure-critical crack size failure surface. Subsequently, it was found that these discontinuities were caused by the discontinuity in the derivative of the ψ function. This paper documents the approach used to smooth the TWC_fail moment-pressure-critical crack size surface by making a ψ function fit that minimizes the difference between J from LBB.ENG2 and J from finite element analyses results. The results of the finite element analyses and fitting methodology are described and the basic equations for the solution are presented. Following this, the new ψ function is applied to cases to evaluate the efficacy of the approach.

Effect of In Plane and Out of Plane Bending on Weld Residual Stresses

When manufacturing cylindrical or conical structures from metal plates, residual stresses originate from both welding and bending. The sequence in which these fabrication steps are carried out is essential as it can radically change the final distribution of residual stresses. To study this effect, detailed welding and bending simulations have been performed on both axial and circumferential X-welds of a 316L stainless steel cylindrical vessel. Bending after welding is shown to reduce residual stresses markedly more than bending before welding and the benefit on critical crack size is illustrated by a Fracture Mechanics analysis.