Issue 50

R. Boutelidja et alii, Frattura ed Integrità Strutturale, 50 (2019) 98-111; DOI: 10.3221/IGF-ESIS.50.10 99 parameters in order to take them into account in a micromechanical modelisation. Stress corrosion cracking (SCC) is one of the important mechanisms in the degradation of steels. This mechanism induces material cracking due to a combined action of a sensitive material, a tensile stress, and corrosive environment (see Fig. 1). In the piping of a boiling water reactor, the sensitive material in the vicinity of welds is the stainless steel AISI 304. Figure 1 : Main types of aging and damage. The sensitivity of this material to cracking by SCC is due to the precipitation of chromium carbide at the grain joints immediately adjacent to the area with lower chromium grade [1]. Zhang and al. have done experimental verifications to determine the initiation time and the propagation rate of IGSCC in sensitized stainless steel type AISI 304 in diluted sulfate solutions [2]. Many researchers have approached the probabilistic analysis of components failure due to SCC based on fracture mechanics [3-10]. Piping component failure probabilities under SCC, including the effects of residual stresses, have been realized by Guedri and al. using Monte Carlo simulation techniques (MCS). The results of these studies have been used to develop the input data for the analysis of failure probabilities [11-12]. The present study is incorporated in a research topic involving the optimal conception and the reliability study of pre- cracked mechanical structures, aiming; in particular, at its application in austenitic steels under complicated solicitations. This paper is structured as follows: the first part presents relevant generalities on reliability, the second part is a general description of the piping reliability model, and the third part presents an application example with an analysis of the results. Finally, a general conclusion synthesizes the analysis methodology and the main results. R ELIABILITY EVALUATION he present section describes probabilistic fracture mechanics calculations that were performed for selected components using the PRAISE computer code. The calculations address the failure mechanisms of stress corrosion cracking and intergranular stress corrosion cracking for components and operating conditions that are known to make particular components susceptible to cracking. Comparisons with field experience showed that the PRAISE code predict relatively high failure probabilities for components under operating conditions that have resulted in field failures. It was found that modeling assumptions and inputs tended to give higher calculated failure probabilities than those derived from data on field failures. Sensitivity calculations were performed to show that uncertainties in the probabilistic calculations were sufficiently large to explain the differences between predicted failure probabilities and field experience. Overview of the PRAISE Code The first version of PRAISE [13] was developed in the 1980s by Lawrence Livermore National Laboratory under contract to NRC, with the initial application to address seismic-induced failures of large-diameter reactor coolant piping. This version of the code addressed failures (small leaks and ruptures) associated with fabrication flaws in welds that were allowed to grow as fatigue cracks until they either caused the pipe to leak or exceed a critical size needed to result in unstable crack growth and pipe rupture. The next major enhancement to the code [14] addressed IGSCC and simulated T