STATISTICAL ANALYSIS AND MONTE CARLO SIMULATIONS OF WARM PRE-STRESSING OF IRRADIATED SMALL SINGLE EDGE NOTCH BEND SPECIMENS

摘要

Warm pre-stressing (WPS) is the process of subjecting a pre-cracked component to a load cycle at a temperature higher than the subsequent re-loading temperature. It is widely acknowledged as being able to enhance the load to fracture, especially in ferritic steels, which exhibit lower shelf cleavage fracture. This enhancement has had a significant impact on the integrity of pressure vessels, particularly during severe loading conditions such a pressurised thermal shock. Experimental data was generated from WWER 440 RPV steel, at UJV Rez, a.s., using small single-edge-notched bend SEN(B) specimens (or commonly referred to as pre-cracked Charpy), irradiated with both low and high fluence levels. The experiments were conducted under different WPS and fracture temperatures. Detailed statistical analyses of the experimental data were performed using a three parameter Weibull distribution to map the toughness scatter. The statistical significance of the enhancement in apparent toughness (due to WPS) was evaluated using the Mann-Whitney test. A WPS model, developed by Chell and co-workers, is reformulated to create a method of undertaking Monte Carlo simulations to study the effects of WPS on brittle fracture and predict the distribution in apparent toughness following different warm pre-stressing cycles. The results obtained from this model are also compared to predictions made by the Wallin approach. Although accurate estimates of the toughness distributions can be obtained, the accuracy of WPS predictions is limited by the sample size.