Markus Niffenegger

On the temperature independence of statistical model parameters for cleavage fracture in ferritic steels

Abstract The work relates to the effect of temperature on the model parameters in local approaches (LAs) to cleavage fracture. According to a recently developed LA model, the physical consensus of plastic deformation being a prerequisite to cleavage fracture enforces any LA model of cleavage fracture to observe initial yielding of a volume element as its threshold stress state to incur cleavage fracture in addition to the conventional practice of confining the fracture process zone within the plastic deformation zone. The physical consistency of the new LA model to the basic LA methodology and the differences between the new LA model and other existing models are interpreted. Then this new LA model is adopted to investigate the temperature dependence of LA model parameters using circumferentially notched round tensile specimens. With the published strength data as input, finite element (FE) calculation is conducted for elastic-perfectly plastic deformation and the realistic elastic-plastic hardening, respectively, to provide stress distributions for model calibration. The calibration results in temperature independent model parameters. This leads to the establishment of a ‘master curve’ characteristic to synchronise the correlation between the nominal strength and the corresponding cleavage fracture probability at different temperatures. This ‘master curve’ behaviour is verified by strength data from three different steels, providing a new path to calculate cleavage fracture probability with significantly reduced FE efforts.

In Situ Biaxial Mechanical Testing at the Neutron Time-of-Flight Diffractometer POLDI

Complex strain paths are often applied to materials during production processes. This paper shows the first successful in-situ biaxial mechanical tests during neutron diffraction performed on a cruciform steel sample and reports on the differences compared to uniaxial deformation. Digital image correlation is demonstrated to be an appropriate tool to monitor spatially resolved the macroscopic straining. The new, modular biaxial machine that will be installed at the neutron diffractometer POLDI is presented.

Probabilistic Pressurized Thermal Shocks Analysis for a Reactor Pressure Vessel

Both deterministic and probabilistic methods are used to analyze the integrity of a reactor pressure vessel (RPV) subjected to pressurized thermal shocks (PTSs). The FAVOR code is applied to calculate the probabilities for crack initiation and failure of the RPV subjected to two transients, by considering crack distributions based on cracks observed in the Shoreham and pressure vessel research user facility (PVRUF) RPVs. The crack parameters, i.e. crack density, depth, aspect ratio, orientation and location are assumed as random variables following different distributions. KI of the cracks with the same depth increases with its aspect ratio. Both KI and KIC at the crack tip increase with crack depth, which is the reason why a deeper crack does not necessarily lead to a higher failure probability. The underclad crack is the most critical crack and the deeper crack is the least critical one in this study. Considering uncertainties of the transients results in higher failure probabilities.

Precracked pipe under waterhammer action

We numerically simulate a full scale test in several computational steps with the finite element method and compare all calculated data with the experimental findings. First, we compute the deflection under static loading and the spectrum of eigenfrequencies of an integer piping, attached to a nuclear reactor pressure vessel (RPV). Then we consider a sudden pipe break at some distance from the vessel, immediately followed by an undamped closure of a check valve close to the break on the RPV side, and calculate the elastic and plastic transient dynamic response of the integer piping part between the RPV and the break. Finally, we consider a circumferential internal surface crack, fairly close to the vessel; after extensive testing of our fracture mechanics calculation procedure we investigate the stress in the crack region under the waterhammer action.

Methodology and Warm Prestressing Effect for Pressurized Thermal Shock Analysis in Nuclear Power Plant

One potential challenge to the integrity of the reactor pressure vessel (RPV) in a pressurized water reactor is posed by pressurized thermal shock (PTS). Therefore, the safety of the RPV with regard to neutron embrittlement has to be analyzed. In this paper, the procedure and method for the structural integrity analysis of RPV subjected to PTS is presented. The FAVOR code is applied to calculate the probabilities for crack initiation and failure by considering crack distributions based on cracks observed in the Shoreham and PVRUF RPVs in the U.S. A local approach to fracture, i.e. the σ*-A* model is used to predict the warm prestressing (WPS) effect on the RPV integrity. The results show that the remaining stress contributes to the WPS effect, whereas the increase of fracture toughness is not completely attributed to the remaining stress. The modeled load paths predict a material toughness increase of 30-100%.

Probabilistic Pressurized Thermal Shocks Analysis for a Reactor Pressure Vessel by Considering Plume Effect

The inner surface of a reactor pressure vessel (RPV) is assumed to be subjected to pressurized thermal shocks (PTSs) caused by the downstream of emergency cooling water. The downstream is not homogeneous but typically in a plume shape coming from the inlet nozzles.In this paper, both deterministic and probabilistic methods are used to assess the integrity of a model RPV subjected to PTS. The FAVOR code is used to calculate the probabilities for crack initiation and failure of the RPV considering crack distributions based on cracks observed in the Shoreham and PVRUF RPVs.The study shows that peak KI of the cracks inside the plume increases about 33% compared with that outside. The conditional probability inside the plume is more than eight orders of magnitude higher than outside the plume. In order to be conservative, it is necessary to consider the plume effect in the integrity assessment.Copyright

Computational Analyses of the Thermal Stress Induced by Coolant Mixing in a T-Joint

In this paper we report computational fluid dynamics and thermal stress finite element analyses of a T-joint component as used in a Swiss nuclear power plant under realistic loading conditions and simplified boundary conditions. In this report the focus is on the thermal stress analyses, and therefore only those fluid dynamics simulations used for thermal stress analyses are presented. The local stress amplitudes, simulated using elastic finite element simulations, are compared to a regulatory fatigue life curve to estimate the local distribution of low cycle fatigue damage and crack initiation probability in the T-joint. The case studies reported here have been selected with the purpose of analyzing the different ways in which the fluid structure interaction induces thermal stress in the mixing tee. In case I only the thermal stress induced by the mixing turbulence is simulated, i.e. with constant mass flow rate boundary conditions. In case II a peak in the flow rate in the main pipe, typical for transient (startup) plant conditions, is addressed and analyzed in detail. Only in the latter case is low cycle fatigue cracking to be expected in the form of non-propagating (i.e. not throughwall) surface cracks or crazing.Copyright