Benoit Tanguy

Cleavage Fracture Micromechanisms Related to WPS Effect in RPV Steel

Since the first investigations four decades ago, a large number of experiments on ferritic steels has confirmed the existence of a warm pre-stress (WPS) effect, which describes the effective enhancement of the cleavage fracture toughness at low temperature following the application, at a higher temperature, of a stress intensity factor (SIF) which exceeds the fracture toughness of the virgin material at low temperature. These experiments allowed for the establishment of the so-called ‘conservative principle’, which states that no fracture will occur if the applied SIF decreases (or is held constant) while the temperature at the crack-tip decreases, even if the fracture toughness of the virgin material is exceeded. In structural integrity assessments involving a prior overload or a thermal transient, such as that of a nuclear pressure vessel subjected to a pressurized thermal shock (PTS) consecutive to a loss of coolant accident (LOCA), such a principle is of great importance in the evaluation of the safety margins. Three main reasons have been advanced to explain the WPS effect: the blunting of the crack tip at high temperature, the formation of high compressive stresses on elastic unloading, and a change in the cleavage fracture micromechanisms induced by plastic deformation. While all these factors certainly contribute to the effective toughness enhancement following WPS, their relative incidence on the fracture risk is not easily established. In this paper, we choose to mainly focus on the cleavage fracture micromechanisms following WPS, as a first step towards better quantifying the individual contributions of crack tip blunting and residual stresses.

Modelling of irradiation embrittlement in the ductile to brittle transition range for an A508 pressure vessel steel

The aging behavior of structural steels used to manufacture nuclear pressure vessels is surveyed using Charpy V-notch specimens located in capsules inside the pressure vessel. The Charpy data are then used to assess the safety integrity of the structures based on semi-empirical relations relating Charpy impact transition curve shifts and the fracture toughness shifts due to irradiation. Using a computational strategy proposed in [1] which combines a deterministic model for ductile fracture and a statistical description of brittle fracture, this work aims at the prediction of the whole Charpy transition curve of irradiated steels. The actual strain hardening behavior of an A508 Cl.3 steel from the french surveillance program is considered in the simulations, contrarily to a previous work where a shift of the un-irradiated stress-strain curve to higher stress values was considered. Comparison with Charpy energy data for two levels of irradiation shows that irradiation possibly also affect brittle fracture. It is also shown that if a low increase of the yield stress is considered, the ductile fracture energy can decrease as a result of a compensation between the increase of dissipated energy due to a higher yield stress and a decrease of dissipated energy due to a faster ductile crack propagation.Copyright

Unified Constitutive Equations to Describe Elastoplastic and Damage Behavior of X100 Pipeline Steel

The deformation and fracture of the high strength steel which is used to fabricate grade X100 steel pipe were investigated at room temperature, on plate and pipe. Anisotropic behaviour was characterized by using tensile tests conducted along different directions. The fracture toughness of plate and pipe materials are compared using compact tension (CT) and Charpy V-notched tests. In both cases, these materials give rise to good fracture properties. The aim of the present study is to establish unified constitutive equations able to describe the elastoplastic and damage behavior of one X100 linepipe steel. The model is then used to simulate three point bending and compact tension tests.