Philippe Bompard

Electron backscattering diffraction analysis of secondary cleavage cracks in a reactor pressure vessel steel

An analysis of secondary cleavage cracks in the A 508 C1.3 bainitic steel has been carried out using electron backscattering diffraction (EBSD) and scanning electron microscopy (SEM). The crystallographic orientation in the vicinity of the secondary cracks was studied. Secondary cleavage crack propagation planes were identified to be {100}, {110}, {112} and {123}. The secondary cracks propagated mostly in the range of one crystallographic grain (bainitic packet) and were arrested on high-angle twist type boundaries or in the upper bainite carbide colonies. The size of cleavage facets on the fracture surface corresponds to the size of the bainitic packets.

Mechanical behaviour of ±55° filament-wound glass-fibre/epoxy-resin tubes—III. Macromechanical model of the macroscopic behaviour of tubular structures with damage and failure envelope prediction

Abstract This series of papers on ±55° filament-wound glass-fibre/epoxy-resin tubes consists of three parts. In the present paper (Part III), the macroscopic mechanical behaviour of the tubular structure is presented. A method for predicting composite tube macroscopic properties from the ply constants is given. A more general analytical method is used for determining ply stresses of a composite tube under a combined load, which will then be compared with 3D finite-element analysis, classical and adjusted laminate theory. Failure envelope prediction is then made according to the micro- and meso-scale model results. The stresses distribution in the tube thickness direction predicted by the present method agrees well with 3D finite-element analyses. Four methods give similar stress distribution results under tensile loading, but different ones under internal pressure loading. For failure envelope prediction, the first-ply-failure theory underestimates the failure load for pressure dominated loading. The effect of possible micro structural damage on the failure load prediction is also discussed and compared with experimental results. In Part I (Bai et al. Compos. Sci. Technol. , 1997, 57 , 141–153), the microstructure, mechanical behaviour and damage initiation mechanisms were presented. In Part II (Bai et al. Compos. Sci. Technol. , 1997, 57 , 155–164), micromechanical modelling of the damage initiation was conducted in order to determine the mechanical conditions under which different microcracking mechanisms occur.

Ductile fracture of duplex stainless steel with casting defects

Abstract Experimental and numerical studies were carried out on CF8M steel with special regard to the influence of casting defects i.e. shrinkage cavities on fracture behaviour. As-received and thermally aged materials were studied. Physical steps of damage in specimens containing casting defects were observed using in-situ tensile tests in a scanning electron microscope. Mechanical properties of duplex stainless steel were characterised using three-point bend tests of three types of specimens — defect free, pre-cracked, and containing shrinkage cavities. These mechanical tests were simulated using Gursons model including crack nucleation. A modelling of the shrinkage cavities is proposed in order to study their influence on the mechanical response of the specimen.

Mechanical behaviour of ± 55 ° filament-wound glass-fibre/epoxy-resin tubes: II. Micromechanical model of damage initiation and the competition between different mechanisms

Abstract This series of papers on ± 55 ° filament-wound glass-fibre/epoxy-resin tubes consists of three parts. In the present paper (Part II), micromechanical modelling of the damage initiation is conducted in order to determine (1) the mechanical conditions under which different microcracking mechanisms occur and (2) the critical σ zz /σ θθ ratios which correspond to the change from interfacial failure to microcracking at porosity. Emphasis is placed on assessing the influence of microstructural defects and the competition between the different mechanisms. The general tendency of damage-envelope prediction by means of micromechanical modelling fits the microscopy observations quite well. The sensitivity of local criteria is also discussed. The correction of the local stress field has been improved by introducing a local stress concentration factor in a mean field theory model (Mori-Tanaka theory). In Part I, the microstructural analyses, mechanical behaviour and damage initiation mechanisms were presented. In Part III, the macroscopic behaviour of the tubing structure with and without damage will be presented. The simulation results will be compared with experimental ones.

Influence of ductile tearing on cleavage triggering in ductile-to-brittle transition of A508 steel

Abstract A large quantitative fractographic study was carried out onan A508 C1.3 pressure vessel steel in the temperature range corresponding to the ductile-to-brittle transition. Fractographic analyses of fractured Charpy V-notch (CVN) and compact tension (CT) specimens revealed a certain proportion of ductile fracture preceding cleavage, even if the specimens were tested at temperatures below the DBTT. The influence of the ductile tearing on cleavage triggering was studied. In particular, the stress concentrations in CT and/or CVN specimens induced by the presence of an ellipsoidal defect representing the cluster of debonded MnS inclusions were calculated by finite element method and related to the fracture probability given by Beremin model.

KIa Crack Arrest Toughness Assessment Using Thermal Shock on Notched Disks

The aim of the study is to validate the KIa -T curve on a thermal shock experiment performed on a notched disk (DTSE) taken from a A533-B type steel. Several experiments have been performed. Non linear thermal analyses were carried out using the finite element method in order to obtain the full thermal field within the specimen during crack propagation. The results obtained are in excellent agreement with the experimental results. The DTSE is also interpretated in static terms to compare the obtained KIa (T) values with the limit curve. Finally, dynamic F.E. simulations allow to estimate the influence of dynamic effects in the DTSE and thus validate the methodology. According to the computations, the crack arrested when dK/da>0 and dKd /da = 0. The comparison between stress intensity factor computed from elastic-static analysis (or dynamic) and RCC-M code demonstrated the conservatism of the approach. Then static analysis is sufficient to analyse the result, since no wave interference with the crack propagation was identified.Copyright

Cleavage Crack Propagation and Arrest Prediction in French PWR Vessel Steel

The integrity assessment of Reactor Pressure Vessels is mainly based on crack initiation. Nevertheless, in the frame of component life extension, crack arrest conception is investigated.This paper presents a local non-linear dynamic model to predict the propagation and arrest of cleavage crack in French PWR vessel steel (16MND5). The propagation criterion used in this model is a Ritchie Knott and Rice (RKR) fracture stress criterion: the crack propagates when the maximum stress ahead of crack tip reaches a critical level, which has been shown to depend on temperature and plastic strain rate.In the first step, the criterion has been identified from crack growth and arrest analysis on CT specimens at different temperatures. Then it was applied to predict the propagation and arrest of cleavage cracks on pre-cracked rings under mixed mode loading, at three different temperatures: −150°C, −125°C and −100°C.2D modeling was performed by using extended element method (XFEM) in CAST3M software. The propagation direction on pre-crack rings under mixed mode loading was determined from maximum hoop stress criterion. Numerical computation showed a good agreement with experiments, for both curved crack paths and crack arrest locations. Furthermore it showed that crack paths and crack arrest also depend on the level of the crack loading at initiation.Copyright

Cleavage Dynamic Propagation Analysis in a Nuclear Reactor Pressure Vessel Steel Using a High-Speed Camera

Initiation stage of cracks is considered as a key issue, but more and more component integrity analyses investigate the crack propagation and arrest possibility. This study deals with physical mechanisms of cleavage crack propagation and numerical computations related to brittle fracture. Dynamic effects, involved in unstable cleavage crack propagation, have to be taken into account to properly depict brittle crack propagation, arrest and possible propagation re-initiation events.Experiments were carried out on thin CT specimens made of 16MND5 PWR vessel steel at five temperatures (−150°C, −125°C, −100°C, −75°C, −50°C). In addition to standard crack gages, an innovative experimental technique has been used to determine crack propagation. By the means of developments on the experimental protocol (improvements of isolation and airtightness of the thermal chamber, optimization of the experimental protocol to eliminate ice in the thermal chamber and in order to have a good acquisition quality), use of a high-speed framing camera was made possible to measure crack propagation on a CT mirror polished surface. This optical device, combined with this optimized experimental process, has allowed the study of straight and branching crack paths with high accuracy. The framing camera (520 000 fps up to 1 100 000 fps) has allowed to have a very accurate estimation of crack speed even up to 1000 m.s−1 and also to detect some phases of crack branching during propagation and phases of arrest-re-initiation. Numerical computations, based on X-FEM and combining a local non linear dynamic approach with a RKR type fracture stress criterion, have been performed to depict experimental crack behavior.This paper describes this innovative experimentation and the interpretation by FE calculations and SEM observations associated with quantitative 3D optical microscopy.Copyright

Cleavage Crack Propagation and Arrest in a Nuclear Pressure Vessel Steel

The integrity assessment of Reactor Pressure Vessels, mainly based on crack initiation, can be completed by studying crack propagation and arrest. Whereas engineering approaches do not take into account dynamic effects, these effects are important in unstable cleavage crack propagation, arrest and possible propagation re-initiation events. This study deals with physical mechanisms of cleavage crack propagation and numerical computations related to brittle fracture in the framework of local approach to fracture.Experiments were carried out on thin CT 25 specimens made of 16MND5 PWR vessel steel at five temperatures (−150°C, −125°C, −100°C, −75°C, −50°C). Two kinds of crack path, straight or branching path, were observed. Branching cracks appear for the highest critical loadings at initiation, that increase the elastic stored energy and the effect of plasticity. The elastic-viscoplastic behavior of the ferritic steel was studied up to a strain rate of 104 s−1 and taken into account in the numerical simulations. The eXtended Finite Element Method (X-FEM) was used in CAST3M FE software to model crack propagation. Numerical computations combine a local non linear dynamic approach with a RKR type fracture stress criterion. The different physical micro-mechanisms, involved in cleavage fracture, were examined by the means of SEM fracture surface analyses at different temperatures and strain rates for the two kinds of crack path.The links of the critical fracture stress with both temperature and strain rate for straight crack path as well as analyses of branching crack phenomena were considered by the means of Scanning Electron Microscopy (SEM) fracture surface analyses, 3D quantitative optical microscopy and FE computations in order to aim at a robust physical justification of the propagation model which has already been developed at CEA in the frame of the B. Prabel PhD.Copyright