Cleavage Crack Arrest Research Articles

3D modeling of cleavage crack arrest with a stress criterion

Thermal shock experiments on precracked discs were carried out in order to study cleavage crack arrest capabilities of one bainitic steel. Two thicknesses of specimen were used. Standard analysis shows no influence of thickness but an influence of the initial crack length on arrest toughness. A local stress criterion based on the maximal principal stress was applied in order to predict brittle crack arrest. 3D finite element models were developed in order to simulate the shape of the crack front. A scatter in critical stress for cleavage crack arrest was introduced through Monte-Carlo simulations and Weibull distribution. The comparison with the experiment is accurate, that shows a good potential of the stress criterion for predicting cleavage crack arrest. The scatter of critical stress influences the regularity of the simulated crack arrest front, but not the crack length. The cleavage crack propagation and arrest could then be simulated by the mean value of the local fracture stress.

Propagation and arrest of cleavage cracks in a nuclear pressure vessel steel

The safety of nuclear structures is crucial while the service time of nuclear power stations is planned to be extended up to 60years. Initiation stage of cracks is still considered as a key issue, but more and more component integrity analyses investigate the crack arrest possibility. This study deals with physical mechanisms of cleavage crack propagation and numerical computations related to brittle fracture. Experiments using standard measuring techniques and a high-speed framing camera system, as well as Scanning Electron Microscope fracture surface analyses were carried out on thin CT specimens made of 16MND5 PWR vessel steel. The elastic–viscoplastic behavior of the ferritic steel has been studied and taken into account in numerical simulations. The eXtended Finite Element Method (X-FEM) is used in CAST3M finite element analysis software to model crack propagation. Numerical computations combine a local non-linear dynamic approach and a fracture criterion based on critical cleavage stress, whereas current standards in the nuclear field use a global static approach to fracture to depict crack initiation and arrest. The links of the criterion with temperature and strain rate are considered thanks to experiments, SEM fractographies and 2D computations in order to get a robust physical model which can be effective for model-based predictions of industrial structures.

Using the X-FEM method to model the dynamic propagation and arrest of cleavage cracks in ferritic steel

Although initiation criteria have been the subject of many publications, the phenomena associated with the propagation and arrest of brittle cracks have not. To be able to predict the dynamic behaviour of cleavage cracks, we made a series of experiments and associated numerical studies. Tests of crack propagation and arrest were carried out on specimens of two different geometries (Compact Tension and compression ring) made of the 16MND5 ferritic steel of which French nuclear reactor vessels are constructed. The elastic-viscoplastic behaviour of this material was characterised and its nature was taken into account in the numerical simulations. The eXtended Finite Element Method (X-FEM) was developed in CAST3M finite element analysis software to enable fine, effective modelling of crack propagation. Propagation models based on principal stress were studied and it was found that critical cleavage stress depended on loading rate. The use of criteria calibrated for Compact Tension specimens gave excellent results in predictive calculations for similar specimens, and also for compression rings in both mode I and mixed-mode. The speed and path of crack predicted with the numerical simulations were in close agreement with the experimental results.

Local approach to fracture for cleavage crack arrest prediction

Crack arrest can complement the crack initiation concept to assess integrity of structures. This paper deals with the application of a local approach model derived from the RKR model to predict cleavage crack propagation and arrest in thermal shock experiments performed on pre-cracked discs made of a low alloy bainitic steel. The numerical procedure takes into account dynamic effects and an accurate behavior law for the material. Simulations with a crack speed criterion help the identification of the critical stress criterion. The results show a good prediction of the crack jump when a 3D modeling is used.

Analyses of cleavage crack arrest experiments: influence of specimen vibration

The crack propagation resistance of a low alloy bainitic steel used for nuclear pressure vessels was studied. Thermal shock experiments were carried out on pre-cracked discs. These tests were analyzed using the finite element method according to various assumptions, from elastic–static to full transient elastic–viscoplastic analysis. The crack propagation was modeled using node release technique. It was shown that crack deceleration at the end of the propagation greatly modifies the stress field at crack tip, due to interactions with structural vibrations. Therefore, it may be inaccurate to transfer the arrest toughness obtained from specimens under static analysis to components.

The CAFE model of fracture—application to a TMCR steel

ABSTRACTCAFE, a combination of Cellular Automata (CA) to describe the material microstructural properties, and Finite Elements to represent the macroscopic strain, allows the evolving states of microstructural change to be analysed in a deforming body. The paper explores the potential of the technique for the assessment of the ductile‐to‐brittle transition behaviour of ferritic steels. The most important features are due to the microstructural control of cleavage. The model was applied to the Charpy test on a TMCR (Thermo‐Mechanically Controlled Rolled) steel. The results include simulations of the full Charpy energy transition curve, the varying amount of scatter in the data, and, in distinction to most other models in the literature, a full description of the state of the fracture surface on each specimen. The results agree reasonably well with the experimental data. The model follows the development of both ductile and cleavage micro‐mechanisms throughout the deformation of the structure or component. As a consequence, it allows cleavage to develop as a progressive point‐to‐point process. The model can, and does, predict the arrest of cleavage cracks by ductile processes. It can be easily generalized to include more sophisticated aspects of a material microstructure.

Brittle-to-ductile fracture transition in Fe–3wt.%Si single crystals by thermal crack arrest

The brittle-to-ductile (BD) transition in fracture in Fe–3wt.%Si single crystals was studied through the arrest of cleavage cracks propagating up a temperature gradient under various loading rates. Through the dependence of the BD transition temperature on loading rate an activation energy for the overall fracture transition was determined to be ΔG ∗=0.096 eV. This is very close to the measured activation energy of 0.1 eV for double kink nucleation on screw dislocations under high effective shear stresses in the range of 500 MPa in dilute solid solution iron alloys of Ni and Si and indicates that the BD transition by crack tip shielding was governed by the mobility of dislocations away from the crack tip. Observations of slip line patterns in the flank regions of the arrested cracks by Nomarski interference contrast microscopy provided direct verification of the expected modes of plastic crack tip shielding.

Selection of crack-tip slip systems in the thermal arrest of cleavage cracks in dislocation-free silicon single crystals

New B–D fracture transition experiments in dislocation––free Si single crystals are described based on arrest of cleavage cracks propagating up a temperature gradient. Combined etch pitting and Berg–Barrett imaging shows that the fracture transitions are accomplished exclusively on planes with the most advantageous conditions for dislocation nucleation from crack-tip cleavage ledges.

Brittle-to-ductile transitions in the fracture of silicon single crystals by dynamic crack arrest

Abstract Experiments have been conducted on the brittle-to-ductile transition of fracture in silicon single crystals through the arrest of cleavage cracks made to propagate on the (110) cleavage planes up a temperature gradient. An activation energy of 1.82 eV has been determined for the transition process based on the dependence of the T BD on an averaged crack velocity, inferred from a jerky mode of crack advance. The dislocation patterns in the arrest zones have been studied in detail by a combination of etch pitting and Berg–Barrett X-ray topographic imaging after the arrest. These observations indicated that the plasticity of the entire arrest process is accomplished by slip activity on a set of two symmetrically placed vertical slip planes in which only one type of dislocation was involved. These planes do not have the highest resolved shear stresses but have the advantage of a very low energy barrier to the nucleation of dislocations from crack tip cleavage ledges. A close correspondence was noted ...