Rachid Bensaada

Bio: Rachid Bensaada is an academic researcher. The author has an hindex of 1, co-authored 1 publications receiving 7 citations.

Caractérisation à la rupture de matériaux ductiles par approche locale

Abstract: This thesis is a contribution to the study of the ductile fracture phenomenon of porous materials. Firstly, a review on the ductile fracture divided in two main parts is presented. The first part concerns the elastic plastic fracture mechanics that is represented by the J-Integral. The other part is dedicated to the micromechanical approach of the ductile fracture. A numerical and experimental study of the AISI 304L thin sheets is proposed. Firstly, the ductile fracture phenomenon is simulated using the GTN and Rousselier models. An hybrid approach combining experimental tests on CT specimens and simulations using the GTN model in order to assess the fracture toughness is proposed. The EWF method is also used with a numerical validation using a cohesive zone modeling. In the third chapter, an identification method for the GTN model parameters identification is proposed. The method is based on the hierarchy of the void volume fraction parameters evolution until the final fracture. In the fourth chapter, the mechanical response of porous materials is studied using the numerical homogenization. The GTN model is extended to take into account a wide range of porous materials. Finally, an experimental validation is proposed.

The Sandia Fracture Challenge: blind round robin predictions of ductile tearing

Abstract: Existing and emerging methods in computational mechanics are rarely validated against problems with an unknown outcome. For this reason, Sandia National Laboratories, in partnership with US National Science Foundation and Naval Surface Warfare Center Carderock Division, launched a computational challenge in mid-summer, 2012. Researchers and engineers were invited to predict crack initiation and propagation in a simple but novel geometry fabricated from a common off-the-shelf commercial engineering alloy. The goal of this international Sandia Fracture Challenge was to benchmark the capabilities for the prediction of deformation and damage evolution associated with ductile tearing in structural metals, including physics models, computational methods, and numerical implementations currently available in the computational fracture community. Thirteen teams participated, reporting blind predictions for the outcome of the Challenge. The simulations and experiments were performed independently and kept confidential. The methods for fracture prediction taken by the thirteen teams ranged from very simple engineering calculations to complicated multiscale simulations. The wide variation in modeling results showed a striking lack of consistency across research groups in addressing problems of ductile fracture. While some methods were more successful than others, it is clear that the problem of ductile fracture prediction continues to be challenging. Specific areas of deficiency have been identified through this effort. Also, the effort has underscored the need for additional blind prediction-based assessments.

Ductile Tearing Resistance of Metal Sheets

Abstract: The concept of R -curves has been adopted to characterise stable crack extension and predict residual strength of thin-walled structures particularly in the aircraft industry. The present contribution uses results of FE simulations of crack extension in panels by the cohesive model to validate analytical procedures for determining J -integral values at large crack extension from measurable quantities, namely the force vs. displacement records. The numerically determined J -integral is taken as the benchmark for the outcome of the analytical formulas. The geometry dependence of J and CTOD based R -curves is investigated and alternative concepts like CTOA and dissipation rate at crack extension are discussed.

Thickness dependence of cracking resistance in thin aluminium plates

Abstract: The influence of thickness on the fracture toughness of aluminium 6082T0 thin plates of 1-6 mm thicknesses was investigated experimentally and numerically from tensile testing of cracked DENT specimens. The critical J-integral, J(c), critical CTOD, delta(CTODc), and essential work of fracture, w(e), are found to increase with thickness and to constitute equivalent measures of fracture toughness at small thickness. For larger thickness, J(c) and delta(CTODc) increase non-linearly with thickness and reach a maximum for 5-6 mm thickness Whereas iv, keeps increasing linearly with thickness. This difference is related to a more progressive development of the necking zone in front of the crack tip when thickness increases: at large thickness, cracking initiates well before the neck has developed to its stationary value during propagation, w(e) is more directly related to the steady-state crack growth resistance. A linear regression on the fracture toughness/thickness curve allows further separation of the two contributions of the essential work of fracture: the necking work and the fracture work spent for damaging. The maximum of the stress triaxiality ratio is shown to constitute a pertinent parameter for characterising how constraint affects cracking initiation in the present context where out-of-plane constraint dominates in-plane constraint. It allows justifying the shape of the J(c)/thickness relationship and results in the proposal of a 3D J(c)/thickness/triaxiality fracture locus. As fracture profiles are macroscopically flat with microscopic dimples and with only very small shear lips along the edges, a local criterion based on the growth and coalescence of voids has been used in order to predict fracture initiation. (C) 1999 Elsevier Science Ltd. All rights reserved.

Homogenization Methods to Approximate theEffective Response of RandomFibre-reinforced Composites

Abstract: In this article a fibre-reinforced composite material is modelled via an approach employing a Representative Volume Element with periodic boundary conditions. The effective elastic moduli of the material are thus derived. In particular, the method of asymptotic homogenization is used where a finite number of fibres are randomly distributed within the representative periodic cell. The study focuses on the efficacy of such an approach in representing a macroscopically random (hence transversely isotropic) material. Of particular importance is the sensitivity of the method to cell shape, and how this choice affects the resulting (configurationally averaged) elastic moduli. The averaging method is shown to yield results that lie within the Hashin-Shtrikman variational bounds for fibre-reinforced media and compares well with the multiple scattering and (classical) self-consistent approximations with a deviation from the latter in the larger volume fraction cases. Results also compare favourably with well-known experimental data from the literature.

Local Approach Studies of the Effect of Load History on Ductile Fracture

Abstract: Local Approach methods provide alternative routes for carrying out fracture mechanics assessments. These methods use a finite element analysis of the cracked component that incorporates a micro-mechanical model of material behaviour in the region of the crack tip. The development of damage, and eventual failure of the material for particular loading conditions, is calculated directly by the micro-mechanical model. A potential advantage of such models is that they can automatically incorporate the effects of loss of constraint and load history. This paper describes studies that compare the predictions of the Gurson-Tvergaard-Needleman (GTN) local approach model of ductile fracture with traditional fracture mechanics parameters. The model is used to consider the effect of load history on ductile fracture initiation in a typical fracture mechanics Compact Tension (CT) specimen. The aim is to investigate the benefits of applying such models to situations where events in the plant loading history can have a significant effect on crack driving force for typical defects that subsequently develop during service. It is anticipated that the results will be used to make improvements to the accuracy of the traditional assessment procedures. The results show that in certain circumstances, for instance where residual stresses are present in the vicinity of a defect, load excursions on the specimen can have a beneficial effect on global load carrying capacity and that load history effects can be captured by adopting the local approach for the assessment of ductile fracture. It is also noted that the trends observed in other local parameters, e.g. J and CTOD, must be treated with caution, probably due to near crack-tip softening associated with the implementation of the GTN model in a region of high stress concentration.Copyright © 2008 by British Crown/MOD