Papers by Author: Moussa Karama

Abstract: A new micromechanical modelling approach for brittle damage in initially orthotropic materials is presented. The proposed strain-based energy formulation allows to derive a fully anisotropic multilinear model for microcracked materials with arbitrary oriented defects. The thermodynamics framework provides a standard procedure for the damage evolution law. The new model explicitly accounts for the interaction between primary and induced anisotropies. Moreover, the very challenging issue of opening-closure effects (unilateral behavior) is addressed in this framework.

Abstract: Thick composites are increasingly used in the design of mechanical structures. Combined with low weight, they are generally resistant structures, which can support importante loads. In addition, depending on the number and nature of the materials used, it is possible to adapt properties for specific applications (damping structures).This work proposes the establishment of a new theoretical model of multilayer beam. The model, which is simple and easy handling, is intended for the subsequent establishment of a finite element. The goals are:improve the refinement of the transverse displacement and transverse shear, avoiding the calculation of transverse shear, the use of correction factors,keep only the usual displacement,test the accuracy of the model compared with models from the literature (for an equivalent single-layer approach).The proposed approach is of the kinematics, the form adopted for the displacement field is justified from a dimensional point of view, by the equations of elasticity. The equations of motion and boundary conditions are obtained by applying the principle of virtual power.The validity of the model is tested on problems for which solutions (obtained by previous theories) exist.

Abstract: This paper deals with an identification methodology of the interfacial fracture parameters to predict the lifetime of a metallic brazed joint. The methodology is based on an experimental-numerical study whereby the optimal parameters are obtained. The experimental data, using the scanning electron microscope analysis, allowed approving that failure of the assembly based AuGe solder seems first to appear near the interfaces. These results were confirmed by micrographs analysis of the solder/insert and solder/substrate interfaces. Then, using shear test results and parametric identification coupled with a finite elements model (FEM) simulation, the damage constitutive law of the interfacial fracture based on a bilinear cohesive zone model are identified. The agreement between the numerical results and the experimental data shows the applicability of the cohesive zone model to fatigue crack growth analysis and life estimation of brazed joints.

Abstract: This study presents the measurement of fields of displacement and deformations on butt joints in hydrostatic depression. The aim of this paper is to observe the general behaviour of the rubber. Various definitions of butt joints were tested in traction. We are interested on the types of fracture and the phenomenon of vacuolization, which occurs in the rubber.

Abstract: This paper presents different interests of non destructive full-field measurement. More precisely, it focuses on the characterization and the comparison of the X-ray tomography and two methods of infrared thermography in order to define the defect detection limits and to precise the specific application fields for each technique on multi-layered and sandwich composite structures. The obtained results are qualitatively and quantitatively analyzed.

Abstract: This work presents a sensitivity analysis of mechanical and geometrical parameters on a composite component under a cyclic loading. A design of experiments approach has been used. This study in improved by a reliability approach where the scatterings of all the characteristic properties are taken into account. The Monte Carlo method where used to compute the reliability. An application has been carried out on a composite rectangular plate with a hole in its center. Where a useful surface responses diagrams of the reliability are determined. A significant nonlinear effect of the fiber volume faction is observed on the behaviors reliability with no significant interactions effect with the others factors.

Abstract: The use of multi-layered composite materials in the confection of industrial pipe connections (elbows, reduction pipes, flanges etc.) is often put off because of the high cost of manufacture. The aim of this study is to replace the multi-layer process (woven reinforcement-organic matrix) by sandwich process with a view to reducing the cost of manufacture. Validation is carried out on different structures: ring, cylinder with and without end effect, elbows and floating flanges.

Abstract: Solder materials are critical packaging compounds and due to usually weakest melting temperature among packaging constitutive materials, thus, they are frequently subjected to a multitude of physical phenomena: creep, fatigue and combined hardening effects. The complexity and interaction of such factors must be considered in suitable way in the mechanical behavior modeling using the appropriate material behavior laws. The choice of the mechanical model depends on several factors such as the complexity of constitutive equations to be integrated, the availability and suitability of implementation in the FE codes, the number of parameters to be identified, the capability of the model to represent the most common physical features of the material… Following these observations and in order to deal with these critical remarks, comparisons between the most common unified viscoplastic models should be done in the local and finite element levels for the decision upon the most efficient model. That is the aim of this paper with application to a tin based solder token as the test material.

Abstract: This paper deals with the advantages of the finite element modeling and design, especially, of delamination test coupons involved in fracture analysis of laminated composite plates. This is shown through two relevant aspects in delamination toughness measuring, say: data reduction and Iso-G delamination front design. Many experimental data reductions are based on beam theories and thus assumes straight delamination front during propagation, which is not true when investigating laminates with general anisotropy. Another aspect is also emphasized, and concern test procedure simplification to avoid displacement measurements. This is done through a direct energy release rate calculations via the crack closure integral method.

Abstract: The work of this study concerns the fine modelling of the thermomechanical and metallurgical behavior of interface ceramic-metal in order to determine the residual mechanical state of the structures during brazing process. For these cases, difficulties mainly arise in the modelling of the solid-solid phase transformations as well as in the modelling of the mechanical behavior of the multiphasic material. Within an original theoretical framework - generalized standard materials with internal constraints – we proposed models for the behavior of multiphasic material. The design of joints in engineering structures and the optimisation of the industrial brazing process require determining and analysing such a phenomenon. In this way, the present work aims at predicting the thermally induced stresses (localisation and level) through numerical simulations and then, at defining the main parameters which influence their development