Fodil Meraghni

Mechanical behaviour of cellular core for structural sandwich panels

This paper deals with the analysis of the mechanical properties of the core materials for sandwich panels. In this work, the core is firstly a honeycomb and secondly tubular structure. This kind of core materials are extensively used, notably in automotive construction (structural components, load floors...). For this study, three approaches are developed: a finite element analysis, an analytical study and experimental tests. Structural members made up of two stiffs, strong skins separated by a lightweight core (foam, honeycomb, tube...) are known as sandwich panels. The separation of the skins by the core increases the inertia of the sandwich panel, the flexure and shear stiffness. This increase is obtained with a little increase in weight, producing an efficient structure to resist bending and buckling loads. A new analytical method to analyse sandwich panels core will be presented. These approaches (theoretical and experimental) are used to determine elastic properties and ultimate stress. A parameter study is carried out to determine elastic properties as a function of geometrical and mechanical characteristics of basic material. Both theoretical and experimental results are discussed and a good correlation between them is obtained.

Implementation of a constitutive micromechanical model for damage analysis in glass mat reinforced composite structures

Abstract A micromechanical model based on a probabilistic approach is implemented in the finite element code CASTEM 2000 to develop numerical simulations that efficiently predict the overall damaged behaviour of random oriented fibre composites. The proposed damage constitutive model is based upon the generalised Mori and Tanaka scheme and Eshelbys equivalence theory. Damage mechanisms occurring at each composite constituent (fibres, matrix and interface) are associated to Weibull probabilistic functions to model their onset and progressive growth at the microscopic scale level. It is obvious that the damaged behaviour of the composite material depends widely on the microscopic material parameters (fibre length, fibre volume fraction, fibre orientation, …). On one hand, the micromechanical model uses homogenisation techniques which enabled us to link these microscopic parameters to the material behaviour and to evaluate explicitly their influences. On the other hand, the implementation of the derived behaviour law into a finite element code enabled us to reflect on the effect of these microscopic parameters on the overall response of a simple composite structure presenting heterogeneous stress fields. In fact, the damage evolution in each constituent (local scale) and the related stiffness reduction are estimated at any material point (integration point) or node of the considered structure subject to a specific loading. Numerical simulations of a composite plate with a hole under in-plane tension were performed to validate the implementation of the behaviour law. Numerical results have been compared to experimental curves and damage evolutions monitored by acoustic emission techniques. Simulations agree well with experimental results in terms of damage onset and growth.

Generalised Mori-Tanaka Scheme to Model Anisotropic Damage Using Numerical Eshelby Tensor

This work presents a new micromechanical model to describe the 3-D elastic and damaged behaviour of randomly oriented fibre composite. The damage mechanisms are described by the progressive introduction of oriented microcracks in a random discontinuous fibre composite. In this work, the homogenisation is performed using a Mori-Tanaka scheme. Whereas, symmetry is not obtained in estimates of the overall stiffness for multiphase systems with heterogeneity of different shapes or orientations. For this reason, in this work two levels of homogenisation are considered. The Mori-Tanaka method is applied on the matrix with microcracks to give an equivalent damaged matrix, which can be anisotropic. The Mori-Tanaka scheme is used for the second time to homogenise fibres embedded in the new equivalent damage matrix. Generally, this new matrix is anisotropic and for this case no explicit formula for the Eshelby tensor (S) has been obtained. Thus, it is necessary to compute numerically the components of this fourth order tensor. Moreover damage mechanisms can be modelled using statistical considerations. The damage onset and growth can appear in the matrix, fibres or interface. Hence, overall damaged behaviour can be modelled and predicted as well as the microscopic damage accumulation process.

Computational micro to macro transitions for shape memory alloy composites using periodic homogenization

In the current manuscript, a homogenization framework is proposed for periodic composites with shape memory alloy (SMA) constituents under quasi-static thermomechanical conditions. The methodology is based on the step-by-step periodic homogenization, in which the macroscopic and the microscopic problems of the composite are solved simultaneously. The implementation of the framework is examined with numerical examples on SMA composite laminates. Complexity of the composite nonlinear response and non-proportional stress state in the SMA appears, even in the case of uniaxial macroscopic boundary conditions. Moreover, under certain conditions, the composite laminate can exhibit a non-convex transformation surface. Additionally, the transformation temperatures at various stress levels under isobaric thermal cycling can be quite different between the composite and the pure SMA.

Micromechanical Modelling Coupled to a Reliability Approach for Damage Evolution Prediction in Composite Materials

This work is based on Mori and Tanakas work combined with statistical tensile strength theories for the computation of the effective properties of composites. In order to describe the entire behaviour of composite materials, statistical local damage criteria are introduced representing interface, fibres amd matrix. The damage accumulation process is described by the microcrack density, which increases according to probabilistic considerations. In fact, the Weibull distribution applied at the microscale level arises as a key model for the strength of composite materials. In addition, the representation of the failure processes of each constituent gives a more accurate prediction of composite material behaviour. Specific results are given for composites reinforced by aligned or randomly oriented fibres and for particulate material called Twintex®, developed by Vetrotex©.

Phase Transformation of Anisotropic Shape Memory Alloys: Theory and Validation in Superelasticity

In the present study, a new transformation criterion that includes the effect of tension–compression asymmetry and texture-induced anisotropy is proposed and combined with a thermodynamical model to describe the thermomechanical behavior of polycrystalline shape memory alloys. An altered Prager criterion has been developed, introducing a general transformation of the axes in the stress space. A convexity analysis of such criterion is included along with an identification strategy aimed at extracting the model parameters related to tension–compression asymmetry and anisotropy. These are identified from a numerical simulation of an SMA polycrystal, using a self-consistent micromechanical model previously developed by Patoor et al. (J Phys IV 6(C1):277–292, 1996) for several loading cases on isotropic, rolled, and drawn textures. Transformation surfaces in the stress and transformation strain spaces are obtained and compared with those predicted by the micromechanical model. The good agreement obtained between the macroscopic and the microscopic polycrystalline simulations states that the proposed criterion and transformation strain evolution equation can capture phenomenologically the effects of texture on anisotropy and asymmetry in SMAs.

Coupled effect of loading frequency and amplitude on the fatigue behavior of advanced sheet molding compound (A-SMC)

This paper presents the experimental results of tension-tension stress-controlled fatigue tests performed on advanced sheet molding compound (A-SMC). It aims at analyzing the effect of fiber orientation, loading amplitude, and frequency on the fatigue response and the related temperature evolution due to the self-heating phenomenon. Two types of A-SMC have been analyzed: randomly oriented (RO) and highly oriented (HO). The coupled effect of the loading amplitude and the frequency has been studied. It has been shown that the couple frequency-amplitude affects the nature of the fatigue overall response which can be governed by the damage mechanisms accumulation (mechanical fatigue) and/or by the self-heating (induced thermal fatigue). For fatigue loading at 100 Hz, self-heating has been observed and yielded to a temperature rise up to 70℃. The latter causes a decrease of the storage modulus related to the β-transition of the vinylester. It has been demonstrated that the self-heating produced a material softening and decreased the fatigue life. SEM observations revealed that the samples tested at 100 Hz, exhibit smooth debonding surfaces due to the induced thermal softening of the matrix whereas more brittle fracture of the matrix surrounding fibers is observed during the fatigue tests achieved at 10 Hz.

Experimental Study of Dynamic Behaviour of Aluminum/Aluminum and Composite/Composite Double Lap Joints

This paper presents an experimental investigation on the behaviour of structural adhesive bonding under quasi-static and moderately high loading rates. It addresses the effects of the loading rate on the strength of the adhesively bonded joints under dynamic tensile. A comparison has been achieved between the strength and the damage of specimens’ made of aluminium and lamina substrates. High rate tests showed ringing in the force/displacement curves.

Three-dimensional constitutive model considering transformation-induced damage and resulting fatigue failure in shape memory alloys

In this work, a constitutive model is developed that describe the behavior of shape memory alloys undergoing a large number of cycles, developing internal damage, and eventually failing. Physical mechanisms associated with martensitic phase transformation occurring during cyclic loadings such as transformation strain generation and recovery, transformation-induced plasticity, and fatigue damage are all taken into account within a thermo-dynamically consistent framework. Fatigue damage is described utilizing a continuum theory of damage. The damage growth rate has been formulated as a function of both the stress state and also the magnitude of the transformation strain, while the complete or partial nature of the transformation cycles is also considered as per experimental observations. Simulation results from the model developed are compared to uniaxial actuation fatigue tests at different stress levels. It is shown that both lifetime and the evolution irrecoverable strain can be accurately simulated.

Experimental Parameters Identification of Fatigue Damage Model for Short Glass Fiber Reinforced Thermoplastics GFRP

In the present work, a new polycyclic fatigue damage model is formulated and applied for short glass fibre reinforced thermoplastics. The model is able to capture experimental trends observed for the considered composites. The damage growth description involves a set of 20 parameters in the case of a complete 3D –structure. In the current paper, it is considered the particular case of a displacement controlled fatigue tensile test involving 4 damage parameters. The present contribution is a first approach of parameter identification. It is considered a least squares sense based cost function and homogeneous fatigue tests performed on a short glass fibre reinforced polyamide. The identified set of parameters appears to be not depending on the adopted initial values. The model as the parameters determined by the minimisation algorithm, are validated on a fatigue test performed with a different loading condition.