Sébastien Mistou

Analysis of Sandwich Composite Beams with a New Transverse Shear Stress Continuity Model

This work presents a new composite beam model based on discrete layer theory. It enables the automatic verification of the continuity of transverse shear stresses by taking into account the Heaviside step function. The transverse shear is represented by a sine function which improves the accuracy of the results on the transverse shear stress. The membrane refinement cosine function improves the warping of the straight section in bending deformations. In order to validate the proposed model, several problems in bending and free vibration are presented. For sandwich composite beams, the proposed new model satisfies exactly and automatically the continuity conditions of displacements and stresses at the interfaces, as well as the boundary conditions.

Effect of Ductile Damage Evolution in Sheet Metal Forming: Experimental and Numerical Investigations

The numerical simulation based on the Finite Element Method (FEM) is widely used in academic institutes and in the industry. It is a useful tool to predict many phenomena present in the classical manufacturing forming processes such as necking, fracture, springback, buckling and wrinkling. But, the results of such numerical model depend strongly on the parameters of the constitutive behavior model. In the first part of this work, we focus on the traditional identification of the constitutive law using oriented tensile tests (0°, 45°, and 90° with respect to the rolling direction). A Digital Image Correlation (DIC) method is used in order to measure the displacements on the surface of the specimen and to analyze the necking evolution and the instability along the shear band. Therefore, bulge tests involving a number of die shapes (circular and elliptic) were developed. In a second step, a mixed numerical–experimental method is used for the identification of the plastic behavior of the stainless steel metal sheet. The initial parameters of the inverse identification were extracted from a uniaxial tensile test. The optimization procedure uses a combination of a Monte-Carlo and a Levenberg-Marquardt algorithm. In the second part of this work, according to some results obtained by SEM (Scaning Electron Microscopy) of the crack zones on the tensile specimens, a Gurson Tvergaard Needleman (GTN) ductile model of damage has been selected for the numerical simulations. This model was introduced in order to give informations concerning crack initiations during hydroforming. At the end of the paper, experimental and numerical comparisons of sheet metal forming applications are presented and validate the proposed approach.

Edge Effects on Sandwich Composite by Analytical, Finite Element, and Experimental Approach

Knowledge of edge effects in composite with honeycomb cores is essential for the design of sandwich structures. This paper deals with the measurement of strain by the application of reflection photoelasticity. Because of the geometry of honeycomb cores a classical method by strain gages is inaccurate and unsuitable. The measurements by photoelasticity is compared to an analytical and numerical results. The final aim of this work is to determine the behavior of such structures by the exact knowledge of its mechanical properties deduced in part from edge effect results.

Impact Resistance of Composite Materials under Biaxial Preloading

The first aim of this study is to analyze the impact behavior of pre-loaded composite. Indeed, a bi-axial load is applied to the composite specimen, in order to keep in touch with a real case of composite fuselage. Then, this pre-loaded specimen is impacted by a pendulum. The used energy and velocity are weak in order to be in the case of low-energy and low-velocity impact. The second aim of this study is to develop and design a pendulum device to be integrated to the bi-axial fatigue loading. Moreover, two Non Destructive Inspections (Sonoscan and InfraRed Thermography) is used in order to establish links between pre-load and induced impact damage.

Micro-Scale Modeling of Carbon-Fiber Reinforced Thermoplastic Materials

Thin-walled textile-reinforced composite parts possess excellent properties, including lightweight, high specific strength, internal torque and moment resistance which offer opportunities for applications in mass transit and ground transportation. In particular, the composite material is widely used in aerospace and aircraft structure. In order to estimate accurately the parameters of the constitutive law of woven fabric composite, it is recommended to canvass multi-scale modeling approaches: meso, micro and macro. In the present investigation, based on the experimental results established by carrying out observations by Scanning electron microscope (SEM), we developed a micro-scale FEM model of carbon-fiber reinforced thermoplastic using a commercial software ABAQUS. From the SEM cartography, one identified two types of representative volume elementary (RVE): periodic and random distribution of micro-fibers in the yarn. Referring to homogenization method and by applying the limits conditions to the RVE, we have extracted the coefficients of the rigidity matrix of the studied composites. In the last part of this work, we compare the results obtained by random and periodic RVE model of carbon/PPS and we compute the relative error assuming that random model gives the right value.

Shear Test on CFRP Full-Field Measurement and Finite Element Analysis

The purpose of this work is to study the Iosipescu shear test and more precisely its ability to characterize the shear modulus of a carbone/epoxy composite material. The parameters influencing this identification are the fibre orientation, the geometry of the notch and the boundary conditions. Initially these parameters were studied through the finite element analysis of the shear test. Then, the measurement of the shear strains was carried out by traditional methods of measurement (strain gauges) but also by optical methods. These optical methods: the digital image correlation and the electronic speckle pattern interferometry (ESPI); allow for various levels of loading, to reach a full-field measurement of the shear strain. This enabled us to study the strain distribution on the section between the two notches. The finite element model enabled us to study the parameters influencing the calculation of the shear modulus in comparison with strain gauges, image correlation and ESPI. This work makes it possible to conclude on optimal parameters for the Iosipescu test.

Visco-Elastic Laws Study for Mechanical Modelization of Super-pressure Balloons

This study deals with the mechanical behavior knowledge of super-pressure balloons in realistic flight conditions in order to determine the flight profile and to develop numerical tools to make this kind of balloons evolve. The aim is then to be able to know the stress-strain state of a complete envelope for in-flight conditions. This is achieved by a finite element modelization, using a commercial code implemented with mechanical behavior laws of envelope materials. Several non linear laws are evaluated and two selected (Kelvin-Voigt+Bingham and spectral models) which are likely to describe the behavior of the material. Laws, Poisson and linear thermal expansion coefficients are determined by uniaxial tensile tests. Results of modelization are compared to experimental results obtained with a dedicated facility called NIRVANA which measures the 3D strain of a balloon sub-system submitted to differential pressure stress. The good agreement between experimental and modelization results validates the law parameters of the best fitted law.

Characterization and Comparison of Defects Detection Limits of Ultrasonic Non Destructive Techniques

This work deals with the Liquid Resin Infusion (LRI) process developed within the research program “FUSelage COMPosite” of DAHER SOCATA. This manufacturing process enables the realization of complex composite structures or fuselage elements in a single phase (mono-material), which considerably reduce connections and relative difficulties. The concern here is the investigation of non destructive testing (NDT) methods that can be applied to LRI-structures in order to define their capacities for defect detection, and especially their associated critical defect size. In aviation industry, the AITM standards require the ultrasonic testing as NDT for composite materials. Therefore the aim of this work is to characterize and compare three different and complementary ultrasonic techniques on composite specimens. Such analysis allows to define the NDT application field of each method in term of defect detection.

Identification of Materials Properties Using Displacement Field Measurement

The aim of this work is to identify parameters driving constitutive equations of materials with displacement field measurements carried out by image stereo-correlation during an unidirectional tensile test. We evaluate two identification techniques. The first one is the virtual fields method which consists in writing the principle of virtual work with particular virtual fields. It is generally used in the case of linear elasticity and it requires a perfect knowledge of the model in terms of boundary condition since the virtual fields used must be kinematically admissible. This method allows to determine parameters by a direct and fast calculation, without iterations. The second method is the finite element model updating method. It consists in finding constitutive parameters that achieve the best match between finite element analysis quantities and their experimental counterparts. This method is more adaptable than the virtual field method but it needs to spend more calculation time.

Mechanical Behaviour of Butt Jointsunder Traction Using the Image Stereocorrelation Technique

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.