Advanced Materials Research Vol. 682

Abstract: A mathematical model for the forced vibration of sandwich structures with a viscoelastic and piezoelectric layers is presented. The active-passive damping is realized by adding piezoelectric sensor and actuator layers to a sandwich viscoelastic structure. The mathematical formulation is developed in a general form in order to take into account for various viscoelastic models in the frequency domain. Frequency dependent Young modulus based on various Maxwells model is used for viscoelastic materials modelling. A numerical method combining the finite element and perturbation methods called asymptotic numerical method is developed for the displacement and frequency dependent problem. Resonance curves for sandwich structures are obtained for various frequency ranges, excitation amplitudes and viscoelastic models. Only some matrix inversions and a few iterations are needed for large frequency ranges.

Abstract: A sandwich structure is obtained from two skins or soles, with good mechanical characteristics, bonded to a core made of a lightweight material of low resistance. Glued to a core made of a lightweight material of low resistance. The strength and modulus of elasticity of the skin condition the bending behavior of a sandwich. Bending, the skins of the sandwich are solicited in traction and in compression, while the core is subjected to shear. Our study focuses on the experimental characterization under flexural load of new composite sandwich combined. The sandwich proposed constituted of two skins armed by metal grids impregnated with epoxy matrix type STR and souls composed of hybrid corrugated cardboard reinforced by fabric. Several configurations of souls made from cardboard cellulosic and fabrics were taken into account. An experimental and numerical investigation is conducted to analyze the behavior of these structures. The results show that the failure mode is strongly influenced by the stacking sequence considered.

Abstract: The purpose of this paper is to estimate the number of cycles for fatigue crack initiation in notched plate under constant amplitude loading through tensile residual stress field of Aluminum alloy plate. Residual stress field was generated by plastic deformation using finite element method. Simulation of fatigue behavior was made on AFGROW code. It was shown that the fatigue crack initiation and propagation were affected by level of residual stress filed. In this investigation, the presence of tensile residual stresses reduces considerably the total fatigue life. Loading parameter namely stress ratio was studied. The decreasing in this parameter reduces the fatigue crack growth rate (FCGRs).

Abstract: The change in crystallographic orientation distribution during deformation by deep drawing in mild steel has been investigated in order to understand their role in initiation and propagation of crack. The results show that the change of grain orientation from the initial recrystallization texture component of {111}<112> to deformation orientation {111}<110> incites the initiation and propagation of transgranular cracking in the region of {111}<112> small grains. Moreover, the transgranular misorientation and local orientation are analyzed in detail to discuss the change from {111}<112> to {111}<110>. The SEM-EBSD technique was used to reveal that change of orientationm, XRD was performed for the characterization of the global evolution of texture for deformed samples.

Abstract: dentifying characteristics of a force generated by non punctual impact is performed to better monitor the health of the impacted structure. This can be achieved through using an implemented structural model. For composite beams, the model can be constructed by means of the finite element method. In this work, the impact is assumed to be a uniform distributed pressure and the impact location is known. Reconstructing the force signal is performed by using regularized deconvolution techniques of the Toeplitz like equation giving the answer in terms of strains as function of the input force. Here, the generalized singular value decomposition based method is used in conjunction with truncation filtering. Quality of the reconstructed force is discussed as function of the mesh size and the mode truncation order.

Abstract: When coupling with temperature is incorporated, the problem of fatigue is formulated within the general framework of thermomechanical fatigue. Considering the special case of steel structures, in addition to variations of material and fatigue parameters with temperature, fatigue damage depends on the phasing existing between the concomitant strain and temperature cycles. In this work, the extended finite element method is used to simulate crack growth under thermomechanical fatigue coupling. Assuming large cycle duration for which temperature variations can be considered to be uniform, this approach is applied in the context of linear elastic fracture mechanics for the particular case of the three dimensional Compact-Tension specimen. The objective is to attempt understanding more closely crack growth mechanism under thermomechanical loading. Characterization of fatigue was assessed as function of phasing and strain restraint.

Abstract: Functionally graded materials (FGMs) are a new kind of composite materials which have a smooth variation of material properties along one or more directions. At each interface, the material is chosen according to specific applications and environment loadings. This paper presents some solutions to study the free vibration of FGM plates made of ceramic and metal. The formulation used is based on Reddys higher order shear deformation plate theory. Material properties are taken to be temperature-dependent, and vary continuously through the thickness direction according to a power law distribution (P-FGM). The plate is assumed to be initially stressed by temperature rise through the thickness. The energy functional of the system is obtained by using energy principles. Free vibration frequencies are then obtained by using a set of characteristic orthogonal polynomials and by applying Ritz method.

Abstract: In the aerospace domain, sandwich plates represent an efficient structural element, providing a high stiffness/weight ratio characteristic. Moreover, when using this structural element, different design parameters and materials of the core can be adopted in order to obtain desired properties. When the dynamic analysis of the spacecraft structure made up of the honeycomb sandwich plates is performed in MSC Patran/Nastran. In this study a three dimensional finite elements of a clamped-free (CFFF) honeycomb plate under dynamic vibrations loading had been analysed. Geometric parameters of hexagonal plate is specific to absorb vibrations, hence the effect of each parameters is crucial to determine the rigidity of plates under a single-point cyclic loading. The effect of honeycomb core thickness, unit cell size, and the materials contribute to determine the rigidity of honeycomb plate. Therefore, all of the analysis results can accommodate bases for structural design and optimum design of the spacecraft structure.

Abstract: Work presented is interested in the characterization of the quasistatic mechanical properties and in fatigue of a composite laminated in jute/epoxy. The natural fibres offer promising prospects thanks to their interesting specific properties, because of their low density, but also with their bio deterioration. Several scientific studies highlighted the good mechanical resistance of the vegetable fibre composites reinforced, even after several recycling. Because of the environmental standards which become increasingly severe, one attends the emergence of eco-materials at the base of natural fibres such as flax, bamboo, hemp, sisal, jute. The fatigue tests on elementary vegetable fibres show an increase of about 60% of the rigidity of elementary fibres of hemp subjected to cyclic loadings. In this study, the test-tubes manufactured by the method infusion have sequences of stacking of 0/90° and ± 45° for the shearing and tensile tests. The quasistatic tests reveal a variability of the mechanical properties of about 8%. The tensile fatigue tests were carried out for levels of constraints equivalent to half of the ultimate values of the composite. Once the fatigue tests carried out for well defined values of cycles, a series of static tests of traction type highlights the influence of the number of cycles on the quasi static mechanical behavior of the laminate jute/epoxy.