Abderrahim El Mahi

Damping Analysis of Sandwich Composite Materials

The article presents an analysis of the damping of sandwich composites, made of PVC foam cores and laminated skins. Damping parameters are investigated using beam test specimens and an impulse technique. Damping modeling is developed using a finite element analysis which evaluated the different energies dissipated in the material directions of the core and the layers of the skins. The results obtained show that this analysis describes fairly well the experimental results. Next, finite element analysis is applied to investigate the influence of different parameters of the core and skins. The finite element analysis considered can be applied to complex shape structures.

Monitoring of damage mechanisms in sandwich composite materials using acoustic emission

This work presents a procedure for investigating local damages created in sandwich composites. The proposed analysis is based on the processing of the recorded acoustic emission hits. Unsupervised pattern recognition analyses (k-means algorithm) associated to Principal Component Analysis (PCA) are the tools used for the classification of the detected acoustic emission events. A cluster analysis of acoustic emission data is achieved and the resulting clusters are correlated to the created damage mechanisms within the sandwich composite. The study is validated on the constituents of the sandwich material (resin, polyvinyl chloride foam and skins) taken separately before investigating the sandwich composite during a three-point bending test. In addition to the advantages given by the use of the multivariable data analysis, results show the impact of the transducers positioning on the analysis of the involved damage mechanisms. An optimized positioning is proposed in order to improve the detection of the acoustic emission activity.

Numerical and Experimental Characterization of the Dynamic Properties of Flax Fiber Reinforced Composites

In this paper, the damping properties of flax fiber reinforced composites were investigated. Throughout a series of resonance vibration tests, the natural frequencies and the modal damping were evaluated. A numerical modelling was also produced by using a finite element model to determine the energies dissipated in each layer of the laminate structure in order to calculate the damping factors. The results obtained for the dynamic properties of flax fiber reinforced composites from experimental data and numerical analysis method show close agreement. The effect of fiber orientations on the damping behavior for this material was investigated. Another part of our work was to insert a thin viscoelastic layer within the flax fiber laminate. The interposition of this viscoelastic layer had a significant influence on the vibration behavior, bending stiffness and damping factors.

Vibration Behavior of Composite Material with Two Overlapping Delaminations

The paper presents an analysis of the vibration behavior of glass fiber reinforced composites with two overlapping delaminations. The effect of delamination length on the vibration parameters is studied. Throughout a series of vibration tests, the change of natural frequencies and modal damping due to delaminations is evaluated. A numerical simulation considering finite element analysis allows to predict the change of natural frequencies for a known damage size. Modal damping was established which evaluates the different energies dissipated in the material layers direction of the fiber reinforced composites. A comparison of the different results was performed. Next, strain energy of layers directions were established by a numerical analysis and discussed.

Effects of debonding length on the fatigue and vibration behaviour of sandwich composite

The aim of this study is to investigate the effects of debonding length on the fatigue and vibration behaviour of sandwich material. The sandwich material used in this study is constructed with glass fibre laminates as skins and with PVC closed-cell foams as core. The tests were conducted using impulse frequency response technique and cyclic fatigue loading in three-point bending with various debonding lengths. The vibration test was used to study the effects of debonding length on modal parameters (frequency, loss factor). The effects of debonding lengths on the stiffness, hysteresis loops and damping were studied during fatigue tests. The different relative change in frequency, loss factor and maximum load is determined. These parameters are compared for vibration and fatigue tests.

Quasi-static and fatigue properties of a balsa cored sandwich structure with thermoplastic skins reinforced by flax fibres

This paper presents the results of several experimental analyses performed on a bio-based sandwich structure. The material studied is made up of thermoplastic skins reinforced by flax fibres and applied to a balsa core. It is produced by a single-step liquid resin infusion process. First, the skin–core interfacial properties were measured and compared with those of the existing materials. Then, several quasi-static and dynamic analyses were performed to study the flexural behaviour of sandwich beams. The elastic and ultimate properties of the skins and core were deduced from these flexural tests. In addition, a failure mode map was developed based on the existing theories to explain the failure modes observed under different boundary conditions. The influence of the statistical spread of the core’s mechanical properties on the failure of sandwich beams was considered. Moreover, fatigue analyses were conducted to determine the fatigue resistance of this bio-based structure. Its behaviour was compared with that of the previously tested non-bio-based sandwich beams. Finally, the fatigue loss factor evolution of this structure under several loading ratios was measured and discussed. This study aims to determine some of the main mechanical properties of this material to see whether or not it might be suitable for industrial applications currently using glass fibre reinforced polymer/foam cored sandwich structures.

Characterization of sandwich beams with shear damages by linear and nonlinear vibration methods

The aim of the present work is to study the vibration behavior of sandwich composites with shear damages on the foams. The effect of the densities of damage on the linear and nonlinear vibration parameters is studied. The sandwich materials used in this study is constructed with glass fiber laminates as skins and with PVC closed-cell foams with density 60 and 100 kg m−3 as core. After a serie of vibration tests, the change of natural frequencies and modal damping due to the damage and the foam densities. For an intact specimen, the resonance frequency was constant when we increase the excitation amplitude. For damaged specimens, the resonance frequencies decrease and the loss factors increase proportionally with the increasing excitation amplitude. The nonlinear parameters corresponding to the elastic modulus and damping were determined for each frequency modes and each damage and foam densities. The results showed that nonlinear dissipative parameters were more sensitive to damage than linear elastic and dissipative parameters.

Tensile Fatigue Behavior of Carbon-Flax/Epoxy Hybrid Composites

Hybridation of carbon fiber composites with flax fiber offer interesting bio-degradability, respect of the environment, reduced cost and important dynamic properties. The purpose of this work is to study the effect of hybridation on the mechanical fatigue behavior of unidirectional carbon-flax hybrid composites. Static and fatigue tensile tests were realized for different laminates made of carbon fibers and carbon-flax hybrid fibers with an epoxy resin. The carbon laminates and two different staking sequences of hybrid laminates were manufactured by hand lay-up process. Monotonic tensile tests were realized to identify the mechanical properties of composites and the ultimate loading. Then, load-controlled tensile fatigue tests were conducted on standard specimens with applied load ratio RF of 0.1. Specimens were subjected to different applied fatigue load level until the failure (60%, 65%, 75% and 85%). Damage was observed early after a few loading cycles. The decrease in the Young’s modulus was depending on the ratio of fibers on the composites. Overall, the stiffness decreases by showing three stages for all studied samples. It has been found that the stress-number of cycle S-N curves show that carbon laminates have higher fatigue endurance than hybrid composites.

Fatigue behavior and damage analysis by the acoustic emission technique of cross-ply laminates under tensile loading

Purpose The purpose of this paper is to study the mechanical behavior in fatigue tensile mode of different cross-ply laminates constituted of unidirectional carbon fibers, hybrid fibers and glass fibers in an epoxy matrix; and to identify and characterize the local damage in the laminated materials with the use of the acoustic emission (AE) technique. Design/methodology/approach The tests in the fatigue mode permitted the determination of the effect of the stacking sequences, thickness of 90° oriented layers and reinforcement types on the fatigue mechanical behavior of the laminated materials. The damage investigation in those materials is reached with the analysis of AE signals collected from fatigue tensile tests. Findings The results show the effects of reinforcement type, stacking sequences and thicknesses ratio of 90° and 0° layers on the mechanical behavior. A cluster analysis of AE data is achieved and the resulting clusters are correlated with the damage mechanism of specimens under loading tests. Originality/value The analysis of AE signals collected from tensile tests of the fatigue failure load allows the damage investigation in different types of cross-ply laminates which are differentiated by the reinforcement type, stacking sequences and thicknesses ratio of 90° and 0° layers.

Effect of Piezoelectric Implant on the Structural Integrity of Composite Laminates Subjected to Tensile Loads

The embedment of sensors within composite structures gives the opportunity to develop smart materials for health and usage monitoring systems. This study investigates the use of acoustic emission monitoring with embedded piezoelectric sensor during mechanical tests in order to identify the effects of introducing the sensor into the composite materials. The composite specimen with and without embedded sensor were subject to tensile static and fatigue loading. The analysis and observation of AE signals show that the integration of a sensor presents advantage of the detection of the acoustic events and also show the presence of three or four types of damage during tests. The incorporation of piezoelectric sensor has a negligible influence on the mechanical properties of materials.