Rachid El Guerjouma

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.

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.

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.

Structural Integrity of Laminated Composite with Embedded Piezoelectric Sensors

Recent progress in sensor technologies, signal processing and electronics made it possible to fulfil the need for the development of in-service structural health monitoring (SHM) systems. An experimental study of local damage materials using acoustic emission (AE) has been conducted to characterize the mechanic behavior of E-glass/epoxy unidirectional composites instrumented by piezoelectric implant. A series of specimens of composite laminates with and without embedded piezoelectric were tested in static loading while constantly monitoring the response by AE technique. The acoustic signals were analyzed by the classification multi-parameters method available on NOESIS (k-means) in order to identify the different damage and to follow the evolution of these various mechanisms for both types of materials (simple and instrumented). The results show that integration of the sensors presents advantages of the detection of the acoustic events and also show the presence of three types of damage during tests. Comparing embedded sensor to sensor mounted on the surface, the embedded sensor showed a much higher sensitivity.

Structural Health Monitoring by Acoustic Emission of Smart Composite Laminates Embedded with Piezoelectric Sensor

The smaller sizes of current electronic devices suggest the feasibility of creating a smart composite structure using piezoelectric implant to monitor in-situ the lifetime of civil and aerospace structures. This study presents a health monitoring of composites materials integrated by piezoelectric sensors using acoustic emission (AE) technique. A series of specimens of laminates composite with and without piezoelectric implant were tested in three-point bending tests while continuously monitoring the response by the AE technique. The analysis and observation of AE signals leads to the identification of the acoustic signatures of four damage mechanisms in laminates materials. One of the major differences between the two types of materials (with and without embedded sensor) is the intense acoustic activity in the integrated material.

Mechanical Behaviour and Damage Evaluation by Acoustic Emission of Sandwich Structure

The aim of the present study is to investigate the mechanical behaviour of sandwich structure under static 4-bending loading. Two type of sandwich structures constituting with the [0/902/0] s cross-ply laminates as skins and with PVC closed-cell as foams which are differing in their densities: 60 and 100 kg m− 3, were considered. The effects of the foam densities on the mechanical behaviour of composites under loading were studied. This work is also interested in identifying and characterizing the local damage in the composites with the use of acoustic emission method (AE). The damage investigation in those materials is reached by the analysis of acoustic emission signals collected from static 4-point bending tests. A cluster analysis of AE data is achieved and the resulting clusters are correlated to the damage mechanism of specimens under loading tests.

Structural noise characterization and flaw detection in austenitic stainless steels using ultrasonic signals, wavelet analysis and significance testing

The aim of this study is to characterize the structural noise in order to better detect flaws in several heterogeneous materials (steels, welding, composites œ) using ultrasonic waves. For this purpose, a continuous wavelet transform is applied to ultrasonic Ascan signals acquired using an ultrasonic Non Destructive Testing (NDT) device. The time‐scale representation provided, which highlights the temporal evolution of the spectral content of the Ascan signals, is relevant but can lead to misinterpretation. The problem is to identify if each pattern from the wavelet representation is due to the structural noise or a flaw. To solve it, a detection technique based on statistical significance testing in the time‐scale plane is used. Typical structural noise signals are then described using an autoregressive model which seems relevant according to the spectral content of the signals. The approach is tested on experimental signals, obtained by ultrasonic NDT of metallic materials (austenitic stainless steel) t...

Nondestructive evaluation of heterogeneous materials using acoustic emission and ultrasound

Heterogeneous materials as Composites and concrete are advantageous as structural components in many applications. However, damage detection in such materials is difficult due to their heterogeneity and anisotropy. Furthermore, conventional non destructive technique as X‐radiographic is mostly not very sensitive to early damage and very time consuming and expensive. The purpose of this contribution is to study the capabilities of several methods, non destructive and very sensitive for damage characterisation, as Acoustic Emission (AE) and ultrasonic for the structural health monitoring of heterogeneous materials as polymer based composite materials and concrete. These materials are instrumented by piezoelectric sensors in order to detect acoustic emission and to measure the ultrasonic velocity. The mechanisms of the damage events and their space‐time localizations are identified from AE. Simultaneously, the longitudinal ultrasonic velocity is measured in situ by transmission through the composites thickne...