Thomas Monnier

Non‐Destructive Evaluation of Damage and Failure of Fibre Reinforced Polymer Composites Using Ultrasonic Waves and Acoustic Emission

Applications of reinforced composites and heterogeneous solids are widespread, spanning technological areas of various aerospace and mechanical industries. A real challenge concerning these materials is their life time prediction when subjected to wide variety of environmental and mechanical loading conditions that can initiate damage and lead to failure. Indeed, damage at the smallest scales drives damage accumulation at larger length scales until some critical local damage state is attained that causes macroscopic failure. A key issue in predicting life time is to characterise distributed volumic and localised damage and to understand the mechanisms of its initiation, evolution and criticality and so, the identification of relevant precursors of failure. To answer to these questions, volumic and guided ultrasonic waves and acoustic emission are of particular interest. As a matter of fact volumic ultrasonic wave propagation is sensitive to homogeneously distributed microcracks and represents in that case a good damage indicator. Guided waves as Lamb waves especially when generated from inside the material using an inserted piezoelectric element offer a specific sensitivity to localised damage as cracks or delaminations. Besides, acoustic emission which corresponds to the energy released by the material during the damage processes is directly related to the damage mechanisms and so can give pertinent information about the damage initiation and development. In this paper, our aim is to show in the one hand the ability of volumic ultrasonic waves to characterise volumic damage of glass epoxy composites under hydrothermal ageing and also the ability of Lamb waves to detect and identify localised damage. In the other hand our purpose is to demonstrate the potentiality of acoustic emission in understanding the damage mechanisms that occurs during a tensile test of polymer fibre composites and to discriminate in real time the different types of damage occurring at the microscopic scale.

Long-term stability of normal condition data for novelty detection

As a technique of diagnosing failure in structures and systems, the method of novelty detection shows considerable merit. The basis of the approach is simple: given measured data from normal condition of the structure, the diagnostic system builds an internal representation of the system normal condition in such a way that subsequent departures from this condition can be identified with confidence in a robust manner. The success or failure of the method is contingent on the accuracy of the description of normal condition. In many cases, the normal condition data may have quite a complex structure: for example, an aircraft may experience a wide range of ambient temperatures in the course of a single flight. Also, the operational loads experienced by the craft as a result of flight manoeuvres may have wide-ranging effects on the measured states. The object of the current paper is to explore the normal condition space for a simple benchmark monitoring system. The said system uses Lamb-wave inspection to diagnose damage in a composite plate. Both short-term and long-term experiments are carried out in order to examine the variations in normal condition as a result of run-in of the instrumentation and variations in ambient temperature. The exercise is not purely academic as the fiber-optic monitoring system is a serious candidate for a practical diagnostic system.

Stiffness Tuning Using a Low-Cost Semiactive Nonlinear Technique

This paper presents a semiactive method for stiffness control of electromechanical systems. The proposed method consists of intermittently connecting a piezoelectric element on an oscillating electrical network. It is demonstrated that such a nonlinear treatment allows an effective control of the stiffness, while requiring less power than the classical methods. Experimental measurements carried out show that such a technique allows controlling the stiffness in a wide range of value, which validates the theoretical predictions.

Application of Lamb waves for the characterization of composite plates

In order to detect and evaluate flaws in thin composite structures, such as skins on aircraft wings or sail boat bodies, Lamb waves are the preferred tool of ultrasonic excitation. In the framework of a European Brite EuRam project, we have been involved in the problem of damage assessment in smart composite plates. Our goal is to predict the system signature and to identify optimal signal extraction routines. Given the wave frequency, thickness and physical properties of the materials, we simulate, using the Local Interaction Simulation Approach (LISA), the propagation of Lamb waves in carbon fiber reinforced plates and their interaction with defects and compare the numerical results with the experimental data.

Self-powered Wireless Health Monitoring Supplied by Synchronized Switch Harvesting (SSH) Method

The development of autonomous wireless sensors and actuators in order to design advanced structural health monitoring (SHM) systems is an exciting challenge for both the industrial and the academic communities. Some studies have dealt with the implementation of wireless devices. Almost all the studies about so-called autonomous systems today still require power supply. It means that a fully self-powered system has not been achieved yet. The present study shows the design of self-powered wireless health monitoring system, for which the energy is supplied by the original method called the synchronized switch harvesting (SSH) method. The piezoelectric elements and electrical circuit, located on the vibrating structure, harvest electrical energy from the direct conversion of mechanical energy vibration. Lamb wave transmission is chosen for the SHM of a composite beam of length around 30 cm. Some piezoelectric elements, used for energy harvesting and signal transmission, are located on the beam. The energy required to wake-up the micro-controller and to achieve two complete transmission cycles is only 1.5mJ. This small amount of energy can be harvested in a short time period for reasonable beam displacement levels. The study details the different trade-off concerning such a self-powered health monitoring device.

Energy efficient method for embedded in-situ structural health monitoring

This article introduces the principles of an ultra low-cost scheme for in situ, embedded structural health monitoring. Based on the interactions of a Lamb wave with the structure, the proposed technique relies in comparing the time period where the sensed signature of the Lamb wave is greater than an user-defined threshold with a reference value. Compared to similar methods based on the monitoring of the structural state evolution, the proposed scheme is ultra low-cost, and therefore suitable for integrated systems that have a limited amount of available energy.

Passive Impact Location Estimation Using Piezoelectric Sensors

As part of Structural Health Monitoring (SHM), the history of a structure has become a crucial element to take into account. This has been shown, for example, by the spectacular accident of the flight Aloha 243 near Hawaï, when a whole part of the fuselage of a Boeing 737 had been torn off. Thus, monitoring impacts has become particularly interesting to give a comprehensive view of the occurrence of structural damage. Typical impact location estimation techniques use structural frequency drifts of a structure. Thus, such methods need an external excitation of the structure, which is unrealistic in most of the cases. As well, huge and possibly long computations, as genetic algorithms or artificial neural networks, are required for such techniques in order to retrieve the impact location. The scope of the this article was to present a passive impact location estimation using piezoelectric elements for a 1D infinite beam. The principles of the proposed technique was the comparison of the vibrational energy extracted by each sensor. From this comparison, the impact location was cost-efficiently estimated. Theoretical development and experimental results showed that this extraction-based force location estimation method performed well, while being very simple.

Damage Assessment in Smart Composite Structures: the DAMASCOS Programme

The DAMASCOS (DAMage Assessment in Smart COmposite Structures) project is a European Union funded program of work bringing together a number of academic and industrial partners throughout Europe. The aim of Damascos is to apply new ultrasonic detection and generation techniques integrated within the structure, together with advanced signal processing to realize damage assessment and ageing characterization in composite structures. This paper describes the background, experimental findings and future applications of the technology as the project moves into its final phase.

Health monitoring of smart composite structures using ultrasonic guided waves

The health of a structure depends on both the homogeneously distributed degradation of its mechanical properties during its life cycle and the presence of localised defects such as cracks or delaminations. The proposed non-destructive health monitoring method allows to recover both kinds of information using ultrasonic waves. To avoid traditional techniques limitations, such as coupling reproducibility for instance, we propose here to integrate a piezoelectric element into the plate-like composite structure. The element dimensions are determined in order to uncouple the frequency ranges of the thickness and radial vibration modes. The thickness mode is used to monitor the homogeneous ageing of the structure through electrical impedance measurement. As for the radial vibrations, they are used to generate and detect Lamb waves, which have the advantage of propagating over long distances and offering specific sensitivity of various modes to different kinds of defects. The present work focuses on this last application and studies the ability of the proposed technique to detect and identify defects such as low speed impact-induced delaminations and cracks incomposite plate-like structures.

Acoustic microscopy of functionally graded thermal sprayed coatings using stiffness matrix method and Stroh formalism

Acoustic microscopy of multilayered media as well as functionally graded coatings on substrate necessitates to model acoustic wave propagation in such materials. In particular, we chose to use Stroh formalism and the recursive stiffness matrix method to obtain the reflection coefficient of acoustic waves on these systems because this allows us to address the numerical instability of the conventional transfer matrix method. In addition, remarkable simplification and computational efficiency are obtained. We proposed a modified formulation of the angular spectrum of the transducer based on the theoretical analysis of a line-focus transducer for broadband acoustic microscopy. A thermally sprayed coating on substrate is treated as a functionally graded material along the depth of the coating and is approximately represented by a number of homogeneous elastic layers with exponentially graded elastic properties. The agreement between our experimental and numerical analyses on such thermal sprayed coatings with different thicknesses confirms the efficiency of the method. We proved the ability of the inversion procedure to independently determine both thickness and gradient of elastic properties. The perspective of this work is the opportunity to non-destructively measure these features in functionally graded materials.