Angelos Filippatos

In situ integrity assessment of a smart structure based on the local material damping

Integration of functional elements into fibre-reinforced host structures provides the possibility for in situ monitoring of the structural integrity of critical components. In this study, a vibration-based monitoring function has been developed that allows the structural integrity identification of critical components. For this purpose, signal analysis algorithms were developed to enable the estimation of damage-dependent modal damping. The analysed smart structure was a carbon fibre–reinforced epoxy composite plate with an integrated actuating/sensing system. The local material damping is a parameter especially sensitive to different failure modes of composites. In order to characterise the changes of this parameter resulting from impact events, dynamical mechanical analysis on intact and damaged specimens made of the composite material was conducted. Based on the dynamical mechanical analysis results, a finite element model of the structure was developed. Then, modal damping ratios for different sizes and locations of damaged regions were numerically determined, and a relation between modal damping and damage-dependent local damping was identified. The deterministic decision trees describing the reverse relationship between online-measured modal damping and damage condition were determined. That was accomplished through the application of information entropy-based data-mining algorithms to the numerically generated learning dataset obtained using the developed finite element model.

A Simulation-Based Monitoring of a Composite Plate Using an Integrated Vibration Measurement System

The unique potential to integrate functional elements into fibre-reinforced components combined with the recent progress in the simulation models of composite materials provides new perspectives for reliability improvement of the next generation components. Such combination is presented on the example of a carbon-fibre reinforced composite plate with integrated vibration measurement and excitation systems. The investigated structure was consolidated in an adapted resin transfer moulding process using additional layers for positioning, contacting and isolating of the active elements. The integrated elements enable an online estimation of the structural dynamic behaviour and its damage-dependent changes.The article considers the identification problem of diagnostic models enabling a precise interpretation of the measured vibration responses. An approach based on the generation of classifiers by means of inductive machine learning algorithms is applied. At the baseline phase, modal properties are measured that correspond to the undamaged state of the structure. Using these experimental data, a simulation model of the structure was fitted by means of a mixed numerical experimental technique and used for the generation of multiple vibration patterns resulting from different mass distributions. The unique combination of experimental and numerical results enables a generation of high resolved learning datasets for machine learning algorithms using a minimum amount of experimental data. The verification of the estimated classifiers by means of the achievable diagnostic performance is firstly conducted theoretically using standardised validation techniques and a high performance is identified. Then, at the inspection phase, the performance of the whole diagnostic system is additionally experimentally confirmed based on the dynamic response resulting from different unseen structural disturbances.

Damping Analysis of Cylindrical Composite Structures with Enhanced Viscoelastic Properties

Constrained layer damping treatments are widely used in mechanical structures to damp acoustic noise and mechanical vibrations. A viscoelastic layer is thereby applied to a structure and covered by a stiff constraining layer. When the structure vibrates in a bending mode, the viscoelastic layer is forced to deform in shear mode. Thus, the vibration energy is dissipated as low grade frictional heat. This paper documents the efficiency of passive constrained layer damping treatments for low frequency vibrations of cylindrical composite specimens made of glass fibre-reinforced plastics. Different cross section geometries with shear webs have been investigated in order to study a beneficial effect on the damping characteristics of the cylinder. The viscoelastic damping layers are placed at different locations within the composite cylinder e.g. circumferential and along the neutral plane to evaluate the location-dependent efficiency of constrained layer damping treatments. The results of the study provide a thorough understanding of constrained layer damping treatments and an improved damping design of the cylindrical composite structure. The highest damping is achieved when placing the damping layer in the neutral plane perpendicular to the bending load. The results are based on free decay tests of the composite structure.

A composite-appropriate integration method of thick functional components in fibre-reinforced plastics

The use of integrated structural health monitoring systems for critical composite parts, such as wind turbine blades, fuselage and wing parts, is an promising approach to guarantee a safe and efficient operational lifetime of such components. Therefore, the integration of thick functional components like sensors, actuators and electronic components is often necessary. An optimal integration of such components should be ensured without material imperfections in the composite structure, i.e. voids and resin rich areas, and failure of the functional components. In this paper, first investigations were undertaken for a basic understanding of the mechanical performance of a fibre reinforced plastic component with integrated functional elements. The influence of different materials and treatment methods for the encapsulation of electronic components was experimentally investigated under static and dynamic loading tests. By means of a parametric finite element model, the effects of an encapsulation and various parameters such as the shape and orientation of the electronic components were examined. Several encapsulation variants were investigated in order to minimise the chance of failure initiations. Based both on experimental and numerical results, a preferred composite integration concept was selected for an electronic board and some first recommendations for an optimal integration were derived.

In-process, non-destructive multimodal dynamic testing of high-speed composite rotors

Fibre reinforced plastic (FRP) rotors are lightweight and offer great perspectives in high-speed applications such as turbo machinery. Currently, novel rotor structures and materials are investigated for the purpose of increasing machine efficiency, lifetime and loading limits. Due to complex rotor structures, high anisotropy and non-linear behavior of FRP under dynamic loads, an in-process measurement system is necessary to monitor and to investigate the evolution of damages under real operation conditions. A non-invasive, optical laser Doppler distance sensor measurement system is applied to determine the biaxial deformation of a bladed FRP rotor with micron uncertainty as well as the tangential blade vibrations at surface speeds above 300 m/s. The laser Doppler distance sensor is applicable under vacuum conditions. Measurements at varying loading conditions are used to determine elastic and plastic deformations. Furthermore they allow to determine hysteresis, fatigue, Eigenfrequency shifts and loading limits. The deformation measurements show a highly anisotropic and nonlinear behavior and offer a deeper understanding of the damage evolution in FRP rotors. The experimental results are used to validate and to calibrate a simulation model of the deformation. The simulation combines finite element analysis and a damage mechanics model. The combination of simulation and measurement system enables the monitoring and prediction of damage evolutions of FRP rotors in process.

Dynamic Behavior of Active Lightweight Compliant Mechanisms with Integrated Piezoceramic Actuators by Under- and Overcritical Periodic Excitation

Integration of active elements as thermoplastic-compatible piezoceramic modules in lightweight compliant mechanisms offers the possibility to actively control its structural behavior by static, dynamic or vibro-acoustic loads. New active lightweight structures with material-integrated structural monitoring, energy-harvesting, or active vibration damping functionalities become possible. Previously, a demonstrator mechanism was designed and built, and its behavior under quasi-static excitation was simulated as a multi-body system, using two-layer cells with torsion and traction springs for compliance and linear motors for excitation. The mechanism was later tested to confirm the simulation results. This paper presents a study of the dynamic behavior of active compliant mechanisms obtained by integration of piezoceramic actuators into fiber-reinforced composite structures. The results of the dynamic mechanical simulation procedure were compared with experimental results for a given demonstrator mechanism. The comparison concluded that the simulation procedure describes fairly accurate the real behavior of the lightweight compliant mechanism, thus it can be used in the development of new active structures.

Simulation and experimental investigation of active lightweight compliant mechanisms with integrated piezoceramic actuators

Compliant mechanisms with integrated actuators can enable new function-integrative structures through the elastic deformation of elements without the use of classical links and joints. For such designs, the mechanical behaviour of the mechanism has to be well known, because external loads, the utilised materials and the geometry of the structural parts influence the deformation performance significantly. In order to speed up the development process of such mechanisms, a tool for the dynamic analysis of compliant movements is necessary before any further FEM simulation and manufacturing. Therefore, the paper presents a simulating procedure for active compliant mechanisms obtained through the integration of piezoceramic actuators into fibre-reinforced composite structures using a double layer model. A new mechanism was designed, simulated, constructed and tested. The comparison between simulation and experimental results confirm the effectiveness of the presented procedure in regard to the design phase of new active compliant structures.

In-process deformation measurements of translucent high speed fibre-reinforced disc rotors

The high stiffness to weight ratio of glass fibre-reinforced polymers (GFRP) makes them an attractive material for rotors e.g. in the aerospace industry. We report on recent developments towards non-contact, in-situ deformation measurements with temporal resolution up to 200 µs and micron measurement uncertainty. We determine the starting point of damage evolution inside the rotor material through radial expansion measurements. This leads to a better understanding of dynamic material behaviour regarding damage evolution and the prediction of damage initiation and propagation. The measurements are conducted using a novel multi-sensor system consisting of four laser Doppler distance (LDD) sensors. The LDD sensor, a two-wavelength Mach-Zehnder interferometer was already successfully applied for dynamic deformation measurements at metallic rotors. While translucency of the GFRP rotor material limits the applicability of most optical measurement techniques due to speckles from both surface and volume of the rotor, the LDD profits from speckles and is not disturbed by backscattered laser light from the rotor volume. The LDD sensor evaluates only signals from the rotor surface. The anisotropic glass fibre-reinforcement results in a rotationally asymmetric dynamic deformation. A novel signal processing algorithm is applied for the combination of the single sensor signals to obtain the shape of the investigated rotors. In conclusion, the applied multi-sensor system allows high temporal resolution dynamic deformation measurements. First investigations regarding damage evolution inside GFRP are presented as an important step towards a fundamental understanding of the material behaviour and the prediction of damage initiation and propagation.