Characterisation of Low Impact Energy Induced Damage in Composite Plates with Embedded Optical Sensors

Fibre reinforced materials are increasingly employed in aerospace, naval and civil structures because of their low weight and high specific strength and stiffness. A major concern is the inherent susceptibility of composite laminates to barely visible damage induced by low velocity impacts. Non-destructive, in-situ inspection techniques are required to continuously control the integrity of such structures. Traditional non-destructive testing methods, like ultrasonic scanning, are time-consuming and require the withdraw from service of the tested part. Vibration-based structural health monitoring methods are reported to be a promising tool to check the structural integrity. In fact, damage leads to a local decrease of the structural stiffness and alters the wave propagation in a laminate. The stiffness modification results in a change of the modal characteristics of the structure. Numerous studies have shown that eigenfrequencies, damping ratios and curvature mode shapes of a laminated composite structure are sensitive to impact induced damage. Inverse numerical-experimental techniques based on modal characteristics have already demonstrated their applicability for the identification of mechanical properties of intact composite laminates. By using an adequate damage model, a similar method may be used for the identification of damage parameters. In this work, a damage identification method based on signals obtained by an integrated sensing system is proposed. Fibre Bragg gratings (FBG), a recent sensor technology, are optical sensors allowing to measure internal strains in composite laminates. This type of sensor can be perfectly integrated in a structure, is maintenance-free and may last for the entire lifetime of a structure. A high rate FBG interrogation system based on intensity modulation is enhanced so that calibrated low-noise strain measurements can be performed with acquisition rates of up to 250 kHz. In this study, the FBG sensors are embedded in carbon fibre reinforced cross-ply plates made of 28 unidirectional plies. The sensors are used to capture the dynamic response of the plate to an impact event and to carry out experimental modal analysis. Moreover, acoustic waves originating from impacts are sensed with a high sensitivity and an acquisition rate of 1 GHz. The experimental results using several instrumented plates demonstrate the efficiency and accuracy of the interrogation system. Monitoring of the structural integrity of the composite plate consists of two stages, first impact localisation and second damage identification. The appearance of an eventual impact is detected by surveying the dynamic response of the FBG sensors and its location is predicted based on waves propagating from the impact to the sensors. The prediction is made via interpolation of a previously determined reference data set produced by non-destructive hammer impacts. The interpolation-based localisation method does neither require the knowledge of the wave propagation velocity nor the exact position of the sensors within the laminate. The prediction accuracy of the localisation method is evaluated with several numerical and experimental validation tests and is shown to be in the order of a few millimetres. The different potential error sources are identified and they are found to be mainly independent on the plate size. Upon the detection and localisation of an impact, an eventual damage is identified using an inverse numerical-experimental optimisation method. A finite element model of the damaged plate is built based on three-dimensional characterisation of the damage pattern using high resolution X-ray computed tomography. The identification method utilises a homogenised damage model with an approximated damage shape and reduced transverse shear moduli. The damage surface and position are identified by minimising the discrepancy between the numerically calculated and experimentally determined eigenfrequency changes using a hybrid iterative and global search algorithm. The initial guess of the damage position required for the optimisation procedure needs to be sufficiently precise. Within this work, it consists of the predicted impact location obtained from the localisation method. The robustness of the algorithm to different initial guesses of the parameters is tested by numerical and experimental examples. The impact localisation and damage identification method is summarised and demonstrated by a comprehensive experiment. Four FBG sensors are employed to detect and localise the impact and to determine the plate's eigenfrequencies. The damage surface is in general underestimated by approximately 20% by the numerical-experimental optimisation algorithm and the distance between the identified and exact damage position corresponds to less than 10% of the damage size

Modal numerical-experimental identification method for characterising the elastic and damping properties in sandwich structures with a relatively stiff core

In this paper, a mixed numerical-experimental identification procedure for characterising the storage and loss properties in sandwich structures with a relatively stiff core is developed. At the computational level, the proposed method is based upon an original structurally damped shell finite element model derived from the higher-order shear deformation theory and, at the experimental level, upon an accurate contact-free measurement setup with a loudspeaker-based excitation and a scanning laser interferometer for capturing the time responses. From the modal information extracted from two specimens with different geometries, the procedure allows the simultaneous estimation of the skin and core constitutive parameters through adequate objective functions measuring the discrepancy between the experimental data and the numerical predictions. For validation purposes, the method is then applied to two test cases for which all the influent properties could be estimated with a fairly good accuracy. (C) 2012 Elsevier Ltd. All rights reserved

Vibration-based characterization of impact induced delamination in composite plates using embedded FBG sensors and numerical modelling

Dynamic strain signals from embedded fiber Bragg grating sensors are used for experimental modal analysis in carbon fiber reinforced polymer plates. Throughout a series of impact tests, the change of eigen-frequencies due to damage is evaluated for different impact energies. A detailed numerical model including the damage pattern obtained from X-ray computed tomography images demonstrates that most of the frequency changes can be explained by a delamination type of damage, whereas the total delamination surface has an affine relation to the absorbed impact energy. A homogenized damage model, including two damage factors, allows to predict the change of natural frequencies for a known damage size. (C) 2011 Elsevier Ltd. All rights reserved

Vibration-based characterization of impact induced delamination in composite plates using embedded FBG sensors and numerical modelling

Dynamic strain signals from embedded fiber Bragg grating sensors are used for experimental modal analysis in carbon fiber reinforced polymer plates. Throughout a series of impact tests, the change of eigenfrequencies due to damage is evaluated for different impact energies. A detailed numerical model including the damage pattern obtained from X-ray computed tomography images demonstrates that most of the frequency changes can be explained by a delamination type of damage, whereas the total delamination surface has an affine relation to the absorbed impact energy. A homogenized damage model, including two damage factors, allows to predict the change of natural frequencies for a known damage size

Low energy impact damage monitoring of composites using dynamic strain signals from FBG sensors - Part II: Damage identification

In part I of this two-paper series, a method for the localization of an impact using dynamic strain signals from fiber Bragg grating (FBG) sensors is presented. In this paper, an inverse numerical-experimental method allowing to identify the damage based on experimentally measured eigenfrequency changes is developed and validated. The damage identification is limited to a region in the vicinity of the impact position predicted by the localization method. The eigenfrequency changes are determined experimentally from dynamic strain signals obtained with embedded FBG sensors and the parameters of a homogenized damage model are adjusted to fit the numerical results to the experimental data. (C) 2011 Elsevier Ltd. All rights reserved

Low energy impact damage monitoring of composites using dynamic strain signals from FBG sensors - Part I: Impact detection and localization

Fiber reinforced composite materials risk to suffer from subsurface, barely visible, damage induced by transverse relatively low energy impacts. This two-paper series presents a method for the localization of an impact and identification of an eventual damage using dynamic strain signals from fiber Bragg grating (FBG) sensors. In this paper, the localization method allowing to predict the impact position based on interpolation of a reference data set is developed and validated. The data utilized in the method are the arrival times of the asymmetric zero order Lamb waves at the different sensors. A high rate interrogation method based on intensity modulation of the Bragg wavelength shift is used to acquire the FBG signals. The localization method allows to predict the impact position with a good accuracy and therefore the inspection of the laminate can be limited to this region. (C) 2011 Elsevier Ltd. All rights reserved

High-speed internal strain measurements in composite structures under dynamic load using embedded FBG sensors

Internal strain measurements in cross-ply carbon-epoxy composite plates under dynamic loads are carried out using embedded FBG sensors. The principle of the FBG interrogation is based on intensity demodulation achieved via a Fabry-Perot filter. To account for the non-linearity of the filter, the system is calibrated and the amplitude of the strain data is validated. Strains are acquired at a rate of 100 kHz with a noise level as low as 2 mu epsilon and used for modal analysis and strain monitoring in low energy impact. The experimental results under impact and modal analysis compare very well with pertinent numerical models and modal analysis obtained from laser vibrometer measurements. (C) 2010 Elsevier Ltd. All rights reserved