Impact Damage In Composite Structures Research Articles

Consequences of thermal aging on the detection of impact damage in composite structures with PZT/FBG guided wave systems

In the context of recent reusable launch vehicles (RLV) developments, SHM could help fast and economic revalidation of RLVs between launches, provided that the transducers keep their functionality and reliability after repeating launches being exposed to harsh flight conditions. This paper studies the detection of barely visible impact damage in composite plates with guided waves (GW) based SHM systems. More precisely, it focuses on how this detection can be affected by the degradation of the sensors potentially induced by the exposition to flight conditions. The aging conditions in this work mainly consist of thermal cycling with high maximum temperature (150°C), expected for RLV structures and close to the operational limit of the thermoset-matrix composite. The response of the sensors to such environmental conditions depends on the sensor itself and on its bonding to the structure. Thus, in this paper two different types of sensors are tested (commonly used PZT transducers and FBG sensors) along with two different bonding methods (classic bonding with a secondary adhesive and cobonding on composite surface during the curing process). The sensors are setup in pairs (PZT-PZT or PZT-FBG) with GW signal paths going through the impact location. After a baseline acquisition, incremental impact tests (with increasing energy) are performed and GW signals are acquired after each impact to assess the damage signature on the waveform. Then after submitting the plate to thermal cycling, GW signals are recorded again to assess its effect on the damage detection performance and on the risk of false alarms (mistaking sensor degradation for structural damage). Sensor degradations are monitored through non-destructive techniques and impedance self-diagnostic measurements for PZT. Additionally to this aging test, the responses of degraded sensors previously studied by the authors are compared with the impact signature to assess how those degraded sensors could harm the impact detection.

Guided Ultrasonic Wave Monitoring of Damage in Anisotropic Composites

Lightweight, carbon fibre reinforced composites are often employed for the manufacture of aerospace components due to their good strength to weight ratio. However, due to poor interlaminar strength, composite components are prone to barely visible impact damage, caused by low velocity impacts, during aircraft operation. Guided-wave-based structural health monitoring (SHM) techniques, using a network of distributed sensors, is an important Structural Health Monitoring (SHM) tool for the detection and localization of in-service impact damage in composite structures. However, the material anisotropy of composite laminates influences guided wave propagation and scattering. These anisotropic effects, if unaccounted for, could lead to inaccurate localization of damage, and potential regions of the structure where guided waves cannot propagate with sufficient amplitude, reducing damage sensitivity. Defect characterization can be improved by considering the scattering characteristics of various damage types for the sparse array signal processing. Guided wave scattering (A0 Lamb wave mode) was investigated around an artificial insert delamination in a quasi-isotropic CFRP panel. Permanent magnets, mounted on an undamaged region of the plate, were also used as scattering targets and compared to the delamination case. Full 3D Finite Element (FE) simulations were performed for both the delamination and magnet cases and compared to wavefield data obtained from non-contact laser Doppler vibrometer (LDV) measurements. Individual ply layers were modelled using unidirectional composite material properties to accurately capture the anisotropy effects. Good agreement was found between the experimental measurements and simulations. Scattered guided wave amplitudes around each damage type show strong directional dependency with energy focusing along the fiber directions of the outer ply layers of the laminate. Distinct scattering behavior was observed for each damage type. Implications of anisotropy and angular scattering on sparse array SHM of different defect types are discussed.

Synergistically improving the edge-on impact damage resistance of composite laminates using multi-scale CNF/Z-pin toughening

Edge-on impact damage of composite structures has drawn significant attention from the engineering and academic communities because of the much more notorious damage caused than that by out-of-plane impacts. However, research focusing on improving the edge-on impact resistance of these structures has been limited. This paper proposed a synergistic toughening approach involving multi-scale carbon nanofiber (CNF) and Z-pin to improve the composite laminates' edge-on impact damage resistance. Through analysis of mechanical response, C-scan, optical and Scanning electron microscopy (SEM) photographs of unreinforced, CNF-reinforced, Z-pin-reinforced and CNF/Z-pin-reinforced laminates, the failure mechanisms of the four types of laminates and the synergistic toughening mechanism of CNF/Z-pin were revealed. The results show that the synergistic toughening of CNF/Z-pin could weaken the negative effects caused by Z-pin implantation and that CNF has the potential to increase the damage initiation energy threshold of the laminate. Furthermore, the toughening effect of Z-pin was significantly greater than that of CNF in damage propagation. Specifically, at 50 J impact energy, the damage area of CNF/Z-pin, Z-pin and CNF-reinforced laminates was reduced by 50.5 %, 39.8 %, and 8.3 %, respectively, compared to unreinforced laminates.

Detection of Barely Visible Impact Damage in Composite Structures Using Backward Sweep Vibro-thermography Technique Utilizing Asymmetry in Local Defect Resonance

ABSTRACT In this article, local defect resonance-based vibrothermography has been studied for different sweep direction and ranges. Two different carbon fiber-reinforced polymer (CFRP) composite plate has been fabricated from vacuum assisted resin transfer molding. In the first plate, a flat bottom hole has been made and two barely visible impact damages are created in other plate. The area of delamination has been determined from phased array ultrasound testing. Laser doppler vibrometry (LDV) has been performed first for different sweep ranges and directions. The CFRP plate is excited with a piezoelectric element at 150 Vpp and the vibration over defect area is captured using a single point laser doppler vibrometer, operating in scanning mode. It has been found that vibration amplitude at local defect resonance (LDR) frequency increases in narrow sweep range compared to a wideband excitation. Again, in both cases, it has been found that backward sweep produces more amplitude compared to forward one due to softening nonlinearity. An asymmetry in LDR frequency is also been observed when the sweep range is further narrowed. An uncooled microbolometer camera is used for reception in case of vibrothermography. Backward sweep is found to be more effective as compared to the forward one and the temperature increment increases in case of narrowband excitation range.

Smart hybrid composite sensor technology to enhance the detection of low energy impact damage in composite structures

This paper introduces novel structural health monitoring (SHM) sensors to improve the detection of low energy impact damage in laminated composites. The sensor is a purposely designed thin-ply hybrid composite, composed of a layer of unidirectional S-glass/epoxy and another layer of unidirectional ultra-high modulus (UHM) carbon/epoxy. The sensor was incorporated onto both the impacted face and back of a substrate plate made from unidirectional T800 carbon/MTM49-3 epoxy prepregs with the stacking sequence of [45/0/90/-45]4S. A series of drop tower tests were conducted on the composite plates with and without the attached hybrid sensing layer, with two different in-plane dimensions and varying energy levels ranging from 3 J to 124 J. The results indicate that the sensors functioned satisfactorily and provided direct correlations between visible and internal hidden damage detected by C-scan. The sensor can be optimized by selecting appropriate material properties and adjusting it to the in-plane dimensions of the substrate.

Analysis of low-energy impact damage to epoxy resin film based on surface damage characteristics

For fiber reinforced epoxy resin composites, when low-energy impact events occur, although the surface damage of the structure is invisible, a large area of delamination might have appeared inside. In order to detect these less noticeable impact damages, epoxy resin is intended to be used as an indicative coating to show obvious surface characteristics when the impact damage occurs. On that basis, the main purpose of this study is to establish the correspondence between the damage patterns of the epoxy resin film and the low-energy impact. First, a three-dimensional progressive finite element model was established and the impact damage mechanism of the film was analyzed. Then both numerical simulation and experimental investigation were conducted for the low-energy impact response of the film, and they were in good agreement with each other. The results showed that the damage characteristics of the epoxy resin film were clearly visible and changed regularly with the impact, as reflected by the fact that the damage area, the number of cracks and the crack distribution obviously depended on the magnitude of the impact energy. Therefore, the established simulation model can be used to predict and obtain systematic damage patterns of the film and offers the potential to evaluate the invisible impact damage of composite structures based on the regular damage patterns if the film is used as the indicative coating.

Application of PZT Ceramic Sensors for Composite Structure Monitoring Using Harmonic Excitation Signals and Bayesian Classification Approach.

The capabilities of ceramic PZT transducers, allowing for elastic wave excitation in a broad frequency spectrum, made them particularly suitable for the Structural Health Monitoring field. In this paper, the approach to detecting impact damage in composite structures based on harmonic excitation of PZT sensor in the so-called pitch–catch PZT network setup is studied. In particular, the repeatability of damage indication for similar configuration of two independent PZT networks is analyzed, and the possibility of damage indication for different localization of sensing paths between pairs of PZT sensors with respect to damage locations is investigated. The approach allowed for differentiation between paths sensitive to the transmission mode of elastic wave interaction and sensitive reflection mode. In addition, a new universal Bayesian approach to SHM data classification is provided in the paper. The defined Bayesian classifier is based on asymptotic properties of Maximum Likelihood estimators and Principal Component Analysis for orthogonal data transformation. Properties of the defined algorithm are compared to the standard nearest-neighbor classifier based on the acquired experimental data. It was shown in the paper that the proposed approach is characterized by lower false-positive indications in comparison with the nearest-neighbor algorithm.

Modeling of a realistic barely visible impact damage in composite structures based on NDT techniques and numerical simulations

In this paper, the authors presented a concept of a construction of a parametric model of barely visible impact damage (BVID) in carbon fiber-reinforced composite structures based on a reconstruction of X-ray computed tomography scans, ultrasonic B- and C-Scans as well as numerical simulations of such a type of impact loading. Considering the results of the experimental tests made it possible to model the resulting BVID with a realistic geometry and the location of delaminations, while the results of the numerical simulations allowed confirming and adjusting the damage extent. The original procedure of the BVID reconstruction from non-destructive testing data was developed and presented in the paper for the purpose of preparing the parametric computer-aided design (CAD) models. Using both numerical and experimental results the generic shapes of BVID for the set of various impact energies and various impactors were achieved. The determined dependencies of changing the damage extent with respect to various energies and impactors made it possible to parametrize BVID. The resulting generic models of BVID were validated on the X-ray computed tomography scans.

A method of predicting visual detectability of low-velocity impact damage in composite structures based on logistic regression model

This paper proposed a new method for quantitative assessment of visual detectability of damage based on logistic regression, using the Probability of Detection (POD) as a criterion. Experiments were performed to establish the massive hit/miss data of visual inspection. Authoritative investigations verified the reliability of the data. The prediction function concluded comprises more than one flaw size parameters, including the depth and diameter of the dents. The results show that the depth and diameter of the dents are pivotal for the evaluation of detectability; the type of detection, the detection distance, and the qualifications of personnel are critical external factors to be considered. This function, with an accuracy rate of nearly 85%, is capable of predicting the visual detection probability of impact damage under various detection environments, which will provide a reference for the damage tolerance design of composite materials and field maintenance in the Non-Destructive Testing (NDT) field.

Analysis of selected parameters in numerical modeling of low-velocity impact damage in composite structures

The barely visible impact damage (BVID) in layered composite structures is one of the critically important factors that affect structural residual life, when detected not early enough. This introduces a necessity of using numerical models for the prediction of the residual life of structures with BVID in order to reduce the number of experiments on real components and make it possible to predict various scenarios of BVID and the resulting structural residual life. Modeling of such a complex phenomenon is usually very challenging, and numerous factors and parameters should be taken into consideration in order to reflect the true damage form. In this study, the authors analyzed selected parameters of numerical modeling of this phenomenon.

Reconstruction of Barely Visible Impact Damage in Composite Structures Based on Non-Destructive Evaluation Results.

The occurrence of barely visible impact damage (BVID) in aircraft composite components and structures being in operation is a serious problem, which threatens structural safety of an aircraft, and should be timely detected and, if necessary, repaired according to the obligatory regulations of currently applied maintenance methodologies. Due to difficulties with a proper detection of such a type of damage even with non-destructive testing (NDT) methods as well as manual evaluation of the testing results, supporting algorithms for post-processing of these results seem to be of a high interest for aircraft maintenance community. In the following study, the authors proposed new approaches for BVID reconstruction based on results of ultrasonic and X-ray computed tomographic testing using authored advanced image processing algorithms. The studies were performed on real composite structures taking into consideration failure mechanisms occurring during impact damaging. The developed algorithms allow extracting relevant diagnostic information both from ultrasonic B-and C-Scans as well as from tomographic 3D arrays used for the validation of ultrasonic reconstructed damage locations, which allows for a significant improvement of the detectability of BVID in tested structures. The developed approach can be especially useful for NDT operators evaluating the results of structural NDT inspections.

Nonlinear elastic imaging of barely visible impact damage in composite structures using a constructive nonlinear array sweep technique

Linear and nonlinear ultrasound imaging methods highlight different damage features: the linear method detects large stiffness changes, while the nonlinear technique identifies small impedance mismatches, such as microcracks or closed delaminations. Typically, nonlinear ultrasound techniques detect damage/defects in materials by measuring higher order harmonics. These harmonics can be difficult to measure due to low magnitude and signal to noise ratios (SNR): hence large excitation amplitudes are needed, which can further complicate the reliability of these methods as equipment nonlinearities can be generated. To overcome these issues, exciting at specific frequencies, known as local defect resonances (LDR), produce a much larger displacements at the damaged regions. However, estimation of LDR is time-consuming, complex and not an easily automated process.A coupled baseline-free linear and nonlinear ultrasonic imaging approach is proposed, using a Constructive Nonlinear Array Sweep excitation and an image subtraction method for identifying damage in layered materials. The signal sweep method uses a narrow band frequency excitation to increase the probability of detection of a LDR frequency. The novel imaging approach was employed using laser vibrometry measurements in various complex composite structures to assess barely visible impact damage, critical for the aircraft industry. The results showed better estimation of impact damage when compared to classical linear or nonlinear ultrasonic methods leading to improved reliability of aircraft inspections.

Real-time monitoring of low-velocity impact damage for composite structures with the omnidirection carbon nanotubes’ buckypaper sensors

A novel, omnidirectional, nanomaterial-based sensor technology which can provide wide area damage detection of composite structures was proposed in this work. The behaviors of the buckypaper sensors subjected to both tensile and low-velocity impact were investigated. The experimental results showed that the rectangle buckypaper sensor has a large range of sensing coefficients from 21.40 to 35.83 at different directions under tensile. However, the circular buckypaper sensor has a steady sensing coefficient of about 155.63. Thus, the circular buckypaper sensor as a kind of omnidirectional sensor was chosen to monitor the impact damage. The low-velocity impact damage of composite structures is characterized by the gauge factor of omnidirectional buckypaper sensors and the results of C-scanning. Omnidirectional buckypaper sensors’ electrical resistance increases with repeated impact loading; composite structure elastic deformation and damage evolution can be identified from resistance change. Experiment results show that structure monitoring based on the omnidirectional buckypaper sensor not only can detect small barely visible impact damage flaws and the damage evaluation of composite structures subjected to impact but also can determine the location of low-velocity impact damage through the analysis of results. Through comparison with C-scan, the results have preliminarily demonstrated that the omnidirectional carbon nanotubes’ buckypaper sensor can serve as an efficient tool for sensing the evolution of impact damage as well as serve structural health monitoring of composite structures.

Localizing impact damage of composite structures with modified RAPID algorithm and non-circular PZT arrays

Detecting and localizing impact damage of composite structures is one of the key expectations towards development of structural health monitoring (SHM) systems. In this paper, a method intended to meet these requirements is presented. The developed method is based on guided waves actuation in a monitored structure. One of the methods used for damage localization with guided waves is the RAPID/PRA algorithm. This algorithm is mostly used for circular arrays of PZT piezoelectric transducers. In the paper a modification of this approach, adopted to be used for more general geometries of PZT networks is presented. Its main improvement is that predicted location of damage is less biased by inhomogeneous distributions of sensing paths, i.e. lines connecting pairs of transducers of a network, than for RAPID algorithm. The developed method was verified experimentally on composite laminated specimens with introduced damage caused by low energy impact. Detailed description of the developed algorithm as well as the results of impact damage localization tests are delivered in the paper.

High-Velocity Impact Damage Behavior of Carbon/Epoxy Composite Laminates

In this paper, the impact damage behavior of USN-150B carbon/epoxy composite laminates subjected to high velocity impact was studied experimentally and numerically. Square composite laminates stacked with [45/0/-45/90]ns quasi-symmetric and [0/90]ns cross-ply stacking sequences and a conical shape projectile with steel core, copper skin and lead filler were considered. First high-velocity impact tests were conducted under various test conditions. Three tests were repeated under the same impact condition. Projectile velocity before and after penetration were measured by infrared ray sensors and magnetic sensors. High-speed camera shots and C-Scan images were also taken to measure the projectile velocities and to obtain the information on the damage shapes of the projectile and the laminate specimens. Next, the numerical simulation was performed using explicit finite element code LS-DYNA. Both the projectile and the composite laminate were modeled using three-dimensional solid elements. Residual velocity history of the impact projectile and the failure shape and extents of the laminates were predicted and systematically examined. The results of this study can provide the understanding on the penetration process of laminated composites during ballistic impact, as well as the damage amount and modes. These were thought to be utilized to predict the decrease of mechanical properties and also to help mitigate impact damage of composite structures.

Modelling and numerical simulations of vibrothermography for impact damage detection in composites structures

The paper investigates modelling aspects related to application of vibrothermography for detection of barely visible impact damage in composite structures. Low-velocity impact tests were performed to introduce multiple delaminations into carbon/epoxy composite plate. Damage severity was revealed using well-established non-destructive evaluation techniques. Vibrothermography was used subsequently to show good agreement with classical damage detection techniques. Following these experimental investigations, numerical simulations were performed to assess feasibility and sensitivity of vibrothermography for impact damage detection. Numerical results were validated using experimental data showing very good qualitative and encouraging quantitative agreement. The study demonstrates that virtual impact damage detection using vibrothermography can be performed as part of structural design to assess sensitivity of the method in real engineering applications. Copyright © 2012 John Wiley & Sons, Ltd.

Impact Damage Detection and Assessment in Composite Panels using Macro Fibre Composites Transducers

Structural health monitoring of composite components is vital to the use of these materials in aerospace applications. Being highly susceptible to impact damage, composite materials can sustain internal damage that is very difficult to detect externally. This paper explores the use of macro fibre composite (MFC) transducers not only to detect acoustic emission (AE) released during the impact event but also as pulse/receivers to further quantify damage. To assess the dual functionality of these transducers, two were adhered to a composite panel and subjected to a number of impacts. A commercial AE system recorded signals during the impact event. After each impact, signals were pulsed and received between the two transducers and a C-scan inspection of the panel was conducted to assess the extent of the damage. Post test analysis demonstrated a clear correlation between the AE signal energy and the increase in delamination size. Furthermore, a cross correlation technique was utilised to compare signals in the undamaged plate with those at varying damage levels. The results demonstrated a decreasing correlation of signals with increasing damage severity. The results have shown that MFC transducers offer a real possibility for identifying and sizing impact damage in composite structures.

Homogenised non-linear soft inclusion for simulation of impact damage in composite structures

This paper presents the development of a homogenised non-linear soft inclusion which captures the geometric and material non-linearity of impact damage zone loaded in tension and compression. The homogenised non-linear soft inclusion can present a conservative worst case damage zone or use experimental data to mimic the behaviour of a particular damage zone in a simple and computationally efficient way that can be used as a structural design tool for composite structures subjected to impact. The development of the non-linear soft inclusion, implemented in an ABAQUS/Explicit VUMAT, is presented at element and coupon level. The non-linear soft inclusion is validated against experimental coupon data and produces a conservative worst case estimate in all cases investigated.

Impact damage and repair of composite structures

AbstractThis paper gives an overview of the work carried out in a GARTEUR (Group for Aeronautical Research and Technology in Europe) program, under the chairmanship of the author, to develop and validate analytical and numerical methods to characterise real impact damage in composite structures, particularly those designed to sustain load in a postbuckled state, and to study the durability of bonded repairs. GARTEUR is an inter-governmental agreement between the seven European countries with the largest direct employment in the Aerospace industry, to mobilise scientific and technical knowledge between the member countries. A number of Action Groups have been launched, since GARTEUR’s inception in the early 1970s, to address specific technical issues of interest to the participating members. The research presented in this paper was performed under Action Group 28 with partners from ONERA, EADS-CCR (France), DLR, AIRBUS-Deutschland, EADS-M (Germany), CIRA (Italy), INTA (Spain), SICOMP, Saab, (Sweden), NLR (The Netherlands), QinetiQ, BAE Systems, Imperial College London and the University of Sheffield (United Kingdom). The Action Group tasks were divided into four Work Elements (WEs): WE1-Prediction and characterisation of impact damage, WE2-Postbuckling with delamination, WE3-Repair and WE4-Fatigue. This paper outlines the main developments and achievements within each Work Element.

Influence of delamination on impact damage in composite structures

This paper describes recent progress on materials modelling and numerical simulation of fibre reinforced composite shell structures subjected to high velocity impacts. A continuum damage mechanics (CDM) model for fabric reinforced composites is applied to model both in-ply damage and delamination failure during impact loading. The CDM model has been implemented in a commercial explicit finite element (FE) code in which a laminate is modelled by stacked shell elements with tied interfaces. Delamination failure is modelled by allowing the interfaces to damage and fracture when a delamination failure energy criterion is reached. The code is applied to predict the response of composite shells subjected to gas gun impact tests by a steel impactor under a range of test conditions. A comparison of structural response and failure modes from numerical simulations and impact tests shows good agreement for the prediction of delamination damage and shell penetration at higher impact energies.