Damage In Composite Materials Research Articles

Nonlinear ultrasonic imaging of damage in composite materials using a higher harmonic and modulated multi-path reciprocal method

Structural health monitoring has become an important factor in the assessment of defects/damage in material components. Ultrasonic methods generally incorporate a sparse array of sensors/transducers as they provide a low number of piezoelectric sensors per area, thus providing savings with regard to system costs and weight. Many structural health monitoring techniques rely on linear ultrasonic effects such as reflections, amplitude changes, time of arrival and wave scattering effects which rely on precise baseline measurements. In this work, a nonlinear ultrasonic method based on a sparse array of surface-bonded ultrasonic transducers was used to evaluate the second harmonic and modulated elastic responses from a damaged medium. A complex composite stiffened panel with barely visible impact damage was evaluated. The points closest to damage are found on the paths between transmitter–receiver pairs through a reciprocal relationship of nonlinear elastic parameters and a statistical approach was used to select a cloud of points so that a two-dimensional image of the damaged region is created. Experimental results revealed that the second-order nonlinear parameter provided accurate damage localisation and imaging and the use of modulation bands further improved imaging accuracy and damage localisation. The maximum error between the calculated and the real damage area centres was only 1.3 mm. The proposed nonlinear elastic multi-path imaging technique based on higher harmonic generation and modulation coupled with a statistical approach provides damage detection without a priori knowledge of the material characteristics, which is in direct contrast to conventional linear ultrasonic methods which rely on precise measurement of elastic wave effects both before and after the initiation of damage.

Damage sensing and mechanical properties of a laminate composite material containing MWCNTs during low-velocity impact

In this work, we present a new criterion, unlike other attempts, to evaluate and quantify the degree of damage of composite material when it subjected to a sudden impact load. Our criterion exploits the high intrinsic electrical conductivity property of the Multi-walled carbon nanotubes (MWCNTs) after dispersing different concentrations of them (0, 0.5, 1.0, 1.5 and 2.0 %) in the epoxy matrix of a glass fibre composite. Following this goal, the low-velocity impact and flexural after impact (FAI) tests on the MWCNTs-glass epoxy (i.e. MWCNTs-GF) nanocomposite were evaluated. At the same time, the changes in its electrical resistance were measured. The results showed that the properties of the self-sensing composites were significantly affected by impact energy. The damage after impact causes an increase in the electrical resistance of the MWCNTs-GF nanocomposite and increases with increased impact energy. In addition, the samples containing a high concentration of MWCNTs showed lower damage sensitivity under all impact energies levels as compared with the samples contain a lower MWCNTs concentration. Therefore, the results presented in this work have shown that it is possible to associate the change in electrical resistance of the MWCNTs-GF nanocomposite with the degree of damages caused by impact load.

Material model for simulation of progressive damage of composite materials using 3D Puck failure criterion

This paper describes the proposal and implementation of a material model in the commercial software Abaqus. The material model is capable of simulating progressive damage of composite materials. Progressive damage simulation is based on a stiffness matrix degradation and the 3D Puck failure criterion. Statistics of the input material parameters are simulated. The predictive capability of the material model was tested by a comparison of the numerical simulations to the tensile test of a laminate.

Active Thermal Testing of Impact Damage in 3D-Printed Composite Materials

Using the method of ultrasonic infrared thermography, it has been shown that 3D printing technology prevents cracking of the edges of access holes in composites during their machining. It is expedient to evaluate the impact strength of composites by the relative change in the thermophysical characteristics in the zones of impact damage, based on the assumption that a higher impact energy leads to the more developed defects and, accordingly, to the greater relative changes in thermal inertia and thermal diffusivity. The impact resistances of Kevlar and carbon fiber composite specimens, as well as their hybrid, have been compared. The highest impact resistance was demonstrated by a hybrid sample of Kevlar and carbon fiber composite, in which the change in thermal inertia and thermal diffusivity after an impact with an energy of 15 J was 4 and 8%, respectively, compared with 10 and 9% for the CFRP and 15 and 11% for the Kevlar.

Influence of the Arrangement of Mechanical Fasteners on the Static Strength and Fatigue Life of Hybrid Joints.

This paper presents the results of experimental research and numerical calculations regarding the static strength and fatigue life of hybrid joints. In the experiments, specimens built as single-lap adhesive–mechanical joints (hybrid joints) were tested. In a two-stage process of the failure of the hybrid joints, the adhesive joint was damaged first. Therefore, it was assumed that the assembly of fasteners closer to the edge of the overlap (beyond the ranges recommended for mechanical joints) limits the negative impact of the peeling phenomenon on the strength and performance properties of hybrid joints. The specimens used in the experiments were prepared from composite elements (i.e., carbon fiber-reinforced polymer (CFRP)), as well as from the aluminum alloy 2024T4. Because the detection of fatigue damage in composite materials is a complex problem, computed tomography was used to evaluate the degradation of the composite material. Experimental and numerical comparative analyses of the static strength and fatigue life of hybrid joints with adhesive and mechanical joints confirmed the assumptions made.

Strain-based theoretical fatigue damage model of woven glass-epoxy fabric composite material

Strain-based theoretical fatigue damage model is developed by utilizing internal variable theory of thermodynamics of continuum damage mechanics approach related to woven glass fabric composite material. Based on the laminate theory of composite engineering, a phenomenological fatigue damage model under decomposition of Helmholtz Free Energy (HFE) is carried out to demonstrate the stiffness degradation, decrease in the Young's modulus of elasticity), damage development corresponding to normalized life, its accumulation, strain evolution, loss in strength and stress reduction. Investigational data from the tensile fatigue test from different literatures were employed to verify the model and the results show that model can describe strain-based theoretical fatigue damage of woven glass fabric epoxy composite material.

Modelling Low Velocity Impact on Structural Composites

Insertion of composite materials into next generation vehicles requires an understanding of their response over the range of relevant loading conditions encountered in the field. This paper focuses on modelling the low velocity, high energy impact on structural composite materials. An experimental procedure, using the four quadrant impact methodology, is presented to characterize composite material response and damage during these events. A model is developed and verified (within 9% for peak force and peak deflection) to predict the composite response to low velocity, high energy events. The model is then exercised to gain a deeper understanding about how mechanical and damage properties influence the structural response of the composite. The model was most sensitive to delamination failure stress, which changed the damage area by a factor of 8 over the range of values examined. Through investigating the effect of boundary conditions, a 67% increase in peak force was found by switching from simply supported to clamped. This work demonstrates a first level analysis and model to predict the composite response to low velocity, high energy loading conditions.

Towards intelligent CFRP composite machining: Surface analysis methods and statistical data analysis of machined fibre laminate surfaces

Many carbon fibre reinforced polymer composite parts need to be edged trimmed before use to ensure both geometry and mechanical performance of the part edge matches the design intent. Measurement and control of machining induced surface damage of composite material is key to ensuring the part retains its strength and fatigue properties. Typically, the overall surface roughness of the machined face is taken to be an indicator of the amount of damage to the surface, and it is important that the measurement and prediction of surface roughness is completed reliably. It is known that the surface damage is heavily dependent on the fibre orientation of the composite and cutting tool edge condition. This research has developed a new ply-by-ply surface roughness measurement methods using optical focus variation surface analysis and image segmentation for calculating areal surface roughness parameters of a machined carbon fibre composite laminate. Machining experiments have been completed using a polycrystalline diamond edge trimming tool at increasing levels of cutting edge radius. Optical surface measurement and µ-CT scanning have been used to assess machining induced surface and sub-surface defects on individual fibre orientations. Statistical analysis has been used to assess the significance of machining parameters on Sa (arithmetic mean height of area) and Sv (areal magnitude of maximum valley depth) areal roughness parameters, on both overall roughness and ply-by-ply fibre orientations. Empirical models have been developed to predict surface roughness parameters using statistical methods. It has been shown that cutting edge degradation, fibre orientation and feed rate will significantly affect the cutting mechanism, machining induced surface defects and surface roughness parameters.

Three-dimensional damage quantification of low velocity impact damage in thin composite plates using phased-array ultrasound

The evaluation of internal damage in multilayered composite materials is of great importance for high reliability-demanding applications, and remains a challenge due to the complex failure modes and mechanism of composite materials. This study presents a volumetric method of three-dimensional size quantification and prediction for low velocity impact damage in thin composite plates using phased-array ultrasound. A set of low velocity impact damages are induced in thin carbon fiber/epoxy resin matrix composite plates using quasi-static indentation tests. A volumetric reconstruction method is proposed to reconstruct a three-dimensional volume from the raw data, allowing for direct damage identification, localization, and quantification. Using the echo amplitude feature of the reconstructed volume, the 6 dB-drop method is employed to characterize the damage size in terms of volumes and areas. An impact size prediction model is established to correlate the impact energy and the damage volume/area. Comparisons are made between the microscopy measurement of the damage cross-section and results obtained using the developed method.

Crack damage identification of a thick composite sandwich structure based on Gaussian Processes classification

Structural damage of composite materials used in aeronautics and aerospace has attracted increasing attention. Efficient and reliable Structural Health Monitoring (SHM) systems are required to provide a probabilistic interpretation of diagnostics. In this study, crack damage identification of a thick composite sandwich structure based on Gaussian Processes (GP) classification is reported by numerical simulations. The goal of the study is to obtain a data-driven probabilistic interpretation of damage detection. The investigation is carried out based on healthy and damaged status of a sandwich panel with a honeycomb core modelled in ANSYS. Instantaneous signals with different frequencies are applied to the structure and finite element analysis is performed to obtain vibration responses in both statuses. Features extracted by Discrete Wavelet Transform (DWT) are used to train and test the GP model to assess the health status of the structure. Impacts of mother wavelet in DWT, likelihood function and inference method, as well as iteration numbers are investigated on the classification accuracy. The pertinence of sensors located at different positions is also investigated. This proposed method is effective for crack-type damage detection in the studied composite sandwich structure. It is expected to be suitable for damage detection of more complex structures.

Investigation of thermal degradation and decomposition of both pristine and damaged carbon/epoxy samples with thermal history

Although the investigation of thermal damage in composite materials has increased, the short-term thermal degradation and decomposition mechanisms in a non-oxidative environment have not been well established, as the reported thermal damage results were heavily influenced by the analyses of delicately collected volatiles and the presence of oxygen. The aim of this study, using scanning electron microscopy, Fourier transform infrared spectroscopy and thermogravimetry, along with kinetics, was to develop a post-mortem solid residual-based microscopic characterising methodology for thermal damage in carbon/epoxy composites induced by simulated lightning strikes. Fibre tufting, resin sublimation and discolouration were found to represent the severity of damage in three damage zones. Whilst fibre tufting was caused not by thermal sublimation but by returning shock waves, resin sublimation and discolouration were dominated by Joule heating. The carbon concentration in the damaged zones was found to radially decrease outwards and vice versa with oxygen. Decomposition started from epoxy dehydration, accompanied by discolouration and formation of water and carbonyl, and released chemical compounds like 2-propenal, acetylene and carbonyl-containing aromatic ether. The successful identification of these unique heating-formed functional groups provided the confirmation of the proposed methodology.

Stability and load-carrying capacity of short composite Z-profiles under eccentric compression

This study investigates the problems of non-linear stability and load-carrying capacity of thin-walled composite Z-profiles under compression. The primary objective of the study was to determine the effect of eccentric load and ply-stacking sequence on the post-critical behaviour and load-carrying capacity of the analysed structures. Experimental and numerical methods were employed, which made it possible to describe structural performance of the analysed composite structures over the full range of loading, including the description of composite material damage. Experimental tests were performed on the physical models of CFRP profiles made by autoclaving. The real structures were subjected to compression until a complete loss of their load-carrying capacity. Simultaneously, numerical calculations were performed by the finite element method. The progressive damage model was used to evaluate the degree of damage in the composite material. The developed numerical models were validated with the results of experimental tests.

A Novel Infrared Thermography Sensing Approach for Rapid, Quantitative Assessment of Damage in Aircraft Composites.

The current necessity of the scientific and industrial community, for reduction of aircraft maintenance cost and duration, prioritizes the need for development of innovative nondestructive techniques enabling fast and reliable defect detection on aircraft fuselage and wing skin parts. Herein, a new low-cost thermographic strategy, termed Pulsed Phase-Informed Lock-in Thermography, operating on the synergy of two independent, active infrared thermography techniques, is reported for the fast and quantitative assessment of superficial and subsurface damage in aircraft-grade composite materials. The two-step approach relies on the fast, initial qualitative assessment, by Pulsed Phase Thermography, of defect location and the identification of the optimal material-intrinsic frequency, over which lock-in thermography is subsequently applied for the quantification of the damage’s dilatational characteristics. A state-of-the-art ultra-compact infrared thermography module envisioned to form part of a fully-automated autonomous nondestructive testing inspection solution for aircraft was conceived, developed, and tested on aircraft-grade composite specimens with impact damages induced at variable energy levels and on a full-scale aircraft fuselage skin composite panel. The latter task was performed in semi-automated mode with the infrared thermography module mounted on the prototype autonomous vortex robot platform. The timescale requirement for a full assessment of damage(s) within the sensor’s field of view is of the order of 60 s which, in combination with the high precision of the methodology, unfolds unprecedented potential towards the reduction in duration and costs of tactical aircraft maintenance, optimization of efficiency and minimization of accidents.

Stability and Load-Carrying Capacity of Thin-Walled FRP Composite Z-Profiles under Eccentric Compression.

This study investigates the effect of eccentric compressive load on the stability, critical states and load-carrying capacity of thin-walled composite Z-profiles. Short thin-walled columns made of carbon fiber-reinforced plastic composite material fabricated by the autoclave technique are examined. In experimental tests, the thin-walled structures were compressed until a loss of their load-carrying capacity was obtained. The test parameters were measured to describe the structure’s behavior, including the phenomenon of composite material failure. The post-critical load-displacement equilibrium paths and the acoustic emission signal enabling analysis of the composite material condition during the loading process were measured. The scope of the study also included performing numerical simulations by finite element method to solve the problem of non-linear stability and to describe the phenomenon of composite material damage based on the progressive failure model. The obtained numerical results showed a good agreement with the experimental characteristics of real structures. The numerical results are compared with the experimental findings to validate the developed numerical model.

A reduced-order modelling for real-time identification of damages in multi-layered composite materials

This work is focused on the detection of interlayer damages in multi-layer composite materials, which use the inverse analysis approach based on the reduced-order models (ROMs). Direct problems including heat conduction through the solid and heat radiation to the space are solved by finite element method (FEM), and the solutions of transient nonlinear analysis are collected to build the snapshot data matrix. The Proper Orthogonal Decomposition (POD) method coupled with two surrogate models are utilized to construct a low dimensional model which can be used to rebuild physical fields. By minimizing the discrepancy of measured and simulated temperatures using ROMs, the optimal parameter values related to damages can be estimated efficiently. Results prove that an excellent estimation on the position of damages can be obtained in the multi-layer materials. The computational efficiency and solution accuracy of two ROMs are compared, which proves that the POD-Kriging method has faster convergence speed than the POD-RBF method, with the same fitness value. Moreover, noise analysis validates that two ROMs have similar stability, whereas the POD-Kriging method is easier to achieve convergence in this type of problems.

3D micromechanical progressive failure simulation for fiber-reinforced composites

In this paper, the recently developed two-dimensional (2D) phase field model is further applied to study micromechanical progressive failure process in three-dimensional (3D) fiber-reinforced composites, exploring deeper insight into the intrinsic failure mechanisms caused by the sophisticated microstructures. Although more challenging, a 3D fiber-reinforced composite modelling is desired since it reveals more clearly physical progressive failure process than a 2D model. In this setting, a phase field model based modelling framework is thus adopted to capture the complicated progressive failure mechanisms in 3D composites, in particular, fiber/matrix interface debonding and kinking, matrix cracks propagation, and delamination between adjacent plies. All kinds of cracks are modelled based on cohesive zone model to obtain a more reliable prediction. This significant development of 3D analysis is a generalization of our previous 2D work, its implementation is however completely different as enhanced by parallel computing in terms of ABAQUS user-defined subroutine, which may target the real applications. Two new energy split schemes are proposed for the modelling of progressive damage in composite materials. Numerical verification and validation are given through the modelling of a unidirectional ply and of an angle ply laminate. The damage initiation, propagation, and interaction captured by the developed method are analyzed, exploring the complicated progressive failure mechanisms of fiber-reinforced composites.

A Real-Time NDT Method Based on FBG Sensor Spectrums to Detect the Transverse Matrix Cracks in Composite Materials

Transverse matrix cracking is probably the first and most dominant mode of damage in composite materials. In this paper, a real-time Non-Destructive Testing (NDT) method was proposed to detect the transverse cracks by Fiber Bragg Grating (FBG) sensors. The experiment of observing the crack growth and recording FBG’s reflection spectrums as well as Finite Element Method (FEM) methods were conducted to help verifying the regularities of the reflection spectrum distortions in the cracking angle-ply [0n/ ± θm/0n] laminates. The results showed that the shape of the spectrum was highly correlated to the crack density of composites. From undamaged state to high crack density conditions, the shape of the spectrum experienced an ‘original-splitting-regressive’ process. Under the assumed ideal periodic strain distribution conditions, the peak width w of the reconstructed spectrums and crack density $$\rho$$ showed very good linearity which provided the possibility for quantitative detection of the crack density of composites.

Composite Material Damage Processes

Composite materials are in use in the shipbuilding industry for a long period of time. Composites appear in vast number of fibre – matrix combinations and can be produced with several different production processes. Due to the specific nature of the composite material structure, the selection of the production process and the limitations in the quality control procedures, composite materials will always be subject to defects and imperfections which may, under certain circumstances, lead to the appearance and propagation of cracks. The size and the shape of the crack, the load type and the stress field in the material surrounding the crack will be crucial for crack growth and crack propagation. This paper reviews the composite material damage processes especially relevant for shipbuilding. The basic principles of composite material fracture mechanics are briefly explained, and finally, mechanisms responsible for the development of damage and fracture of composite materials are presented. This paper has emerged from the need to summarize information about composite material fracture and failure mechanisms and modes relevant for the shipbuilding industry.

Data-Driven Approaches for Characterization of Delamination Damage in Composite Materials

Composite materials have been widely used in the aerospace industry and are critical for safe operations. However, the delamination, caused by cyclic loads and corrosive service environment, poses a serious threat to the structural integrity of composite laminates. The acoustic emission technique has been adopted to assess the structural integrity by characterizing damage location, type, and size. This article proposes and compares data-driven prognostic methods to quantify the delamination area efficiently and accurately. To address the problem of insufficient inspection data, the prediction model adopts the path length across the delamination area as the target value. The delamination area can then be estimated with the predicted path length based on formulated geometric relationships. This solution will augment the model training datasets, and consequently, avoid the overfitting problem during the training process. Experimental results on composite coupons demonstrate that the proposed ensemble learning-based model outperforms other state-of-the-art methods in terms of prediction accuracy and efficiency.

Degradation and Damage of Composite Materials in Marine Environment

IIn spite the fact that composite materials have been in use in the maritime sector for over a half of century, classification societies regulations tend to limit the usage of composites at the larger scale. One of the reasons for such strict class rules is a lack of comprehensive analytical and numerical models representing the behaviour of composites in the sea environment. Understanding the process of degradation and damage of composite materials assisted by sea environment a crucial step in building such a model. This paper aims to give a critical review of the research advancements in assessments of the sea environment influence on the degradation of mechanical properties of composites with a special emphasis on developed models of processes containing water and moisture entering composite inner structure. The list of major references in the last five years is given and suggestions for future research are discussed.