Damage In Laminates Research Articles

Experimental analysis and simulation of low-velocity impact damage of composite laminates

The capability to accurately analyse the response of multi-directional composite laminates during impact events is of high importance for the design of lightweight aircraft structures. In this work, both experimental and numerical analyses are performed covering a large design-space of laminates for all aspects from on-set of damage and barely visible impact damage up to clearly visible impact damage and full penetration of the laminates. The impact tests are simulated using a sophisticated three-dimensional continuum damage model, combined with an automated meso-scale model generation algorithm for ply-by-ply, material/fibre-aligned meshing of laminated composite coupons. To assess the accuracy of the predictions, an extensive validation test program of several configurations and impact energies has been performed, thus demonstrating that the simulations are capable of accurately predicting the damage and failure mechanisms under low-velocity impact loading. Not only the evolution of impact loads and energy dissipated are numerically analysed, but the competition of the dominant failure mechanisms from low impact energy and full penetration cases are also macroscopically replicated.

Impact Damage Detection Using Chirp Ultrasonic Guided Waves for Development of Health Monitoring System for CFRP Mobility Structures.

When impact damage occurs in carbon fiber-reinforced plastic (CFRP) structures, it is barely visible but may cause significant degradation in the mechanical properties of the structure. Hence, a structural health monitoring (SHM) system that can be installed in CFRP mobility structures and is sensitive to impact damage is needed. In this study, we attempted to establish an SHM system based on ultrasonic guided waves, which are generated by inputting a broadband chirp signal into a film-like piezoelectric actuator. The relationship between impact damage size and maximum time-of-flight (ToF) delay was investigated for three types of CFRP plates: woven, non-woven, and hybrid laminates. As a result, it was found that the maximum ToF delay increased linearly with an increase in the damage size for all CFRP laminates. Moreover, the amplitude of the A0 mode was found to be significantly affected by the damage length in the wave propagation direction. Thus, this SHM method using chirp ultrasonic waves can quantitatively evaluate the size and extent of the impact damage in CFRP laminates.

Effect of drilling-induced defects on progressive damage of open-hole composite laminates under compression

In this study, the effect of drilling-induced defects on open-hole compression progressive failure and buckling and post-buckling behavior of composite laminates was investigated numerically. First, drilling-induced defects were identified from X-ray computed tomography data using image processing software, from which delamination and fiber pull-out defects were quantified on the specimen. These defects were then incorporated into finite element mesh, and numerical analyses simulating ASTM open-hole compression tests were performed in Abaqus. Hashin failure criterion and energy-based linear softening law were employed to determine in-plane failure initiation and evolution. Interlaminar failure was accounted for by cohesive zone modelling inserting cohesive elements at delaminated interfaces. The numerical model with defects was validated against experimental results and very good agreement was obtained. Next, simulations were performed on both defect-free and defective open-hole composite laminate models to study the effect of drilling-induced defects on compressive strength and progressive failure behavior. Finally, analyses were performed with buckling deformation allowed and the effect of buckling and post-buckling on compression failure behavior was investigated.

Compressive damage and heat release of composite laminate with circular prefabricated defect, part II-numerical method

In this paper, the compressive damage and its corresponding heat dissipation of carbon fiber reinforced polymer (CFRP) composite laminates with prefabricated defects are investigated by means of the numerical method. First, the relationship between the temperature change and the damage of laminate is derived. Second, both the damage evolution and the corresponding energy dissipation of composite laminates with the prefabricated interlaminate defect are simulated. The numerical results are consistent with the experimental results. Finally, influences of the circular prefabricated defect’s size are discussed by numerical models. These results play important roles in analyzing and evaluating damage of the composite laminates, which is significant for the damage detection and damage thermo-response analysis in engineering applications of composites.

Probabilistic ultrasound C-scan imaging of barely visible impact damage in CFRP laminates

Ultrasonic pulse-echo C-scan imaging is a widespread method for detecting and characterizing defects in fiber reinforced polymer composites. However, the accurate assessment of a complex distributed damage cluster, like barely visible impact damage, in multi-layer and heterogeneous composites is not straightforward. For reliably estimating the remaining load carrying capacity and/or remaining useful lifetime of a damaged composite, a proper and complete damage assessment is of utmost importance.In this paper, a statistical time-energy gating approach is proposed in view of obtaining improved ultrasonic pulse-echo imaging of impacted composites. The majority of virgin A-scan signals are first clustered by analyzing their back-wall echoes. Next, using the principle of maximum likelihood, a Rice distribution is matched to the instantaneous amplitude in order to estimate the natural variability in the local energy of the virgin response signals. The resulting time-varying reliability interval provides an effective means to identify signals coming from defects or inhomogeneities, and as such to robustly assess defect parameters. The proposed probabilistic imaging procedure is demonstrated on various carbon fiber reinforced polymer laminates with barely visible impact damage. The obtained results are benchmarked by conventional ultrasonic C-scan imaging in through-transmission mode as well as in pulse-echo mode using the classical time gating approach. In contrast to the classical time gate method, the proposed statistical time-energy gating procedure successfully extracts and quantifies the full extent of the complex impact damage cluster. Further, the good noise resistance of the proposed probabilistic imaging method is demonstrated for a wide range of signal-to-noise ratios.

Effect of hole interaction in the initiation of damage in laminates

In open-hole laminates, when a second hole is made, the stresses and the damage evolution are modified. The main failure mechanisms in laminates subjected to compressive loads are fibre microbuckling and matrix breakage. In this work, the influence of the presence of a second hole in these mechanisms is analysed for two distances between holes. The Serial/Parallel Mixing Theory was used to model the elastic behaviour of the material and a Continuum Damage Model to represent the fibre microbuckling and matrix failure. The equations were implemented in a FEM code developed by the authors. The proposed model has been validated with results from the literature for a laminate with two holes positioned transversally to the applied load. In this problem, it was shown that the growth of the fibre damage is different from that of the matrix.  The laminate is damaged mainly in the zone between the holes. When the distance between holes is reduced, both types of damage around the holes increase.

Experimental study of the appearance of the damage in composite laminates

The scale effect in Composite Materials, consisting in the delay in the first damage appearance as ply thickness decreases, is, at present, a phenomenon under study due to the use of ultrathin plies (whose thickness may reach 20 microns) in the most leading industries. Based on this, the appearance of the first damage is experimentally analysed in this work for [04,90n,04] conventional laminates where n=1,2,4,8,16,32. The objectives are to study damage mechanisms occurrence as the thickness of the weakest layer varies when subjecting the laminate to uniaxial tension and to check if the scale effect is present. Acustical techniques have been employed for damage detection and optical and electronical microscopy have been used for its observation. The conclusions lead to well-diferenced mechanisms of damage for the different thicknesses analysed: high thicknesses show unstable damage resulting in an explosive failure of the whole laminate, whereas low thicknesses show a more stable damage, it being less significant for the laminate integrity. In addition, an scale effect is detected, since the load at which the failure is generated decreases as the thickness increases. In future works the same analysis will be presented for ultrathin plies.

Fatigue Damage Evaluation of Quasi-Isotropic CFRP Laminates Based on Electrical Resistance Change and Cross-Sectional Observation

In recent years, the demand for CFRP is steadily growing because of its high specific strength. Since CFRP laminates show complex fracture modes, an accurate damage detection technique is anticipated to be developed. In this study, we aimed to detect the micro damage of CFRP laminates in the early stage of fatigue. We focused on the DC potential difference method because it is a convenient measurement one using the conductivity of CFRP. The potential difference decreased in the early stage of fatigue test and turned to increase at the very end of fatigue test. The cross-sectional observations on fatigued specimens confirmed that transverse crackings and interlaminar delaminations occurred during the descending process of the potential difference. The results of this study indicated the need to focus on the decrease of potential difference to detect the early stage of fatigue damage in CFRP laminates.

Significantly improving mechanical properties of epoxy resin-based carbon fiber-reinforced plastic composites via introducing oxazolidinone segments

Herein, the oxazolidinone (OX) segments have been successfully introduced into carbon fiber-reinforced plastic (CFRP) composites via the bulk polymerization of the isocyanate and epoxy (EP) groups to significantly promote the mechanical properties. Through the bulk polymerization, the OX segments in epoxy resin (OX epoxy resin) have been formed, verified via the Fourier transform infrared (FT-IR) spectroscopy. Afterward, in order to obtain the cured OX epoxies, the as-prepared OX epoxy resin has been mixed with the pure EP resin for further curing. Thermogravimetric analysis (TGA) and dynamic mechanical thermal analysis (DMTA) have shown that the cured OX epoxies have exhibited improved thermal resistance and higher Tg than the cured neat epoxies. After introducing OX segments, the flexural strength, flexural modulus, and impact strength of the cured OX epoxies have been promoted by 30.1%, 12.1%, and 82.9%, respectively. Moreover, the mechanical properties of CFRP composites, fabricated from OX epoxy resin and T1100 carbon fibers, have been improved as well and in particular, the flexural strength, flexural modulus, impact strength, interlaminar shear strength, and compressive strength after the impact of the composites have been improved by 25.9%, 8.4%, 23.6%, 35.5%, and 23.8%, respectively. Furthermore, the impact damage of the composite laminates has been analyzed via Ultrasonic C-scan.

Evaluation of Early Fatigue Damage of CFRP Laminates by DC Potential Difference Method

Evaluation of Early Fatigue Damage of CFRP Laminates by DC Potential Difference Method

Predicting the Delamination Mechanisms of Multidirectional Laminates Using the Energy Release Rate Obtained from AE Monitoring

This study investigates delamination damage mechanisms during the double cantilever beam standard test using the strain energy release rate. The acoustic emission parameter is used to replace the original calculation method of measuring crack length to predict delamination. For this purpose, 24-layer glass/epoxy multidirectional specimens with different layups, and interface orientations of 0°, 30°, 45°, and 60°, were fabricated based on ASTM D5528 (2013). Acoustic emission testing (AE) is used to detect the damage mechanism of composite multidirectional laminates (combined with microscopic real-time observation), and it is verified that the strain energy release rate can be used as a criterion for predicting delamination damage in composite materials. By comparing the AE results with the delamination expansion images observed by microvisualization in real time, it is found that the acoustic emission parameters can predict the damage of laminates earlier. Based on the data inversion of the acoustic emission parameters of the strain energy release rate, it is found that the strain energy release rate of the specimens with different fiber interface orientations is consistent with the original calculated results.

Predicting the Delamination Mechanisms of Multidirectional Laminates Using the Energy Release Rate Obtained from AE Monitoring

This study investigates delamination damage mechanisms during the double cantilever beam standard test using the strain energy release rate. The acoustic emission parameter is used to replace the original calculation method of measuring crack length to predict delamination. For this purpose, 24-layer glass/epoxy multidirectional specimens with different layups, and interface orientations of 0°, 30°, 45°, and 60°, were fabricated based on ASTM D5528 (2013). Acoustic emission testing (AE) is used to detect the damage mechanism of composite multidirectional laminates (combined with microscopic real-time observation), and it is verified that the strain energy release rate can be used as a criterion for predicting delamination damage in composite materials. By comparing the AE results with the delamination expansion images observed by microvisualization in real time, it is found that the acoustic emission parameters can predict the damage of laminates earlier. Based on the data inversion of the acoustic emission parameters of the strain energy release rate, it is found that the strain energy release rate of the specimens with different fiber interface orientations is consistent with the original calculated results.

Predicting the upper-bound of interlaminar impact damage in structural composites through a combined nanoindentation and computational mechanics technique

Low-energy/speed impact damage of composite laminates is still challenging to simulate due to difficulties in measuring some key material properties. The present study develops an integrated numerical and experimental method for predicting interlaminar impact damage. A nanoindentation technique for measuring the stiffness properties of composites at a small length scale (nanometers) is leveraged to determine damage to composite laminates due to fast (microsecond) projectile impact. Specifically, nanoindentation is employed to measure the contact stiffness of an indenter and aerospace carbon/epoxy IM7/977-3 laminates with four different stacking sequences. Then, through a technique that combines nanoindentation and computational mechanics, an equivalent impact force approach is proposed to predict the upper-bound of interlaminar impact damage at impact energy levels of 5 and 10 Joules. Drop-weight impact experiments are conducted to validate the prediction results. In practical applications, estimating the upper-bound of damage is important for conservative and efficient damage tolerance designs, especially for thick composite laminates.

Multiple Impact Damage in GLARE Laminates: Experiments and Simulations.

Experiments and finite element simulations for multiple impact were performed on GLARE 5-2/1 and aluminum 2024-T3. Experiments were conducted on aluminum 2024-T3 and GLARE 5-2/1 at diverse impact energies to produce BVID (barely visible impact damage) and CVID (clearly visible impact damage). The finite element model was developed for multiple impact analysis using ABAQUS software and was confirmed by comparing the finite element analysis outcomes with experimental results. The two- and three-dimensional failure criteria model was applied to predict multiple impact behavior such as load-time history, maximum deflection-impact energy history, and damage progression. In addition, a user subroutine VUMAT was created to represent a three-dimensional progressive failure and was linked with ABAQUS. FEM results showed good agreement with experimental data.

Experimental and Numerical Study on Long-Life and Progressive Damage in Laminates in Open-Hole Compression Tests

The strength theory design accuracy of carbon fiber reinforced plastics (CFRP) composite structures are integral to CFRP composite structures product weights along with production costs. In order to further improve the CFRP composite structure strength theory design accuracy in this paper, it developed an ATM/SIFT analysis method by combining strain invariant failure theory (SIFT) and Accelerated testing methodology (ATM). The developed implementation scheme of ATM/SIFT includes: three parts of accelerating material strength test, component strength main curve characterization, structures life prediction, among which the material life prediction is achieved by using Python code to customize user material subroutines (VUMAT), through secondary development on the ABAQUS platform. Based on the developed ATM/SIFT method, it analyzed the compressive gradual failure mechanism and life of IM600/TDE-85 composite laminated plates open pore structures. Finally, it predicted the long-term static load compression life of IM600/TDE-85 composite laminate open-hole structures by using ATM/SIFT method and compared this prediction results with test results.

Mechanical model and numerical simulation of composite laminates subjected to low velocity impact

A mechanical model of laminated plates subjected to low velocity impact is proposed in this paper. The model studies the attenuation of impactor velocity affected by air resistance, intra-laminar damage initiation based strain, intra-laminar damage evolution based on damage parameters and delamination damage. Based on this mechanical model and ABAQUS platform, the experiment of Shi is numerically simulated. It is found that the predicted contact force, energy absorption and delamination damage of laminates are in good agreement with the experimental results, indicating that the established mechanical model can be used to predict the mechanical response and damage characteristics of composite laminates subjected to low velocity impact.

Compressive damage and heat release of composite laminate with circular prefabricated defect, part I-experimental method

This paper focuses on the compressive delamination and its corresponding heat release of carbon fiber reinforced polymer (CFRP) composite laminates with circular delamination defects. First, the compressive process of the composite laminates with the prefabricated polytetrafluoroethylene film at the center between the outermost and sub-outermost layers is observed and recorded by CMOS (complementary metal-oxidesemiconductor) camera which is used for image acquisition in digital image correlation (DIC) and the infrared thermal imager. Second, both the deformation fields and the temperature fields before and after surface damage of laminates are analyzed. Finally, the influences of the circular prefabricated defect’s size on the strength and the energy dissipation are investigated. This work will provided an important experimental basis in evaluating the internal damage of laminate composite.

Effect of preload and shim types on the mechanical properties of composite-aluminium bolted joints

The influence of both preload and the presence of shim types on the mechanical properties of composite-aluminium single-bolt, single-lap joints were studied in this paper. The load-displacement curve and surface strain field of joints in different shim types and preloads were obtained through tensile experiments. A progressive damage model was established using the UMAT subroutine in ABAQUS. A hybrid failure criterion and a linear continuous degradation model were used to describe the progressive damage of composite laminates. The results show that for joints with no shim and for those with various types of shims, the tensile stiffness, peak load and initial damage load could be reduced when the preload is insufficient or too large. Compared with joints with no shims or with peelable fibreglass shims, joints with liquid shims required a larger preload to achieve the best mechanical properties. As the proportion of peelable fibreglass shim increased, the tensile stiffness and peak load continued to increase in joints with a mixed shim of liquid and peelable fiberglass shim. Shims can serve as tension bearings, but have little effect on the initiation and development of bearing failure.

On the importance of finite element mesh alignment along the fibre direction for modelling damage in fibre-reinforced polymer composite laminates

A finite element mesh, aligned along the fibre direction, has often been purported to yield more accurate results in the modelling of damage in composite structures constructed from unidirectional fibre reinforced polymer laminates. However, there has been a lack of a systematic assessment of this approach. This issue is addressed through modelling a selection of test cases; (i) end notched tension (ENT) coupons, (ii) simple tensile/compressive specimens, (iii) open-hole tension (OHT), (iv) low velocity impact (LVI) and (v) compression-after-impact (CAI) of laminated plates. In each case, two models were constructed, one using a uniform mesh, aligned with the global coordinate system, and independent of the fibre direction, and the other where each ply mesh was aligned along the fibre direction. In both cases the local material axes in each ply were correctly represented. Results show that a finite element mesh aligned along the fibre direction plays an important role in the prediction of damage, particularly in the presence of a crack. However, when matrix crack paths are not established a priori, or may not be the dominant damage mode, fibre mesh alignment is unnecessary. Rather, long-established approaches of refined meshes, robust damage models and well-defined material data and boundary conditions, are shown to be sufficient requirements.

Damage Detection of Cross-Plied CFRP Laminates Based on Rectangular Differential Pulse Eddy Current Sensors

ABSTRACT Carbon fiber reinforced polymer (CFRP) laminates are extensively used in aviation due to their excellent material properties. Damages resulted from manufacturing or usage seriously undermine the safety of aircraft structures. Therefore, the efficient detection of the damage in CFRP laminates is important. In this paper, three kinds of common damages including impact damage, crack damage and delamination damage are detected based on pulsed eddy current (PEC) with rectangular differential sensors. Firstly, to overcome shortcomings of traditional PEC sensors, an optimized rectangular differential sensor is proposed in the study. Then, a normalization method is introduced to process the differential signal obtained with the sensor. Finally, the relationship between the size of the damage and the normalized differential signal is investigated. The investigation results show that the rectangular differential sensor can detect the three types of damages. When a new damage object is detected, the measurement step of a reference signal is omitted. The peak value of the normalized differential signal increases with the increase in damage size. The new PEC method has been proved in the study.