Showing papers on "Composite laminates published in 2019"

Structural vibration-based classification and prediction of delamination in smart composite laminates using deep learning neural network

Abstract: This paper proposes a Convolutional Neural Network (CNN) based approach for the classification and prediction of various types of in-plane and through-the-thickness delamination in smart composite laminates using low-frequency structural vibration outputs. An electromechanically coupled mathematical model is developed for the healthy and delaminated smart composite laminates, and their structural vibration responses are obtained in the time domain. Short Time Fourier Transform (STFT) is employed to transform the transient responses into two-dimensional spectral frame representation. A convolutional neural network is incorporated to distinguish between the damaged and undamaged states, as well as various types of damage of the laminated composites, by automatically extracting discriminative features from the vibration-based spectrograms. The CNN showed a classification accuracy of 90.1% on one healthy and 12 delaminated cases. The study of the confusion matrix of CNN provided further insights into the physics of the problem. The predictive performance of a pre-trained CNN classifier was also evaluated on unseen cases of delamination, and physically consistent results were obtained.

Damage and failure mechanism of thin composite laminates under low-velocity impact and compression-after-impact loading conditions

Abstract: Impact resistance and damage tolerance are of great significance in the design of composite structures. This study investigated the damage and failure mechanism of thin composite laminates under low-velocity impact and compression-after-impact (CAI) loading conditions. Four levels of impact energy were included in the test matrix. Delamination induced by low-velocity impact was captured using ultrasonic C-scan, and a three-dimensional (3D) digital image correlation (DIC) system was employed to measure full-field displacement during the CAI tests. Infrared thermography was also used to online monitor the thermal field variation of the test specimen during the impact and CAI process. The cross sections of typical tested specimens were inspected using an optical microscope and a scanning electron microscope (SEM). A 3D damage model that considers both interlaminar and intralaminar damage was proposed to study the complex damage and failure mechanism. Excellent correlation was obtained between the experimental results and the numerical results. The experimental results obtained from various tests and the results from the numerical simulation were combined to provide a new and deep insight of damage evolution and failure mechanisms under low-velocity impact and CAI loading conditions.

Low velocity impact behavior of interlayer hybrid composite laminates with carbon/glass/basalt fibres

Abstract: This work investigates the effects of carbon/glass/basalt hybridization and fabric structure on the low velocity impact resistance of fibre reinforced plastic composites. Interply hybrid specimens used in the study were fabricated in a sandwich-like stacking sequence using a vacuum assisted resin infusion molding technique. Low velocity impact tests were carried out to study the effects of hybridization and fabric structure on the impact resistance of composite laminates. A continuum damage mechanical model was developed and validated for non-hybrid woven fabric laminates at different impact energy levels. Residual damage characteristics were identified using a 3D surface scanning system and an X-ray computed tomography (CT) method. On the basis of the experimental results, numerical simulation was also conducted to explore the damage mechanisms of the hybrid laminates. The study showed that: (a) hybrid laminates with carbon fibre as the core exhibited superior impact resistance for sandwich-like stacking sequence; (b) similar impact behaviors appeared for carbon laminates hybridised with either basalt or glass fibre; (c) for basalt fibre, weave fabric composite laminates exhibited better energy absorption capability and deformation resistance than cross-ply laminates reinforced in unidirectional fabrics.

Interfacially reinforced carbon fiber/epoxy composite laminates via in-situ synthesized graphitic carbon nitride (g-C3N4)

Abstract: Carbon fiber composite laminates were interfacially reinforced through in-situ synthesis of g-C3N4 on the carbon fibers. The introduced g-C3N4 greatly improved the roughness, functional groups and wettability on the carbon fiber surface and markedly enhanced the interfacial properties of composite laminates. The surface free energy of carbon fibers was increased by 67.81%. Interlaminar shear strength and interfacial shear strength of composite laminates were increased from 51.84 to 72.09 MPa and 44.62–73.41 MPa, respectively. The significantly enhanced interfacial properties enabled the mechanical performance of composite laminates to reach a superior state. Tensile strength and bending strength were increased by 19.54 and 10.51%, respectively. The total absorbed energy of impact experiment was also enhanced from 1.14 to 1.78 J. Meanwhile, dynamic mechanical properties and hydrothermal aging resistance were also ameliorated significantly. The improved interfacial properties and mechanical properties were ascribed to the increased mechanical interlocking, enhanced chemical bonding and ameliorated wettability created by g-C3N4.

Hygromechanical properties of 3D printed continuous carbon and glass fibre reinforced polyamide composite for outdoor structural applications

Abstract: The additive manufacturing of structural composites is a disruptive technology currently limited by its moderate mechanical properties. Continuous fibre reinforcements have recently been developed to create high performance composites and open up encouraging prospects. However, to increase their use, deeper understanding of the relationship between process and induced properties remains necessary. In addition, to apply these materials to engineering applications, it is of high importance to evaluate the effect of environmental conditions on their mechanical performances, particularly when moisture-sensitive polymer is used (PolyAmide PA for instance) which is currently lacking in the literature. This present article aims to investigate in more detail the relationship between the process, the mechanical behaviour and the induced properties of continuous carbon and glass fibres reinforced with a polyamide matrix manufactured using a commercial 3D printer. In addition, their hygromechanical behaviour linked to moisture effect is investigated through sorption, hygroexpansion and mechanical properties characterization on a wide range of relative humidity (10–98% Relative Humidity RH). The printing process induces an original microstructure with multiscale singularities (intra/inter beads porosity and filament loop). Longitudinal tensile performance shows that the reinforcing mechanism is typical of composite laminates for glass and carbon. However, the rather poor transverse properties are not well fitted by the Rule Of Mixture (ROM), thus underlining the specificity of the printing-induced microstructure and an anisotropic behaviour in the material. Non-negligible (5–6%) moisture uptake is observed at 98% RH, as well as orthotropic hygroscopic expansion of PA/carbon and PA/glass composites. The consequences of various moisture contents on mechanical properties are studied, showing a reduction of PA/carbon stiffness and strength of 25 and 18% in the longitudinal direction and 45 and 70% in the transverse direction. For PA/glass composites, we obtain a reduction in strength of 25% in the longitudinal direction, along with a 80% reduction of stiffness and 45% in strength in the transverse direction. A wetting/drying cycle underlines reversible phenomena in the longitudinal direction and mainly non-reversible degradation in the transverse direction.

Internal damage evaluation of composite structures using phased array ultrasonic technique: Impact damage assessment in CFRP and 3D printed reinforced composites

Abstract: The application of non-destructive techniques for the evaluation of internal damage in composite materials is a challenge due to their complexity. The aim of this study is to analyse the ability of ultrasonic technique to identify and evaluate manufacturing defects and internal damage in composite laminates. Artificial object inclusions of different shapes, sizes and materials were embedded in carbon fibre reinforced epoxy (CFRP) laminates. Comprehensive investigation of non-destructive evaluation using phased array ultrasonic testing to trace and characterize the embedded defects is presented. Phased array ultrasonic technique was able to precisely locate most of the artificial inclusion in the composite laminates. Nerveless, the shape and size of the inclusions were not accurately determined due to the high signal attenuation and distortion characteristics of the carbon fibre epoxy composite. Furthermore, a major concern affecting the efficient use of composite laminates is their vulnerability to low velocity impact damage. The dynamic behaviour of composite laminates is very complex as there are many concurrent phenomena during composite laminate failure under impact loading. Thus, the practicality of the previous results is demonstrated by the evaluation of impacted carbon fibre reinforced epoxy laminates using ultrasonic testing. The influence of laminate thickness and impact energy on impact damage is also investigated. The performance of phased array ultrasonic testing for the inspection of composite laminates with barely visible impact damage is evaluated. In addition, fibre-reinforced thermoplastic composites are becoming more significant in industrial applications due to their excellent mechanical performance and potential recycling. In this study, impact damage in 3D printed continuous glass fibre reinforced thermoplastic laminates is also analysed. Phased array ultrasonic testing is performed and the amount of damage is quantified in terms of delaminated area.

An experimental and numerical investigation on low-velocity impact damage and compression-after-impact behavior of composite laminates

Abstract: This study investigated the damage and failure mechanism of composite laminates under low-velocity impact and compression-after-impact (CAI) loading conditions by numerical and experimental methods. Ultrasonic C-scan, DIC and SEM methods were combined to give a new and deep insight of damage evolution and failure mechanisms in composite laminates. A novel three-dimensional damage model based on continuum damage mechanics was developed to investigate the impact and CAI behavior with consideration of both interlaminar delamination damage and intralaminar damage. The maximum-strain failure criterion and an improved three-dimensional Puck criterion, which was physically-based, were employed to capture the initiation of fiber and matrix damage respectively and a bi-linear damage constitutive relation was used for characterization of damage evolution. The interlaminar delamination damage was simulated by the interfacial cohesive behavior. Good correlation between numerical and experimental results demonstrated the effectiveness and rationality of the proposed numerical model. The effects of impact energy level and multiple impacts were discussed.

Flexural performance and cost efficiency of carbon/basalt/glass hybrid FRP composite laminates

Abstract: This study investigates the interply hybridization of carbon fibre reinforced polymer (CFRP) composite laminate to improve the flexural performance and cost efficiency. Carbon layers were replaced partially by basalt and/or glass fibres to explore the effects of hybrid ratio and stacking sequence on the flexural behavior and material usage. Hybrid laminates were manufactured by vacuum assisted resin transfer molding (VARTM) process. Three-point bending tests were carried out to characterize the flexural properties and failure mechanisms of the hybrid composite laminates. The fracture surfaces were examined by scanning electron microscopy (SEM). The results showed that flexural strength and modulus of the hybrid laminates decreased with the increase in the hybrid ratio of basalt fibres ranging from 0 to 50%; however negligible effects on flexural properties were observed when hybrid ratio increased further up to 75%. For the hybrid samples, a higher flexural modulus can be obtained by placing carbon layers on the both tensile and compressive sides symmetrically; and a higher flexural strength can be achieved by placing basalt or glass fibre through a sandwich-like stacking sequence with a hybrid ratio of 50%. The finite element modeling and classic laminate theory (CLT) analysis were also conducted through validation against the experimental results, which enabled to reveal the details of strain, damage and fracture under bending. The study exhibited a better material efficiency for glass/carbon hybrid laminates in terms of strength/cost and modulus/cost ratio; and the benefits of such cost efficiency of hybridization were discussed in depth for potential engineering applications.

A new model for fatigue life prediction based on infrared thermography and degradation process for CFRP composite laminates

Abstract: In this paper, a new fatigue life prediction methodology is proposed by combining stiffness degradation and temperature variation measured by InfraRed Thermographic (IRT) camera. Firstly, the improved thermographic method is used to determine the fatigue limit by using the data of stabilized temperature rising. Following this, a two-parameter model is proposed to characterize the stiffness degradation of CFRP laminates with the increase of cycle numbers. After the calibration parameters and the calculation of the normalized failure threshold stiffness, the whole S - N curve can be obtained in a very short time. The proposed model is applied to both the experimental data of triaxially braided CFRP laminates from literature and those of unidirectional CFRP laminates obtained from our fatigue tests. Results show that predicted S - N curves have a good agreement with traditional tests. The principal interests of this model could be listed as follows: (i) it is a more general criterion applicable to different materials; (ii) it has more physical senses; (iii) it allows the determination of the total S-N curve for composite materials in a short time.

An investigation on composite laminates including shear thickening fluid under stab condition

Abstract: Shear thickening fluids have been extensively utilized in composite laminate structures to enhance the impact resistance in the last decade. Despite the contribution of shear thickening fluids to t...

Influence of impactor shape on low-velocity impact behavior of fiber metal laminates combined numerical and experimental approaches

Abstract: Fiber metal laminates (FMLs) are fabricated with composite laminates and metallic alloys. This paper systematically investigates the influence of impactor shape on the low-velocity impact response of FMLs by combining the experimental and numerical methods. A series of impact experiments are conducted to study the low-velocity impact behavior of different FMLs under various impactor shapes and impact energies. The commercially available digital image correlation measuring system is applied to scan the exterior deformations of FMLs after impact. Subsequently, a progressive damage model based on the Hashin and Yeh delamination failure criteria is employed to characterize the damage evolution and failure mechanisms of composite materials through a user defined subroutine (VUMAT) in ABAQUS/Explicit. The study's results demonstrate that the impact behavior of FMLs is strongly dependent on impactor shape, impact energy and metal layer distribution. For the complete penetration case, the smooth perforation resulted from shear effect occurs in FMLs impacted by blunter impactor, while the petaloid cracks caused by tensile tearing damage appears for sharper impactor case in the process of penetration. The predicted results for different impact cases show a good comparison with experimental results in terms of impact loads, total absorbed energies and damage morphologies, which indicate that the numerical simulation could be a helpful tool to evaluate the impact response of FMLs.

Simulation of the Mechanical Response of Thin-Ply Composites: From Computational Micro-Mechanics to Structural Analysis

Abstract: This paper provides an overview of the current approaches to predict damage and failure of composite laminates at the micro-(constituent), meso-(ply), and macro-(structural) levels, and their application to understand the underlying physical phenomena that govern the mechanical response of thin-ply composites In this context, computational micro-mechanics is used in the analysis of ply thickness effects, with focus on the prediction of in-situ strengths At the mesoscale, to account for ply thickness effects, theoretical results are presented related with the implementation of failure criteria that account for the in-situ strengths Finally, at the structural level, analytical and computational fracture approaches are proposed to predict the strength of composite structures made of thin plies While computational mechanics models at the lower (micro- and meso-) length-scales already show a sufficient level of maturity, the strength prediction of thin-ply composite structures subjected to complex loading scenarios is still a challenge The former (micro- and meso-models) provide already interesting bases for in-silico material design and virtual testing procedures, with most of current and future research focused on reducing the computational cost of such strategies In the latter (structural level), analytical Finite Fracture Mechanics models—when closed-form solutions can be used, or the phase field approach to brittle fracture seem to be the most promising techniques to predict structural failure of thin-ply composite structures

The effect of voids on matrix cracking in composite laminates as revealed by combined computations at the micro- and meso-scales

Abstract: Voids are an important type of manufacturing defects in fiber-reinforced composites. Matrix cracking is sensitive to the presence of voids. Although this cracking occurs at the ply scale, its dynamics is strongly affected by ply’s microstructure, in particular, fiber distribution, fiber content, and the presence of voids. In the current study, a computational approach to simulate the influence of intra-laminar voids on cracking in composite laminates is developed. The approach combines finite element models of two scales: a micro-scale model, where the fibers and voids are modeled explicitly, and a meso-scale model, where the cracking phenomenon is captured on the ply scale. The micro-scale model, incorporating plasticity and damage in the matrix, provides input for the meso-scale model, which simulates the progressive cracking by means of the extended finite element method. The methodology is applied to investigate the effect of voids on the density of transverse cracks in cross-ply laminates in function of the quasi-static tensile load. Different sizes and contents of voids, which are chosen based on experimental micro-computed tomography data, are simulated. The numerical experiments show that the presence of voids leads to earlier start of the cracking, with the crack density evolution less sensitive to voids.

Highly conductive ultra-sensitive SWCNT-coated glass fiber reinforcements for laminate composites structural health monitoring

Abstract: In the present study, hierarchical single-wall carbon nanotube covered Glass fibres (GF-CNT) are developed for self-sensing Structural Health Monitoring (SHM) of epoxy laminate composites. The unidirectional (UD) CNT-modified glass fibers were fabricated by a versatile and scalable wet-chemical blade coating deposition process under ambient conditions. GF-CNT were employed to manufacture UD damage sensing composite laminates. Scanning Electron microscopy (SEM) revealed an extremely homogeneous CNT nanolayer consisting of highly entangled CNT networks covering fully the GF surfaces. A comprehensive electrical characterization of the manufactured laminate was carried out, revealing a strongly orthotropic response in terms of electrical resistivity. The damage sensing capability of the new developed “smart” reinforcement material was verified taking advantage of mode I Double Cantilever Beam (DCB) tests carried out on specimens with a pre-delamination. The electrical resistance, measured during the tests, exhibited a pronounced increase proportional to the delamination growth. The experimental data were also compared with the assessment of a predictive analytical model, showing a very satisfactory agreement.

Modification on glass fiber surface and their improved properties of fiber-reinforced composites via enhanced interfacial properties

Abstract: For fiber reinforced composite laminates, properties of laminates can be significantly affected by interfacial properties. In this work, a kind of phthalonitrile containing aromatic ether nitrile linkage (PEN-BAPh) was applied to modify glass fibers (GFs). Cured PEN-BAPh on GFs surface were investigated by Fourier transform infrared spectroscopy (FT-IR) and Ultraviolet–visible spectroscopy (UV–Vis). Scanning electron microscope (SEM) and thermogravimetry (TGA) testing were also applied to confirm the modification. Then, modified fiber-reinforced phthalonitrile-based resin composite laminates were fabricated. As the results indicated that all of the composite laminates showed improved flexural strength, flexural modulus and glass transition temperature (Tg = 296.8–299.6 °C). Improved interfacial adhesion was also studied via interlaminar shear strength (ILSS), impact strength test and monitoring the fracture surfaces. Stable dielectric constants, relative low dielectric loss and outstanding thermal stability (T5% > 350 °C) were obtained for designed composite laminates.