Carbon Fiber Reinforced Plastic Laminates Research Articles

Analytical prediction of the local Young’s modulus in CFRP laminate with ply discontinuity: a variational approach

Carbon fibre reinforced plastics (CFRPs) with continuous fibres are strong and stiff. However, ply discontinuities can introduce stress concentrations, potentially damaging the material and adversely affecting its mechanical properties. Evaluating the impact of ply discontinuities on the mechanical properties of CFRP laminate is, therefore, crucial. Extending the applicability of a variational stress analysis based on Hashin’s original approach, this study presents a method to predict local Young’s moduli in unidirectional CFRP laminate with ply discontinuity. The complementary energy for the specific length of the laminate in the longitudinal direction was used to derive the local Young’s moduli. An experimental study and a finite element analysis (FEA) were conducted to validate the results. In the experimental study, strain gauges of various lengths were placed on the ply discontinuity area; the measured Young’s moduli varied with the gauge length. The numerically derived local effective Young’s moduli showed a relatively positive agreement with the experimental and FEA results, especially for lower values of applied stress. For more accurate predictions, future analyses should consider the impact of delamination on the Young’s moduli.

Mechanical properties and damage behavior of tapered unidirectional CFRP laminates

An understanding of damage mechanisms induced by drop-off plies in tapered composite laminates is crucial. This study investigated the mechanical properties and damage behaviors of asymmetric tapered unidirectional carbon fiber-reinforced plastic laminates. Damage observations of laminates with simultaneous tapered and staggered tapered structures under monotonic and cyclic tensile loads were done by an optical microscope and X-ray radiography. Based on the results, matrix cracks in the resin pocket and delamination occurred earliest in the simultaneous tapered specimen due to a greater stress concentration in a single large resin pocket in the structure. Intralaminar damages in the staggered tapered specimen with a shorter step spacing occurred mainly in the lower dropped ply as there was a more significant interaction between the neighboring steps in the specimen. The drastic decrease in the stress concentration after the occurrence of damages in the neighboring step(s) then suppressed the occurrence of intralaminar damages in the upper dropped ply. This is in contrast to the specimen with a longer step spacing where the intralaminar damages occurred in both dropped plies due to the distributed stress distribution. Delamination propagated between the dropped plies and continuous (belt and core) plies for both staggered laminates regardless of the length of the step spacing at higher applied stress levels. The results showed that internal ply-drop configuration and step spacing in staggered tapered structures contribute to significant differences in the mechanical properties and damage behavior in the laminates.

Effect of Stitching, Stitch Density, Stacking Sequences on Low-Velocity Edge Impact and Compression after Edge Impact (CAEI) Behavior of Stitched CFRP Laminates.

Low-velocity edge impact and compression after edge impact (CAEI) behavior of stitched carbon-fiber-reinforced plastic (CFRP) laminates were experimentally investigated in the paper. Five groups, including three stacking sequences (P1, P2, P3) and two stitch densities (stitch space × stitch pitch is 10 mm × 10 mm and 15 mm × 15 mm) of stitched/unstitched CFRP laminates, were prepared by the VARTM technique and subjected to low-velocity edge impact and compression after edge impact experiments. The damage of CFRP laminates was detected by optical observation and micro-CT. The effects of stitching, stitch density, stacking sequences and impact energy on properties of edge impact and CAEI were discussed. The results show that the damage of edge impact of stitched laminates is smaller than that of unstitched laminates. The main failure mode of CAEI of the unstitched laminates is delamination and that of the stitched laminates is global buckling. The addition of stitches can effectively improve the edge impact resistance and damage tolerance of CFRP laminates. Compared with the unstitched laminates with the same stacking sequence, the peak impact force of the laminates with stitch density 15 mm × 15 mm increases by 5.61-12.43%, and the increase in residual compression strength is up to 5-20.9%. The peak impact force of the laminates with stitch density 10 mm × 10 mm increases by 8.1-31.4%, and the increase in residual compression strength is up to 24.2-27%. Compared with the other two stacking sequences (P1 and P2), the stacking sequence P3 has excellent resistance of edge impact and CAEI properties.

Synthesis, characterization and cutting performances of gradient gradual changing (GGC-) MCD/SMCD/NCD/UNCD diamond coatings with tailored interfacial integrity on CFRP drilling tools

Drilling holes is a common mechanical processing method in the high-precision machining of carbon fiber reinforced plastic (CFRP) material. Diamond coatings can be deposited on the drilling tools to enhance the tool life and improve the machining quality. However, traditional multilayer diamond coatings, such as MCD/NCD etc., easily suffer from interfacial separation in actual applications due to the sharp interfaces of diamond layers. Novel gradient gradual changing (GGC-) structure without clear interfaces is applied to tailor the architecture of diamond coatings, which can lead to superior interfacial integrity and the reduction of sp2 phase content at the transition between distinct diamond layers. Various diamond films, containing monolayer diamond films (MCD, SMCD, NCD, UNCD) and gradient gradual changing multilayer diamond films (GGC-MCD/SMCD/NCD, GGC-MCD/SMCD/NCD/UNCD), were fabricated on WC-Co samples. The deposition of tailored gradient gradual changing coating interfacial architecture can be accomplished by controlling the linear variation of carbon concentration and gas pressure simultaneously. The complementary characteristics of high coating-substrate adhesive strength, superior frictional and wear resistance, good interfacial integrity and favorable crack propagation resistance are concentrated on GGC-MCD/SMCD/NCD and GGC-MCD/SMCD/NCD/UNCD coatings compared to monolayer MCD, SMCD, NCD and UNCD coatings. Moreover, because of the existence of UNCD layer, GGC-MCD/SMCD/NCD/UNCD coating performs lower surface roughness, frictional coefficient and higher surface smoothness than GGC-MCD/SMCD/NCD coating. Besides, the cutting performances in high-speed drilling experiments of CFRP laminates using different types of diamond coated drilling tools were compared systematically, showing that the dominant wear modes are abrasive wear and adhesive wear, and the GGC-MCD/SMCD/NCD/UNCD coated drilling tool exhibits the longest tool lifetime and best machining quality because of the overall interfacial integrity and high surface smoothness.

Laser microdrilling of CFRP with a nanosecond-pulsed high-power laser

The global demand for air travel and air transport is expected to further increase in the next couple of years, and so the environmental protection will also increasingly come into focus again. In the aviation sector, this means not only saving fuel and reducing emissions but also reducing the noise pollution caused by aircrafts. For this purpose, sound-absorbing acoustic liners are used, which consist of a sandwich structure with one perforated skin layer. To reduce weight, these outer layers are often made of carbon fiber-reinforced plastic (CFRP). For manufacturing these CFRP skin layers, laser drilling offers specific benefits, but when it comes to industrial applications, further process improvements are necessary. Although laser microdrilling is a well-known process in laser materials’ processing, it has not yet been sufficiently investigated for the processing of carbon fiber-reinforced plastics. This study investigates the quality and efficiency of laser microdrilling with a 1.5 kW nanosecond-pulsed high-power laser for percussion drilling and helical drilling of thin CFRP laminates. The efficiency was evaluated in terms of drilling depth, the amount of energy required to remove a specific volume of material, and the time required to remove a specific volume of the material. The quality evaluation focused on the heat-affected zone and the hole taper. The efficiency and quality results were set in relation to higher-level parameters such as energy fluence in the laser spot, pulse overlap, or total energy applied. This facilitates future transferability to similar laser processes that may use laser machines and system technology with deviating specifications.

An integrated modeling of barely visible impact damage imaging of CFRP laminates using pre-modulated waves and experimental validation

Nonlinear ultrasonic techniques have been broadly used for detecting barely visible impact damage (BVID) in carbon fiber reinforced plastic (CFRP) from both numerical and experimental approaches. However, the interaction between the BVID and nonlinear ultrasonic is not considered in the currently available numerical models and BVID needs to be more accurately modeled. This work presented an integrated three-dimensional (3D) finite element (FE) model for imaging BVID using pre-modulated wave (PMW) technique, in which the 3D Hashin damage criterion and energy-based damage evolution are introduced to predict intralaminar and interlaminar damage during the impact process. The non-contact PMW tests based on a laser scanning Doppler vibrometer (LSDV) are adopted to experimentally validated the proposed numerical methodology. The results show that the BVID predicted by the FE model agrees well with the experiments. Then, the damage contour images based on frequency spectra of modulation are illustrated to identify the presence of BVID, and damage severity is quantified based on the proposed maximum response amplitude (MRA). It is found that the location of BVID can be identified by the PMW approach numerically and experimentally and the sum of MRA of sidebands exhibits the best sensitivity of the variation of BVID with different energies.

Finite element method of a progressive intralaminar and interlaminar damage model for woven fibre laminated composites under low velocity impact

A novel three-dimensional damage model based on continuum damage mechanics (CDM) is established for the analysis of damage and fracture in woven carbon fibre reinforced plastic (CFRP) laminates. The damage model was developed in LS-DYNA, a nonlinear explicit finite element software, and the model was incorporated with the user-defined material. The multiple dimensions damage model was applied to predict the intralaminar damage initiation and propagation. A cohesive zone model was used to simulate the delamination induced by interlaminar cracks. The simulation results of four different impact energies (5 J, 8 J, 10 J, and 15 J) were analysed by impact responses. Moreover, the results of different damage modes and fracture modes of two representative ply orientations ([0]8 and [45]8) are discussed in detail.The impact peak force, energy absorption, and maximum crack length in the results of simulating woven carbon fibre laminated composites with different ply orientations are within 5% of the experimental results. Accordingly, the established model can accurately predict the crack evolution of [0]8 and [45]8, thus demonstrating that the proposed methodology is more suitable for damage simulations of woven CFRP laminates with different ply orientations under low velocity impact than existing methods.

A global interactive attention-based lightweight denoising network for locating internal defects of CFRP laminates

Carbon fiber reinforced plastic (CFRP) has become one of the main structural materials for aerospace vehicles. However, some internal defects are prone to occur and have potential to cause significant losses of life and property. Currently, the detection of internal defects for CFRP mainly relies on ultrasonic, and other technologies, while they have disadvantages of low efficiency, and poor adaptability. Therefore, this paper explores a novel method to locate internal defects of CFRP laminates by analyzing vibration signals. Firstly, a signal acquisition scheme is designed. Then, a global interactive attention-based lightweight denoising network (GIALDN) is designed to analyze vibration signals and locate internal defects of CFRP laminates. In GIALDN, the threshold denoising method is used to eliminate noise-related features and improve feature discrimination; a global interactive attention module is designed, which makes the network pay more attention to the valid features while realizing the global interactive connection and obtains the rich contextual features; combining with the convolution layer of de-pooling strategy and multi-layer convolution using the residual connection, the backbone of the network is formed. Finally, an experimental platform is established to test the performance of GIALDN. Results show that the location accuracy of GIALDN can reach 98.68%, which is more than 15% higher than those of VGGnet11 and FaultNet, and is also superior to those of LSTM, RNN, Rsenet18, SEresnet18 and Densenet121. Lastly, the location accuracies of GIALDN on CFRP laminates with the same thickness and different stacking sequences are investigated and a good model applicability can be observed.

Voids formation characteristics in typical epoxy-based CFRP laminates during low-temperature annealing in the air or N2 atmosphere

The feedstock recycling of carbon fiber reinforced plastics (CFRPs) is typically conducted by either thermal decomposition or dissolution methods for resin separation. A common issue in the recycling processes is that neither oxygen nor solvents can easily penetrate dense CFRP; therefore, penetrating diffusion paths such as voids is expected to improve the separation efficiency. This study presents voids formation characteristics by low-temperature annealing as pre-treatment in typical epoxy-based cross-ply CFRP laminates. The void characteristics were evaluated by electrical treatment and observations after the annealing in the air or N2 atmosphere. Annealing in air resulted primarily in ply delamination and matrix cracking through slight oxidative reactions. By contrast, annealing in the N2 atmosphere denatured the epoxy resin, resulting in the formation of voids and swelling that reached the interior of the specimens. The void characteristics could be controlled by the atmosphere in the low-temperature annealing, leading to the penetrating diffusion paths.

Finite element model for simulating entropy-based strength-degradation of carbon-fiber-reinforced plastics subjected to cyclic loadings

In this study, a novel model for simulating strength and fracture energy reductions based on the stress and strain histories of a carbon-fiber-reinforced plastic (CFRP) ply is developed, thus allowing us to comprehensively simulate the versatile failures including fatigue failure for CFRPs. The strength and fracture energy reductions are defined as functions of entropy generation. The entropy value is obtained from the absolute temperature and dissipated mechanical energy, such as the area of the inelastic hysteresis loop. Strength and fracture energy reductions are introduced into the conventional Hashin’s criterion. An algorithm for this model and some numerical validations are presented herein. Through a simplified CFRP laminate model with frequency dependence, a delayed failure of 90°layer under cyclic displacement conditions is numerically simulated.

A detailed study on the damage evolution and failure assessment of single-lap hybrid joints in CFRP laminates under tensile loading

Hybrid joints have potential to improve the joint strength and efficiency compared to the bonded or bolted joint. Several studies have been performed to understand the influence of various design parameters on the hybrid joint strength. However, very few studies have been reported on the complex failure mechanism of the hybrid joint. In this study, a detailed analysis is carried out to understand the failure mechanism of the hybrid joint in Carbon fiber reinforced plastic (CFRP) laminate having [0/+45/90/−45]s layup sequence under tensile loading both experimentally and numerically. Analysis of a simple bonded and bolted joint configurations are also included for comparison purposes. The acoustic emission (AE) technique is utilized for the damage assessment and the 3D digital image correlation (DIC) technique is used to capture the whole field strain around the bolt region. A detailed fractographic study using a digital optical microscope is also carried out to critically ascertain the presence of different damage modes. An in-house instrumented bolt is realized to measure the load transfer through it upon loading. A finite element based progressive damage model (PDM) along with the cohesive element is also developed to predict the damage evolution and failure mechanism for all the joint configurations.

Experimental and numerical investigations on the impact behaviour of pristine and patch-repaired composite laminates.

The present paper investigates the impact behaviour of both pristine carbon-fibre-reinforced-plastic (CFRP) composite laminates and repaired CFRP laminates. For the patch-repaired CFRP specimen, the pristine CFRP panel specimen has been damaged by cutting out a central disc of the CFRP material and then repaired using an adhesively bonded patch of CFRP to cover the hole. Drop-weight, impact tests are performed on these two types of specimens and a numerical elastic-plastic, three-dimensional damage model is developed and employed to simulate the impact behaviour of both types of specimen. This numerical model is meso-scale in nature and assumes that cracks initiate in the CFRP at a nano-scale, in the matrix around fibres, and trigger sub-micrometre intralaminar matrix cracks during the impact event. These localized regions of intralaminar cracking then lead to interlaminar, i.e. delamination, cracking between the neighbouring plies which possess different fibre orientations. These meso-scale, intralaminar and interlaminar, damage processes are modelled using the numerical finite-element analysis model with each individual ply treated as a continuum. Good agreement is found between the results from the experimental studies and the predictions from the numerical simulations. This article is part of the theme issue 'Nanocracks in nature and industry'.

Flexible Mechanoluminescent SrAl2O4:Eu Film with Tracking Performance of CFRP Fracture Phenomena.

Non-destructive testing of carbon-fiber-reinforced plastic (CFRP) laminates with bidirectional fiber bundles (twill-weave) using a mechanoluminescence (ML) technique was proposed. The dynamic strain distributions and fracture phenomena of the CFRP laminates in the tensile testing were evaluated by the fabricated ML sensor consisting of SrAl2O4:Eu (SAOE) powder and epoxy resin. The ML images for the ML sensor attached to the CFRP laminates with bidirectional fiber bundles gave a net-like ML intensity distribution similar to the original twill weave pattern. Specifically, it was found that the ML intensity on the longitudinal fiber bundle, which is the same as the tensile direction, is higher than that on the transverse fiber bundle. This indicates that the ML sensor can visualize the load share between fiber bundles in different directions of the CFRP laminate with high spatial resolution. Meanwhile, the ML sensor could also visualize the ultrafast discontinuous fracture process of the CFRP laminates and its stress distribution. The amount of SAOE powder in the ML sensor affects the tracking performance of the crack propagation. A higher SAOE amount leads to a fracture of the ML sensor itself, and a lower SAOE amount leads to poor ML characteristics.

The Effect of the Setting Force on the Fatigue Resistance of a Blind Rivet Nut Set in CFRP

Weight reduction is often a key factor in modern mechanical design, therefore materials such as carbon fibre reinforced plastics (CFRP) are increasingly used and integrated within multi-material structures using adhesive technologies, which require high effort and are difficult to disassemble. The capability of a blind rivet nut (BRN) to join different materials without these disadvantages has created a growing industrial interest in the fastener. However, installing a BRN in CFRP laminate induces a significant stress concentration in the plate, which potentially causes damage. Given that ‘damage free’ joints are demanded by the industry, the BRN is often not considered as a suitable joining technique. In the present research, an experimental campaign is performed to investigate the fatigue resistance of a BRN joint in CFRP. It is demonstrated that the resulting compressive stress after installing a BRN can enhance the fatigue resistance of the specimen. The results increase the potential of the BRN as a fastener for CFRP.

Application of machine learning for improved accuracy of simultaneous temperature and strain measurements of carbon fiber-reinforced plastic laminates using an embedded tilted fiber Bragg grating sensor

Tilted FBG (TFBG) sensors exhibit capability for simultaneous measurement of temperature and strain. However, improving the measurement accuracy is difficult. This study examined the applicability of machine learning techniques for improving the accuracy of simultaneous measurement of temperature and strain of a TFBG sensor, and of a carbon fiber-reinforced plastic (CFRP) composite laminate using an embedded TFBG sensor. Temperature and strain were ascertained using machine learning methods, i.e. deep neural networks (deep NN) and support vector regression (SVR), from multiple peaks in the transmission spectrum. The results obtained using the machine learning methods were compared directly to those found using a conventional method with the simultaneous linear equation of strain and temperature cross-sensitivity by extracting two peaks from the transmission spectrum. Results demonstrated that the accuracy was improved significantly using deep NN method with Bayesian regularization compared to the conventional cross-sensitivity method.

Digital image correlation (DIC) based damage detection for CFRP laminates by using machine learning based image semantic segmentation

Vision-based damage detection in carbon fiber-reinforced plastic (CFRP) composites can be interfered by such factors as surface texture, stains and lighting. A digital image correlation (DIC) based surface strain monitoring technique, on the other hand, enables to track the change of strain distribution. It is promising to develop a new approach for online structural health monitoring (SHM), in which the DIC strain contours can be scrutinized automatically and the results are no longer substantially subjected to human interference. In this study, a convolutional neural network (CNN) based image semantic segmentation technique is proposed for pixel-level classification of DIC strain field images. A DeepLabv3+ encoder-decoder architecture combined with different feature extraction networks is investigated. The training dataset and validation of the model are obtained through finite element (FE) simulation. The images of quasi-static axial tensile strain field obtained from 2D-DIC are used to test the accuracy and efficiency of the trained CNN model. It is found that use of a pre-trained ResNet-50 CNN model as the backbone network of DeepLabv3+ architecture through a transfer learning algorithm can make the semantic segmentation results reach a mean intersection over union of 0.9236. The prediction accuracy of the semantic segmentation model trained from the FE data is comparable with that of the model trained from the experimental data, which demonstrates that the proposed machine learning approach for DIC measurement is cost-effective.

Temperature distribution analysis of carbon fiber reinforced polymer composites during self-resistance electric heating process

Self-resistance electric (SRE) heating for carbon fiber reinforced plastics (CFRP) is an alternative out-of-autoclave curing methodology that has high efficiency and low energy consumption. However, thanks to the heating mechanism of inside-out conduction, which is significantly different from that of the traditional outside-in heating process, the temperature distribution and its homogenization strategy still need further investigation. This work focuses on analyzing the affecting factors of the temperature uniformity of CFRP laminates during the SRE curing process utilizing a multi-physical numerical computation model and experimental verification. It is found that mold sizes have the most significant impact due to the non-uniform center-to-edge heat absorption of the mold. The maximum temperature difference during the untreated process could be lower than 20 K if the excess size proportion of the mold is smaller than 10%. Thermal barriers made of aerogel homogenize the temperature distribution during the dwell stage, reducing the maximum temperature difference by 84.18%. Considering that the mold size is sometimes unchangeable, the existence of auxiliary heating elements compensates the heat dissipation from the self-heated laminate and reduces the maximum temperature difference by 79.81%. Based on analysis results, a temperature homogenization strategy is proposed, where the maximum temperature difference is reduced by 86.15%.

Thermal-damage suppression of composite material by anti-lightning NCF/CIPCF film

Lightning strike protection (LSP) design is significant for carbon fiber reinforced plastic (CFRP), which is sensitive to lightning strikes. An anti-lightning film reinforced by nickel-coated carbon fibers/carbonyl iron powders conductivity framework (NCF/CIPCF) is designed for lightning protection of CFRP laminates. Representative volume element (RVE) models of NCF/CIPCF film are established according to random sequential addition (RSA) method. Material properties are obtained based on homogenization theory. This numerical method is validated by calculations of electrical resistivity and thermal conductivity. Subsequently, lightning damage analysis of CFRP laminates with different protections is conducted based on thermal-electric coupling method and temperature dependent ablation criteria, which aims to evaluate the suppression effects of lightning damage by this film. Results show that damage area and depth of CFRP laminates are effectively controlled by NCF/CIPCF film, which is slightly superior to aluminum at the same weight. NCF/CIPCF anti-lightning film is light and easy to repair and replace, which can be considered as an excellent and promising method in LSP design of aircraft.

2D slowness visualization of ultrasonic wave propagation for delamination detection in CFRP laminates

Visualizing the propagation of Lamb waves generated by scanning laser injection is a promising and effective method for the nondestructive inspection of carbon fiber reinforced plastic (CFRP) structures. We evaluate the use of two-dimensional time-slowness maps of the acquired data for defect detection in CFRP laminates. The slowness maps can be constructed from the differences in wave propagation characteristics between two adjacent laser injection points. Using a CFRP plate, in which an artificial delamination is located at the center, we show that the delamination can be clearly observed as a hotspot in the two-dimensional slowness map. The visible hotspot is due to the dispersion of the lowest antisymmetric (A0) mode of the Lamb wave: the A0 mode slows down at the delaminated region. We also demonstrate that our method can be applied to an L-shaped structure. Two-dimensional slowness images can be useful for developing and applying practical inspection methods based on laser ultrasonic waves.

Laser ablation of CFRP surfaces for improving the strength of bonded scarf composite joints

Repairing damaged composite parts using scarf technique requires a careful selection of treatment methods for composite surface. Laser treatment is one of the emerging techniques to treat the milled composite surface by unlocking various levels of morphological changes and, thus, optimizing joint strength. However, laser parameters, e.g., energy density (fluence), should be carefully determined to ensure the acceptable structural recovery. Here, the influence of CO2 laser with relatively high fluence (ablation effect) on the surface characteristics (roughness, morphology, wettability) and scarf joint strength with associated failure modes of unidirectional (UD) and quasi-isotropic (QI) carbon fiber-reinforced plastic (CFRP) laminates is studied. We found that the ablation effect using CO2 laser at 3.6 J/m2 was considered safe for UD laminates as their joint strength was comparable with that treated by manual sanding. The ablation at higher fluence (8.4 J/m2) reduced the joint strength in UD laminates due to severe damage occurred in 0∘ fibers that triggered adhesive failure. In QI laminates, 3.6 J/m2 laser fluence could improve joint strength since the cohesive failure was activated in off-axis plies (90∘, +45∘, −45∘).