CFRP Structure Research Articles

Domain generalization-based damage detection of composite structures powered by structural digital twin

This research addresses the challenge of generalizing deep learning models for different CFRP composite structures in the task of fatigue damage detection. To overcome this challenge, knowledge distillation is employed to enhance the generalizability of deep learning models. A teacher network processes continuous wavelet transform images using Fourier transform and neural networks, while a student network distills the teacher network. This framework improves the models' generalization performance by transferring knowledge from the teacher network to the student network. Additionally, soft gradient boosting is utilized to further enhance the generalizability. By constructing a main sub-network and multiple parallel auxiliary sub-networks within the teacher network, the student network mimics the main sub-network to achieve improved accuracy in the target domain and prevent overfitting. To augment limited datasets of real CFRP monitoring signals and help to learn domain-invariant features, structural digital twin technology is leveraged to generate simulated monitoring signals, which enables the models to capture domain invariant information, significantly enhancing its performance of fatigue damage detection across different structures. Damage detection based on the generalization results between multiple Layups demonstrates a test accuracy exceeding 80 % when the monitoring data of the target CFRP structure is unavailable during training. Therefore, the cross-structure damage detection ability of the proposed approach is well proved.

Multiscale simulation and experimental studies on modal damping of ultra‐thick CFRP laminates

AbstractThe vibration characterization of ultra‐thick laminates widely used in aerospace and marine industries was different from that of thin laminates. However, modal testing and simulation methods were generally limited to relatively thin laminates. To explore the vibration behavior of ultra‐thick laminates, a novel multiscale simulation technology based on the representative volume modal strain energy (RVMSE) method is presented for predicting modal damping in this paper. The adaptive Greedy‐Based Generation (GBG) algorithm is used to create a 3D random distribution of fibers for composites. Modal shape information of the structure is obtained according to the Lanczos algorithm, which is used as an input parameter for the microscopic model. This is followed by homogenization based on the volumetric analysis performed on representative volume elements (RVEs) to determine the strain energy in six directions. The calculation of modal damping is performed by using the modal strain energy (MSE) method and the damping properties of the six directions. The fast rate of convergence and high accuracy of the method are demonstrated through different examples. The vibration response predictions produced by novel RVMSE methodology are shown to closely match experimental measurements, providing scope to expand the application of this approach to more complex, ultra‐thick laminate components.Highlights The RVMSE method uses a higher level of homogenization to overcome the limitations of the traditional MSE method for calculating the damping of ultra‐thick CFRP laminates. The modal parameter error is controlled to less than 10%, and the experimental results fill the gap of the damping data of ultra‐thick CFRP structure. The proposed RVMSE model improves the accuracy of dynamic response prediction for ultra‐thick laminates.

Effect of bondline thickness on the mechanical performance of CFRP laminate with asymmetric damage repaired by double-sided adhesive patch

As a common CFRP repair method, double-sided adhesive patch has been widely used in the automotive and aeronautic industry. More specific research is thus needed to help better understanding the effect of repairing parameters on the mechanical performance of CFRP structures after patch bonding. In this paper, experimental and Finite Element Modelling (FEM) efforts were made on the adhesive repaired CFRP laminate with different bondline thicknesses. Artificial damage was prefabricated in the centre of the CFRP laminate, which was then repaired through double-sided adhesive patch. Quasi-static tensile tests were conducted on the repaired specimen up to failure, to obtain the load-displacement curves. With a bondline thickness of 0.5 mm, the repaired CFRP structure reached its maximum peak load, increased by 25.3% and 26.4% compared to 0.2 mm and 1.0 mm bondline thicknesses, respectively. SEM observations were used to analyse the influence of adhesive thickness on the fracture modes. Combined with Cohesive Zone Model (CZM) for the adhesive layer and Hashin damage criterion for the CFRP, the FE model of the CFRP laminate repaired with double-sided adhesive patch subjected to tensile loading was established. Finally, the damage evolution as well as the failure modes in the parent laminate and adhesive layer with different bondline thicknesses were compared and analysed. It was revealed that bondline thickness can effectively affect the mechanical performance of CFRP laminate after adhesive patch repair.

High-fidelity simulation model for moisture-induced deformation of CFRP structure considering anisotropic nonlinear moisture absorption-desorption characteristics

This study developed a high-fidelity simulation model for predicting moisture-induced deformation of polymer-based composite structures. First, to identify the anisotropic moisture diffusion in unidirectional carbon fiber/epoxy composites, coupon-level tests were conducted for each principal material direction (i.e., fiber-parallel, in-plane transverse, and out-of-plane transverse directions). In addition, the moisture-induced strain in the fiber and the transverse directions of unidirectional composites was evaluated using an optical fiber sensor. Nonlinear strain against moisture content was identified in each direction. Next, the strain history in the tube element for the metering truss structure was monitored using an optical fiber sensor during the moisture absorption-desorption cycle. Finally, a prediction of the strain behavior was provided using a finite element analysis (FEA) model. The model accurately reflected the complex moisture absorption-desorption characteristics (e.g., directional dependence, nonlinearity, and irreversibility) obtained from the coupon tests. The prediction was compared with the monitoring data and agreed with the experiment.

Experimental analysis of entrance and exit damage mechanism affected by the structural dynamic deformation characteristics during drilling of thin-walled CFRP

Due to lack of effective backup when drilling aircraft thin-walled CFRP structure, serious dynamic structural deformation including vertical and in-plane deformation usually occur, which extremely changes the cutting behavior and materials removed mechanism, causing severe entrance and exit damages. Therefore, this paper proposed a dynamic deformation measurement experiment in vertical and horizontal direction at various processing parameters to analyze their effects on cutting mechanism and damages distribution. Results show the vertical and in-plane deformation occurrence are highly dependent on the actual positions of tool-CFRP system. Namely elastic deformation, local deformation and drawback reflection appear many a time in different stages, causing obvious sudden change of thrust force and different initiation and propagation characteristics of damages. With the feed rate increasing and workpiece thickness decreasing, the maximum displacements of every stages all increase and some thinner workpiece displacement exceed several times the plate thickness. The larger in-plane deformation will nonuniformly distribute along borehole exit due to the different cutting mechanism and strength of different fiber cutting angles. The larger elastic deformation and drawback action in vertical direction will change the out-ply material remove mechanism into mainly puncture rather than only cutting. Under the joint action of deformation in two directions, the loose surface damages composed of delamination and uncut fiber will gather heavily at exit.

Preparation and Testing of Carbon Fiber Reinforced Composites Embedded with Lithium-Ion Polymer Batteries

Vehicle lightweight promises to drive down the maintenance quality and increase energy efficiency in transportation with the safe and strength performance. The use of fiber reinforced polymer composite materials structural battery vehicles is recognized as an essential enabling technology for achieving high bearing capacity and energy storage capacity. At the same time it improves the energy storage capacity, the overall structural energy storage and space utilization of new energy vehicles. In this study we bring an idea to fabricate composites with integrated lithium-ion pouch batteries. The whole preparation process was carried out at room temperature to prevent thermal damage on battery. Experimental results indicate that the cyclic working voltage and capacity of CFRP structure battery are basically consistent with the monomer lithium-ion polymer battery which obtained from Xinwei battery testing equipment. Therefore, the fabrication process does not damage the lithium-ion polymer battery.

Analysis of Plastic Improvement and Interference Behavior in Current-Assisted Riveting of CFRP Laminates.

In order to improve the joint performance of a titanium alloy rivet connecting aircraft CFRP structure and promote the wide application of ordinary titanium alloy rivets in the aviation field, the ductility of a Ti45Nb rivet was improved using a current-assisted method in this paper. Through experiments, the mechanical behavior and temperature during the riveting process were monitored, and the variation rules of interference and damage were studied in detail. The results show that a current within 16.5 A/mm2 can effectively reduce the riveting pressure requirement, and the maximum engineering stress is reduced by nearly 22%. As the current density increases, the softening effect is obvious, but as the processing time increases, the softening effect has an upper threshold. The current-assisted method can significantly increase the interference fit level, and the uniformity of riveting can be improved by nearly 30%. The outlet burr height of a joint obtained by new technology meets the relevant standards. When the current density is too large or the action time is long, the damage pattern and mechanism at different depths of hole have obvious regional differences.

Key issues and development trends of mechanical connection relating to aviation CFRP structure performance

Key issues and development trends of mechanical connection relating to aviation CFRP structure performance

Weight optimization in composite fibre reinforced plastic fairings

This abstract describes the weight optimization process followed by ALESTIS AEROSPACE in a secondary faring structure, designed and manufactured in composite material, focused in detail in the Belly Fairing structure of the A350 XWB A/C. The obective of this abstract is to enumerate and to decribe the weight optimization processes of this kind of structures, vieweing in detail following aspects: Optimum floatability definition in the joint between panels and structure Optimum design in sandwich panels Method to obtain out of plane loads for the joint between sandwich panel and structure Allowable determination for CFRP structure, based on finite element models correlated with mechanical tests It is also described in ths abstract the process followed to obtain test allowables and the methods developed to perform the weigth optimization in panels. Results obtained evidence that the Certification requirements are fulfilled for the complex design of a fairing such as Belly Fairing from A350XWB, manufactured with carbon fibre meterial with an optimum weigth.

Determination of Detection Probability and Localization Accuracy for a Guided Wave-Based Structural Health Monitoring System on a Composite Structure

The capabilities of detection and localization of damage in a structure, using a guided wave-based structural health monitoring (GWSHM) system, depend on the damage location and the chosen sensor array setup. This paper presents a novel approach to assess the reliability of an SHM system enabling to quantify localization accuracy. A two-step technique is developed to combine multiple paths to generate one probability of detection (POD) curve that provides information regarding the detection capability of an SHM system at a defined damage position. Moreover, a new method is presented to analyze localization accuracy. Established probability-based diagnostic imaging using a signal correlation algorithm is used to determine the damage location. The resultant output of the localization accuracy analysis is the smallest damage size at which a defined accuracy level can be reached at a determined location. The proposed methods for determination of detection probability and localization accuracy are applied to a plate-like CFRP structure with an omega stringer with artificial damage of different sizes at different locations. The results show that the location of the damage influences the sensitivity of detection and localization accuracy for the used detection and localization methods. Localization accuracy is enhanced as it becomes closer to the array’s center, but its detection sensitivity deteriorates.

A comparison study of water diffusion in unidirectional and 2D woven carbon/epoxy composites

AbstractThe use of CFRP composites is significantly increasing in the aerospace, automotive, and marine industries, particularly in safety critical primary structures. This work presents a newly developed experimental approach to investigate the directional diffusion of water in CFRP composites with the use of Fick's law. The approach is used to study the effect of fiber architecture on directional diffusion rates, with a particular focus on the role of fiber waviness in the diffusion process. A comparison of water diffusion is made in three different fiber architectures: Unidirectional (UD), plain weave, and twill weave. The specimens were fully immersed in 90°C purified water until their maximum moisture saturation was achieved, with some specimens being selectively exposed from the edges only to obtain the directional diffusion coefficients. The water penetration process into the CFRP structure initiate from the micro‐cracks and defects. The experimental work of this study shows sharp mass increases within the first stage followed by an equilibrium stage where saturation is present. The interfacial region is found to be a critical parameter where detachment of the interfacial fiber/matrix bonding is observed further demonstrating the potential effect of different fiber architecture in this region. UD fiber architecture showed ~20% higher diffusion coefficient in the Dx,y direction compared with plain and twill woven architectures. The weave patterns in 2D woven fiber architectures are therefore believed to play a key role on the moisture ingress mechanism and subsequently contributed in slowing down the capillary process in the interfacial region. This has implications for materials development and selection for CFRP composites used in moist environments.

Energy-absorption assessments of perforated CFRP tube induced by inward-splaying trigger with different trigger radius

For different purposes of engineering design, CFRP structure inevitably be processed with open holes, resulting in the discontinuity of fiber or matrix and reducing structural performance. To improve crashworthiness, the crushing behaviors of perforated CFRP tube with different trigger radius based Inward-Splaying Triggers (IST) are studied. The finite element method with the non-linearly progressive damage model is adopted for verifications and numerical investigations. The improve effect and trigger mechanisms of IST on the perforated tube is revealed by comparing with original tube. To extensively reveal the energy-absorption (EA) behaviors, the analyses of parameters including trigger radius, radius and position of open-hole are investigated. It is found that increasing open-hole radius can aggravate hole circumferential fracture and position of open-hole has significant effect on EA of tube with IST. By comparisons, any perforated tube with IST has higher total EA than intact tube. It is indicated that much improvements of EA are attributed to the fact that a transition from hole circumferential fracture to progressive crushing can be achieved by IST, leading to more adequate energy dissipation of materials. Several crucial conclusions from the parameter analyses are drawn for guiding the perforated structural design and improving crashworthiness.

Influence of the aluminum die casting process on PEEK layered CFRP in the manufacturing of a hybrid connection

Lightweight design is used as a key technology in a variety of fields to achieve significant weight reduction. Due to its low density in combination with high strength, CFRP and aluminum are predestinated lightweight materials. However, the joining of this material combination is still performed using common technologies like adhesive or mechanical joining. A major challenge at the joining of this material combination is the corrosion behavior. Therefore a novel approach of a hybrid connection between CFRP and aluminum was developed by using the HPDC process of aluminum. During efficient HPDC process, a CFRP structure is recast with aluminum (AlSi10MnMg). To resist the shortly appearing temperature of 700 °C from the liquid aluminum melt, the CFRP is coated with a layer of PEEK. This leads to a material transition, which spatial separates CFRP and aluminum and a hybrid connection with tensile shear strengths up to 22 MPa. Within this work, the influence of the several processing steps on the PEEK layer and the structure of the CFRP was investigated. DSC and TGA results show no significant change on thermal properties of the PEEK. Imaging investigations show the appearance of heat induced volatilizations in the PEEK layer of the hybrid specimens after the HPDC process.

Determinating the Volume Content of a Polymeric Matrix in CFRP Structures Using a Laser-Ultrasonic Method

An acoustic method for a quantitative assessment of the volume content of a polymeric matrix in CFRPs is proposed and realized experimentally. For this purpose, a laser-ultrasonic method based on the thermo-optical excitation of broadband pulses of longitudinal acoustic waves is used. A formula for calculating the volume content of the polymeric matrix in a CFRP from experimentally measured phase velocities of longitudinal acoustic waves in this material is proposed. Distributions of the volume content of the matrix in a control CFRP specimen and in a CFRP structure made under similar conditions are obtained. It is shown that the laser-ultrasonic method allows one to detect the areas with excess and deficient matrix content in CFRP specimens and areas of CFRP constructions and to quantitatively assess the spread of matrix content. The spread between the maximum and minimum matrix content observed depends on the shape of the investigated area of the structure. This method can be used for the quality control of composite structures before their exploitation.

Optimisation design of CFRP passenger car seat backplane based on impact characteristics

The newly revised GB 13057-2014 put forward high requirements for the dynamic occupant protection of passenger car seats. Due to the high density of steel, there is currently not much space for lightweight through structure improvement. In this paper, the quasi-static and dynamic mechanical properties of CFRP and corresponding simulations are used to study the mechanical behaviour of CFRP and verify the accuracy of simulation. It is now applied to the composite seat backplane structure design, and combined with the CFRP moulding process and the energy absorption characteristics of the car seat backplane to initially design the CFRP seat backplane material and structure. At the same time, considering the designability of the material itself, the ply optimisation technology under multi-condition is used to design the material and structure of the CFRP passenger car seat backplane. The new CFRP passenger car seat achieves significant weight reduction while meeting regulatory requirements.

Process Monitoring and Modeling of Aircraft CFRP Structure

Process Monitoring and Modeling of Aircraft CFRP Structure

Damage detection of CFRP structure based on electrical impedance tomography

Damage detection of CFRP structure based on electrical impedance tomography

Scalable Monitoring System for the Localization of Damaging Events in Thin-Walled CFRP Structures Based on Acoustic Emission Analysis and Neural Networks

A constant challenge for the design and operation of CFRP primary structures is their sensitivity towards impact loading. This can lead to the formation of externally invisible delaminations which endanger the structural integrity. In practice, this circumstance is encountered with elaborate inspections or conservative design. Structural Heath Monitoring (SHM) systems offer the potential for permanent monitoring and represent an alternative approach that has drawn more attention in the last decade. The biggest barriers to market entry for this technology are system costs and reliability. This study is dedicated to these two points with the development of a low-cost system with which representative acoustic emission sources can be located reliably in a complex CFRP structure. The implementation is carried out using acoustic emission analysis, which represents a promising solution for the integral monitoring of primary structures. It is based on the detection of acoustic waves that are released during crack initiation and growth and propagate over large areas in thin-walled structures as Lamb waves. The challenges of source localization in thin-walled CFRP structures lie in the consideration of wave dispersion, anisotropic material properties, variable component geometry and interfaces. In this thesis, this complexity is captured by training a neural network. For this purpose, artificial sources are used which imitate acoustic emissions of typical damaging events in the material in frequency and mode content. The demonstrating component is an omega profile equipped with a network of piezoelectric sensors that is designed for reliable localization within a defined window. Signal processing takes place on a single-board computer which, together with a digital oscilloscope, completes the measurement chain. The system represents a modular, low-cost approach that can be transferred to other applications by adapting the hardware and training.

DETERMINATION OF THE CONNECTION BETWEEN THE STRUCTURE OF CARBON FIBER SAMPLES AND THE PARAMETERS OF ACOUSTIC EMISSION SIGNALS DURING SIMULTANEOUS STATIC AND THERMAL LOADING

The effects of positive and negative temperature and static load on the main informative parameters (structure coefficient, partial energy, location) of acoustic emission (AE) signals, which determined the mechanism for changing the structure of carbon fiber and the beginning of its destruction, are investigated. Tests of specimens of carbon fiber T800, made of nine monolayers with laying [±45/90/О3/90/±45], size 600x100x0.9 mm. Each sample was subjected to static loading and the effects of positive (+20, +40, +60, +80, +100 °С) or negative (–20, –40, –60, –80 °C) temperatures in the area of the concentrator in the form of a 12 mm diameter hole. Using fractography, changes in the structure of carbon plastic from the applied static load and temperature and changes informative parameters were analyzed. It was shown that the lamination of the material with simultaneous exposure to static load and temperatures from –80 to –20 °C and from +60 to +100 °C corresponded to an increase in the structural coefficient and partial energy, which caused an energy shift in the frequency range 125…250 kHz. Under the same static loads, but temperatures of +20 and +40 °C, informative parameters took on minimal values, that meant the energy was shifted to the frequency range 250…500 kHz, characterized the crumbling of the matrix and breaking of the fibers without lamination of the CFRP. Under all temperature conditions, the location of the signals began in the hole area and spread in the direction of the static load. The detection the disturbance of the CFRP structure by the informative parameters of AE signals makes it possible to reduce the risk of emergency situations during working of the composite construction.

Fracture Behavior and Strength Evaluation Method of CFRP Structure with Edge-on Impact Damage

Fracture Behavior and Strength Evaluation Method of CFRP Structure with Edge-on Impact Damage