Carbon Fiber Reinforced Plastics Structures Research Articles

Herringbone-Bouligand CFRP structures: A new tailorable damage-tolerant solution for damage containment and reduced delaminations

In this work, we design, prototype, test and analyse the first high-performance Herringbone-Bouligand microstructure (with Carbon Fibre Reinforced Plastic (CFRP)) inspired to the high-impact-resistant mantis shrimp's dactyl club. To this end, we devised the first prototyping procedure to manufacture point-by-point tailorable Herringbone-Bouligand CFRP microstructures; this was based on the micro-moulding of uncured CFRP prepreg, and led to mimicking features of the club microstructure never achieved before with CFRPs. We investigated the damage tolerance of the prototyped Herringbone-Bouligand CFRP laminates, compared against ‘classical’ Bouligand CFRP laminates, using quasi-static indentation tests. Our test results show that the Herringbone-Bouligand microstructure resulted in delayed onset of delaminations, reduced in-plane spreading of damage, increased energy dissipation capability, and in the containment of damage within the tailored Herringbone-Bouligand region. We conclude that Herringbone-Bouligand CFRP microstructures offer an excellent tailorable damage-tolerant solution with great potential for composite applications where resistance to through-the-thickness loads is paramount.

Temperature dependence of statistical fatigue strengths for unidirectional carbon fiber reinforced plastics under tension loading

The formulation for time- and temperature-dependent statistical static and fatigue strengths for carbon fiber reinforced plastics laminates is newly proposed based on the physically serious role of resin viscoelasticity as the matrix of carbon fiber reinforced plastics. In this study, this formulation is applied to the tensile strength along the longitudinal direction of unidirectional carbon fiber reinforced plastics constituting the most important data for the reliable design of carbon fiber reinforced plastics structures which are exposed to elevated temperatures for a significant period of their operative life. The statistical distribution of the static and fatigue strengths under tension loading along the longitudinal direction of unidirectional carbon fiber reinforced plastics were measured at various temperatures by using resin-impregnated carbon fiber reinforced plastics strands as specimens. The master curves for the fatigue strength as well as the static strength of carbon fiber reinforced plastics strand were constructed based on the time–temperature superposition principle for the matrix resin viscoelasticity. The long-term fatigue strength of carbon fiber reinforced plastics strand can be predicted by using the master curve of fatigue strength.

A stochastic material model to describe continuous mode conversion

AbstractDeterministic material models are not able to comprehensively describe phenomena observed in experiments. The continuous mode conversion (CMC) of propagating waves in carbon fiber reinforced plastic (CFRP) plates is an example of this. The mechanism is similar to the conversion in the presence of defects. If a symmetrical mode while propagating through a plate‐like structure encounters a damage, which is non‐symmetric with respect to the waveguide thickness, the symmetrical mode converts into an asymmetric mode at the obstacle. The conversion is usually bounded to the location of the obstacle. In contrast, the CMC occurs even in damage free CFRP structures and manifests itself in conversion taking place permanently or continuously. The reason for this phenomenon is the irregular distribution of fiber and matrix material. With the extension to the stochastic context, it is possible to work out an enhanced material model which is able to reproduce the observed wave behaviour.

Research on Finite Element Model Modification of Carbon Fiber Reinforced Plastic (CFRP) Laminated Structures Based on Correlation Analysis and an Approximate Model.

Carbon fiber reinforced plastic (CFRP) laminated structures have been widely used in modern engineering due to their excellent material properties, especially in the aerospace and shipping industries. This requires a high-accuracy finite element model of CFRP laminated structures. However, it is difficult to master the mechanical properties of CFRP structures comprehensively and accurately due to influences from multiple aspects, such as dispersion of material properties, uncertainty of manufacturing technologies, etc. Therefore, a finite element model modification method of CFRP laminated structures based on correlation analysis and an approximate model was proposed. Aiming at minimizing the difference between the analysis model and the measured inherent frequency, the proposed method improves the finite element modeling accuracy of CFRP laminated structures, by iterative optimization based on a global optimization algorithm. In order to solve the problem of high spatial dimension and slow searching in modification of CFRP laminated structure models, the Pearson correlation analysis method was used to screen the material parameters which exert significant impacts on frequency characteristics to reconstruct the searching space. Based on significance parameters, an approximate response model of the CFRP laminated structural system was established. Meanwhile, the modeling accuracy of different orders of response surface models (RSM) and a radial basis function (RBF) neural network model was analyzed, and the best approximate modeling scheme was obtained. The approximate model was updated based on the multi-island genetic algorithm (MIGA) to modify the finite element model of the CFRP laminated structure model. The maximum error and mean error of the updated model are 1.47% and 0.45%. It was proved that the material parameters modified by the proposed method are applicable to simulation analysis of the CFRP laminated structure.

Effect of random vibration-assisted vacuum processing on void development and interfacial properties in composites

Voids and delamination are viewed as significant defects in the manufacturing process of carbon fiber-reinforced plastics structures and have been demonstrated to reduce the final performance of co...

Self-resistive electrical heating for rapid repairing of carbon fiber reinforced composite parts

Carbon fiber reinforced plastics have been widely applied in aircraft structures, which require more rapidly repairing process when carbon fiber reinforced plastics structures are damaged. A novel method of self-resistive electrical heating to rapidly repair carbon fiber reinforced plastics parts was investigated, which utilizes the electrical current passing through the carbon fiber to generate heat and directly cure the whole carbon fiber reinforced plastics patches. A portable self-resistive electrical repairer with automatic temperature control unit was developed. Repair experiments of self-resistive electrical and the electrical blanket heating were conducted, in which the heating temperature distribution field was observed by the thermal infrared imager. The degree of cure of different samples was characterized, and the cross-section morphology and void content of different samples were assessed. Flexural and inter-laminar shear tests were carried out. The experiment results showed that the flexural and interlaminar shear strength recovery ratio of self-resistive electrical repaired samples attained to 85.48% and 65.89%, which was the same or even slightly higher level as the blanket repairing process, but only cost 62% repairing period and 58% repairing energy of the later one. Finally, from the experimental results, the heat transfer behavior of different processes was analyzed.

Polyaniline-based all-polymeric adhesive layer: An effective lightning strike protection technology for high residual mechanical strength of CFRPs

Carbon fiber reinforced plastics (CFRPs) are vulnerable to lightning strikes due to their low electrical conductivity and low heat resistance. A lightning strike can damage CFRP structure catastrophically. Most common lightning strike protection (LSP) technology, consists of expanded metal foils/films on top of composite structures. This technology possesses disadvantages such as increased weight, galvanic corrosion, expensive integration and repair costs. In the present study, authors introduce a novel, easy to apply and all-polymeric LSP material. A doped intrinsic conductive polymer i.e. Polyaniline (PANI) dispersed in a thermosetting cross-linking polymer divinylbenzene (DVB) has been used to prepare an adhesive layer of 0.25–0.4 mm thickness. CFRP structure coated with PANI-based LSP layer, when tested against simulated lightning impulse current of 100 kA, demonstrated effective dissipation of the current, rendering almost 100% safety to the CFRP structure. PANI showed the capability to create a 3-D conductive network due to its self-assembling property, which makes it superior compared to its counterpart carbon/metal nano-filler based LSP technologies. Almost 100% residual strength of PANI-LSP protected CFRPs is reported in this work.

Damage degree prediction method of CFRP structure based on fiber Bragg grating and epsilon-support vector regression

The assessment of structural damage is of great significance for ensuring the service safety of carbon fiber reinforced plastics (CFRP) structures. In this paper, the damage degree prediction method of CFRP structure based on fiber Bragg grating and epsilon-support vector regression was studied. The structural dynamic response signals were detected by fiber Bragg grating sensors. Then, the Fourier transform was used to extract the dynamic characteristics of the structure as the damage feature, and the damage feature dimensionality was reduced by using the RReliefF algorithm. On this basis, the damage degree prediction model of CFRP structure based on epsilon-support vector regression was established. Finally, the method proposed in this paper was experimentally verified. The results showed that the epsilon-support vector regression model can accurately predict the damage degree of unknown samples, and the absolute relative error of 27 experiments was less than 10% for 30 testing experiments. This paper provided a feasible method for predicting the damage degree of CFRP structures.

How surfaces of carbon fiber reinforced plastics with thermoset matrices need to be treated for structural adhesive bonding

ABSTRACT In this research study two very different surfaces of the same type of a carbon fiber reinforced plastic (CFRP) are deliberately prepared to compare characteristics of the created surfaces and the achieved strength of the adhesively bonded joints. Within this approach, explanatory power of state-of-the-art surface analysis methods, applicability of destructive tests and adhesion mechanisms for high-strength adhesive bonding of CFRP are discussed. One CFRP surface was prepared with a highly optimized atmospheric pressure plasma treatment (APPJ). Another CFRP surface was created with a highly optimized and adjusted vacuum blasting process. Initial CFRP surface and treated CFRP surfaces were characterized with surface analysis methods X-ray Photoelectron Spectroscopy (XPS), Atomic Force Microscopy (AFM), contact angle measurements and Field-Emission Scanning Electron Microscopy (FE-SEM). Mechanical strengths of adhesive joints were determined with three different destructive test methods (single lap shear, double cantilever beam, centrifugal adhesion test) for all surfaces. Interestingly, the very different surfaces led to adhesive joints with comparable mechanical strength. After a comprehensive discussion, the author isolates relevant adhesion mechanisms for adhesive bonding of surfaces of thermoset polymers. The study concludes with proposals for necessary future research efforts to achieve the aimed reliable high-strength adhesive joints for safety relevant CFRP structures.

Damage detection method of CFRP structure based on fiber Bragg grating and principal component analysis

Damage detection is very important to ensure the safety of carbon fiber reinforced plastics (CFRP) structure. In this paper, a damage detection method of composite structure based on fiber Bragg grating sensors and principal component analysis was proposed. Firstly, the non-damaged CFRP structure was excited by active excitation, and its dynamic response signal was monitored by FBG sensors. The FBG sensors were attached to the surface of the CFRP structure. And then, the frequency response was extracted as the damage feature by using the Fourier transform method. After that, principal component analysis was used to reduce the dimension of damage feature. The principal component model of the non-damaged state only based on the non-damaged state samples was established and the reconstruction error threshold was determined as the index of damage detection. Thus, structural damage can be detected by determining if the reconstruction error of the new sample was greater than the reconstruction error threshold. Finally, the damage detection system of CFRP structure was constructed to validate the proposed method. The experimental results showed that the method proposed in this paper can accurately identify whether the structure was damaged only based on the non-damaged state samples. This paper provided a feasible method for the damage detection of CFRP structure.

Residual crashworthiness of CFRP structures with pre-impact damage – An experimental and numerical study

This study investigated the effect of transverse pre-impact damage on the load bearing capacity and failure behavior of square carbon fiber reinforced plastic (CFRP) tubes for axial crushing. The CFRP tubes were impacted transversely in different levels of impact energies to generate initial damage, and then the specimens were further crushed axially to evaluate the relation between transverse impact energies/positions and the residual axial crashworthiness. A finite element (FE) model was also developed to simulate the complex damage behavior of the CFRP tubes under these two different loading processes, based upon the continuum damage model (CDM) with user-defined material subroutine in Abaqus. A combined failure mode was observed in the transverse pre-impact tests, in which delamination was combined with partial or complete fiber breakage when increasing the impact energy from 10 J to 30 J. In the axial compression tests, two typical failure modes with circumferential fracture near the pre-impact position were identified for the damaged tubes, exhibiting significant difference from the progressive folding failure seen in undamaged tubes. Further, the damaged tubes yielded up to 38%, 58.5% and 58.3% reduction in terms of the peak load, mean load and energy absorption respectively in comparison with the specimens without pre-impact damages. It is also found that the residual crushing capacity decreased with increase in the transverse pre-impact energy; nevertheless the residual axial crushing properties were insensitive to single or double impacts on different circumferential positions. The failure modes of fiber breakage, delamination and matrix crack were investigated in detail by using the FE analysis.

Laser-Based Surface Modification of Microstructure for Carbon Fiber-Reinforced Plastics

Bonding repair is a powerful feature of carbon fiber-reinforced plastics (CFRP). Based on the theory of interface bonding, the interface adhesion strength and reliability of the CFRP structure will be directly affected by the microscopic features of the CFRP surface, including the microstructure, physical, and chemical characteristics. In this paper, laser-based surface modification was compared to Peel-ply, grinding, and polishing to comparatively evaluate the surface microstructure of CFRP. The surface microstructure, morphology, fiber damage, height and space parameters were investigated by scanning electron microscopy (SEM) and laser confocal microscopy (LCM). Relative to the conventional grinding process, laser modification of the CFRP surface can result in more uniform resin removal and better processing control and repeatability. This decreases the adverse impact of surface fiber fractures and secondary damage. The surface properties were significantly optimized, which has been reflected such things as the obvious improvement of surface roughness, microstructure uniformity, and actual area. The improved surface microstructure based on laser modification is more conducive to interface bonding of CFRP structure repair. This can enhance the interfacial adhesion strength and reliability of repair.

Effectiveness and Shape Design of CFRP Structural Sections in Roof Trusses

Roof trusses have always been constructed using traditional materials such as timber and steel which have been used in industrial, agricultural and residential structures. This study aims at obtaining further effectiveness which can be attained when Carbon Fibre Reinforced Plastic (CFRP) is used as the structural section materials of a roof truss. A CFRP truss section is compared with the same 350W (AISC 2005) steel truss section subjected to same loading conditions Results indicate that the steel truss withstand deflections more than the CFRP truss in both linear and non linear analysis. The CFRP truss was 77% lighter that the steel truss. It was also concluded that the shape of sections used in steel can be adopted for CFRP structures.

Lamb Wave Interaction at Delamination and Debondings Due to Impact Damage in Complex Stiffened CFRP Structures

The increased usage of carbon fiber reinforced plastics (CFRP) for primary aerospace structures involves dealing with the susceptibility of composite laminates to impact loads as well as the occurrence of barely visible impact damages. One special case among impact sources is the so-called blunt impact, which may cause damage primarily to the internal structure. Therefore, the assessment of debonding of stiffening elements in CFRP structures poses an attractive application case for structural health monitoring by guided ultrasonic waves. Wave propagation phenomena at impact damages as well as the signal processing utilized to extract a damage related feature (i.e., damage index (DI)) contribute to the sensitivity, and thus, to the reliability of structural health monitoring (SHM) systems. This work is based on data from the EU-funded project SARISTU, where a generic CFRP door surrounding fuselage panel with an integrated sensor network has been built and tested by introducing a large number of impact damages. Wave interaction of delaminations and stringer debondings of different size and morphology in omega-stringer stiffened structures are examined to highlight the factors contributing to the sensitivity. Common damage indicator formulations for the use with imaging algorithms, such as the reconstruction algorithm for the probabilistic inspection of damage (RAPID), are applied on data from various damage cases. Furthermore, the difference in detectability of delaminations and debondings as well as the implications on imaging algorithms is examined.

Dynamic Mechanical Properties of Typical CFRP Laminate Under High-impact Compressive Loads

Because of its outstanding mechanical properties, carbon fiber reinforced plastic (CFRP) is widely used in structures bearing high-impact loads. In this study, based on the bridging model, the dynamic constitutive equations and yield criteria concerning the strain rate effect of composite material are used to analyze the optimal design and compressive strength of a CFRP laminate under high-impact compressive loads. The mechanical properties of laminates under different strain rates were calculated and compared with experimental results to verify the viability of improving the bridging model, where our theoretical results agreed with them. Structural design schemes based on this were analyzed and a ply scheme satisfying our requirements selected. Using this scheme, samples were manufactured and subjected to quasi-static and medium strain rate compressive tests, and the results agreed with theoretical calculations. This study can help design optimal CFRP structures for penetrator case under high-impact compressive loads and predict their properties.

Modeling for CFRP structures subjected to quasi-static crushing

Carbon fiber reinforced plastic (CFRP) composite materials demonstrate significant promise to further improve weight to performance in automotive engineering. Nevertheless, design of CFRP components for crashworthiness criteria remains rather challenging and typically requires laborious trial-and-error processes. This study aims to promote computational design of CFRP structures by establishing effective constitutive model that is implemented in the commercial finite element code Abaqus/Explicit. Two different numerical models (namely, the single layer shell model and the stacked shell model) were developed to simulate experimental crushing tests on the square CFRP tube. The effects of key parameters for these two FE models were analyzed, respectively. The comparisons of numerical results with experimental data indicated that the 9 layers stacked shell model is capable of reproducing experimental results with relatively higher accuracy. Based on the validated modeling approach, crushing behaviors of several CFRP thin-walled structures with different cross sectional geometries and thicknesses were further explored. The failure modes and key indicators in relation to the structural crashworthiness were investigated for identifying a best possible sectional configuration. It is found that the circular tube shows superior specific energy absorption capacity of all different tubal configurations with the same wall thickness, meaning that the tube with circular section is of good potential asa crashworthy CFRP structure.

Weld-bonded stainless steel to carbon fibre-reinforced plastic joints

This paper investigates a resistance spot welded reinforced adhesive (weld-bonded) joint between 304 stainless steel to carbon fibre reinforced plastic (CFRP), where welds are made both with and without the reinforcing carbon fibres present. Successful welds with the fibres present could only be produced with high electrode pinch forces, which helped reduce contamination of the weld nugget. Similar joint strengths were achieved in both cases, however the joints without fibres exhibited an increased strain to failure. Both joints were significantly stronger than either an adhesive joint or a comparable bolt reinforced adhesive joint. These techniques provide an alternative for joining thin metallic components to CFRP structures where increased strength and integrity is required.

Nondestructive testing of contaminated CFRP surfaces with the BonNDTinspect\xae system

Composite materials are already being used in the mass production of structural components in the automotive industry, particularly at the BMW Group. Adhesive bonding is generally considered to be the best technique for joining CFRP (carbon fiber reinforced plastic) light-weight structures. The conventional NDT (nondestructive testing) methods currently being used focus on the detection of material defects, e.g. debonding. These methods give little information about the surface properties or bond quality. A new ENDT (extended nondestructive testing) method is the BonNDTinspect system, based on a patent held by the Fraunhofer IFAM. An ultrasonic atomizer nozzle creates a water-aerosol and the small water droplets are sprayed onto the surface. Depending on the surface properties (surface energy or contamination state), the aerosol will form wide or narrow drops on the surface. We determined a test for certain contaminations, including release agents, oil, and fingerprints, the detection of which is critical to ensure the performance of adhesively joined CFRP structures. The BonNDTinspect system is an inline-capable NDT technique that is suitable for distinguishing surface states for adhesive bonding of CFRP. We verify this statement with destructive tests, including the single-lap shear test. It was found by using an extension of the evaluation criteria that it is possible to detect contamination such as water-soluble release agent, CFRP dust and fingerprints. The investigated contamination with hydraulic oil allows only a clear differentiation between cleaned and contaminated. A contamination with corundum is not detectable by the BonNDTinspect system alone.

Intrinsic Aluminum CFRP Hybrid Composites Produced in High Pressure Die Casting with Polymer Based Decoupling Layer

Combining aluminum and carbon fiber reinforced plastic (CFRP) has been a key focus in realizing lightweight concepts. Manufacturing technologies for high load-bearing and ultra-lightweight CFRP structures have reached a high level of innovation. The same goes for near-net-shape high pressure die casting (HPDC) aluminum components, which can be mass-produced in a highly efficient manner. Yet for hybrid composites of these materials, the solutions to date have relied on conventionally mechanical or adhesive joining techniques. The direct joining of these two materials is problematic, due to their electrochemical intolerance and the resulting corrosive degradation. The joining technology therefore is at the center of this challenge. The DFG-sponsored joint research project of Fraunhofer Institute for Manufacturing Technology and Advanced Materials IFAM, Fiber Institute FIBRE, and Bremen Institute for Mechanical Engineering BIME, all three institutes located at the campus of the university in Bremen, aims at combining aluminum and thermoplastic CFRP into an intrinsic hybrid composite. This is to be achieved in a single-step primary shaping process, avoiding conventional joining techniques like adhesive bonding or riveting. To this end, CFRP structures are to be recast with aluminum, creating an electrochemically decoupling layer between the two materials. This decoupling layer can therefore be considered as a key factor for realizing hybrid composites. It also needs to have a high process reliability and be long-term and mechanically stable. Polyetheretherketone (PEEK) thermoplast was identified as a suitable material for that purpose, given its stability at high temperatures and electrochemical insulation effect. First test results show the possibility of incorporating CFRP accordingly by HPDC, resulting in a continuous intact decoupling layer of PEEK. The trend indicated that different thermal treatments as well as different aluminum thicknesses of the hybrid casted sample influence the joint strength. On average, in tensile shear tests a joint strength approximately in the range of current single lap adhesive bonds could be achieved.

Dynamic crash responses of bio-inspired aluminum honeycomb sandwich structures with CFRP panels

Abstract Nature has provided us with extraordinary resources to tackle design challenges facing in modern society nowadays. The multistate structures inspired by animal shell have proven effective to improve the impact resistance of composite laminate. This study aims to identify the crash responses and crashworthiness characteristics of bio-inspired sandwich structures composed of carbon fiber reinforced plastic (CFRP) panels and aluminum honeycomb. The crash responses, failure mode as well as the effects of core side length, height and impact velocity on peak load and energy absorption were explored herein. The differences of crashworthiness characteristics between the CFRP aluminum honeycomb sandwiches and bare CFRP panel were quantified. Two typical load-displacement relations, namely single-peak and double-hump curves, were observed in the tests. It was noted in the energy-displacement curve, where the slopes corresponding to the failure stages of the upper and lower face-sheets, were greater than that in the honeycomb failure stage, indicating that the bare aluminum honeycomb was of lower energy absorption capacity than the CFRP face-sheet. By comparison, the honeycomb filling was an effective way to improve the impact resistance of CFRP structure, yielding higher energy absorption and lower peak load during the impact. It was also found that the crashworthiness characteristics were more sensitive to the core length than to the core height; and the specific energy absorption (SEA) varied insignificantly with the increase in the core height. It was noted that the peak load, absorbed energy and SEA increased significantly under high impact velocity.