CFRP Structures Research Articles

Study of equi-energetic effects on the low-velocity impact and compression after impact response of carbon fiber composite tube structures

This study investigates the equi-energetic low-velocity impact (LVI) and compression after impact (CAI) response of tubular CFRP structures. Various layups and geometries are employed to understand the effects of impact energy, configurations, and damage on the CAI behavior. Low mass-high velocity (LMHV) impact results in greater absorbed energy than high mass-low velocity (HMLV). Impact damage formation, especially circumferentially, significantly affects CAI strength. Braided tubes show higher impact resistance, but weaker compressive strength compared to unidirectional/twill tubes. A new analytical model provides accurate prediction of CAI strength. This study emphasizes the importance of impact energy and lay-up configurations in CFRP design.

Mechanical properties of CFRP and shrinkage compensating concrete actively confined and strengthened short columns under freeze–thaw cycles

AbstractThis paper investigates the use of CFRP precast tubes filled with shrinkage‐compensating self‐consolidating concrete (SSCC) for strengthening columns. Compared to ordinary self‐consolidating concrete (OSCC), SSCC can address the stress hysteresis issue caused by the poor cooperation between CFRP and concrete structures, while also enhancing the mechanical properties of columns. Experimental and theoretical studies were conducted to examine the tensile mechanical properties of CFRP materials and the axial compressive mechanical properties of CFRP precast tubes strengthened with OSCC or SSCC under freeze–thaw cycles. Various variables were considered, such as freeze–thaw cycles, types of strengthening concrete, and the number of CFRP layers. The results indicate that freeze–thaw cycles do not affect the ultimate tensile strength of CFRP but significantly reduce its ultimate tensile strain. SSCC‐strengthened specimens exhibit higher load‐carrying capacity and better ductility compared to OSCC‐strengthened specimens. In addition, compared to the strengthening of two‐layer CFRP and SSCC, the strengthening of one‐layer CFRP and SSCC more effectively demonstrates the advantage of the post‐tensioning effect. However, the ductility coefficient of the columns decreases with an increase in the number of freeze–thaw cycles, regardless of the number of CFRP layers or type of strengthened concrete. Modified theoretical model accurately predicts the normalized peak stress and strain of the strengthened specimens.

SLSR-Based Microwave NDT Array Sensor for Surface Cracks Inspection on Coated CFRP Structures

SLSR-Based Microwave NDT Array Sensor for Surface Cracks Inspection on Coated CFRP Structures

Interlaminar fracture behaviour of emerging laminated-pultruded CFRP plates for wind turbine blades

Laminated pultruded composite plates are gaining interest for use in wind turbine blades due to their excellent structural performance with affordable cost. However, there is limited understanding of their fracture properties. The present work explores the interlaminar fracture behaviour of pultruded composite plates, bonded through resin infusion, to form thick CFRP structures. Mode-I, −II, and mixed-mode (I/II) tests were performed to obtain fracture properties at different mixed-mode ratios. Mode I crack propagation exhibits stick–slip behaviour, resulting in brittle failure in a few steps, while mode II provides more stable crack propagation along with cohesive failure. The mixed-mode fracture patterns follow the trend of the mode-mix ratios, in which higher mode-mix ratios (more mode II) induce more stable crack propagation. Benzeggagh-Kenane and power law criteria were compared regarding their prediction of crack initiation toughness given a mode mix ratio, and a linear relation between the mixed-mode I/II fracture toughness components could exist at interfaces of laminated pultruded plates. Meanwhile, applicability of testing standards and the effect of manufacturing-induced defects on fracture properties are thoroughly discussed. The results show that existing standards provide sufficient support for characterising fracture properties of bonded pultruded plates; and that manufacturing-induced defects can be detrimental to crack propagation and cause more brittle behaviour in mode I dominant cases, while beneficial effect of defects by toughening the interface was exhibited in mode II dominant cases.

Bonding and fracture properties of SPI-CNTs/epoxy adhesive on SPI-CNTs deposited CFRP composites

Adhesive bonding is frequently used to connect CFRP structures due to its lightweight, easy implementation and high production efficiency. However, the bonding performance of adhesive is far from satisfactory as dimension and complexity of CFRP structures increase. In this paper, a new bonding strategy for CFRP structures via introducing soy protein isolate modified carbon nanotubes (SPI-CNTs) both in the epoxy resin adhesive and on the bonding parts' surface is proposed. Results showed that SPI can effectively improve CNTs' dispersion on the two areas, notably enhancing the bonding performance of the adhesive. When 0.3 wt% SPI-CNTs was added in the adhesive, its adhesive shear strength increased to 12.76 MPa, which was 72 % higher than the unmodified CNTs/epoxy adhesive. Based on this concentration, the SPI-CNTs was deposited uniformly on the surface of the CFRP adherends via bath sonication and resulted in a maximum shear strength of 15.59 MPa, which was 22.2 % greater than the untreated ones. Based on the above condition, the Mode I fracture toughness G1c and the Mixed-Mode I/II fracture toughness Gc could reach a maximum of 0.31 and 0.44 N/mm respectively, which were 117 % and 97 % higher than the pure epoxy adhesive. The present work shows a new pathway for the effective bonding of CFRP structural components used in the aerospace industry.

SHM System Architecture based on Ultrasonic Guided Waves for Aircraft Application Scenarios

Structural health monitoring (SHM) based on ultrasonic guided waves is a novel technology using permanently attached actuator and sensor networks, data acquisition and data evaluation systems to enable in-service inspection of aircraft structures. A modular and decentralized SHM system is presented that can be adapted to various aircraft application scenarios. The central element of the SHM system is a robust sensor array based on piezocomposite technology that can be efficiently integrated into the manufacturing process of CFRP structures (via co-bonding). The sensor array consists of integrated piezoceramic transducers, temperature sensors, electrical cables and integrated electronic components. The decentralized system architecture of the individual electronic components reduces the installation and cabling effort during the assembly of aircraft structures as well as the system weight. Integration and installation aspects as well as various system architectures are discussed using an Airbus A350 door surround structure. Furthermore, an outlook of damage detection under representative flight loads is given.

STRAIN GAUGE TESTING OF 3D-PRINTED CFRP SANDWICH STRUCTURE

In this article, the authors present composite sandwich-type CFRP structures and a study of their properties by strain gauge testing. The paper presents the modeling of a parameterized elementary unit serving as the core of a 3D printed structure using Fused Deposition Modelling (FDM) technology. The properties of these structures with different outer layers made of pure epoxy resin and resin with 10% and 20% carbon fiber powder were then investigated. Based on the results of the strain gauge tests, material models were reconstructed for each resin layer, which can be used in computer FEM studies of more complex components. As an application example, a strength analysis of the driver's seat of a Greenpower car made with printed sandwich structures coated with carbon fiber powder resin was conducted.

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.

Passive imaging in composite plates using Green’s function reconstruction from the diffuse field

Aiming at the problem that enables damage detection in real-time without any active excitation sources, we propose a new passive detection way based on diffuse field for structural health monitoring of CFRP structures. In this work, the wave propagation was first simulated with a k-space pseudo-spectra method in CFRPs. A diffuse field can be generated after multiple scattering from fibers and boundary reflections. After that, the reconstruction of Green’s function was investigated through different parameters including the number of noise sources, pulse bandwidth, transducer distance, etc. The delamination inspection process was explored by contrasting the reconstructed Green’s function with the referenced Green’s function on the composite plate. Finally, a probability distribution-based imaging algorithm was further applied for delamination imaging.

Prediction of creep failure life for unidirectional CFRP with heat-resistant epoxy resin as matrix exposed to high temperature under tension load

The accelerated testing methodology (ATM) developed by the authors is the methodology for predicting statistically long-term life of CFRP structures based on the viscoelasticity including the time-temperature superposition principle for matrix resin. The formulae of ATM are expressed as the product of static strength at room temperature, its dispersion, and the viscoelastic parameter of matrix resin that changes with time and temperature history. This methodology ATM was applied to the prediction of long-term creep failure life for unidirectional CFRP with heat-resistant epoxy resin as matrix under heat degradation in this study. Resin impregnated CFRP strands with heat-resistant epoxy resin were molded using a filament winding system as virgin specimens of unidirectional CFRP. Then, heat-degraded specimens were prepared by exposing the virgin specimens under acceleration conditions determined based on the time-temperature superposition principle for chemical deterioration assuming the condition of practical temperature 110°C and period 10 years. Second, the creep strengths of virgin and heat-degraded CFRP strands were statistically predicted based on ATM and the effect of heat degradation on the long-term life of CFRP strands was evaluated. As results, it was cleared that when the creep strength of CFRP strands undergoes thermal aging at the practical condition, in addition to the strength decrease to time due to the viscoelasticity of the resin, a comparable decrease in static strength and increase in this scatter occur.

Optimization Research on Defect Localization in Ultrasonic Images of Anisotropic and Multilayer CFRP Structures

Optimization Research on Defect Localization in Ultrasonic Images of Anisotropic and Multilayer CFRP Structures

Strain rate-dependent failure modelling of impact damage in laminated CFRP structures

This work focuses on the development, implementation, and validation of a constitutive model for unidirectional composites, aiming to numerically simulate impact damage in laminated structures. The model incorporates a failure criterion that accounts for the influence of strain rate on failure initiation and distinguishes between four failure modes. Failure initiation criteria are 3D phenomenological-based criteria according to the previous work of Puck and Schürmann (1998) and Pinho (2006). Additionally, a mesh-objective damage propagation model is implemented to simulate the impact damage. The impact simulations are performed using Abaqus/Explicit, with the constitutive model implemented through the user subroutine VUMAT. Validation of the strain rate effects on failure initiation is performed using strain rate-dependent bi-axial failure curves for IM7/8552 and AS-4/3506. The analyses indicate that the properties governing the strain rate dependence of failure initiation are consistent. Therefore, the hypothesis was tested in which the same strain rate scaling approach could be used for CFRP materials for which only static properties are available. This hypothesis was tested in high and low-velocity impact simulations at laminated IMS60/977–2 CFRP plates. The results demonstrate that the model accurately replicates the experimentally observed impact-induced contact forces and effectively captures the damaged state in the impacted plate.

Simulation and analysis of delamination damage propagation in CFRP structures

For aerospace vehicles, composite materials are used in large-scale integral panels due to their excellent material properties. In the structure of vehicles, this kind of composite material panel is connected to the internal structure through fasteners. During the composite material molding process and the process of drilling connection holes, some local delamination sometimes occurs, this delamination is hardly detected in time, it can cause great damage to the compressive strength of the structure. In this paper, the ABAQUS/Explicit solver is introduced to analyze the progressive damage of interlaminar separation with cohesive elements, the accuracy difference between different damage modes with typical solvers is verified. Then based on the self-compiled three-dimensional Hashin failure criterion subroutine, the numerical simulation is carried out on the compressive strength of the composite laminate under different pre-delamination damage ranges, and the compressive strength of the single-fastener connection structure with pre-delamination, indicates the fiber damage, matrix damage and delamination damage, in order to explore its failure mode and residual strength law.

Advances in Automated Eddy-Current Characterisation of Carbon Fibre Composites

Ply wrinkles in CFRP laminates are common and can lead to significant failure in composite structures if not properly assessed during the design, manufacturing, and service phases. Evaluating wrinkle geometries can be difficult as they are influenced by multiple factors, such as asymmetry, wavelength, and amplitude, which impact the structure's mechanical performance. High-frequency eddy current testing has proven effective for detecting in-plane waviness and out-of-plane wrinkles. However, the complex nature of CFRP structures and electrical anisotropy may hinder the effectiveness of an eddy current in-line monitoring system if not taken into account. In this study, an advanced modelling approach is employed to simulate the structural variation of the conductivity tensor. A designated wrinkle area, characterized by reduced electrical conductivity, is integrated with structural modelling to replicate a realistic coupon sample containing a wrinkle defect. Four proposed configurations are used to virtually test the samples, and their detection capabilities are analyzed quantitatively.

Tensile, Compressive, and Flexural Characterization of CFRP Laminates Related to Water Absorption

CFRP structures are often exposed to humid environment resulting in water absorption and causing property degradation. Water swelling and its effect on tensile, compressive, and flexural properties were investigated according to ASTM standards. Fracture modes were evaluated by analyzing micrographs of fracture areas. The specimens were cut from twill wave CFRP composite plates fabricated using a vacuum infusion technique. Some of them were immersed in water prior to being mechanically tested. It was found that tensile strength, as well as compressive, and flexural strength and moduli decreased due to water swelling, but fracture strain was found to increase due to water swelling. The most severely affected by water swelling is flexural strength (decreased by 25.72%), and the least is compressive modulus (decreased by 1.89%). Tensile specimens underwent fibre breakage followed by matrix cracking, compressive and flexural specimens showed fibre buckling followed by kinking and crushing where flexural specimens failed in their compressive side. In conclusion, water absorption has a bad impact on the composite strength.

Improving the crashworthiness of CFRP structures by rubbery nanofibrous interlayers

Crashworthiness is the capability of a component to dissipate impact energy throughout its deformation and failure. Composite materials are used to produce crashworthy components to ensure vehicle safety, thanks to their ability to dissipate a high energy amount while maintaining a low weight. The present work investigates the integration of rubbery nanofibers within the laminate interlayers to enhance crush performance. Three different thicknesses (10, 20, and 40 µm) of nanofibrous mats made by nitrile butadiene rubber and polycaprolactone (NBR/PCL) blends were produced by single-needle electrospinning technique and integrated into the laminates during the hand-layup. Mechanical properties of the nano-modified laminates are compared to the reference configuration: the effect of the interlayers is evaluated by Double Cantilever Beam (DCB) and End-Notched Flexure (ENF) interlaminar fracture tests. At the same time, the specific crush energy absorption (SEA) is measured by the compression of self-supporting corrugated specimens. Results show that NBR/PCL nanofibers significantly increase the interlaminar fracture toughness (up to +254% for Mode I and +47% for Mode II), which ultimately helps to improve the total SEA up to +8.2%. The best SEA enhancement is achieved already with a 10 µm nanofibrous membrane while integrating the highest thickness mat has a detrimental effect.

A novel two-level approach to defect detection in braided CFRP using air-coupled ultrasonic testing

Air-coupled ultrasonic testing and C-scan technique has been increasingly applied to the braided CFRP structures owing to its non-destruction, non-contact and high visualization characteristics. Due to the noise, structural vibration, and airflow in the process of detection, the accuracy of defect identification is easily deteriorated. To address this issue and further determine the relationship between the ultrasonic acoustical pressure attenuation and structural parameters, a novel two-level identification method based on the modified two-dimensional variational mode decomposition (2D-VMD) has been proposed. In the first level, C-scan images have been sparsely decomposed into ensembles of modes by 2D-VMD method. Then, the modes have been screened by mutual information method to realize the reconstruction of new image in the second level. Experimental results have shown that the proposed method has the good ability to identify defects with a minimum detectable diameter of 1–2 mm. It has been noted that the ultrasonic acoustical pressure attenuation has become remarkably higher in the twill weave CFRP than the plain weave CFRP and the ratio of pressure attenuation between two weave types of CFRP has decreased with the defect depth increase. Meanwhile, shadows around defects in C-scan images have been suppressed to a great extent. It has been demonstrated that the capability of denoising has enabled the developed method with the accurate detection in terms of the shape, size, depth and weave type. With these advantages, the proposed method has provided valuable insights into the development of an effective method for defect detection of braided CFRP structures.

Numerical Analysis of Thermoplastically Welded CFRP Structures

Carbon fibre reinforced thermoplastic polymers (CFRTP) are of particular interest to the aerospace industry. The possibility of thermoplastic welding as a joining method makes CFRTP an enabler in the pre-installation of systemic functionalities and cabin elements. This can be achieved by dust-free joining. In this work, thermoplastically welded joining zones are numerically analysed and evaluated for their failure behaviour. Finite element models for the evaluation of the peel strength (L-pull) are defined. In particular, the respective beginning of the damage as well as the damage propagation within the thermoplastic joining zone are of interest in order to identify the critical regions and to derive possibilities for design improvements.

Excitation and Reception of Higher-Order Guided Lamb Wave's A1 and S1 Modes in Plastic and Composite Materials.

Featured Application The developed new method will allow use for more detailed analysis and characterization of defects that cannot be detected with A0 or S0 modes. Contemporary technologies are employing composite plate materials developed by using various innovative materials (nanostructures, mica structures, etc.). Application of higher-order modes could allow better detection and characterization of defects characteristic of planar plastic and composite structures, mainly due to shorter wavelength. However, excitation of higher-order modes meets many problems, especially in the case of the air-coupled technique, and is not sufficiently investigated. This is relevant in the cases of paper, high-density polyethylene (HDPE), membranes, GFRP, GLARE, CFRP and other composite structures. The objective of the paper was investigation of the excitation and reception of higher-order guided Lamb wave modes in plastic and composite plates. Therefore, it is appropriate to develop new non-contact ultrasonic measurement methods based on the excitation and reception of guided waves for the study of such objects. The obtained results clearly demonstrate the possibility to excite and receive efficiently different higher-order guided Lamb wave modes with very different phase velocities. The presented comparison of the experimental results with the simulation results showed a good agreement. The combination of air-coupled excitation and non-contact reception enables a non-destructive evaluation and characterization of moving plastic objects and composite structures.

Controllable energy-absorption behaviors of the perforated CFRP tube with an adhesively bonded CFRP patch

A controllable crushing failure mode is very crucial for the energy-absorption (EA) of CFRP structures under the crushing load. However, the structural instability caused by middle-height fracture always occurs in the perforated CFRP structures, leading to low energy-absorption capacity. In this study, a controllable energy-absorption behavior of the perforated CFRP tube was induced by an adhesively bonded CFRP patch. Through numerical investigations, it was found that this method is very effective in reducing the stress concentrations around the hole and then avoiding the middle-height fracture at the initial stage. Hence, effects of adhesive bonding strength, ply-number of patch and single- or double-sided patching on the EA of the perforated CFRP tube were studied for further understanding of patching enhancement mechanisms. It was concluded that the EA of tube increases with the increase of adhesive bonding strength and ply-number of the patch, but the increment by increasing the ply-number of the patch is limited to the adhesive bonding strength. The double-sided patching was proved to be more effective in improving EA and specific energy-absorption than any single-sided patching even with more layers. Finally, the failure and strengthening mechanism from middle-height fracture to progressive crushing failure was analyzed to reveal the controllable energy-absorption behavior.