Carbon Fiber Reinforced Polymer Laminates Research Articles

Elasto-Plastic Analysis of Two-Way Reinforced Concrete Slabs Strengthened with Carbon Fiber Reinforced Polymer Laminates

This study explores a technique for enhancing the punching strength of reinforced concrete (RC) flat slabs, namely carbon fiber reinforced polymer (CFRP). Four large-scale RC flat slabs were fabricated, to assess the efficacy of this strengthening method. One slab served as a reference and the three other specimens were strengthened with CFRP, as a method of external strengthening. These slabs, featuring identical overall dimensions and flexural steel reinforcement, underwent testing until failure, under the influence of concentrated patch loads. A concrete plastic damage constitutive model (CDP) was developed and employed to examine the strength of two-way RC slabs. Additionally, to enhance the strength of existing RC slabs, carbon fiber reinforced polymer (CFRP) strips are affixed to the tension surface of the sections. The research begins with the calibration of a numerical model, based on data from laboratory tests. The objective of this study is to constrain the plastic behavior of two-way RC slabs reinforced with CFRP, with a focus on establishing an optimal elasto-plastic analysis, aimed at controlling concrete damage plasticity using CFRP, and employing a plastic limit load multiplier. Subsequently, a series of numerical simulations, incorporating different variables, are conducted to investigate shear behavior. The numerical results indicate that an increase in the strengthening ratio has a significant impact on shear strength. Finite element simulations are carried out using Abaqus CAE®/2018.

Effects of long-term exposure of NSM CFRP-to-concrete bond to natural and accelerated aging environments

Carbon fiber reinforced polymer (CFRP) composites can be used to strengthen existing reinforced concrete (RC) structures. The CFRP laminate can be bonded to RC structure using epoxy adhesive via near-surface mounted (NSM) strengthening technique. However, existing literature generally lacks data about durability of NSM CFRP-to-concrete bond. In this study, strengthened concrete elements were exposed to laboratory-controlled environments (at approximately 20 °C/55 % RH, and water immersion at 20 °C) and natural field environments (to promote natural aging induced mainly by carbonation, high temperatures, freeze–thaw attack, and airborne chlorides) for up to four years. Durability tests were conducted yearly for the bond and its constituent materials. The highest bond strength degradations were nearly 12 % and 9 % for the specimens immersed in water and those exposed to freeze–thaw attack, respectively. Besides, environmental conversion factors of 0.88 and 0.93 were derived from a database of existing accelerated, and natural aging data from the present work, respectively.

Dynamic behavior of axially loaded masonry walls strengthened with different innovative techniques under explosion loading

In the realm of construction engineering, brick masonry stands out as a pivotal structural element, renowned for its widespread application. Given the load-bearing nature of masonry structures, the ramifications of wall failure under extreme loads induced by explosions are profound. Particularly concerning is the out-of-plane failure of walls, which occurs without warning and affords scant time for assessment of their response. Brick masonry, characterized by its brittleness and susceptibility to cracking, presents inherent challenges in this regard. Recent examples, such as the Beirut explosion in 2020 and the Nashville bombing in 2020, vividly illustrate the devastating consequences of inadequate blast resistance in masonry structures. These incidents serve as poignant reminders of the imperative to fortify masonry constructions against explosive threats. While ample research exists on the behavior of free-standing braced and unbraced masonry walls subjected to explosive loading, there remains a notable gap in understanding the response of strengthened masonry walls exposed to blast loading, especially when carrying varying axial loads. This study addresses this gap by investigating different strengthening methodologies, namely (1) Cement-mortar plaster, (2) Ferro-cement, and (3) Carbon Fiber-Reinforced Polymer (CFRP) laminate, aimed at enhancing the anti-blast performance of unreinforced masonry walls. The experimentation focuses on a specific unreinforced masonry wall measuring 5000 × 2800 × 230 mm3, constructed from clay bricks and subjected to axial compressive loads of 51, 30, and 9kN/m, corresponding to the loads on the 1st, 2nd, and 3rd storey walls of a 3-storey masonry building, respectively. Utilizing a sophisticated finite-elements code, Abaqus, equipped with an explicit module and Concrete Damage Plasticity model, the blast simulations are meticulously conducted. The walls are meticulously modeled through detailed micro-modeling techniques. Results showed that the utilization of CFRP on the explosion-facing surface of the wall resulted in a remarkable reduction in damage, with the highest decrease of 64% achieved under the lowest axial compression. Conversely, the minimum decrease of 61.71% was recorded under maximum axial compression, highlighting the efficacy of CFRP in mitigating damage caused by explosions on walls. This research sheds light on effective methods to enhance the blast resistance of masonry walls, offering valuable insights for improving structural resilience and informing building codes. Its findings are crucial for ensuring the safety and security of infrastructure against explosive threats, ultimately safeguarding lives and property.

Experimental study on the mechanical property of carbon fiber reinforced polymer with the combined application of digital twin and reduced order algorithm

AbstractRecognizing the anisotropy of mechanical property transfer between carbon fiber reinforced polymer (CFRP) layers, a unidirectional reduced‐order model (UROM) is proposed to comprehensively express the mechanical properties of each layer. A crucial focus is on effectively analyzing the real‐time mechanical properties of CFRP specimens, particularly in accurately identifying and predicting the interlaminar mechanical states of anisotropic CFRP laminated specimens. Moreover, the multi fidelity surrogate (MFS) is employed to replace the computationally intensive model, amalgamating high‐fidelity sensor data with low‐fidelity data obtained from the UROM. The UROM MFS is used to reduce the amount of data in the same layer of CFRP laminates, while preserving the characteristics between different layers and preserving the interlaminar information of CFRP laminates. Finally, the stress prediction results of the UROM MFS and the unreduced MFS model were compared and analyzed. The real‐time response speed of the u‐ROM MFS digital twin (DT) model was increased by 658%. The UROM MFS DT model effectively captures the mechanical properties of each layer of CFRP laminates, enables quick calculation of model parameters, and provides accurate predictions.Highlights First, the interlayer mechanical property of CFRP is analyzed with the combined application of experimental data, DT model, and UROM MFS. Second, MFS is employed to merge high‐fidelity sensor data of CFPR laminate with low‐fidelity data obtained from UROM. Third, the digital twin framework for CFPR laminate with high response speed and calculation accuracy is proposed.

Repair Analysis of Overlay Woven Fabric CFRP Laminates

The increase in aerospace composites usage for structural components demands advanced repair analysis. Overlay repairs of carbon fiber-reinforced polymer laminates offer an alternative that is easier to perform and less time-consuming to produce than the widely used tapered scarf repair and stepped lap. Composite specimen manufacturing was based on both twill carbon/epoxy prepreg and wet lay-up. The repair was performed with both prepreg and wet extra plies to the parent prepreg structure. However, the design of overlay joints must be carefully investigated to avoid generating stress concentration regions at free edges. This study examined specific extra ply terminations' impact on peak stresses in the adhesive bond line. Linear finite element analysis was performed to conduct a maximum principal stress study with a focus on three joint design parameters: ply material, overply effect, and stacking sequence. FEA accurately predicted experimentally observed responses and provided further insight into the failure behavior of the structure. Results showed that overlay joints have a strong sensitivity to ply material type, the number of overply, and stacking sequence. The introduction of overplies provided protection and stiffness at joint tips, and an overply material behavior was identified. The location of 0̊ plies in the composite laminates was highlighted as an important factor. The analysis was then extended to three-dimensional FE models for verification. In conclusion, results showed that high-stress concentration in overlay joints can be mitigated with the introduction of overplies and appropriate changes in joint design parameters to reduce stress peaks at joint tips and corners.

High-Resolution Ultrasound to Quantify Sub-Surface Wrinkles in a Woven CFRP Laminate.

Carbon fiber reinforced polymer (CFRP) composites are popular materials in the aerospace and automotive industries because of their low weight, high strength, and corrosion resistance. However, wrinkles or geometric distortions in the composite layers significantly reduce their mechanical performance and structural integrity. This paper presents a method for non-destructively extracting the three-dimensional geometry, lamina by lamina, of a laminated composite. A method is introduced for fabricating consistent out-of-plane wrinkled CFRP laminate panels, simulating the in-service wrinkle observed in industries that utilize thick structure composites such as the vertical lift or wind power industries. The individual lamina geometries are extracted from the fabricated coupon with an embedded wrinkle from captured ultrasonic waveforms generated from single-element conventional ultrasonic (UT) scan data. From the extracted waveforms, a method is presented to characterize the wrinkle features within each individual lamina, specifically the spatially varying wrinkle height and intensity for the wrinkle. Parts were fabricated with visibly undetectable wrinkles using a wet layup process and a hot press for curing. Scans were performed in a conventional immersion tank scanning system, and the scan data were analyzed for wrinkle detection and characterization. Extraction of the layers was performed based on tracking the voltage peaks from A-scans in the time domain. Spatial Gaussian averaging was performed to smooth the A-scans, from which the surfaces were extracted for each individual lamina. The extracted winkle surface aligned with the anticipated wrinkle geometry, and a single parameter for quantification of the wrinkle intensity for each lamina is presented.

Enhancing CFRP laminates with plasma jet arrays: A study of interlaminar mechanical properties

AbstractThis study delves into the efficacy of plasma jet array treatment in bolstering the interlaminar toughness of Carbon Fiber Reinforced Polymer (CFRP) prepregs, a crucial parameter for their deployment in high‐performance domains. Employing high‐frequency pulsed microsecond plasma jet arrays for surface modification, this research not only demonstrated a 19.49% upsurge in peak load capacity but also a remarkable 29.94% enhancement in Mode I interlaminar fracture toughness, as evident from Double Cantilever Beam (DCB) tests. Furthermore, the surface wettability improvements were underscored by a substantial decrease in water contact angle from 108.33° to 31.45° and a total surface energy augmentation from 12.20 to 64.16 mN/m, post a 1‐min plasma treatment. X‐ray Photoelectron Spectroscopy (XPS) results indicated an appreciable rise in oxygen functionalities, suggesting enhanced resin‐fiber bonding. Additionally, Atomic Force Microscopy (AFM) and Scanning Electron Microscopy (SEM) analyses revealed notable decreases in surface roughness and microscale defects, contributing to smoother surfaces conducive to better adhesive bonding and reduced manufacturing defects. These multifaceted improvements, stemming from alterations in surface chemistry and topography, underpin the significant potential of plasma treatment in advancing CFRP laminate applications requiring superior durability and mechanical resilience.Highlights A plasma jet arrays method to enhance CFRP's interlaminar properties was studied. The influence mechanism of plasma modification was revealed in detail. The interlayer toughening mechanism of CFRP after plasma modification is proposed.

Study of the tensile mechanical properties and failure mechanisms of CFRP screwed joints

Removable joint technology is commonly used in composite laminates for various load-bearing structures. However, existing research primarily focuses on bolted joints, there is relatively limited research on screwed joints in composite materials. This study investigates the influence of connected layer thickness and hole diameter on the tensile behavior of threaded joints in carbon fiber-reinforced polymer (CFRP) laminates. After fabricating different CFRP screwed joint specimens, tensile tests were conducted. The digital image correlation (DIC) technique captured the deformation process. The experiment results indicate a significant increase in load-bearing capacity with the increase in diameter. For instance, joints with an 8 mm diameter exhibited a load-bearing capacity of 10.82 kN. The increase in the connected layer thickness correspondingly enhanced the load-bearing capacity of the joint. The joint with a thickness of 7 mm had the highest load-bearing capacity of 8.83 kN. Besides, with the increase in the thickness of the connected layer, the failure mode transitioned from shear failure in the connected layer to screw pull-out. The tilt angle of the screw during the pull-out process also decreases with the increase in the connected layer thickness. Strain and out-of-plane displacement measurements under ultimate load conditions verify these observations.

Multi-scale “core-sheath” structure of electrospun veils to enhance the interlaminar fracture toughness and in-plane properties of CF/poly(phthalazinone ether ketone) laminates

Carbon fiber reinforced polymer laminates' weak interlaminar properties limit application in high load-bearing structural components. Interleaving electrospun polymer veils or nanofillers between layers has been shown to improve laminate interlaminar fracture toughness (ILFT). In this work, we constructed a “core-sheath” reinforced veil based on a bionic principle inspired by the “tree-root system” by combining ZnO nanowires with electrospun nanofibres to achieve uniform nanocomponent distribution and directional modulation. The effect of “core” structures with different thermal properties and “sheath” structures with different dimensions on the mode I and mode II interlaminar fracture toughness were investigated. It was found that mode I and mode II ILFTs impose different requirements on the interlaminar reinforcement structure due to the different loading modes. The intact nanofiber morphology and longer ZnO nanowires are more advantageous for mode I ILFT. Mode II ILFT, however, is more dependent on the adhesion of the reinforcing veil to the matrix. Mode I and mode II ILFTs values increased by up to 75.6 % and 118 %, respectively, when enhanced with suitable reinforcement veils.

A multiscale experimental study of CFRP-UHPFRC interfacial bond behaviour: Failure mechanisms and a fibre volume dependent bond-slip law

Carbon fiber reinforced polymer (CFRP) materials are being increasingly used with ultra high performance fiber reinforced concrete (UHPFRC) to construct composite structural members making use of their unique properties. As the interfacial bond behaviour between CFRP and UHPFRC plays a key role in the design of such members, it was investigated in detail by multiscale experiments of single shear pull-off tests of 26 CFRP-UHPFRC bonded joints with different bond lengths and widths of CFRP laminates and 0.5–4% volume fraction (Vsf) of straight high-strength steel fibers (length = 13 mm, diameter = 0.2 mm). 12 CFRP-normal strength concrete (NSC) bonded joints were also tested for comparison. The surface strain evolution and the interfacial debonding process were captured and analysed by the digital image correlation (DIC). The complex microscale failure mechanisms, such as the mortar cracking, the CFRP-UHPFRC interfacial debonding, and the steel fibers’ slipping and pulling out of the mortar, were visualised and analysed by the 3D micro X-ray Computed Tomography (μXCT) scanning. It was found that the debonded layer of mortar under the laminates was much thinner than that in the CFRP-NSC bonded joints, due to the lack of coarse aggregates in UHPFRC. The peak pulling forces for the specimens with Vsf = 0.5%, 1%, 2%, and 4% were 9%, 21%, 28%, and 29% higher than those without steel fibers, respectively. An empirical bond strength model and an interfacial bond-slip model were proposed based on the test results, accounting for Vsf, the material properties, and the laminate to UHPFRC width ratio. Finally, the proposed bond-slip model was applied in finite element modelling to simulate the flexural behaviour of an FRP-UHPFRC composite deck in the literature, with the interfacial debonding failure and the ultimate loads successfully predicted.

Research on chloride ion permeability resistance of concretes bonded with CFRP sheets under dry-wet cycles

Considering the widespread application of carbon fibre-reinforced polymer (CFRP) as a protective material for reinforced concrete (RC) structures, this study focuses on the ability of CFRP laminates with different numbers of layers bonded to the concrete surface to reduce the ingress of chloride ions. Variations in the free chloride concentration distribution in concrete before and after CFRP bonding were explored systematically. Nonlinear fitting was applied to the experimental data to establish a calculation model for the diffusion of free chloride ions in concrete with CFRP bonded to the surface, which was subsequently validated. Based on the proposed model, the reduction in the surface chloride concentration of concrete after bonding different numbers of CFRP layers was quantified as the reduction value. Moreover, recommendations for the durable design of RC structures bonded with CFRP in wet-dry cyclic environments are provided. The results indicate that the surface chloride concentration in concrete increases continuously with increasing wet-dry cycles, eventually reaching a steady state. Compared with the concrete specimens without CFRP bonding, the bonding of one, two, or three layers of CFRP led to reductions in the stable surface chloride concentration of concrete by 70.8%, 82.6%, and 84.9%, respectively. And no obvious convection zone was observed in the concrete after CFRP bonding. The validation results of the proposed calculation model demonstrated its high accuracy, making it suitable for durable design and service life prediction.

Revealing low temperature-mechanical coupling failure mechanisms in CFRP laminates with in-situ observations

The number of layers and ply orientations determined the service performance of carbon fiber-reinforced polymer (CFRP) materials. Under low-temperature service conditions, the unclear weakening mechanism of mechanical properties of CFRP aggravates the failure risks of aerospace structures. In present work, the unclear correlation between layers with alternating orientations and the tensile-failure mechanisms of CFRP, the low temperature-mechanical coupling micromechanical behavior, and the failure mechanism of CFRP was experimentally revealed by using a horizontal testing instrument integrated with a low-temperature loading module and real-time optical imaging function. Based on various temperatures in a range from −40 °C to room temperature (RT) and two strain rates, the gradually decreasing trends of tensile strength, Young's modulus, and elongation after fracture with decreasing temperature are obtained. Three failure modes including fiber brittle fracture, fiber pull-out, and interface debonding are proposed to reveal the low temperature-mechanical coupling failure mechanism. Low-temperature asynchronous shrinkage of carbon fiber and epoxy resin promoted the transition from fiber brittle fracture to delamination at the interface. The proposed instrument could establish the approximatively actual service conditions involving low temperature and mechanical load.

Enhancing the Longitudinal Compressive Strength of Freeform 3D-Printed Continuous Carbon Fiber-Reinforced Polymer Composite Laminate Using Magnetic Compaction Force and Nanofiber Z-Threads.

Low fiber-direction compressive strength is a well-recognized weakness of carbon fiber-reinforced polymer (CFRP) composites. When a CFRP is produced using 3D printing, the compressive strength is further degraded. To solve this issue, in this paper, a novel magnetic compaction force-assisted additive manufacturing (MCFA-AM) method is used to print CFRP laminates reinforced with carbon nanofiber (CNF) z-threads (i.e., ZT-CFRP). MCFA-AM utilizes a magnetic force to simultaneously levitate, deposit, and compact fast-curing CFRP prepregs in free space and quickly solidifies the CFRP laminate part without any mold nor supporting substrate plate; it effectively reduces the voids. The longitudinal compressive test was performed on five different sample types. ZT-CFRP/MCFA-AM samples were printed under two different magnetic compaction rolling pressures, i.e., 0.5 bar and 0.78 bar. Compared with the longitudinal compressive strength of a typical CFRP manufactured by the traditional out-of-autoclave-vacuum-bag-only (OOA-VBO) molding process at the steady-state pressure of 0.82 bar, the ZT-CFRP/MCFA-AM samples showed either comparable results (by -1.00% difference) or enhanced results (+7.42% improvement) by using 0.5 bar or 0.78 bar magnetic rolling pressures, respectively.

Experiments and modelling of competitive failure behaviour of CFRP stepped-lap repairs with different design parameters

Stepped-lap bonding has been preferably applied in composite repairing. Nevertheless, the design of repairs presents challenges due to the complex failure mechanism. This paper aims to investigate the mechanical performance and failure mechanism of Carbon Fibre Reinforced Polymer (CFRP) repairs with stepped-lap under tensile loading. Two design parameters are studied, i.e. step numbers and slope combinations. Furthermore, the two different configuration methods of Cohesive Zone Model (CZM) are creatively employed in the developed numerical models, i.e. the adhesive interface adopts cohesive contact, while zero-thickness cohesive elements are inserted in the interlayer between the first two plies of CFRP to simulate possible delamination. Simulations are verified against experimental failure loads, as well as damage evolutions in the repaired adhesive interface, ply interlayer and CFRP laminate. Results showed that the tensile strength of stepped-lap repairs increased with step numbers. The three-step stepped-lap repair with a slope combination of 1/20–1/20–1/20 bears the strongest tensile strength and greatest load-bearing capacity. This work reveals the competitive failure mechanism in adhesively repaired composites and offers a reliable prediction approach for the design of bonded CFRP repairs in practical aeronautic applications.

Low-frequency vibration assisted self-pierce riveting (LV-SPR) of carbon fiber reinforced composite and aluminum alloy

Self-pierce riveting (SPR) has been widely applied to join carbon fiber-reinforced polymer (CFRP) composites and high-strength metallic plates in the automotive and aerospace fields. However, the CFRP is often damaged by rivet piercing owing to its brittleness as well as the relatively small interlocking that forms between the rivet leg and high-strength plate owing to the hard deformation characteristics. Therefore, in this study, a novel low-frequency vibration-assisted self-pierce riveting (LV-SPR) technology is proposed, which utilizes the vibration effect in softening the metal and reducing interfacial friction at room temperature. In riveting experiments involving CFRP and 5052 aluminum alloy utilizing 37Cr4 semi-hollow rivets, LV-SPR exhibited a significant reduction of 68.2% in the riveting force compared to the traditional SPR process. The decreased riveting pressure in LV-SPR effectively mitigated the CFRP damage by 36.2%, which was caused by a reduction in the interfacial friction force between the rivet and CFRP laminate. Moreover, owing to the improved deformation capacity of the rivet and alloy plate by the vibration softening effect, the lateral expansion of the rivet leg in the aluminum alloy was enlarged by 36.4% compared to that using the traditional self-piercing riveting (T-SPR) process. Microscopic characterization revealed that vibrations notably promoted grain refinement and enlarged the subgrain structures of the rivets and alloy plates. Finally, by superposing oscillations, the shear strength of the connection joint increased by 13.5% compared to the T-SPR joint. The proposed LV-SPR was validated as an effective technique to increase the connection strength of high-performance plates, which can efficiently improve the structural safety and promote the widespread application of CFRP/alloys in automobiles and aircrafts.

Mechanical recycling of carbon fibre reinforced polymers. Part 1: influence of cutting speed on recycled particles and composites properties

ABSTRACT Scientific advances and technological requirements to develop carbon fibre-reinforced polymers (CFRP) with excellent strength-to-weight ratios led to the high consumption of CFRP composites. The mechanical recycling of CFRP is a simpler, more economical, and environmentally responsible solution for effectively recovering this structural material that contains epoxy resin and carbon fibre. CFRP laminates were placed on a Computer Numerical Control (CNC) and milling cutting at 1100, 1800, and 2500 rpm. The recovered CFRP particles were mixed with acrylonitrile butadiene styrene (ABS) using the melt intercalation approach. Recovered CFRP particles increased the molecular mobility and reduced the thermal stability of ABS. The main differences between the ABS and the composites were a more pronounced necking region in the ABS than in the composites and a notable reduction in strain. The strain of the ABS was 27.58%, while in the composites, it was 4.29, 4.02, and 3.51%, depending on the cutting speed. Thus, ductility decreased up to 87% in the composites. This work’s successful CFRP mechanical recycling method provides epoxy powder, individual carbon fibres, and CFRP particles, opening a research field of great economic and environmental relevance in developing new materials.

Bottom-up stochastic multiscale model for the mechanical behavior of multidirectional composite laminates with microvoids

This study for the first time develops a coherent and efficient bottom-up stochastic multiscale modeling approach, which aims at revealing the impact of microvoids on the mechanical properties and damage mechanisms of multidirectional (MD) carbon fiber-reinforced polymer (CFRP) composites. To this end, void defects within the composites are characterized using X-ray micro-computed tomography to construct a high-fidelity representative defective volume (RDV). Subsequently, the influence of voids on the microscale failure modes and macroscopic mechanical properties are analyzed by using micromechanics-based finite element (FE) method. Finally, a stochastic multiscale analysis method is developed to predict the mechanical response of MD CFRP laminates and validated by tensile and three-point bending experiments. The results indicate that the proposed framework can accurately predict the mechanical behavior and damage mechanisms of MD CFRP composites. This framework is believed to be a promising method to provide valuable guidance for reliability design and optimization of CFRP composites.

Adaptive ultrasonic full-matrix imaging of internal defects in CFRP laminates with arbitrary stacking sequences

The phased-array-based ultrasonic full-matrix imaging of carbon fibre-reinforced polymer (CFRP) laminates with total focusing method (TFM) is promising yet challenging due to the layered configuration and strong anisotropy. Traditionally, the precise a priori knowledge of ultrasonic ray paths in anisotropic CFRP laminates are required for accurate TFM imaging, making the inspection process complicated and time consuming. In this work, an adaptive ultrasonic-imaging method is proposed for arbitrarily anisotropic CFRP laminates through full-matrix capture (FMC) and TFM. Firstly, the elastic constants are derived from the FMC data, upon which the 3D group-velocity profiles of ultrasonic waves in CFRP laminates with arbitrary stacking sequences can be calculated. The anisotropic effect is then adaptively corrected for TFM. To verify the effectiveness of proposed method, simulations and experiments are conducted on different anisotropic CFRP laminates with mimicked defects, such as delamination, cracks, voids, etc. The simulated and experimental results well agree with each other, indicating that the proposed method can accurately reconstruct the internal defects of different types. This work provides a convenient and comprehensive avenue towards the ultrasonic inspection of arbitrarily anisotropic CFRP laminates, which markedly benefit the quality control of CFRP components in various fields.

Damage evolution of CFRP laminates by normal and oblique impact erosion of pulsating water jets

Impact direction is one of the critical factors affecting the waterdrop erosion behavior of materials. In this paper, rain erosion damage behavior of a typical carbon fiber reinforced polymer (CFRP) laminate is investigated as a function of the impingement angle (0° as normal impact and 15°, 30°, 45° as oblique impact deviated from the normal) and the relative fiber orientation (parallel PA and perpendicular PE to the fiber direction of the surface layer) under the waterjet velocity of 320 m/s and the impact frequency of 50 Hz via a pulsating waterjet erosion test rig. After continuous impact of multiple waterjets, the typical damage of CFRP laminates is mainly composed of the central erosion crater and the upheaval of the surrounding surface layer. The onset and evolution of damage are mainly driven by the shear action of lateral jetting and stress concentration of hydraulic penetration exerted at the surface irregularities. There is no significant difference in the damage modes between the normal and oblique impacts, except for the asymmetric distributions of matrix cracking in PA-orientation and delamination in PE-orientation caused by the angle inclination. With the impingement angle increasing, the volume loss and erosion depth decrease while the incubation period increases mainly due to the decreases of water hammer pressure and contact area. The PA-orientation can cause lower incubation period and larger erosion mass loss than PE-orientation before the surface ply is penetrated, with the underlying mechanism related to the distinct tensile properties of CFRP laminates along longitudinal and transversal directions and the asymmetric properties of fiber–matrix interface. According to the fitting results of erosion curves, it is possible to describe the incubation period and the erosion rate of CFRP laminates under oblique impact using a two-branch equation, and Springer model can give reasonable prediction for the average incubation period and PA-orientation erosion rate of a certain oblique impact occasion with known 0° impact erosion results.

Implementation and precision of CFRP geometric configurations before adhesive bonding repair based on 1064 nm infrared pulsed fiber laser ablation

Before the adhesive bonding repair of the aircraft’s CFRP (Carbon Fiber Reinforced Polymer) structure, the common practice is to remove the damaged area material and form a predetermined repair geometric configuration using the grinding method according to the repair design shape. The geometric accuracy, dimensional accuracy, and surface characteristics of the geometric configuration have a significant impact on the adhesive bonding and repair effectiveness in CFRP structure repair. In the study, a 1064 nm infrared pulse laser was used to selectively and controllably remove the three layers of cured prepreg (fiber and resin) on the surface of the CFRP laminate structure. Based on the analysis of geometric accuracy and surface morphology, the effectiveness of CFRP adhesive bonding repair was verified. The results showed that, under the condition of undamaged CFRP fibers and thorough removal of the surface resin, the laser achieved a dimensional accuracy of ± 50 μm and a thickness accuracy of ± 20 μm for CFRP geometric configuration. The angular accuracy of rectangular configuration was less than ± 0.15°, and the roundness accuracy of circular configuration was greater than 0.995. The mechanical validation of the adhesive bonding repair indicated that the tensile strength recovery rate of CFRP repair structure after laser treatment was 84.7 %, and the failure mode was cohesive failure.