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• A hybrid generation method of gaps and transverse strips was used to prepare sinusoidal wrinkling defects. • An analytical model based on the sine function was proposed to characterize the shape of wrinkles. • The sensitivity of failure strain to out-of-plane wrinkles is higher than that of compressive strength. • Out-of-plane wrinkles have little effect on interlaminar shear strength, but it can change the location and severity of the delamination failure. Out-of-plane wrinkling is a typical defect in the manufacturing process of carbon fiber reinforced plastics (CFRP). The cross-ply composite laminates containing various wrinkles were designed and prepared based on a hybrid generation method of gaps and transverse strips. An analytical model based on the sine function was proposed to characterize the shape of wrinkles, and the severity of the defect was characterized by the wrinkle angle. With the aim to investigate the compression and out-of-plane shear failure performance of CFRP with wrinkles, experimental and numerical studies were conducted. Results show that the existence of wrinkled zone can significantly reduce the compressive strength of laminates and exacerbate delamination during compression failure, a maximum of 26.9% and 2.8% drop in the compressive and interlaminar shear strength was reported in the most severe experimental conditions. Meanwhile, it is indicated that wrinkles have little effect on interlaminar shear strength, but it can change the location and severity of the delamination failure. In addition, the failure mechanism of several models with different levels of wrinkling defects was studied through numerical analysis techniques. It is confirmed that the numerical model can effectively predict the failure intensity and failure modes. These findings are of great significance for the accurate evaluation of the mechanical properties of composites containing wrinkling defects. [ABSTRACT FROM AUTHOR]

Highlights • Aircraft panel deformation caused by assembly variation and combined load is studied. • Analytical solutions of shell equations are proposed by the Fourier–Galerkin method. • Potential functions are introduced to solve the displacement formulae. • Coordinate transformation model is proposed to convert non-ideal boundary condition. • Experiment of panel assembly has been performed to measure actual deflections. Abstract Based on the Kirchhoff-Love's assumptions, shell equations with non-ideal boundary conditions are established to describe the deformation of thin-walled aircraft panel resulting from positioning variation and clamping force in the assembly process. The shell equations are simplified by the theory of functions of a complex variable, and the analytical solutions are presented by the Fourier series approach and Galerkin method. Meanwhile, the potential functions are introduced to calculate the particular solution, which successfully avoids employing specialized displacement functions with different types of loads. A coordinate transformation model is also developed to transform the actual displacement and rotation boundary constraint in a Cartesian coordinate system into the non-ideal boundary in the curvilinear coordinate system. To verify the general applicability of the proposed method, analytical calculation, the finite element (FE) simulation and experiment in real application have been performed taking into account simply supported, ideal boundary and non-ideal boundary conditions, respectively. The results have shown the good performance of the accuracy of presented solution. Graphical abstract Image, graphical abstract [ABSTRACT FROM AUTHOR]

This paper focuses on revealing the evolution mechanism of micro curing residual stress and its impacts on the mechanical responses of medium-temperature cured composites (T700/7901) under out-of-plane tension. An integrated multi-scale framework is established, the macro-scale thermal-chemical and micro-scale stress–strain models are set up to predict the development of the micro residual stress during curing, and the obtained stress field is further employed as the initial boundary conditions in the micromechanical analysis to explore its effect qualitatively and quantitatively. Experimental and analytical methods are taken to verify and validate the current model. Besides, the impacts of different curing cycles on the micro residual stress and out-of-plane tensile strength are also discussed. The evolution of the micro curing residual stress in T700/7901 composites is intuitively revealed, and the results of the micromechanical model show that the out-of-plane tensile strength increases by nearly 14.4% due to the occurrence of the process-induced stress. Both the interfacial damage and matrix plastic deformation are delayed owing to the residual stress. The work builds the connection between the curing process and mechanical responses of unidirectional composites and demonstrates the necessity of considering curing residual stress in designing and evaluating composites. [ABSTRACT FROM AUTHOR]

The compressive strength drops sharply within 30 mm thick, but slowly when exceeding 30 mm thick. Delamination and fiber misalignment dominate the failure mechanism of ultra-thick laminate. Fiber waviness and voids contribute to the effect of thickness change on compressive strength. Higher interlaminar stresses S13 and S33 at the free edge induced premature delamination. As ultra-thick laminates are widely used in the field of aerospace and marine as the main load-bearing structure, extreme service conditions lead to increasing thickness of composite structures. However, the effect of the increase in thickness on the compressive properties of ultra-thick laminates is still unclear. Herein parameters including fiber volume fraction, fiber waviness, void content and free edge effect that affect the compressive strength with increasing thickness are studied. The results show that the fiber volume fraction of ultra-thick laminates does not change significantly with the increase of thickness due to the adoption of the zero-bleeding process, while the fiber waviness and void contribute to the thickness effect on compressive strength. On the other hand, the high interlaminar stress of the free edge contributes to the premature delamination of the specimen, which dominates the compressive strength of the ultra-thick laminate. Finally, compared with the thin laminate (2 mm), the compressive strength of ultra-thick laminate drops sharply (nearly 28%), but when the thickness increases from 30 to 70 mm, the drop of compressive strength is not obvious (only 10%). When the material and curing process are consistent, the weak interlaminar properties of ultra-thick laminates does not change significantly with the increase of thickness, which leads to the insignificant decrease in compression strength of ultra-thick laminates with the increase of thickness. The proposed empirical relationship between the compressive strength and thickness of quasi-isotropic laminates can provide important theoretical basis and experimental reference for the design of ultra-thick composite structures. [Display omitted] [ABSTRACT FROM AUTHOR]

Z-pinning is an effective method to improve the delamination resistance of composite laminates. The microstructure and manufacturing parameters have a great influence on the properties of z-pin composite laminates. In order to explore the influence of microscopic characteristic parameters on material properties and damage analysis, a parameterized RVE finite element model algorithm for automatic establishment of z-pin reinforced composites was proposed. The algorithm involves avoiding in-plane fibers to pin, generating fiber waviness, and assembling pin-fiber–matrix. Moreover, the statistical distribution of fiber misalignment angle can also be controlled. The developed program can generate RVE models with different parameters efficiently and quickly, such as fiber volume fraction, pin volume fraction, pin diameter, RVE size and fiber waviness. The statistical verification of the distribution of fiber misalignment angle can satisfy the probability distribution modeling of fiber angle. The generated model is divided into meshes, and finite element calculation can be realized. This work is of great significance to study the effects of microscopic characteristics on the properties and to optimize the design of z-pin reinforced composites. [ABSTRACT FROM AUTHOR]

The recycling of waste carbon fibers (CF) has emerged as a crucial research area for both environmental protection and economic sustainability. To sufficiently exert the potential and extend the application life cycle of CFs, here we propose an eco-friendly and efficient electrostatic method for rapid recycling of tightly entangled reclaimed carbon fibers (rCFs) by spark-driven disentanglement and depolymerization. Our research indicates that the spark discharge generated by fiber induction under low-voltage conditions can excite the shock waves and Joule heating effects, resulting in the separation of individual rCFs and the removal of surface organic matrix. Furthermore, the constructed electric field can synchronously and continuously orient the dispersed rCFs to form a vertical alignment in a short time with good maneuverability and directionality for reutilization. The results demonstrate that this one-step disentanglement, depolymerization, and alignment method achieves a high-quality recycling process with 100% dispersion efficiency, minimal damage, cost-effectiveness, uniform distribution, upright orientation, and high-speed embedding performance. Our findings reveal the potential of the electrostatic strategy for the high-quality recycling of rCFs, in line with the principles of the circular economy. The proposed method offers broad prospects for application in continuous in-line production and material manufacturing, and is a step towards sustainable and cost-effective CF recycling. [Display omitted] • A one-step disentanglement, depolymerization, and alignment method for the recycling of virgin rCFs. • Spark-driven shock wave for disentangling tightly entangled rCF clumps. • Spark-driven Joule heating effect for removal of residual organic matrix binding due to reclamation defects. • Alignment orientation of rCFs under electrostatic field attraction. • Recycling with low-energy consumption, uniform distribution, upright orientation, and high-speed embedding performance. [ABSTRACT FROM AUTHOR]

• A semi-automatic calibration process is proposed to improve the efficiency and accuracy of hand-eye calibration. • A hand-eye calibration postures generation model is developed to automatically drive the robot and machine tool to the target postures for calibration. • A machine-learning method is extended to process the profile data collected by laser profilometers, which is faster, more robust and accurate than existing methods. Hand-eye calibration is a vital step in the laser profilometer integrated measurement system. However, the widely applied standard sphere calibration process requires extensive laborious human work and expertise. To standardize and streamline the hand-eye calibration process for Industry 4.0, a semi-automatic calibration process was proposed, containing a calibration postures generation model and an intelligent circle detection method based on machine learning. With the proposed process, the measurement system can collect required data with minimal human participation and automatically processes them, which can significantly improve hand-eye calibration accuracy and efficiency. Additionally, an exhaustive study of the influence of the number of postures on the calibration accuracy of the industrial robot and automated fibre placement (AFP) machine demonstrates the proposed model's effectiveness. Moreover, the current study's results can also be excellent tools for further hand-eye calibration research. Our code and dataset are made publicly at: https://github.com/tangyipeng100/hand_eye_cali_circle_segmentation. [ABSTRACT FROM AUTHOR]

The gap is one of the most common defects during the AFP process, which has a nonnegligible impact on the mechanical properties of laminates. This paper proposes a novel three-dimension reconstructive gap model based on the in-process serial layer scanned point clouds, which can characterizes the gap features inside the laminate and pre-determine the mechanical performance before the subsequent procedures. The measured point cloud will be transformed into eligible and processible formats and then be used to generate a finite element grid using the element-based material property assignment method. The local material properties and fiber orientation are defined by previously assigned classes and grid geometries respectively. The effect of gap size on the in-plane tensile strength and damage evolution of laminates is investigated and both the experimental and simulation results find that the ultimate strength and stiffness show a significant decrease with the increase of gap size. [ABSTRACT FROM AUTHOR]

The fluctuation in tow tension is critical for Automated Fiber Placement (AFP). However, it has not been widely studied. In this paper, a tension variation equation was developed to represent the tension fluctuation during AFP, and experiments were used to demonstrate the validity of the equation. According to the equation, the degree of bending rigidity, power-law friction, and extensibility influence on the tension change was obtained. Additionally, the effects of bending rigidity and extensibility can be disregarded during AFP. Moreover, automatic wire feeding and laying trials were carried out separately, and proper tensions (T = 3 N) for wire feeding and layup under experimental conditions were obtained. Therefore, the equation provides theoretical support for the structural and parameter optimization of the AFP. [ABSTRACT FROM AUTHOR]

[Display omitted] • High charging efficiency at low current by a shield pre-charger with low particle accumulation. • High collection efficiency by a narrow-gap electrostatic precipitator with enamel coated discharging plates. • Collection efficiency was 2.5 times higher than conventional ESPs at 1.2 m/s with 0.2 ppm ozone for continuous operation. We designed and evaluated a two-stage electrostatic precipitator with a shield pre-charger and a narrow precipitator for indoor air cleaning. Unlike the conventional ESP-type air cleaner, the ion generation and particle charging regions were separated by a perforated plate with bias voltage. The electrical energization was achieved by connecting a large resistor in series with the shield electrode. A narrow-gap plate-plate precipitator was placed after the pre-charger for a strong electric field and a high specific collection area. The performance of the shield pre-charger, the narrow precipitator and their integration in lab-scale of 62 m3/h and pilot-scale of 500 m3/h were evaluated. The overall collection efficiency was 98.6 % and 75.5 % for gas flow velocities of 0.4 m/s and 1.2 m/s, respectively. The ozone concentration was around 0.2 ppm for a 24 h continuous operation. Our study showed a close relationship between particle charging and shield voltage with charging current, and a relatively stable performance under high gas flow velocity. The particle collection efficiency was 2.5 times higher than that of a conventional ESP-type air cleaner of similar size at a gas velocity of 1.2 m/s. [ABSTRACT FROM AUTHOR]

• A layered pre-compaction scheme has been developed for curing ultra-thick composite laminates. • A novel uniaxial test method for ultra-thick composite laminates is proposed, and a heavy-duty compression test device is built. • High-modulus specimens have lower strength than low-modulus specimens due to premature delamination. • The test and simulation data of ultra-thick laminates are valuable reference for the design of ultra-thick structure. The damage mechanism of ultra-thick laminates widely used in aerospace and marine industries was obviously different from that of thin laminates. However, compression testing and simulation methods were generally limited to relatively thin laminates. To explore the compressive failure behavior of ultra-thick laminates, Firstly, a layered pre-compaction scheme was proposed to ensure the curing quality of ultra-thick laminates. A heavy-duty compression test equipment (accuracy reached 0.001 mm) was then developed, and a novel compression test method for ultra-thick composite laminates was proposed. The results showed that delamination played a key role in the failure behavior of ultra-thick laminates. The low interlaminar normal strength of ultra-thick laminates (55% of the 2 mm thin laminate) led to premature delamination, and wrinkle defects seriously reduced the compressive strength (11%). The compressive behavior for the matrix-dominant failure specimen predicted by the Puck criterion was the smallest (0.07%), while Tsai-Wu 3D predicted the smallest error (5.1%) for the delamination-dominant specimen. Finally, these findings are helpful for the design of ultra-thick composite structures. [Display omitted] [ABSTRACT FROM AUTHOR]

[Display omitted] • Stochastic microstructures containing randomly distributed fibers with non-circular cross-sections are generated. • A novel deep learning multimodal fusion model is proposed for predicting the mechanical properties of carbon fibers. • The prediction accuracy is significantly improved via multisource heterogeneous data compared with single data type. Recently, deep learning methods have become one of the hottest topics in predicting material properties, however, one bottleneck in current research is the simultaneous analysis of heterogeneous data. In this study, a deep learning fusion model is developed for the first time to predict the material properties of carbon fiber monofilament using textual (macroscopic properties of composites and matrix) and visual (two-point statistics of microstructures) data. For this, 1200 stochastic microstructures are generated using the greedy-based generation (GBG) algorithm. Then, the statistical representations of microstructures are determined using two-point statistics and the macroscopic properties are calculated based on a micro-scale finite element (FE) simulation. Finally, the visual and textual data are fed into the convolutional neural network (CNN) and multi-layer perceptron (MLP) fusion model for predicting the mechanical properties of carbon fibers. The developed hybrid CNN-MLP fusion model achieves encouraging average testing R2 of longitudinal modulus, transverse modulus, in-plane shear modulus, major Poisson's ratio, and out-of-plane shear modulus of carbon fibers with values of 0.991, 0.969, 0.984, 0.903, and 0.955, respectively. Thus, the proposed strategy provides a promising framework for predicting material properties via multisource heterogeneous data and is expected to accelerate the smart design and optimization of materials. [ABSTRACT FROM AUTHOR]

[Display omitted] • A novel greedy-based algorithm is proposed to generate random fiber distributions. • The generated anisotropic RVEs are statistically equivalent to real composites. • The effects of anisotropy on transverse mechanical properties are discussed. A novel adaptive greedy-based generation (GBG) algorithm is proposed to generate 2D random distribution of fibers for unidirectional composites. Inspired by greedy algorithm, the "fitness function" is introduced to better overcome the jamming limit of the traditional hard‐core method than most published algorithms and generate representative volume element (RVE) with high fiber volume fraction adaptively. The good agreement of statistical analysis with experiments suggests that the generated anisotropic RVEs is statistically equivalent to real composites. The influence of anisotropy on mechanical properties and progressive failure process is further revealed based on finite element method (FEM) using a progressive damage model. The results show that the predicted transverse elastic and strength properties are both anisotropic, which is not consistent with the traditional transverse isotropic hypothesis. The proposed algorithm can generate RVEs with high fiber volume fraction (up to 80%) and provide anisotropic RVEs statistically equivalent to real composites, and thus can serve as a promising method to guide the design and analysis of composites. [ABSTRACT FROM AUTHOR]

Developing low-cost and high-performance interlaminar toughening strategies for the advanced CFRP products has great engineering significances. Combining features on practical automated fiber placement and well-recognized 3D reinforcement technologies in the field of CFRP composites, herein, we alternately introduced a reinforcement ply in the manner of translaminar enveloping to generate interlocking and binding action into conventional lay-ups. Remarkably, more than 1700% for mode I (DCB) and 500% for mode Ⅱ (ELS) propagation fracture toughness improvements were experimentally investigated. The evolution of interlaminar toughening behaviors was specifically revealed. Multiple fracture energy absorption stages, successively including delamination tolerance, partial interface pull-out and through-thickness ruptures, reflect long-acting mode I toughening capacity. Under the multi-interface synergistic shear-delamination mechanism, the mode Ⅱ crack growth gets far more effectively suppressed. Meanwhile, the in-plane tensile and compressive strength could be maintained acceptably. We also gave insights into the technical superiorities of this original ply-level reinforcing work. • Translaminar enveloping ply was designed for CFRP interlaminar toughening. • Over 1700% for G I c and 500% for G I I c improvements were investigated. • Toughening mechanisms based on the interlocking and binding action were revealed. • In-plane tensile and compressive strength could be maintained acceptably. • Prospects of this work in high-performance CFRP applications were highlighted. [Display omitted] [ABSTRACT FROM AUTHOR]

[Display omitted] • Nearly-complete disentanglement of carbon fiber clump with turbulent-flow-assisted pneumatic dispersion. • Collection and re-launch of conductive carbon fiber by a modified precipitator with dielectric polarization. • Over 97% of embedded fibers are aligned within 60° to 90° of the azimuthal angle with free-angle flocking. Short-chopped carbon fiber is a main recycled product of carbon fiber materials that shows great advantage of anisotropy on mechanical or functional properties; however, their considerable agglomeration leads to limitations of separating and controlling individual fiber's placement and orientation to fine-tune properties of final composite materials. Here we report a turbulent-flow-assisted electrostatic collection and alignment method that results in exceptional disentanglement efficiency, good collection and launching efficiency with great orientation performance in the gaseous phase. The pneumatic dispersion shows a nearly complete disentanglement efficiency (97 %) at a pressure reduction of 0.5 MPa, which ensures full utilization of electrostatic property for each disentangled fiber. By energization of the dielectric layer at different voltage levels, re-entrainment of conductive carbon fiber was avoided, while the collection and launching efficiency could reach 62 % and 47 %, respectively. By fast-frame imaging, the disentanglement, migration, landing and launching process was studied. The electrostatic polarization for launching leads to free-angle flocking that over 97 % of short fibers were controlled to be aligned within azimuthal angle from 60° to 90° regardless of launching angle. The combined disentanglement and alignment offer an attractive material system for recycling short-chopped carbon fibers and fabricating reinforced or functionalized composite materials. [ABSTRACT FROM AUTHOR]

Carbon fiber composites produce a large amount of scrap during their life cycle, to recycle and fully utilize these short-chopped carbon fibers (SCFs), present work proposes a method to disperse agglomerated SCFs using pure electrostatic forces, while using dielectric polarization to recycle and emit SCFs. SCFs are disorderly distributed or agglomerated into pellets in their natural state, and they need to be sufficiently charged and oriented for reliable collection. A novel plate-plate structure is proposed to maintain a higher load voltage to disperse and emit a larger amount of SCFs. The charging and orientation mechanisms of SCFs were investigated with high-speed camera. The dispersive characteristics of SCFs were statistically evaluated with Ripley's K function and Two-point spatial correlation. And SCFs were emitted by two different modes and the effects of embedment were studied. The results show that large-scale SCFs can be uniformly dispersed and sufficiently charged to 2 × 10−2C/kg in the electrostatic field and oriented to move under the driving of dielectric polarization. Moreover, the direction of embedded SCFs can be well controlled. This study describes the behavior pattern and motion characteristics of SCFs in the electrostatic field and provides a new method for the pollution-free, low-cost, efficient recycling of recycled fibers. It has broad application prospects in the field of carbon fiber reinforced polymer composites and is also significant for the pure electrostatic cross-scale control of micron-sized objects. [Display omitted] • A novel plate-plate structure is designed to recycle short-chopped carbon fibers. • Uniform dispersion of agglomerated fibers using an electrostatic field. • Fibers are charged to 2 × 10−2 C/kg and directionally embedded into the substrate. • Structure has better dispersion performance with the ZrO 2 dielectric layer. • Fibers were embedded into various types of targets using two different modes. [ABSTRACT FROM AUTHOR]

The use of depth cameras, such as a laser profilometer, in defect detection during the automated fiber placement (AFP) process is of great importance as they improve AFP's layup quality and efficiency. However, the collected point clouds are large and of heterogeneous densities. Several works have projected 3D point clouds onto 2D images, but the method has only led to loss of key geometric details. In this study, we proposed a two-stage segmentation method for collected point clouds during the AFP process, named AFP-Seg, to realize AFP in-process inspection. In the first stage, the point clouds fused with collected laser lines and sampling position data are fed into a semantic segmentation network, and the semantic label of each point can be obtained, and in the second stage, point clouds with specified semantic labels are clustered using the post-process algorithm. Through the AFP-Seg method, information about defects regarding types, sizes, and positions are acquired eventually. Compared with our previous method, the AFP-Seg method can reduce the data processing time by 33–66% and obtain more robust and better inspection results. Furthermore, it can be well integrated into a real-time AFP in-process inspection system and easily adjusted based on actual engineering inspection specifications. [ABSTRACT FROM AUTHOR]

In this paper, an experimental study using three groups of different process combinations is first performed to explore the interaction of process parameters with respect to wrinkle formation during tow steering in automated fiber placement (AFP). Furthermore, a process-dependent theoretical model for wrinkle formation is proposed, in which the process dependency is only taken into account for the shear tackiness force based on sensitivity analysis. Closed-form solutions for predicting both the critical steering radius and the stress distribution coefficient are derived. The required model parameters are obtained by experimental tests. In parallel, the correlation relationship between the tangential tackiness force and process parameters is established through the response surface methodology. Thereafter, the model-predicted values for two different steering cases are validated by steering trials. A good agreement between the model-predicted value and the experimental results is observed as the difference is only 2.2% and 3.5%, respectively. Finally, the influence of process parameters on the critical steering radius is discussed and analyzed. It is demonstrated that both the lay-up efficiency and quality for the specific steering constraint could be improved by adjusting the process parameters based on the process-dependent theoretical model for wrinkle formation. • A process-dependent theoretical buckling model is proposed to study the effect of process parameters on wrinkle formation. • The stress distribution coefficient α is dependent on the tensile and compressive modulus of the prepreg tow. • An inter-ply shear test is utilized to measure the shear tackiness force and quantify the effect of process parameters on it. • The tangential tack behavior should be incorporated in the theoretical prediction of the critical steering radius. [ABSTRACT FROM AUTHOR]

• Using the offset distance of the tow as the variable of the wrinkle defect. • Establishing a complete recognition algorithm for extracting wrinkle contours. • Quantifying the influences of wrinkles by the wrinkle smoothness S. This paper studies the influence of wrinkles caused by tows' offset on the bending performance of composite laminates during Automated Fiber Placement (AFP) manufacturing. In this paper, the different distances of tows' offset are set to prepare laminates containing various wrinkles and obtain the characteristics of different wrinkles through the feature recognition algorithm. Then a three-point bending test is conducted to investigate the influences of different wrinkles on the flexural strength and failure mode of laminates. The results indicate that wrinkle defects reduce the flexural strength of the laminate by about 12% to 15%, and the flexural strength is positively correlated with the wrinkle smoothness S. As the wrinkles increase, the failure mode of the laminate gradually changes from local collapse under the punch to instantaneous shear failure in the wrinkle area, and the degree of warping and delamination increases. Therefore, avoiding wrinkle defects can effectively maintain flexural strength and reduce the extent of warping and delamination. [ABSTRACT FROM AUTHOR]

• A concurrent multiscale model for studying in-plane damage of Z-pinned composite. • A novelty is to generate spatial fiber distribution in the fiber reinforced zone. • Microbuckling fibers near Z-pin play an important role in in-plane damage failure. Most of the unit cell models for Z-pinned composite laminates (ZCL) are mesoscopic in scale, where the fiber reinforced zone is modeled as homogeneous but without detailed characterization and failure analysis of the fibers and matrix. This paper proposes a new concurrent multiscale unit cell model to investigate the in-plane micro-damage evolution of ZCL. The novelty worth mentioning is the proposed element-based method to generate random microbuckling spatial fibers in the fiber reinforced zone, where the microbuckling is caused by fiber waviness and fiber crimp, and then the mesh of fiber, Z-pin, matrix, and the interface between Z-pin and matrix is regenerated. In-plane tensile tests (ASTM D3039) are conducted for verification. There is good agreement between experiment and simulation, only with deviations of −0.97% and 2.69% for tensile modulus and fracture initiation stress, respectively. The research indicates that in-plane micro-damage initiates from the interface between Z-pin and matrix. The stress concentration and higher local fiber volume fraction, caused by the microbuckling spatial fibers clustered near the Z-pin, play a major role in the evolution of interface damage, crack initiation and growth. [ABSTRACT FROM AUTHOR]

The insertion of finer Z-pins is a key issue for utilization of their reinforcement potential in Z-pinned composite laminates. This paper aims to reveal the insertion mechanisms of the ultrasound guided insertion process of Ø 0.11 mm Z-pins. Taking into account the fibre deformation, fibre–matrix interface, needle tip cutting and needle-prepreg contact, a new mechanical model is proposed to predict the change of insertion force during the insertion process. It is found that the application of ultrasonic vibration can minimize frictional effect and reduce fibre fracture, thus significantly decreasing the insertion force. The traditional two-phase insertion mechanism is transformed into single-phase insertion mechanism due to ultrasonic vibration. Based on this model, the effects of process parameters (including vibration amplitude, vibration frequency, feed rate and needle geometry) on the insertion force can be determined. Then Z-pin insertion experiments are performed on the prepreg sample to verify the proposed model. Results show that the model can capture the insertion mechanisms during the ultrasound guided insertion and predictions are in excellent agreement with the experimental measurements. [ABSTRACT FROM AUTHOR]

This research aims to predict and manage material temperatures on complex curved surface structures during automated fiber placement (AFP). A thermal analysis model is developed by incorporating the information of path points as well as the heating time into the radiative heat transfer model. In it, the heating time is determined by the layup speed which is formulated using the S-shaped speed forward planning method. Finally, the thermal analysis algorithm is integrated into a three-dimensional (3D) finite element (FE) model to predict and regulate material temperatures on complex curved structures. In this paper, the thermal analysis method is exemplarily applied to a C-beam. The predicted material temperature under the condition of the constant power is validated by the AFP experiments. A good agreement between the temperature-predicted value and the experimental results is observed. Moreover, the power design method based on laying temperature analysis is proposed to manage the material temperatures within the optimal laying specification. [ABSTRACT FROM AUTHOR]

The two-stage electrostatic precipitator (ESP) is widely used for indoor air cleaning field, while both the Particle Image Velocimetry (PIV) observations and well-characterized quantitative descriptions are probably scarce. In this work, we propose to use vortex analysis, which is the main quantitative method for flow field, and the relative residence time method (RRT) to evaluate the electrohydrodynamic flow (EHD) in a plate-plate ESP. The effects of electrical energization, pre-charger polarity and primary gas flow velocity are studied. Two vortex indexes sort the particle flow as five patterns and increase almost linearly with applied voltage. Four final particle statuses, penetrated, collected, stagnant and back-mixed are clarified by RRT method. The data generated by RRT are utilized to derive the PIV index for further analysis of the ESP performance. The trend, which rises first and drops later, suggests PIV index is expected to pave a new way to clarify the relationship between the ESP flow patterns and particle collection efficiency. [ABSTRACT FROM AUTHOR]

This paper develops an ultrasound guided Z-pinning process, which can insert finer Z-pins of Ø 0.11 mm (in comparison with traditional Ø 0.28 mm - 0.50 mm) with minimal in-plane damage and substantially improve the interlaminar performance of Z-pinned composite laminates. The experimental results show that for unidirectional fibre/epoxy laminates reinforced by 0.1 vol% of Ø 0.11 mm carbon Z-pins, the in-plane strength reduction is only 1.31% in tension and 3.46% in compression, respectively, while the Mode-I fracture toughness can be improved by 11 times in comparison with the unpinned specimens. The results can be attributed to the small damage to the microstructures of the pinned area, where long and narrow resin-rich zone with small in-plane fibre waviness and neglectable out of plane fibre crimp are represented. The interlaminar toughening mechanism of fine Z-pinned laminates is elucidated by the bridging law with the establishment the DCB testing model based on the traditional Z-pin reinforcement mechanism. [Display omitted] • An ultrasound guided Z-pin insertion process is developed to insert Ø 0.11 mm carbon Z-pins into composite laminates. • The in-plane strength reduction of fine Z-pinned laminates is only 1.31% in tension, and 3.46% in compression. • The Mode-I fracture toughness is improved by 11 times in comparison with the unpinned specimens. [ABSTRACT FROM AUTHOR]

• Analysis of voids in variation layup temperatures are conducted experimentally and numerically. • A microscale model containing interlaminar and interlaminar domain is established. • The crack propagation behavior under out of plane tensile load is further analyzed at the macroscale. The focus of this paper is to understand the effect of voids on the damage mechanism of the composite under out-of-plane tensile load with different lay-up temperature conditions. A micro-scale model for ply-by-ply structure is established containing intralaminar and interlaminar domain in which voids are generated by a random irregular voids generation algorithm to explore the role of voids (intra-ply voids and inter-ply voids) on the transverse damage behavior of the composite. Then, a macro-scale model containing elements with and without void is established to capture crack propagation behavior in the specimen. Analysis of voids in variation layup temperatures are conducted experimentally and numerically. The results show that with the increasing lay-up temperature, the porosity increases from 1% to 4.5%, and the void distribution changes from the intra-ply voids dominant to the inter-ply voids dominant, while the out-of-plane tensile strength decreases by about 30%. [ABSTRACT FROM AUTHOR]

[Display omitted] • A new method to generate the random fiber distribution of carbon fiber composite. • Combined a Delaunay triangulation-based adaptive shaking module with algorithm. • Realize the parameter coordination of fiber volume fraction and fiber spacing. This paper presents a new method for generating the random fiber distributions of carbon fiber reinforced plastic with high volume fraction. The novelty is that an adaptive fiber shaking module is developed on the basis of Delaunay triangulation and combined with the proposed algorithm. The adaptability is to realize coordination between the parameters of fiber volume fraction and fiber spacing, in order to overcome the jamming limit and eliminate the unreasonable resin-rich zone that may appear at the boundary of the microstructures. The maximum fiber volume fraction reached is 67.43%. Statistical analysis is conducted on the generated microstructures, and there is a good agreement by comparing it with the statistical results of random sequential expansion algorithm and completely spatial random pattern. Finite element analysis is also performed to predict the elastic properties of the generated microstructures. It shows that a maximum absolute error is only 7.354% in elastic property by comparing with the experimental and simulation results in literatures. The proposed algorithm contributes to the further researches on the strength and micro-damage evolution of carbon fiber reinforced plastic. [ABSTRACT FROM AUTHOR]

• The influence of fiber cutting at the composite grid intersection on the compressive behavior of laminate is studied. • The varied compressive performance of laminate induced by different fiber cutting methods is considered. • The reduction in compressive strength and dominant failure modes are accurately predicted and revealed. For thick composite grid structure, automated fiber placement (AFP) must be adopted to alternating cut fiber at the composite grid intersection to avoid out-of-plane fiber bending. In the present work, an experimental and numerical study has been carried out to understand and predict the influence of different cutting methods at the composite grid intersection on the compressive failure performance of laminate. Five kinds of test specimens with different cutting ratios were firstly manufactured and tested. A maximum of 67% drop in the compressive strength was recorded for the most severe cutting ratio. A High-speed camera was used to clarify the compressive failure progress of cut specimens. The dominant failure mechanism was classified into fiber fracture failure and inter-ply delamination. Further, numerical simulations were carried out to inform the deformation and failure mechanism of cut specimens. The FEM results correlated well with experiments in terms of location of dominant failure mode and strength knockdown for five different cutting configurations. These findings offered certain practical reference for the design and manufacture of thick AFP composite grid structures. [ABSTRACT FROM AUTHOR]

• A new 3D RVE modeling method for z-pinned laminates is proposed. •.The microstructures of RVE model are regenerated by the DT algorithm. • The micro-damage initiation evolution of the RVE is investigated and verified. Z-pinning is an effective method to increase the delamination resistance of composite laminate. To optimize the design of z-pinned reinforced composite material, the paper proposes a new three-dimension RVE model for z-pinned laminates based on the manufacturing process, which involves the evolution of microstructures inside the laminate. First, an insertion and curing simulation is conducted by the finite element method to obtain an initial three-dimension microstructure RVE grid. Then, use the Delaunay triangulation method to regenerate the grids of resin-rich zone, matrix layer and steel z-pin. Finally, the regenerated grid is transformed into a finite element RVE model. Local material properties and local fiber orientation are determined by the Chamis model and grid geometries, respectively. Periodic boundary conditions are defined to confirm deformation compatibility and continuous stress. The RVE model is used to investigate the effects of z-pinned process parameters on the in-plane compressive strength and micro-damage evolution of laminates, and is eventually verified by the experimental results. [ABSTRACT FROM AUTHOR]

• The selection criteria of tow width and roller • The accessibility analysis standard Automated fiber placement (AFP) is an important process used for fabrication of composite structures. Path planning is one of the important steps in the process of AFP. The rationality of the path is directly related to the quality of the composites component. However, the generation of the placement path is constrained by a variety of factors, including geometry characteristics of curved surface, prepreg tow deformation performance, compaction roller deformation characteristics and capabilities of AFP machine. Unreasonable planning parameters selection will lead to local placement path failure or even entire path planning failure. Thus, this paper deals with making some necessary analysis about the placement suitability before path planning and then selecting the appropriate parameters, including the tow width series, compaction roller specifications and AFP machine type. The selection of suitable planning parameters can reduce the unreasonable risk of path planning and then improve the efficiency of path planning. [ABSTRACT FROM AUTHOR]

Automated fiber placement (AFP) is an advanced composite manufacturing technology which is widely used in the production of complex shaped aircraft components. However, on the surface with large curvature, the fiber compaction performance is poor, and the direction deviation and the geodesic curvature of the path increase sharply with the offset distance, resulting in various placement defects. To solve this problem, the influence of the surface curvature on the path performance is studied, and a two-stage sector partition path planning method for AFP on complex surface is proposed in this paper. The constraints of the direction deviation, turning radius and the compaction state of the fiber are comprehensively considered in this method. By analyzing the compaction feature, the distance between the roller and the surface is used to evaluate the compaction condition and an initial partition criterion is established according to the normal curvature of the surface along the roller axis. Then, the quantitative relationship between path performance and offset distance is derived, and the precise sector is determined under the constraints of placement direction and turning radius. The path performance constraints are fully considered in the path generation process, so the iterative process of evaluation–modification is effectively avoided. Simulations and experiments are then carried out on the winglet surface and the results show that compared with the parallel method, the proposed method can significantly improve the direction deviation, turning radius and compaction quality of the placement paths at the cost of gaps/overlaps. Satisfactory placement paths can be obtained with reasonable constraints on the complex surface. • A new path planning method using two-stage sector partition is established for complex surface. • The direction deviation, turning radius and the compaction state are comprehensively considered. • Initial partition criterion is the fiber number estimated by the surface curvature along roller axis. • The quantitative relationship between path performance and offset distance is established. [ABSTRACT FROM AUTHOR]

Automated fiber placement is especially suitable for manufacturing composite components with curved surfaces, in which case the uneven compaction pressure distribution is an important factor affecting layup quality, but it has not been widely explored. In this paper, a theoretical model of the compaction pressure distribution for layup on irregular curved surface is established by analyzing the contact between the compaction roller and prepreg layers. Based on the model, a numerical algorithm for calculating the pressure distribution in the whole placement process is proposed. Then the pressure distribution around four path points of a winglet mold is obtained and validated by the subsequent experiment. The results show that the model can be used to predict the compaction pressure distribution before the placement and analyze the layup quality and defects from the perspective of compaction pressure. [ABSTRACT FROM AUTHOR]

• Microstructure-based and analysis of automated layup and forming of prepreg laminates are conducted experimentally and numerically. • A two-step approach for constructing a more realistic geometrical model is presented. • The out-of-plane tensile properties show a strong correlation with gap size. This study concerns the effect of AFP process-induced microstructure and imperfections on the failure response of composite laminates subjected to out-of-plane tensile loading. Microstructure-based description and analysis of automated layup and forming of laminates are firstly conducted by experimental observation. Further, two failure modes are experimentally obtained for understanding the fracture mechanisms of the AFP laminates under out-of-plane tensile loading. In order to capture transition zones of the random gaps and their developments, a two-step approach for constructing a more realistic geometrical model is presented, thereby establishing an AFP random gap model to account for the deformation and failure mechanisms of the out-of-plane tensile specimens. Numerical results show a good agreement with experiments in terms of material modulus and strength and in predicting the failure behavior. In addition, parametric studies show significant knockdown in out-of-plane tensile properties due to gaps, specifically with larger gap sizes showing higher knockdown. [ABSTRACT FROM AUTHOR]

A new path generation method is proposed to alleviate the contradiction between direction deviation and fiber wrinkles on complex surface. A critical radius index is first presented to characterize the fiber wrinkle constraint. Then, a mathematical model is established to describe the variation of the direction deviation and steering radius of the path. The existence criteria of the placement direction range that can satisfies the two constraints simultaneously is determined. And the placement direction at each path point is obtained considering the path performances at both the current point and the subsequent point. The comparative study with other two common path generation methods is carried out on an inlet surface. The evaluation results show that the steering radii of the paths generated by the proposed method are all larger than the critical radius, while the direction deviation is constrained within 10°. [ABSTRACT FROM AUTHOR]

This study concerns the buckling and post-buckling performance of composite stiffened panels with sub-stiffening structure subject to compression. The buckling response of the composite stiffened panel is first predicted and verified by experimental data available from the literature. Then sub-stiffeners are introduced into the composite stiffened panel. The distribution and stacking sequence of sub-stiffeners are optimized to improve the critical buckling load without adding weight. Further, to investigate the underlying mechanism of sub-stiffeners involved panels deformation and failure during compressive post-buckling process, different skin-stiffener interfacial parameters are considered. Results show that the introduction of sub-stiffeners to composite T-stiffened panel causes a significant improvement of buckling load (+122.66%) and induces reestablishment of the stress concentrated zone which leads to different local failure forms with different interfacial properties. In addition, the collapse load of sub-stiffened panel becomes more sensitive to interfacial parameters of skin-stiffener interface due to the presence of sub-stiffeners. [ABSTRACT FROM AUTHOR]

A dual-machine horizontal automatic drilling and riveting system for aircraft panel assembly is presented in this paper. Two cooperative machines are adopted, like a two-robot system, to achieve the unobstructed workspace, so it can be applied in an efficient assembly line. Different from the current deign wisdom to ensuring the riveting accuracy by increasing the machine stiffness, which detrimentally results in higher masses to be moved and sophisticated reinforcements being used, this paper proposes a disturbance observer (DOB) based design method of riveting force controller for adaptive riveting with high accuracy driven heads, reducing the dependence of automatic riveting on high stiffness of machine. This method simplifies the force-induced deformation error compensation problem to a force control problem, which has rarely been studied before in literature. The separate layout leads to weak machine stiffness in the whole workspace and the stiffness varies with machine pose greatly, which makes it difficult to establish an accurate force-induced deformation error compensation model. To make the riveting machine precisely generate the desired force under such stiffness-varying conditions, a robust control algorithm is designed: the disturbance observer is introduced to address the robust stability against model parameter variance caused by the change of system stiffness with pose, and the feedforward controller can achieve the high-bandwidth force control to ensure the riveting efficiency. The model identification experiments are carried out for different machine stiffness. Validation riveting experiments have been conducted to verify the proposed controller on the dual-machine riveting system. The results show that, compared with previous displacement control, the proposed force controller can ensure the consistency of driven heads through accurate force control. [ABSTRACT FROM AUTHOR]

The cymbal transducer had more improved performances than the moonie transducer, but as with the moonie end caps, flexural motion of the cymbal end caps would cause high-tangential stresses, which could lead to loss of mechanical output energy. So, a new-style design, called slotted-cymbal transducer, is presented in this paper. The slotted-cymbal transducer released the tangential stresses by adopting a new-style, radial-slotted cymbal end caps. Through theoretical calculation and finite element analysis in collaboration with measurements, the slotted-cymbal transducer was demonstrated to possess high performances of displacement and energy transmission. This design would decrease the fundamental resonance frequencies of systems by different numbers of slots, and it would have more potential applications of low frequency and large displacement.