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In automated fiber placement (AFP), the fiber tension is an important process parameter. Sharp fluctuations in fiber tension are unacceptable in AFP process. The focuses of this article are on fiber tension modeling and control methods for AFP. Firstly, the mechanical model of fiber transport based on frictional loss is established, and the control objective, which should really be focused on in the tension system, is pointed out. Then, identification experiments are carried out to obtain the unknown parameters in the mechanical model. In addition, a dynamic model of fiber tension based on a passive dancer roller is established and linearized by curve fitting. Finally, a variable‐parameter Proportion‐Integration‐Differentiation (PID) controller is established to achieve stable control of the fiber tension. The experimental results show that the proposed control method can accurately stabilize the actual tension at the target value and has a lower control error in the face of strong velocity fluctuations than the traditional tension control system. Highlights: The fiber tension at the end of the tow transfer path cannot be measured directly by the sensor in automated fiber placement.The tension model predicts the fiber tension at various points in the tow path.The control method using the tension prediction model is much more accurate than the traditional method. [ABSTRACT FROM AUTHOR]

The layup quality of prepreg fibers onto complex surfaces is the most concerned issue in automated fiber placement (AFP), which is intimately related to the contact stress in tractive rolling contact. An analytical model of tractive rolling contact is proposed in this paper to clarify the defects mechanism during the AFP process, which considers the mold curvatures, mechanical properties of contacted objects, and pose information to make it suitable for irregular surfaces and arbitrary conditions. Classical Hertz theory in this model is expanded to 3D contact area, and multi‐scale deformations are united in the accurate calculation of contact stress under geometric constraints. Based on the model, the phenomena of adhesion, microslip, and the transition between them in AFP are systematically analyzed. For engineering applications, a discrete processing strategy is proposed to quickly obtain the local curvatures of the mold. Additionally, an algorithm is designed to calculate the contact stress of any path. Experimental results on an aircraft component demonstrate the accuracy of the proposed model in restoring the contact stress distribution during AFP. In conjunction with specific layup information, this model exhibits utility in forecasting laying defects and parameter optimization. Highlights: An analytical modeling approach based on Hertz theory is proposed to calculate the contact stress in automated fiber placement.A model‐based algorithm is designed and combined with the proposed discrete scheme to quickly restore the contact stress distribution.Contact states related to stress are systematically investigated and defects analysis based on computational mechanics results is illustrated. [ABSTRACT FROM AUTHOR]

Robotic fiber placement systems, which have larger workspaces, better dexterity, and easier installations, are less expensive alternatives to the conventional fiber placement machines. To overcome the disadvantage of poor path accuracy, sensor-based path correction can be implemented online by reference to the edge of a previously deposited tow or a guide line on the tooling surface. In fiber placement operations, the end-effector with a compaction roller, acting like a unicycle on the surface, is subject to the nonholonomic constraints when moving along a predefined path to provide proper guidance and pressure for the tows. If the constraints are ignored in path correction, the corrective motion with a lateral velocity component will result in improper tow guidance that induces undesired and uncontrolled tow steering, leading to deviations and wrinkles. In this paper, the nonholonomic constraints of the robot end point is discussed for the first time in online path correction, and a novel method is proposed under the constraints to achieve online path correction without damaging lay-up quality. The path correction problem is analyzed and simplified using twist decomposition. Then the instantaneous motion of the roller is derived by modified vector pursuit algorithm. Afterwards, an iterative algorithm is proposed based on screw theory and differential kinematics for consecutive external control of correction in robot joint space. The implementation of the method is tentatively explored by applying visual sensing based on directional light source and external control to a robotic fiber placement system. Simulations and experiments have showed that the large initial path deviations can be effectively reduced and the roller motions along the corrected paths satisfy the nonholonomic constraints. • The end point nonholonomic constraints are discussed for the first time in online path correction of robotic fiber placement. • The path correction problem is simplified using twist decomposition. • The desired instantaneous motion is derived by modified vector pursuit algorithm. • An iterative algorithm is proposed for consecutive external control of correction in joint space. • The implementation is explored using visual sensing and external control. [ABSTRACT FROM AUTHOR]

The performance of fiber‐reinforced composites is not only determined by their constituents, but also by the spatial distribution of inclusions. The traditional random distribution algorithm only considers the randomness of its relative position, and every newly generated fiber has to be checked with all the existing fibers, which requires a large amount of calculation. In this paper, inspired by the spreading process that the fibers are not arranged randomly, but the forces acting on the upper and lower rows of fibers. A random fiber distribution algorithm based on partition randomness is proposed, and a microscopic model of in‐plane fibers with random distribution is established. The maximum volume fraction of fibers generated by the algorithm is 75%, and the generation time is less than 20s. Statistical characterization and finite element analysis of the structure generated by the algorithm are compared with the experimental data to verify the randomness and effectiveness of fibers. The algorithm overcomes the jamming limit of the traditional hard‐core method, and can efficiently generate random microstructure models with different volume fractions, which can be a promising alternative for modeling and analysis of unidirectional fiber‐reinforced composites. [ABSTRACT FROM AUTHOR]

Prepreg tack plays a crucial role in dominating the part quality during the automated fiber placement (AFP) process. This study aims to study the effect of process parameters on the tackiness between prepreg plies for two kinds of prepreg materials and find the optimal combination for high resultant tackiness. Moreover, the surface characteristics are utilized to evaluate the tack‐determining mechanism. For this proposal, a shear test method based on the theoretical model is first developed to measure the tackiness between prepreg plies. Thereafter, the influence of each process parameter including lay‐up speed, compaction force, and material temperature on the tackiness is investigated and analyzed through the single‐factor method. The results show that process parameters exhibit distinct impacts on the tackiness for two kinds of prepreg systems while the tackiness force of the EH918 prepreg is much lower than that of the M21C prepreg. It is because the resin layer on the surface of the EH918 prepreg is much thinner and even in comparison with that of the M21C prepreg. Finally, the response surface method is utilized to investigate the interaction among process parameters and find the optimal combination for high tackiness. It is shown that different prepreg materials present the distinct optimal combination for high resultant tackiness. These results are significant for controlling the tackiness of prepreg materials by tailoring process parameters to satisfy the specific manufacturing requirements. [ABSTRACT FROM AUTHOR]

For automated fiber placement onto molds with complex surfaces, uneven compaction pressure distribution limits tows number in a single sequence and affects layup quality. Compaction roller has a direct influence on the pressure distribution, but the relationship between the two has not been widely explored. In this paper, the segmented compaction roller is used, and a theoretical model of compaction pressure distribution for layup onto general surfaces is established by analyzing the contact between the roller and prepreg layers, followed by experimental validation. Based on the model, single-point pressure uniformity and whole-path pressure uniformity are proposed to quantitatively evaluate the pressure distribution. Furthermore, pressure distribution and pressure uniformity of segmented roller and common roller are compared, as well as the influence of the two pressure distribution on layup quality. The results show that the established model can predict pressure distribution and provide a basis for analyzing layup defects, and segmented rollers apply evener compaction pressure and help to improve layup quality. [ABSTRACT FROM AUTHOR]

This paper presents a method for measuring, identifying, and compensating the pose errors of a six-axis gantry automated fiber placement (AFP) machine. Due to the particularity of the elastic pressure roller (EPR) of the end-effector, a calibration method of the pressure roller coordinate system is proposed. In order to quickly measure the pose of the EPR, a special pose measurement tool is designed. Based on the screw theory and homogeneous transformation matrix, ideal forward and inverse kinematic models are established and verified on the actual placement path. According to the geometric errors and the offset errors of the machine coordinate system obtained by the analysis, the pose error transfer model of the AFP machine is established. Considering the position and orientation accuracy of the EPR, a comprehensive optimization objective function is established, and all the pose errors are identified synchronously using the Levenberg-Marquardt optimization algorithm, which greatly reduces the pose errors of the AFP machine. Finally, the G codes are modified by combining the iterative method and the inverse kinematic model to realize the compensation of the pose errors. To comprehensively verify the effect of pose error compensation, path control points are randomly selected on the placement paths of the fuselage panel in four directions to compensate the pose errors. The experimental results show that the method proposed in the article can effectively improve the pose accuracy of the AFP machine, so as to meet the requirements of placement accuracy. [ABSTRACT FROM AUTHOR]

Automated fibre placement (AFP) machine in-process layup inspection is essential for composite structure fabrication efficiency and quantity improvement. The laser profilometer scanning technique is one of the automated in-process inspection techniques. Laser profilometers can capture a large number of intensive high-resolution point clouds during the layup. To date, few studies have been published about processing point clouds at high speed for in-process layup defect detection and layup feature measurement. In this study, an algorithm called the cross-sectional line-processing algorithm was proposed, and a testbed was constructed to validate the algorithm. The algorithm processes each laser line captured by the laser profilometer and clusters the processed results together. Finally, the defect features and layup features can be segmented and recognized from the collected point cloud. Layup defect types can be recognized, and the dimensions of the layup features can be measured. The experimental results show that this approach can process at most 200 laser lines (about 160 000 points) per second, and the overall defect type recognition accuracy rate reaches about 78%, which meets basic AFP in-process inspection requirements. [ABSTRACT FROM AUTHOR]

In the automated fiber placement process, the continuous placement paths need to be discretized into a finite number of path points because the laying head cannot continuously trace the predetermined curved path. However, the discretization of the placement path, which is a spatial curve, will inevitably introduce error. In this paper, an improved path discretization algorithm is proposed for the fiber placement of complex double-curved structures. Firstly, the discrete error was decomposed into normal direction and binormal direction, and they are correlated with the laying process and their influences on the laying quality are discussed, respectively. Secondly, the relationship between the binormal error and the overlap of the tow is analyzed with differential geometry, and the influence of the normal error on laying force is discussed by the pressure experiment and the finite element method. Finally, the improved path discretization algorithm has been verified on double-curved surface and compared with the traditional path discrete algorithms. The results showed that the number of discrete path points decreases by 45.8% on average compared with the chordal deviation discretization algorithm and by 63.1% compared with the equal-arc discretization algorithm. [ABSTRACT FROM AUTHOR]

During automated fiber placement, insufficient compaction pressure leads to low bond strength between layers and may cause defects like bridging. The compaction roller contacts the prepreg and affects the pressure distribution directly. In this article, a model for analyzing the adaptability of rollers to molds is established. Then, compaction pressure distribution on prepreg tows is analyzed by finite element simulation and verified by pressure sensitive film experiment. Based on the simulation results, a calculation method of pressure uniformity is proposed. Moreover, the design parameters of the roller are optimized with an aim of improving pressure uniformity. The results show that the pressure uniformity can be improved significantly by design optimization of the roller, which indicated that higher bond quality between layers is achieved. [ABSTRACT FROM AUTHOR]

The fiber tension should be kept constant during the automated placement of fiber prepreg. The velocity of the fiber placement end-effector moving on complex aircraft panel mould surface varies rapidly, which greatly disturbs the precision of tension control. This paper proposes a tension control strategy combining active control and passive control. The pay-off motor controls the fiber tension directly and a passive dancer roll is designed theoretically as the equipment for attenuation of tension disturbance to realize the real-time compensation of low-frequency velocity variations. The nonlinear model of tension control system, which includes the dynamics of the passive dancer roll, is established, and the effect of dancer roll parameters on its disturbances attenuation performance is analyzed. The controller is designed using the H ∞ mixed sensitivity method. An experimental tension control precision about 2 % is obtained at stable placement speed on the automated fiber placement (AFP) machine. The experiments also indicated that the tension would not vary over 1 N at a maximum acceleration of 4 m / s 2 . [ABSTRACT FROM AUTHOR]

The non-geodesics using variable slippage coefficients along the winding trajectories are gaining increasing attention for composite pressure vessels. The aim of this paper is to evaluate the influence of several slippage coefficient distributions on fiber trajectories and the resulting mass of toroidal pressure vessels. The distributions of slippage coefficients are represented in terms of three functions satisfying C 1 continuity. The governing equations for designing non-geodesics on a toroid are formulated using the given functions of slippage coefficients. For each function, the initial fiber angle and the shell thickness are considered as the design variables, and the minimum mass is taken as the objective. With the aid of the classical lamination theory and the SQP algorithm, the optimal fiber trajectories are respectively determined for the given three slippage coefficient distributions, and the corresponding masses of the toroids are then obtained and compared to each other. The results indicate that toroids produced using variable slippage coefficients show improved performance than toroids using constant slippage coefficients; this is mainly due to the maximum utilization of the laminate strength. It is also revealed that the weight efficiency of toroidal vessels can be significantly improved using the quadratic distribution function of fiber slippage coefficients. [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] • 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]

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]

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 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]