Volume 22, Issue 2 - Special issue: Automated composites manufacturing

In automated fibre placement (AFP) process, gaps and overlaps parallel to the fibre direction can be introduced between the adjoining tapes. These gaps and overlaps can cause a reduction in strength compared with pristine conditions. Finite element modelling is an effective way to understand how the size and distribution of such gaps and overlaps influences the strength and failure development. Many modelling work showed that out-of-plane waviness and ply thickness variations caused by gaps and overlaps play an important role in inducing the strength knock-down; however, there has been a lack of effective way to explicitly model the ply waviness, which constrained the relevant research. In this work, 3D meshing tools were developed to automatically generate ply-by-ply models with gaps and overlaps. Intra-ply and inter-ply cohesive elements are also automatically inserted in the model to capture the influence of splitting and delamination. Out-of-plane waviness and ply thickness variations caused by gaps and overlaps are automatically modelled. Models with various sizes and distribution of gaps and overlaps were built to predict the reduction of strength as a function of the magnitude and type of the defects. Results of gap and overlap models will be used to guide future experimental characterization of simulated AFP process defects, manufactured by hand layup from pre-preg tape.

With the increasing demand for carbon fiber-reinforced parts in the aerospace industry, automated manufacturing methods such as automated fiber placement (AFP) are being established. To utilize AFP’s strong potential on multiple-curved surfaces such as chamfered sandwich structures, it is important to analyze and control emerging effects like gaps, bridging, and steering. An analytical relationship between path generation and effects of the AFP manufacture of chamfered sandwich structures have been found with good agreement to experimental tests. The deformation behavior of an industrially used compaction roller has been analyzed numerically and experimentally. The results show a nonuniform deformation behavior on flat and curved surfaces and the limited deformability of the compaction roller.

The fiber steering capability of automated fiber placement machines offers the designers more room to fully exploit the directional properties of composite materials. Circumferential stiffness tailoring by fiber steering can considerably increase the bending-induced buckling performance of laminated composite cylinders. The potential structural improvement resulting from fiber steering depends on different design parameters such as the number of plies considered for fiber steering in a laminate. In this study, the buckling performance improvement of a variable stiffness (VS) composite cylinder is investigated for different percentages of plies considered for fiber steering in a multilayered composite laminate. A surrogate-based modeling along with a multi-step optimization is used in the design procedure of this study. The improvements in the buckling performance are shown and verified using finite element analysis in ABAQUS software. The mechanisms leading to buckling performance improvement of VS composites are also investigated and presented for different percentages of fiber-steered plies.

Variable stiffness laminates can be manufactured using curvilinear fiber paths. A curvilinear fiber path is generally defined based on the plate size and has a curvature that is dependent on the plate size. In practice, however, the fiber path must satisfy manufacturing constraints, such as the minimum turning radius imposed by the automated fiber placement machine, thereby limiting the possible amount of fiber steering. In this work, we studied the effect of the plate size on the structural properties of a plate manufactured with curvilinear fibers. We considered four plate sizes, which were designed by a constant curvature fiber path. We optimized the plates for both maximum buckling load and in-plane stiffness. The results showed that the in-plane stiffness of the plate was not controlled by the plate size, whereas the buckling load was highly affected by the curvature of the fiber path. Hence, the potential of a buckling load increase reduced for plate sizes smaller than the minimum turning radius. In addition, for a given maximum curvature of the fiber path, the influence of a complex layup on the buckling load was marginal.

Developing reliable processes is one of the key elements in producing high-quality composite components using an automated fiber placement (AFP) process. In this study, both simulation and experimental studies were carried out to investigate fiber steering and cut/restart under different processing parameters, such as layup rate and compaction pressure, during the AFP process. First, fiber paths were designed using curved fiber axes with different radii. Fiber placement trials were then conducted to investigate the quality of the steered fiber paths. Furthermore, a series of sinusoidal fiber paths were fiber placed and investigated. Moreover, a six-ply laminate with cut-outs in it was manufactured in the cut/restart trials. The accuracy of the fiber cut/restart was compared at different layup rates for both one- and bi-directional layups. Experimental results show that it was possible to layup steered fiber paths with small radii of curvature (minimum 114 mm) designed for this study when the proper process condition was used. It was observed from the cut/restart trials that the quality of tow cut was independent of layup speed; however, the accuracy of tow restart was related to the layup speed. The faster the layup speed, the less accurate was the tow restart.

In order to investigate the effect of autoclave process on the stiffness properties of thermoplastic composite truncated cones made by automated fiber placement technique (AFP), two short truncated cones were made out of advanced thermoplastic composite material (carbon fiber/PEEK). Then, the truncated cones were tested by impulse excitation of vibration to get experimental natural frequencies. Tensile and compression test coupons were made out of the same material using hand layup and autoclave process, and stiffness properties were characterized experimentally. Furthermore, finite element analysis was performed to extract theoretical natural frequencies using material properties obtained from coupon testing. Comparison between the natural frequencies obtained using the finite element method and experimental modal analysis was conducted. Furthermore, the two truncated cones were treated inside the autoclave, and changes in the thickness and density were measured. After autoclave treatment, experimental modal analysis was repeated to investigate the possible changes in natural frequencies. The finite element model was also updated upon the changes in thickness and density of the truncated cones, and consequently, comparison between the theoretical and experimental natural frequencies was carried out again. Also, micrographs of two truncated cones were taken before and after autoclave treatment to examine the autoclave effect on the quality of the samples.

The high specific strength and stiffness characteristics of composite materials such as carbon fiber-reinforced plastic (CFRP) allow a significant weight reduction of the structural machine components such as automobile driveshafts. But high material cost and rather low productivity of the established manufacturing processes (e.g., filament winding) often inhibit the use of CFRP components in a high-volume car series. In this paper, a novel composite driveshaft system based on a profiled CFRP tube is presented. This system is designed to be produced by a continuous pultrusion process to achieve a significant reduction of the manufacturing costs. A cost assessment study was conducted to quantify the benefit of the developed continuous manufacturing process. In comparison with the state-of-the-art filament winding process, a cost reduction of 36% for the composite shaft body can be obtained. Moreover, the proposed fiber layup processes – braiding and continuous winding – offer the potential to manipulate the reinforcement architecture to maximize material utilization without reducing the manufacturing efficiency. This potential is investigated and validated by experimental tests. A difference in the load bearing capacity of more than 100% between different reinforcing architectures is shown.

The production rates of carbon fiber reinforced plastic (CFRP) parts are rising constantly which in turn drives research to bring a higher level of automation to the manufacturing processes of CFRP. Resin transfer molding (RTM), which is seen as a production method for high volumes, has been accelerated to a high degree. However, complex net-shape preforms are necessary for this process, which are widely manually manufactured. To face these challenges a new concept for the manufacturing of carbon fiber preforms with a form-flexible gripping, draping and joining end-effector is presented and discussed. Furthermore, this paper investigates the application of this concept, describes the initial build-up of a demonstrator, focusing on material selection and heating technology, and discusses test results with the prototype. This prototype already validates the feasibility of the proposed concept on the basis of a generic preform geometry. After a summary, this paper discusses future in-depth research concerning the concept and its application in more complex geometries.