Two-stage model of prepreg bonding interface formation in automated fiber placement process

While Automated Fiber Placement (AFP) of thermoset matrix composites are widely used in the aviation industry, there is little conclusive research on the relationship between the physical model of bonding interface formation process and the actual bonding strength between prepreg layers formed in AFP process. Although massive amounts of experimental data on prepreg tack have been achieved from existing research, engineers are unable to use these data as a decisive criterion in choosing process parameters. In this research, a prepreg layup physical model based on reptation model and viscoelastic mechanical model is built, in which the bonding interface formation process is divided into two stages, namely, diffusion and viscous stage. Layup-peeling experiments are conducted via a special designed high-speed layup experimental platform so that practical AFP process parameters can be imitated, and a logarithmic curve of layup velocity-peeling energy under different layup pressure is achieved. The slope of the logarithmic curve and the surface morphology of the sample after peeling prove the correctness of the established model. Simultaneously, the experimental data proves that when prepreg is peeled off, the transition from the cohesive failure mode to the interface failure mode occurs at the laying speed between 100 mm/s and 200 mm/s. These results can be used as a reference for choosing AFP process parameters to realize the balance between good bonding quality and harmless separation of adjacent prepreg layers.

In Situ Consolidation of Thermoplastic Prepreg by Generating Harmonic Oscillations on the Consolidation Roller

Automation technologies such as Automated Fiber Placement (AFP) or Automated Tape Laying (ATL) are widely used in the aerospace industry today. However, these processes can still be further improved for higher productivity. Fiber-reinforced plastics allow the production of components with extremely high specific strength and stiffness. Regarding the automated manufacturing processes, the thermoplastic tape placement offers efficiency improvements compared to the nowadays more commonly used thermoset tape placement, especially through the substitution of the expensive and time-consuming autoclave process. The consolidation of thermoplastic Prepregs is achieved with an elastic or rigid roller according to the current state of the art. The Prepregs must be consolidated precisely on the substrate or on top of each other. The most important process parameters for high-quality laminate structure with low porosity are the control of heat source, consolidation force, consolidation roll speed, and tape tension. The efficiency of the AFP process can generally be improved by increasing the speed of the consolidation roller. By increasing the speed of the consolidation roller, porosity is increased and mechanical properties of the laminate are reduced significantly due to the short contact time between consolidation roller and Prepregs. This study investigates a process that can reduce these challenges by increasing the contact time and force duration of the consolidation roller on the Prepregs. The consolidation roller in this study is additionally to be driven by the harmonic oscillations. The new method allows the consolidation roller to oscillate forward and backward during the fiber placement process. This creates another force vector in addition to the compressive force of the consolidation roller and increases the bonding strength between the layers.

The Effects of Processing Parameters on the Wedge Peel Strength of CF/PEEK Laminates Manufactured Using a Laser Tape Placement Process

Manufacturing thermoplastic composites (TPC) with excellent mechanical properties requires advanced methods with reduced costs and better overall efficiencies. In this study, fiber-reinforced thermoplastic polymer composite laminates were manufactured using an automated fiber placement (AFP) manufacturing technology. The effects of processing temperature (from 320 ℃ to 500 ℃), lay-up speed (from 20 mm/s to 260 mm/s), consolidation force (from 100 N to 600 N), and prepreg tape tension (from 0 N to 9 N) on the quality of the resulting laminates manufactured using the laser AFP system were investigated. The interlayer bond strength was characterized using wedge peel tests on samples prepared with different process parameters. The studies were complemented by measurements of the thermal properties of the composites using different scanning calorimetry. The optimized process parameter windows were determined to be 360 ℃ to 400 ℃ for the irradiation temperature, 140 mm/s to 160 mm/s for the lay-up speed, 100 N for the consolidation force, and 3 N to 5 N for the prepreg tape tension, respectively. The microscopic analysis of the cross-sections and peel-damaged surfaces revealed that the different distributions of the resin matrix resulting from the different processing parameters affected the interlayer strength. These results may provide an important reference for manufacturing TPC used in aerospace, defense, and automotive applications.

Compaction pressure distribution and pressure uniformity of segmented rollers for automated fiber placement

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.

Study of Allowable Interlaminar Normal Stress Based on the Time–Temperature Equivalence Principle in Automated Fiber Placement Process

Automatic fiber placement (AFP) is a type of labor-saving automatic technology for forming composite materials that are widely used in aviation and other fields. In this process, concave surface delamination is a common defect, as existing research on the conditions for this defect to occur is insufficient. To predict the occurrence of this defect, the concept of allowable interlaminar normal stress is proposed to define its occurrence conditions, and based on this concept, probe tests are carried out using the principle of time–temperature equivalence. Through the laying speed/allowable normal stress curve measured in the probe experiment, the physical meaning of allowable normal stress is discussed. At the same time, the measured curve is quantitatively analyzed, combined with viscoelastic theory and the molecular diffusion reptation model, and the dominating effect in the formation of a metal/prepreg layer and prepreg/prepreg layer is determined. Finally, the experimental data are used to guide the parameter selection in an automatic placement engineering case and prove its correctness.

Study of technological defects arising during automated laying of prepregs (review)

Technologies of automated laying out of fibers (Automated Fiber Placement — AFP) and tapes (Automated Tape Laying — ATL) are used in the manufacture of blanks from polymer composite materials (PCM) for subsequent autoclave or other type of molding. The frequency of defect formation upon laying depends on the complexity of the certain part and the technological parameters (the tape width, speed of laying and idle time, temperature, deviations from the calculated trajectory, etc.). We present a review of the main defects thus formed along with the analysis of the reasons for their appearance and their effect on the physical and mechanical properties of composite materials. It is shown that the defects change the thickness and reduce the strength of the PCM under tension and compression. Moreover, the inaccuracy of fiber placement when using the AFP technology has a negative effect on the quality of the materials. The results presented can be used in the further study of the effect of defects on the PCM properties and reasons for defect formation, as well as in developing promising universal computational program that can provide reduction of the number of corresponding experimental tests.

Effects of processing parameters on the performance of carbon fiber reinforced polyphenylene sulfide laminates manufactured by laser-assisted automated fiber placement

In situ consolidation of thermoplastic composites can be realized through laser-assisted automated fiber placement (AFP) technology, and the properties of composites were significant affected by the process parameters. In this work, the effects of process parameters on the properties of continuous carbon fiber–reinforced polyphenylene sulfide (CF/PPS) composites manufactured by laser-assisted AFP were investigated. Four-plies CF/PPS prepreg was laid under the combination of different process parameters and the morphology, void content, crystallinity, and inter-laminar shear strength (ILSS) of the composites were characterized. It turned out that the resin distribution on the surface of the composites could be significantly improved by increasing the laser temperature and compaction pressure. The highest crystallinity of the composites reached 46% at tool temperature of 120°C while the value was only 18% when the tool temperature was 40°C. Meanwhile, with the increasing compaction force ranging of 500–2000 N, the void content of the composites decreased obviously. The ILSS was evaluated through double notch tensile shear test. The results indicated that the mechanical properties of the composites were dominated by void content rather than crystallinity.

Wrinkle Formation and Initial Defect Sensitivity of Steered Tow in Automated Fiber Placement

This paper aims to study the wrinkle formation of a prepreg with initial defect during steering in automated fiber placement (AFP). Wrinkle formation has a detrimental effect on the mechanical properties of the final product, limiting the AFP applications. A theoretical model for wrinkle formation has been developed in which a Pasternak foundation and a Koiter imperfection model are adapted to model viscoelastic characteristics of the prepreg tack and initial defect of the prepreg, respectively. The initial defect is defined as a slight deviation of the tow’s mid-plane from a horizontal shape. The initial defect is generated in the tow by moving the tow through the guidance system, pressure of the roller, and resin tackiness. Galerkin method, along with the finite difference method (FDM), are employed to solve the wrinkle problem equation. The proposed method is able to satisfy the different boundary conditions for the wrinkle problem completely. The numerical results show that increasing the initial defect leads to a decrease in critical load and an increase in critical steering radius. To validate the theoretical model, experimental results are presented and compared with model-predicted results. It is shown that the model is well able to capture the trends and values of wrinkle formation wavelengths obtained from the experiment.