Author: [Bruce Zhou]
Affiliation: [Jota Machinery Composites Material Prepreg Solution]
Corresponding Author: [jotamachinery@gmail.com]
Published : December 17 , 2025
Abstract
Automated Fiber Placement (AFP) enables high-rate manufacturing of complex composite structures through precise deposition of narrow prepreg tows along curved paths. However, steering-induced defects—particularly tow wrinkling—remain a critical limitation to part quality and productivity. Tow wrinkling primarily arises from differential length requirements between the inner and outer edges of a steered tow, leading to compressive instability when in-plane deformation mechanisms are insufficient to accommodate the mismatch. This paper examines the mechanisms governing tow wrinkling in AFP through a synthesis of geometrical modeling, physics-based analysis, and experimental observations reported in recent literature. The influence of steering radius, nip-point temperature, compaction pressure, and substrate condition on wrinkle formation is analyzed. Quantitative results show that wrinkle amplitude and density increase rapidly with decreasing steering radius, while elevated temperature and pressure promote adhesion and shift deformation from out-of-plane wrinkling toward more benign in-plane modes. The findings provide practical guidelines for AFP process optimization, supporting defect prediction and mitigation in curved layups.
Keywords
Automated Fiber Placement; Tow Wrinkling; Steering Radius; Composite Manufacturing; AFP Process Parameters
1. Introduction
Automated Fiber Placement has become a cornerstone technology in aerospace composite manufacturing due to its ability to deposit material with high precision, repeatability, and productivity. By placing multiple narrow prepreg tows under controlled heat and compaction, AFP enables the manufacture of large, complex structures with reduced labor and improved quality consistency compared to manual layup.
Despite these advantages, AFP is inherently sensitive to path curvature. When tows are steered along curved trajectories, the inner edge of the tow must accommodate a shorter path length than the outer edge. If this differential length cannot be absorbed through elastic stretching or in-plane deformation, compressive instability develops, often manifesting as tow wrinkling. These out-of-plane defects compromise structural integrity, surface quality, and downstream processing.
Tow wrinkling remains one of the primary factors limiting minimum steering radius, tow width selection, and achievable production rates in AFP. Understanding the governing mechanisms and identifying effective mitigation strategies are therefore essential for expanding AFP design envelopes and reducing scrap and rework.
2. Literature Review
Wrinkling phenomena have been studied extensively in fields such as thin-film mechanics, elastic foundations, and laminated composites. In AFP, wrinkling is distinguished by its occurrence during deposition of uncured prepreg tows under combined thermal and mechanical loading.
Previous AFP studies classify tow deformations into three main categories: elastic strain, in-plane deformation (waviness or bunching), and out-of-plane deformation (wrinkling or folding). Among these, out-of-plane wrinkling is the most detrimental, as it introduces local fiber misalignment and thickness variation that persist after cure.
Geometrical analyses demonstrate that wrinkle severity increases as steering radius decreases and as tow width increases. Physics-based models further show that tow stiffness, foundation compliance, and adhesion conditions govern whether compressive strain is relieved through in-plane mechanisms or results in buckling. Experimental studies using optical measurement techniques have confirmed these trends and highlighted the role of process parameters such as temperature and compaction pressure.
3. Methodology
The analysis presented in this paper is based on a synthesis of three complementary approaches reported in the literature:
- Geometrical modeling, where wrinkle patterns are predicted from tow width, steering radius, and path geometry, assuming dominant out-of-plane deformation.
- Physics-based modeling, treating the tow as an anisotropic fiber bundle supported by a compliant foundation, capable of undergoing both in-plane and out-of-plane deformation.
- Experimental measurement, including Stereo Digital Image Correlation (DIC) during placement and post-placement surface profilometry to quantify wrinkle wavelength, amplitude, and spatial distribution.
Process parameters such as nip-point temperature, compaction force, placement speed, and substrate condition are varied to evaluate their influence on wrinkle initiation and growth.
4. Results
4.1 Effect of steering radius
Experimental and modeling results consistently show that tow wrinkling severity increases with decreasing steering radius. For flat-surface steering, wrinkle amplitudes increase from approximately 0.14 mm at infinite radius to over 0.6 mm at a radius of 305 mm. Wrinkle wavelength remains relatively constant, typically around 100–106 mm, indicating that curvature primarily affects amplitude rather than periodicity.
4.2 Influence of temperature and compaction
Elevated nip-point temperature and increased compaction pressure improve tow adhesion to the substrate, promoting in-plane deformation modes and delaying out-of-plane wrinkling. Optimal parameter ranges—approximately 400–445 N compaction force and nip-point temperatures below 93°C—have been shown to reduce the areal percentage of out-of-plane defects by up to 50% on cylindrical tools.
4.3 Substrate effects
Substrate imperfections, such as gaps or overlaps in underlying plies, act as nucleation sites for wrinkles. Once initiated, wrinkles tend to propagate along the steered path, and subsequent thermal exposure can further increase wrinkle amplitude. This highlights the cumulative nature of defects in multi-ply AFP laminates.
5. Discussion
The results demonstrate that tow wrinkling is fundamentally a geometric instability driven by path-induced compressive strain. Process parameters influence wrinkling indirectly by modifying adhesion and foundation stiffness, thereby controlling the balance between in-plane and out-of-plane deformation mechanisms.
Higher temperature and compaction do not eliminate compressive strain but enable the tow to accommodate it through waviness or bunching rather than buckling. However, excessive heating or pressure can introduce other defects, emphasizing the need for a controlled process window rather than extreme parameter values.
From a manufacturing perspective, these findings support the use of predictive tools that combine geometrical models with process-dependent material behavior. Such tools enable offline path planning and parameter selection to avoid critical curvature regions or adjust processing conditions dynamically.
6. Conclusion
Tow wrinkling in automated fiber placement arises primarily from length mismatches during steering and represents a key limitation to AFP productivity and design freedom. Based on the reviewed modeling and experimental evidence, the following conclusions can be drawn:
- Wrinkle amplitude increases rapidly with decreasing steering radius, while wavelength remains relatively constant.
- Elevated nip-point temperature and moderate-to-high compaction pressure promote adhesion and shift deformation toward in-plane modes.
- Substrate quality plays a critical role, with small imperfections acting as wrinkle initiation sites.
- Effective defect mitigation requires balancing productivity with controlled thermal and mechanical conditions rather than maximizing individual parameters.
These insights provide a foundation for predictive AFP process design and improved defect control in curved composite structures.
References
Wehbe, R. Tow-Path Characterization for Automated Fiber Placement. PhD Dissertation, University of South Carolina, 2020.
Wehbe, R., Gürdal, Z., Harik, R. Geometrical modeling of tow wrinkles in automated fiber placement. Composites Science and Technology, 2020.
Beakou, A., et al. Fiber instability during automated placement of composite tows. Journal of Composite Materials, 2015.
Nguyen, C. D., Krombholz, C. Influence of process parameters on prepreg adhesion in AFP processes. ECCM Proceedings, 2016.