Title: Development of an In-Situ Mixed-Mode Tackiness Characterization Technique for Uncured Thermoset Carbon/Epoxy Prepreg Tows at Automated Tow Placement Relevant Timescales

Authors: Debrup Chakraborty, Karan Kodagali, Subramani Sockalingam, Michael A. Sutton

DOI:

Abstract: The in-situ bond strength and toughness (IBST) or tackiness of uncured carbon/epoxy thermoset prepreg tows is a critical parameter that governs the quality of adhesion of automated fiber and tow placement (AFP/ATP) processes. Lack of adhesion at short contact times results in defects such as wrinkles and tow folds, especially along curved placement paths where tows undergo locally mixed-mode compression shear loading during deposition. While recent studies have made advances in the characterization of Mode I IBST for short (10-50 ms) and long (0.1-10 s) contact timescales, the in-situ mixed-mode tackiness response of prepreg tows remains relatively unexplored for its direct relevance to curved-path manufacturing. A novel experimental technique is presented for quantifying the mixed-mode IBST of uncured IM7-G/8552 carbon/epoxy thermoset prepreg tows at short ATP relevant contact millisecond (ms) timescales. The test fixture employs two rigid platens with a single tow attached to each surface at an angle of 45°, a micronscale Teflon insert to ensure a crack initiation point under controlled mixed-mode separation. The experiments are carried out at bonding temperature Tb = 40°C, contact pressure Pc = 230 kPa, and three contact times tc =10 ms, 30 ms, and 1 s. A constant debonding rate of dbr = 5 mm/s is used in all experiments. Force-displacement data is used to derive fracture mechanics-based tractionseparation relations for each condition. The results demonstrate that mixed-mode peak traction (σmax) and critical energy release rate (GMC) increase with contact time (tc), consistent with previously reported Mode I trends. In particular, the mixed-mode IBST values are all higher than the corresponding Mode I values for all contact times examined, which indicate enhanced resistance to interfacial debonding due to additional shear contributions. These findings indicate that tackiness characterization with shear loading better describes the adhesion condition that is encountered upon tow steering and curved-path placement during AFP/ATP processing. The development of this experimental method provides a simpler means into measurement of shorttime, mixed-mode tackiness of uncured thermoset prepregs relevant to real time curved-path ATP manufacturing conditions. Through a direct comparison of Mode I and mixed-mode IBST, this research highlights the role of mixed-mode adhesion in tow fold defect suppression and wrinkle prevention. This serves as a basis for process optimization, defect prediction, improved quality of the bond, and maximum structural performance of composite laminates fabricated by automated placement techniques.

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Conference: SAMPE 2026

Publication Date: 2026/04/27

SKU: 15

Pages: 14

Price: $28.00