Experimental investigation of prepreg slit tape wrinkling during automated fiber placement process using StereoDIC

Abstract

For the first time, an experimental investigation is presented quantifying out-of-plane wrinkle formation and in-plane deformations occurring in prepreg slit tape specimens during automated fiber placement (AFP). The AFP process is conducted on 6.35 mm wide individual prepreg tows that are placed along a straight path and steered along circular paths with radii of curvature, ρ = 2540 mm, 1270 mm and 305 mm. Full-field shape, displacement and strain measurements are obtained using StereoDIC in three successive stages; (i) immediately after lay-up of the tow, (ii) one hour after lay-up and (iii) after reheating of the as-placed tow by traversing the AFP head close to the tow path and exposing the specimen to the same levels of heating as employed during the initial lay-up (without compaction pressure). Measurements obtained in stage (i) showed that (a) all tows exhibited local wrinkling at locations where the underlying substrate had defects (e.g., gaps, overlaps), (b) excluding the substrate defect locations, tows steered with ρ = 2540 mm showed no evidence of additional out-of-plane wrinkling after placement, (c) tows steered with ρ = 1270 mm exhibited several small amplitude wrinkles initiated at locations outside the substrate overlap regions and (d) tows steered with ρ = 305 mm incurred significant additional localized wrinkling along the inner radius of the tow with an increase in both the amplitude and frequency of the wrinkles. In addition, higher variability in the wrinkle wavelength is observed for ρ = 305 mm. Measurements obtained in stages (ii) and (iii) showed an increase in wrinkle amplitude, indicating that wrinkle amplitude is a function of both time and temperature for an as-placed tow.