Session: 03-13-01: Testing and Characterization

Paper Number: 152765

152765 - The Effect of Waviness Defects on the Viscoelastic Properties of Carbon-Epoxy Composites 

Carbon fiber-reinforced polymeric (CFRP) composites are lightweight materials used in load-bearing engineering applications (e.g., aircraft and space structures) due to their excellent mechanical properties. Ply waviness is a common manufacturing defect in CFRP composites that develops when plies are draped around a curved surface (e.g., hemispheres and joggled geometries) due to changes in the plies path lengths that cause out-of-plane deformation of adjacent plies. It is also a defect commonly found in stepped or tapered laminates, laminates with ply drops, and in laminates fabricated from automated fiber placement (AFP), prepreg hand layup and resin transfer molding. The effects of ply waviness defects on the strength reduction of composites have been widely studied, and the primary failure modes nucleating near the waviness region have been identified. However, the influence of these defects on the time- and temperature-dependent response, which arises from the viscoelastic properties of the polymer matrix, has not yet been investigated.

This research investigates the effect of waviness defects on the viscoelastic properties of CFRP composites. To this end, experimental testing via dynamic mechanical analysis was performed. Specimens were subjected to single cantilever cyclic bending, with temperature and frequency sweep tests conducted to study changes in storage modulus, loss modulus, and damping factor (tan delta). For temperature sweeps, initial temperature was 25℃ and ramped up to 200℃ at 3℃/min and held at a frequency of 1.0 Hz. For frequency sweeps, initial frequency was 0.1 Hz and ramped up to 10.0 Hz at 10 points per decade and held at room temperature. All tests were subjected to a strain amplitude of 0.01% at 65 psi. Three different sample types were analyzed: 1) woven composites, 2) composites with embedded ply waviness in a predominantly unidirectional laminate, and 3) cross-ply composite laminates with no ply waviness defects. Optical microscopy images of all samples were taken prior to testing to characterize the waviness defects and identify any other manufacturing-induced defects, such as voids or resin-rich regions.

The results show that for all samples, both the storage modulus and loss modulus exhibit notable dependence on temperature, particularly near the glass transition temperature of the thermoset matrix. However, while the storage modulus showed no dependence on frequency, the loss modulus demonstrated only minimal frequency dependence. The damping factor remained consistent across all composites during temperature sweeps. In contrast, specimens with embedded ply-waviness defects exhibited the smallest changes in damping factor during frequency sweeps. In conclusion, compared to the control samples, it was observed that the waviness defects studied in CFRP composites have minimal effect on the viscoelastic properties measured.

Presenting Author: Paulina Diaz-Montiel University of San Diego

Presenting Author Biography: Paulina Díaz-Montiel, PhD, is an Assistant Professor of Mechanical Engineering at the University of San Diego. Dr. Díaz-Montiel’s expertise is in structural mechanics and composite materials. Her work combines manufacturing, testing, experimental mechanics, and computational methods for studying the effect of manufacturing-induced defects on the progressive failure and sustainability of composites materials and structures. Dr. Díaz-Montiel received her PhD in Structural Engineering from the University of California San Diego - San Diego State University joint doctoral program. She holds an MS in Aerospace Engineering from San Diego State University and a BS in Mechanical Engineering from CETYS University (Mexicali, México).