Date of Award

Fall 2023

Document Type

Open Access Dissertation

Department

Mechanical Engineering

First Advisor

Subramani Sockalingam

Abstract

Traditional unidirectional (UD) carbon fiber-reinforced polymer matrix composites manufactured via automated fiber placement (AFP) are widely used in aerospace structures. While heterogeneous at the micro-scale, these materials are homogeneous at the meso-scale (ply-scale). The major limitation is its limited toughness, poor damage tolerance/impact resistance capability and the inability to sufficiently redistribute stresses, resulting in strength-toughness tradeoffs, making them susceptible to impact damage. From previous studies, there is an understanding that fiber architecture has a first order effect on the damage tolerance. Introducing new discontinuous/heterogeneous meso-scale architectures, through an optimal merger of material and structure, can result in unprecedented property improvements due to multiple interacting deformation mechanisms as demonstrated in biological/biomimetic architected materials. Interestingly, controlling/inducing multiple deformation/failure mechanisms via spatial distribution of in-plane anisotropy (i.e., heterogeneity in fiber orientations) and boundaries / interfaces is largely unexplored. In recent years, a specialized AFP layup procedure was introduced to mimic a woven-like architecture using UD prepreg tows. In this modified procedure, tows are intentionally skipped during placement leaving gaps. These gaps are filled in the subsequent passes to produce pseudo-woven meso-architectured composites (MAC). The unique architecture is macroscopically heterogeneous with discontinuous and spatially varying fiber orientations both in-plane and through thickness resulting in multiple interfaces and an expanded design space. This dissertation explores the incorporation of MAC laminates manufactured using AFP to enhance the damage tolerance of unidirectional carbon fiber reinforced polymer composites. An extensive experimental study has been carried out on hybrid carbon fiber reinforced laminates (consisting of MAC and traditional UD sub-laminates) compared against a baseline traditional unidirectional quasi-isotropic control laminate under high velocity impacts of 250-400ft/s, low velocity impacts of 15-55 J, open hole tension and open hole compression. C-scans and digital image correlation techniques are employed to better understand the experimental response. The hybrid MAC laminates demonstrate a significant 45% reduction in back-face surface damage, 19.5% less back face deflection and an 18% increase in penetration velocity when compared to quasi-isotropic control laminate under high velocity impacts. Under a low velocity impact, the hybrid laminate configurations exhibit increased damage resistance with up to 37% higher critical delamination load and increased damage tolerance with 26% higher residual compressive strength after an impact of 55 J compared to control laminate. Additionally, the hybrid MAC laminates demonstrate a 7% increase in OHT strength and up to a 16% reduction in strains near the open hole while the OHC response is found to be similar. To better understand the operating mechanisms in MACs, a python script is developed to mimic the AFP layup procedure to determine the spatial variations in the fiber angle and stacking sequence and determine the representative unit cell. Furthermore, finite element models are developed to better understand the damage propagation mechanisms in these complex heterogeneous materials. The studies demonstrate that improved impact performance in MACs is due to crack deflection, damage diffusion and stress redistribution mechanisms induced by the heterogeneous composite architecture and provides insights into the fundamental deformation and failure mechanisms during impact onto these complex.

Rights

© 2024, Karan Kodagali

Download

Share

COinS