PhD Oral Exam - Meisam Kheradpished, Mechanical Engineering

When studying for a doctoral degree (PhD), candidates submit a thesis that provides a critical review of the current state of knowledge of the thesis subject as well as the student’s own contributions to the subject. The distinguishing criterion of doctoral graduate research is a significant and original contribution to knowledge.

Once accepted, the candidate presents the thesis orally. This oral exam is open to the public.

Abstract

Automated Fiber Placement (AFP) is a promising technology for manufacturing high-quality, large-scale structural components with complex geometries. However, a major challenge with AFP is the formation of manufacturing-induced defects, such as wrinkles, waviness, and tape folding, that occur during the steering process. These defects arise from the mismatch in length between the inner and outer edges of the prepreg tape when it is placed along a curved path. Such defects can degrade the mechanical properties of the part, leading to a reduction in its quality. Therefore, minimizing or eliminating these defects is crucial to improve the final product's overall quality. This thesis aims to analytically predict the in-plane and out-of-plane defects occurring at a steered tape in the AFP process, and the ultimate goal is to propose some solutions for reducing and eliminating the steering-induced defects.

A thorough experimental investigation was conducted using a variety of process parameters and steering radii to enhance our understanding of the defect formation during the steering of thermoset prepreg tows. Based on the experimental observations, two micro and macro models were presented to predict planar and non-planar deformations at steered tapes. According to the analytical results, it was then shown that interlayer bonding plays a significant role in the generation of defects in the AFP process. Consequently, a systematic series of experiments and finite element analysis was performed to enhance the interlaminar bondings at the AFP process. At the end according to all experimental and analytical analysis, a novel compaction roller is designed and manufactured to provide variable pressure distributions and contact length based on the geometry of the part, unlike traditional rollers, to reduce and minimize the defect formation during steering.