Theoretical and Experimental Investigation of Ultra High-Strength Joining in Steel-Short Fiber Reinforced Composites
Author(s)
Hong, Sungjin
Advisor(s)
Ahn, Sung-Hoon
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Abstract
The increasing demand for lightweight, high-performance, and fuel-efficient structures in industries such as automotive and aerospace has driven significant research into hybrid metal-composite materials. However, effectively joining these dissimilar materials remains a primary obstacle. Traditional methods like adhesive bonding and mechanical fastening suffer from limitations in environmental durability, process efficiency, and strength. While modern laser-textured joining has shown promise, it is critically deficient in two key areas: weak joint strength under cross-tension loading. Furthermore, existing multi-step joining processes are often complex and ill-suited for high-volume industrial production.
This work presents a theoretical and experimental investigation into a novel, laser textured injection molded Joining (LIJ), which overcomes these fundamental limitations. The LIJ method integrates advanced laser texturing; employing a custom angular and wobble-beam system to machine deep, undercut grooves; with a single-step, in-situ injection molding process. A fiber-reinforced thermoplastic is molded directly onto a textured steel substrate, establishing a robust, fastener-free mechanical interlock. A comprehensive experimental optimization demonstrated the process's efficacy, achieving a maximum cross-tension strength of 13.8 MPa, a result that dramatically exceeds previously reported values. The process also yielded an exceptional lap-shear strength of 41.7 MPa. Failure analysis and durability testing are performed to analyze the behavior of the joint in both manufacturing and service environments.
To predict and understand the joint's performance, a multi-faceted modeling approach was employed. A computationally efficient, reduced-order thermo-phase field model was developed to successfully simulate the formation of line-scan grooves, capturing complex recast and narrowing phenomena. For the more complex "wobble" geometry, a hybrid data-driven model was built, which accurately predicts the groove morphology from laser inputs. Finally, a study combining finite element analysis with a novel chemical polishing validation experiment revealed the fundamental source of the joint's high strength. The model quantitatively proves the existence of a multi-scale interlocking mechanism, wherein the joint's integrity is a synergistic product of mesoscale interlocking (overall groove architecture) and microscale interlocking (surface roughness).
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Date
2025-12
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Text
Resource Subtype
Dissertation (PhD)