Session: 12-12-02: Modeling of the Fracture, Failure, and Fatigue in Solids

Paper Number: 144005

144005 - Fatigue Fracture and Strength Improvement of Adhesive Joint Studied by Experiment and Molecular Dynamics Simulation 

Introduction

In recent years, it is required to reduce the weight of transportation in order to reduce carbon dioxide emissions. Metallic materials are being replaced by lighter and stronger composite materials, and “multi-material technology”, which combines multiple materials with different properties to create materials and structures with comprehensively superior characteristics, is being studied. Adhesive bonding technology, which uses adhesives instead of joint components, is a promising method for joining different materials. While expectations for reduction of the weight are increasing, adhesive bonding still has issues in terms of strength and safety, and its reliability is still insufficient. In addition, the bonding area of transportation is subjected to cyclic loading. Therefore, it is essential to improve the fatigue strength of adhesive joints from the viewpoint of designing multi-material structures and enhancing long-term reliability. In this study, we focused on modification of the surface of the adherend, and experimentally evaluated the increase in fatigue strength of adhesive joints by increasing the chemical bonding strength of the interface with the silane coupling agent (SCA) treatment, and further investigated the failure mechanism by molecular dynamics (MD) simulations.

 

Methods

In the experiments of this study, aluminum alloy was used as the adherend, and commercial two-component mixed epoxy resin was used as the adhesive. Quasi-static tension and Fatigue tests were conducted on mirror specimens, which were mirror-polished aluminum/epoxy resin adhesive joints, and on SCA specimens, which were aluminum/SCA/epoxy resin adhesive joints that were treated with SCA in addition to mirror polishing.

In addition, MD simulation was used to investigate the effect of SCA and observe the fatigue fracture behavior inside the epoxy resin in order to elucidate the fatigue fracture mechanism of the adhesive joints. The model used in the simulation was the same two-component mixed epoxy resin as in the experiment. For the effect of SCA treatment, SCA layer was created to understand physical and chemical bonding at the interface. In the fatigue simulation, the stress ratio was kept constant, and the maximum stress was varied to perform “low-cycle fatigue” with high stress and low cyclic loading and “high-cycle fatigue” with low stress and high cyclic loading.

 

Results and discussions

SCA treatment induces the strength improvement, due to the increases in interfacial strength between Al alloy and epoxy adhesive. Thus, it is found that SCA is useful for strength improvement, resulting in cohesive failure of epoxy adhesive resin. Next, the results of fatigue test showed that SCA treatment increased the adhesive fatigue strength and improved the adhesive stability at the adhesive interface. In addition, SCA specimens showed void growth in the adhesive during high-cycle fatigue tests. According to the results, it was predicted that the fatigue failure mechanism of the adhesive joints differed depending on the number of fatigue loading cycles.

In the MD fatigue simulation, it is found that SCA treatment induces SCA layer to encourage chemical bonding at interface, resulting in the strength increasing and interfacial fracture stability. For the fatigue test, it was found that the void volume increased rapidly in a short time in low-cycle fatigue, while it increased gradually over time in high-cycle fatigue. Therefore, it was clear that the growth rate of void volume differs depending on the number of fatigue loading cycles. In low-cycle fatigue, deformation failure was caused by crack propagation, while in high-cycle fatigue, voids were generated discretely, grown by cyclic loading, and eventually fracture.

Conclusion

In this study, SCA treatment of adhesive bonding and fatigue failure mechanism of adhesive joints were investigated, experimentally and numerically. Experimental investigation showed that SCA treatment improved the mechanical properties of adhesive joints. Furthermore, void growth was found inside the adhesive of the SCA specimen in the high-cycle fatigue test. Numerical investigations using molecular dynamics simulations revealed that the growth rate of void volume and the deformation behavior around defects differ depending on the number of fatigue loading cycles. This study will contribute to the construction of an energy-efficient society and the realization of a carbon-neutral society through applications such as the improvement of adhesive bonding technology used in transportation.

Presenting Author: Masayoshi Ogawa Chuo University

Presenting Author Biography: Mr. Masayoshi Ogawa is a graduate student at Chuo University in Japan. He received her B.S from Chuo University in March 2024. He is an expert in computational mechanics of polymer and adhesive materials. He was awarded by Japan Society of Materials Science (JSMS), Kanto Branch, in 2023 and Best Poster Awards from The Adhesion Society of Japan in 2024.

Authors:

Masayoshi Ogawa Chuo University
Akihiro Shinozaki Chuo University
Yuichi Hosoya Chuo Univesity
Yuzuki Kawashima +81-3-3817-1829
Akio Yonezu Chuo University