Abstract: Various testing methods for adhesive strength are prescribed in JIS; however, no testing method has been established for conditions in which a singular stress field does not exist. In general, material strength should be evaluated using smooth specimens that provide a uniform stress distribution. In adhesive strength evaluation, however, all bonded specimens inherently contain a singular stress field, and its intensity (ISSF) varies depending on the adhesive layer geometry. This is the fundamental reason why adhesive strength depends on adhesive layer configuration.
In this study, a protruded butt joint is proposed to obtain a constant interfacial stress distribution. Using this configuration, the ideal adhesive strength under a uniform interfacial stress condition without local stress concentration was clarified. While the adhesive strength of conventional butt joints strongly depends on the adhesive layer thickness h, the proposed joint exhibits thickness-independent strength, remaining constant at a critical stress of σ_B=47.7MPa.
Furthermore, for the JIS butt joint, it was found that the failure mechanism can be classified at a boundary thickness of h=0.1mm into failure governed by internal stress and that governed by the singular stress field. When fracture is controlled by the singular stress field, failure is considered to initiate at a distance of r_B=14.7" " μ"m" from the adhesive edge, determined by evaluating the region (process zone) where the average stress reaches the inherent joint strength σ_B=47.7MPa. This value agrees well with previously reported fracture initiation locations in square-column butt joints, supporting the validity of the proposed evaluation method.
The key contribution of this study is the proposal of an adhesive test specimen in which the singular stress field is eliminated, enabling identification of an ideal maximum strength independent of adhesive geometry, namely the ideal adhesive strength. This finding clarifies the mechanical essence of adhesive strength and the influence of singular stress fields, and is expected to contribute to improving the reliability of adhesive joint design.

Biography: Professor Kazuhiro Oda is a faculty member in the Mechanical Engineering Program, Faculty of Science and Technology, Oita University. He received his Ph.D. in Engineering from Kyushu Institute of Technology in 1995. His research interests include strength of materials, elasticity, and fracture mechanics, with a particular focus on stress analysis and singular stress fields at dissimilar material interfaces and strength design of adhesive structures. After completing his doctoral studies in 1995, he served as a Research Fellow of the Japan Society for the Promotion of Science (JSPS) and later joined Tokuyama College of Technology, where he held positions as Associate Professor and Professor. In 2012, he moved to Oita University as Professor in the Faculty of Engineering (now the Faculty of Science and Technology). He is currently engaged in research on advanced strength evaluation methods for adhesive joints and stress intensity factor analysis for orthotropic dissimilar materials. Professor Oda also serves as Special Assistant to the President for Industry–Academia Collaboration, promoting partnerships between the university and industry. He was a Board Member of the Society of Materials Science, Japan (2020–2024). With numerous publications and contributions, he is recognized as one of the leading researchers in fracture mechanics–based design of dissimilar material joints. In 2025, he received the Society of Automotive Engineers of Japan’s Technical Paper Award.