Study on Fatigue Life Evaluation of Adhesive Structure Based on Fatigue Characteristics of Adhesive

In recent years, the usage of adhesives in the joints of automobile bodies has been expected. Researches on estimating fatigue life are necessary to establish design criteria for adhesive joints. If the fatigue life of bonded joints can be estimated based on the evaluation of fatigue life of a simple-shaped specimen, it will lead to an increase in the efficiency for the design and development stage and cost reduction. In previous researches, it was clarified that interfacial failure was restrained by treating surface of the adherent, and the emergence of low adhesive strength was restrained by the cohesive failure rate at over 40 %. In case of cohesive failure, the strength of bonded joint depends on the strength of an adhesive, allowing the fatigue characteristics of the adhesion specimen to be reproduced by using the fatigue characteristics of the adhesive bulk specimen. In this study, FEM analysis was carried out in order to identify the location of stress concentration and to clarify the relationship between stress value at stress concentration zone and the load value within the adhesive layer of an adhesive specimen. The fatigue test of the adhesive bulk specimen and the adhesive specimens was carried out and the crack initiation life of the adhesive specimen was estimated by converting the stress axes of S-N curve using the analysis results. As a result, in T-peel adhesive specimen, the stress range at crack initiation cycle of the adhesive specimen has been estimated to be higher than the stress range at the same cycle of the adhesive bulk specimen by using average stress at the position where the stress averaged in the thickness direction of the adhesive layer is maximum as a representative value (method 1) in both the maximum principal stress criteria and the Mises stress criteria, and the estimated fatigue life could be conservative. In overlapping adhesive specimen, the stress range at crack initiation cycle of the adhesive specimen has been estimated to be excessively higher than the stress range at the same cycle of the adhesive bulk specimen by using method 1 in both the maximum principal stress criteria and the Mises stress criteria. Compared to method 1, the stress range at crack initiation cycle of the adhesive specimen has been estimated to be closer than the stress range at the same cycle of the adhesive bulk specimen by using average stress averaged in the thickness direction of the adhesive layer at the nodal position where maximum stress is generated (method 2).