Estimation of adhesive stress for bonded patches to strengthen stop-hole repair

Bonded patch repair methods for fatigue cracks have gained attention due to advantages such as rapid application and avoidance of cross-sectional loss. This study focuses on stop-hole repair strengthened by bonded steel patch plates. The method offers simple installation and reduces the likelihood of crack reinitiation from the stop-hole. The fatigue life after repair is governed by two mechanisms: crack reinitiation at the stop-hole and cohesive failure of the adhesive. In previous studies, stress concentration factors affecting stop-hole fatigue strength have been investigated, which are related to crack reinitiation. On the contrary, adhesive stress, which is related to cohesive failure of the adhesive, has not yet been fully investigated. Finite element (FE) analysis was conducted to evaluate shear, normal, and maximum principal stresses in the adhesive, and theoretical formulation were developed to estimate these stresses. Two repair scenarios were considered: a central patch repair model, in which patch plates were bonded at the crack center, and an eccentric patch repair model, in which patch plates were bonded eccentrically to avoid interference with gusset plates. The developed formulation showed that the maximum principal stress in the adhesive ranged from 0.20 to 0.26 MPa for central patch repairs and from 0.22 to 0.28 MPa for eccentric patch repairs, depending on plate width. The predicted stress distributions of the adhesive showed good agreement with the FE results. These findings provide useful insights into the fatigue design of stop-hole repairs with bonded patches, enabling more precise consideration of adhesive cohesive failure.