Study For The Effectiveness Analysis On Tensile Strength Of Adhesively Bonded Repaires To Composite Laminates

As fiber-reinforced composite materials have been widely used in aircraft structures, the task of repair issues for damaged composites is becoming more difficult. Laminates and laminated structures are the most common structural forms of composites, while adhesively bonded repairs are the most common repair techniques used in permanent structural repairs. In order to restore designed strength of laminates furthest and evaluate the static strength effectiveness of adhesively bonded repairs to composite laminates, ultimate strength of undamaged laminates and repaired laminates need to be predicted. Based on investigations of the failure mechanisms of laminates and adhesive layers, both FEM and analytic method for effectiveness analysis on tensile strength of repairs have been studied.(1) FEM. Firstly, UMAT for 3-D solid elements of composite laminated structures was designed and validated. In UMAT, 3D strain-based Hashing failure criteria and Ye delamination failure criteria, along with stiffness degradation rules established in the framework of continuum damage mechanics, were used to define constitutive relations of composites pre- and post- damage. Material softening behavior and stiffness degradation often present severe convergence difficulties. Therefore, a viscous regularization scheme was used to alleviate these difficulties. Then, 3D cohesive elements were used to simulate complex failure modes and damage evolution. After modeling, calculation and experimental verification, the FEM could evaluate the tensile strength effectiveness of adhesively bonded repairs accurately.(2) Analytic method. Firstly, based on 3D layered media theory and progressive damage analysis, analytic method for failure analysis of the composite laminates was studied. Then, based on Hart-Smith models and Ahn models, analytic method for failure analysis of adhesively bonded repaired laminates was studied. In repaired laminates, progressive damages of outer repair regions in parent plates and repair patches were analyzed. In addition, the 3D equivalent stiffness of repair regions in parent plates and repair patches was considered. The software for static strength effectiveness analysis of adhesively bonded repairs to composite laminates was developed according to the analytic method. After analysis, the engineering requirements can be fully satisfied by the calculated results. The software can be helpful for aided designs of composite laminates repairs.The database used in structural repair support system for civil airplane was also researched. Database technology is used in the system to manage data in the process of structural damage assessment and repair decision. Effective repair cases, which were calculated and validated using the analytical software, can be collected into the database for aided designs of composite laminates structural repairs in civil airplane.