Analysis of adhesively bonded repairs in composites: Damage detection and prognosis

The increasing use of high performance composite materials, such as carbon–fibre reinforced plastic (CFRP) composites, in commercial and military aerospace applications has led to increased interest in composite repair technologies. Their growing use has risen from their high specific strength/stiffness properties and improved fatigue life, when compared to the more conventional materials [1]. During its service life, an aircraft is subjected to several structural and aerodynamic loads. These loads can cause damage or weakening of the structure that may affect its load carrying capability. The restoration of structural efficacy by repair or reinforcement of the damaged or weakened part to assure continued airworthiness of an aircraft has thus become an important issue in recent years. Adhesively bonded repairs are the most common type of repair carried out with composite materials [2]. This technology provides an alternative to mechanically fastened repairs, which often introduce undesirable stress concentrations, that may affect performance. There are two types of bonded patches that can be used to repair structural damage; external bonded patches and scarf-type bonded patches. External bonded patches can provide some recovery with respect to the component’s strength, enabling a fast and easy to implement application procedure. In addition, scarf repairs offer great advantages compared to external patch repairs since they provide higher stiffness by matching ply to ply the original structure and by reducing stress discontinuities in the repaired region. This repair technique is often applied where surface smoothness is essential since aerodynamic disturbance is minimized. In the current study, the performance of both bonded repair techniques applied to CFRP laminates was assessed under uniaxial tensile loading. The specimens were manufactured from commercially available carbon–epoxy pre-impregnated tapes and two different laminates were used in the study: quasi-isotropic woven M21/HTA carbon–fibre epoxy and quasi-isotropic unidirectional M21/T700 carbon–fibre epoxy specimens. A 3D finite element analysis was performed to determine the stress field in the optimum repaired configuration and the results were compared with the experimental observations. The behaviour of a scarf repaired composite panel was monitored using two types of on-line damage analysis: Ultrasonic guided waves (Lamb waves) analysis and full-field measurement methods based on Digital Image Correlation (DIC) analysis. The correlation of the results of both techniques with the early stage damage was performed and conclusions about the recovered strength through the scarf repair and damage evolution were deduced. Finally these results were compared with off-line techniques such as ultrasonic C-scanning and X-ray radiographs in order to identify damage location and extent after loading.