Abstract
Composite doublers are frequently used to repair both metallic and polymer-matrix composite aircraft structures. To assure the structural integrity of the aerostructures that contain the bonded doublers requires NDE techniques that are capable of determining bond quality. If the doubler is to work properly, the integrity of the total bond that comprises the joint between the doubler and the underlying structure is essential. Factors that affect joint integrity include variations in the properties of either of the adherends, the adhesive, or the presence of contaminants on the surfaces of the adherends as they are being joined. These joints are frequently inspected using resonant ultrasound methods, thermal wave techniques, or a tap test. These methods will generally detect complete disbonds in the joint. However, if there is a weak bond caused by improper curing of the adhesive, use of an out-of-date adhesive, or contaminants on the surface, it will not be detected by these conventional techniques. We report our initial findings in developing a nondestructive inspection method suitable for detecting regions containing weak bonds. This method includes applying a load to the doubler and measuring the resulting strain distribution on the surface of the doubler. Specimens were fabricated with known weakened areas using staged and cured film adhesive with reduced adhesive properties in these areas. A model for the specimen was developed using finite element methods. The results of the experiments and the finite element analysis (FEA) are presented and compared. The results of the experiments, although preliminary, were encouraging since all of the defects were detected using an electronic speckle interferometry (ESPI) technique. (Author)