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NDE for Adhesive Bond Strength

Advancements in manufacturing and assembly methods to reduce cost and increase rate are pushing for the application of adhesive bonding in composite primary structure applications. In these cases the certification of the structure requires total confidence in the as manufactured strength of the adhesion in the joint. Unfortunately, there are no standard nondestructive evaluation (NDE) methods that can return a value for the bond strength. A number of studies have been performed to attempt to correlate NDE measurement parameters to strength. These measurement methods seek such parameters as bulk modulus, shear modulus, nonlinear coefficients, density, porosity, acoustic attenuation or transmission, etc. andor some combination of these characteristics. In certain cases of bond degradation a correlation can be made. However, the problem remains for the detection of the general weak bond condition where the loss of strength may occur due to any effect such as contamination, out of process conditions or service environmental degradation. Stress wave loading from a laser generated shock has been successful as a localized test of bonded joint strength. This laser bond inspection (LBI) stress wave testing uses strong mechanical waves to create a tension load on the bond. If the bond is weak the joint will fail at the test point. This failure can be detected by sensors in the test instrument or by post test NDE such as ultrasound. While the stress wave method is not strictly nondestructive, when they are performed at a proof testing energy level they are nondestructive to strong bonds but destructive to weak bonds. Recent developments with LBI have involved calibrated composite to composite bond strength standards and composite to metal bonds.

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