Non-destructive testing of adhesive bonds using Fokker bond tester

The design concept and capabilities of the Fokker bond tester as an ultrasonic resonance instrument for nondestructive evaluation of adhesive bonds are outlined. The instrument is designed to compare the acoustical properties of an unbonded facesheet and a bonded joint, which is performed by first placing the transducer on an unbonded facesheet considered to have zero cohesive strength and then coupling the transducer to a bonded joint, thereby inducing loads in the adhesive layer. Change in loading from the unbonded sheet to the bonded joint causes a resonance frequency shift displayed on the A-scale and a change of amplitude indicated on the B-scale of the device. This change is related to the dynamic mechanical properties of the joint and is therefore an indirect measure of the strength of the joint. Correlation curves are examined for sheet-to-sheet and sheet-to-core bondings. Statistical reliability analysis reveals that the Fokker bond tester is capable of predicting bond strength within 95% confidence limits. The device can evaluate bond porosity, which could not be determined by previous testing methods. A device limitation is lack of sensitivity to surface conditions.