Use of the Fokker Bond Tester on Joints with Varying Adhesive Thickness

The operation of the Fokker bond tester has been investigated. The instrument monitors one of the natural frequencies of the system comprising a piezoelectric crystal coupled to the joint under test. The resonant frequencies of two different sizes of transducer coupled to both plain plates and adhesive joints have been investigated both theoretically and experimentally, the measured values being in line with the predictions. It has been concluded that the method is satisfactory for the location of disbonds in a multi-layer construction but that it cannot distinguish between adhesive modulus and adhesive layer thickness. Both of these parameters affect the adhesive stiffness and hence the measured resonant frequencies. However, they have different effects on the cohesive strength of the adhesive so unless independent measurements of, for example, thickness are made, erroneous strength predictions may be obtained. Also, the measured frequencies are independent of adhesive stiffness over a wide range of typical stiffnesses, so changes in adhesive properties will not be measured in these cases.