Higher Order Nonlinearity Measurement Techniques in Thin Adhesive Layer
In a recent paper  a nonlinear method was suggested to evaluate adhesive joint quality and it was pointed out that nonlinear measurements are more directly correlated to mechanical strengths than linear ones. In that paper a new dynamic acousto-elastic measurement technique was introduced and shown to be sensitive enough to measure both first and second order nonlinearities at load levels of 15% of the ultimate strength. At this stress level special geometry of the adhesive joint sample is required in order to adopt the sample to a high power loading machine. Lower stress levels however can be generated by low frequency high power ultrasonic transducer which would replace the loading machine and make the technique truly a nondestructive one. The objective of this paper is to introduce a more sensitive measurement technique to measure both first and second order nonlinearities in a thin adhesive layer at stress level of 1–2% of the ultimate strength.