Ultrasonic evaluation of adhesive bond degradation by detection of the onset of nonlinear behavior

Quantitative nondestructive evaluation (QNDE) of the degradation of adhesiv e bonds remains one of the most challenging problems in QNDE. The objective of this research was to approach this problem by the detection of nonlinea rity due to bond deterioration. The paper starts with experimental observat ions of the reflection of ultrasonic signals by adhesive bonds. The specime ns for these tests had been subjected to cyclic loading which was expected to cause bond deterioration. Differences in the reflections could, however, be observed only if the adhesive bonds were subjected to static loads simu ltaneously with the ultrasonic testing. The higher the number of fatigue cy cles, the lower the required load to display the differences in signals wit h unloaded cases. The second part of the paper presents a theoretical expla nation of these ultrasonic measurements based on the postulate of nonlinear stress-strain behavior of the deteriorated bond. The ultrasonic tests prov ide the slope of the stress-strain curve and the results can therefore be u sed to determine the deviation of the stress-strain curve from linear behav ior. For the higher numbers of fatigue cycles, this deviation, which is ind icative of bond deterioration, starts at smaller load values.