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The primary objective of this study was to demonstrate the effectiveness of various strain measurement techniques at
detecting the disbonding of a composite repair patch and then using this information to validate a new capacitance based
disbond detection technique. The instrumented repair patch was parametrically designed with the help of Finite Element
Analysis (FEA) software to have a stress concentration at its tip. This stress concentration was designed to produce a
disbond during fatigue testing, without the need for the introduction of any foreign material to create an artificial disbond
condition. The aluminum substrate was grit blasted and the instrumented patch was bonded using FM®73 adhesive, and
was cured following the recommendations of the manufacturer. The geometric characteristics of the patch followed
standard repair guidelines for such variables as material selection, taper angles and loading conditions, with the
exception of the area designed for premature disbond. All test specimens were inspected using non-destructive testing
technique (ultrasound pulse echo) to guarantee that no disbonding had occurred during curing of the specimen. The
specimens were placed under fatigue loading to induce a disbond condition between the aluminum substrate and the
patch. The specimens were cyclically loaded and strain gauges bonded to strategic locations on the aluminum and
composite patch surface to be able to measure changes in surface strains as the disbond progressed. A Digital Image
Correlation (DIC) system was also used to measure full field strains over the gauge length of the coupon. The DIC
results were compared with the strain gauge data and were used to provide a qualitative measure of the load transfer in
the bonded specimen, which clearly demonstrated the change in surface strain that occurred as the composite patch
disbonded from the aluminum substrate. Thermoelastic Stress Analysis (TSA) was also used to measure surface strains
on the composite patch. Thermoelastic stress analysis proved to be the most sensitive technique for experimentally
monitoring the disbond process in real time. Failure analysis of the specimens using optical microscope techniques was
performed to determine the type of failure between the patch and the substrate. The results of this work will serve to test
the different types of sensors available for the design and manufacturing of a "Smart Patch" for aircraft structure
applications.
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