AN EVALUATION OF TEST METHODS FOR DETERMINING ADHESIVE SHEAR STRESS-STRAIN PROPERTIES

While the aviation industry is increasingly making use of composite structure to realize reduced air vehicle cost and weight, obtaining certification of adhesively bonded structure has been difficult to achieve to date because of adhesive bonding concerns. Specifically, the material qualification methods for the adhesives and the certification approach for adhesive joints are not well-established, limiting bonded joint acceptance. The current American Society for Testing and Materials (ASTM) industry standard for the measurement of structural adhesive shear stress-strain properties, ASTM D5656, employs the use of an antiquated contact extensometer (KGR-1). Since the method employs contact measurement, the system compliance is quantified and subtracted from the data through the use of a metal standard. Consequently, measurement error is greatly dependent on the precision and accuracy of the extensometer placement for the metal standard and the test specimen. The complications involved with this test method often lead to unnecessary conservatism in joint design. Furthermore, in the last few decades, improvement in image processing with microcomputers has caused non-contact measurement techniques to become increasingly popular in the experimental test community. The objective of this effort was to evaluate a variety of strain measurement techniques, including digital image correlation (DIC), moire interferometry, and contact extensometers, and generate comparative adhesive property data with the overall goal of providing an improved test method approach.