81 Chapter 9 Application of Digital Image Correlation to the Thick Adherend Shear Test Jared Van Blitterswyk, David Backman, Jeremy Laliberté, and Richard Cole Abstract The purpose of this study was to develop and validate a novel method for measuring shear deformation during the Thick Adherend Shear Test (TAST), to determine in situ mechanical properties of an adhesive under tensile shear loading, using two-dimensional (2D) Digital Image Correlation (DIC). Shear strains were optically measured using DIC from the bond line and also from adherend deformations; both were compared against measurements made using the ASTM D5656 standard (KGR-1 extensometers). The results from 17 TAST specimens showed that all techniques were in good agreement, however, both DIC techniques had significantly lower variance on measured mechanical properties compared to the KGR-1 extensometers. The use of correlated adherend deformations was the preferred technique for deriving plastic shear strains, and overall produced the lowest scatter on mechanical properties. This study demonstrates the potential for the use of 2D DIC as a more precise, and time-efficient alternative to the KGR-1 extensometers for room temperature in situ characterization of adhesives in shear. Keywords Adhesives • Mechanical properties • Single-lap joint • Digital image correlation (DIC) • KGR-1 extensometer • Experimental test methods 1 Introduction Bonded joints are seeing increasing application in the aircraft industry for their many advantages over mechanically fastened joints. One of the main challenges facing aircraft designers relates to demonstrating compliance with certification requirements. Certification of highly-loaded adhesive bonded joints typically requires a detailed characterization of the stress–strain response of the adhesive and a statistically-based set of material strength allowables. The large volume of testing required to develop such bases for material design allowables necessitates an experimental methodology that is credible, precise, repeatable, and time and cost efficient. The thick adherend single-lap joint specimen specified in the ASTM D5656 standard [1], is the accepted methodology for measuring in situ adhesive shear properties. The use of thick aluminum adherends is most effective for measuring the response of an adhesive in shear due to enhanced bending rigidity along the bonded overlap, which reduces peel stresses and joint rotation. These considerations restrict the use of other thin adherend single-lap joint test methods, such as ASTM D1002 [2] and ASTM D3165 [3], to “apparent” strength tests of bonded joints, as peel stresses having significant influence on the overall joint strength [2–5]. ASTM D5656 [1] requires the use of the KGR-1 extensometer system to measure relative adherend deformation across the bond line of Thick Adherend Shear Test (TAST) specimens that are subjected to a tensile shear load. The KGR-1 extensometer is a specialized mechanical device, tailored for the TAST specimen, which can resolve sub-micron, relative, uniaxial displacements. The KGR-1 mounting pins span the bond line and measure relative shear displacements of the adjacent adherends. The measured displacements consist of both adhesive shear and adherend elongation. To isolate the strain in the adhesive, the adherend deformations are approximated based on KGR-1-measured displacements on a solid aluminum specimen of the same geometry as the bonded joint, commonly referred to as a “dummy specimen”. J. Van Blitterswyk • J. Laliberté Department of Mechanical and Aerospace Engineering, Carleton University, 1125 Colonel By Drive, Ottawa, ON, Canada K1S 5B6 D. Backman (*) • R. Cole National Research Council Canada, Aerospace Portfolio, 1200 Montreal Road, Ottawa, ON, Canada, K1A 0R6 e-mail: david.backman@nrc.gc.ca © The Society for Experimental Mechanics, Inc. 2017 G.L. Cloud et al. (eds.), Joining Technologies for Composites and Dissimilar Materials, Volume 10 Conference Proceedings of the Society for Experimental Mechanics Series, DOI 10.1007/978-3-319-42426-2_9
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