In-Situ Interface Bond Strength Determination in Thick Adhesive Bonded Box Beam

摘要

A box-beam structure, consisting of adhesive-bonded spar caps and shear webs, isa primary load-bearing structural element in a large composite wind turbine blade. Inevaluation of a wind turbine blade structure, a scaled box beam is often used to examineand determine the structural strength and failure modes of a full-scale wind turbine. Inthis study, a series of scaled composite-sandwich box beams were designed andfabricated for rigorous investigation of their strength characteristics under 4-pointbending. Damage and failure modes observed during the bending tests of the box beamsexhibited progressive development of local buckling of spar caps and final adhesivebond-line failure. Computational mechanics analyses revealed local nonlineardeformation and stress distributions in the box-beam spar caps, shear webs and thickadhesive bond layers. The results also indicated the presence of large normal tensilestresses acting at the interfaces of shear web and adhesive layer and the spar cap andadhesive. In this paper, an investigation of in-situ adhesive joint tensile strength wasconducted. Adhesive joint specimens were taken from the box beams tested underbending. Due to the very small dimensions of the selected sample regions whichconsisted of a bonded spar-cap laminate-adhesive-shear web joint, tab extensionsconsisting of (0)n laminate were attached for proper load application. Initial tensile testresults from the specimens without the gage-section reduction failed prematurely at verylow stresses with very large scatter. Consequently, tensile specimens with dog-boneshaped gage section geometry were designed and tested. By varying the gage regionlocations in the specimen, the true adhesive bond failure occurred at shear web/adhesiveand spar cap/adhesive interfaces. The in-situ bond strength of the composite laminateand the adhesive joint in the box beam was determined. The experimental results of thein-situ bond strength were used in analytical modeling and evaluation of the box-beamstrength and failure mode prediction.