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

Abstract

A box-beam structure, consisting of adhesive-bonded spar caps and shear webs, is a primary load-bearing structural element in a large composite wind turbine blade. In evaluation of a wind turbine blade structure, a scaled box beam is often used to examine and determine the structural strength and failure modes of a full-scale wind turbine. In this study, a series of scaled composite-sandwich box beams were designed and fabricated for rigorous investigation of their strength characteristics under 4-point bending. Damage and failure modes observed during the bending tests of the box beams exhibited progressive development of local buckling of spar caps and final adhesive bond-line failure. Computational mechanics analyses revealed local nonlinear deformation and stress distributions in the box-beam spar caps, shear webs and thick adhesive bond layers. The results also indicated the presence of large normal tensile stresses acting at the interfaces of shear web and adhesive layer and the spar cap and adhesive. In this paper, an investigation of in-situ adhesive joint tensile strength was conducted. Adhesive joint specimens were taken from the box beams tested under bending. Due to the very small dimensions of the selected sample regions which consisted of a bonded spar-cap laminate-adhesive-shear web joint, tab extensions consisting of (0)n laminate were attached for proper load application. Initial tensile test results from the specimens without the gage-section reduction failed prematurely at very low stresses with very large scatter. Consequently, tensile specimens with dog-bone shaped gage section geometry were designed and tested. By varying the gage region locations in the specimen, the true adhesive bond failure occurred at shear web/adhesive and spar cap/adhesive interfaces. The in-situ bond strength of the composite laminate and the adhesive joint in the box beam was determined. The experimental results of the in-situ bond strength were used in analytical modeling and evaluation of the box-beam strength and failure mode prediction.