Failure Modes and Strength of Composite Box Beam Structures

Abstract

This paper presents a study conducted to investigate potential failure modes and strength of the main structural element of a composite wind turbine blade, i.e., the box beam formed with blade spar caps and shear webs. The box beam, a critical element of the wind turbine blade, is usually constructed by adhesively bonding the blade shear webs onto the blade spar caps. Several composite box beam specimens were fabricated and their structural integrities were evaluated using 3-point and 4-point bending tests. The blade spar caps and shear webs were fabricated with glass fabric composites using a vacuum assisted resin infusion molding (VARIM) process. They were bonded together with an epoxy-based adhesive. Box beams with different geometries were designed, fabricated and tested. Bending tests of the composite box beams were conducted using an in-house designed and constructed servo-hydraulic test system with a loading capacity of 25 kips and a displacement range of ±4 inches. Based on the composite structural model and numerical simulations of the beam deformation, strain gages were placed on strategic locations to measure the strains during the bending tests. LVDTs were also used to monitor the composite beam deflections. Strain gage and LVDT signals as well as loads and actuator position were recorded, using an in-house built computer data acquisition system. The bending tests were run under a displacement-controlled mode in order to capture damage initiation, propagation and the final failure of the box beam. After the tests, damage and failure modes were examined macroscopically. Finite element analyses were conducted to determine the strength and buckling behavior of the composite box beams and to evaluate the effect of debonding between the spar caps and the shear webs on box beam structural performance. Correlation between bending test results and numerical simulations were obtained.