Bonded Joint Analysis Methodology for the AH-64D Composite Main Rotor Blade

The design of efficient bonded joints is one of the most challenging issues in aircraft structures. Adequate failure criteria and analysis methodologies are still the subject of extensive research. Several factors influence the initial failure of a bonded joint. The analysis of bonded joints increases in difficulty when the joint is between dissimilar materials, loaded under high cyclic loads for a large number of cycles, and it is exposed to the environment. Such is the case for the root end bonded joint of the Composite Main Rotor Blade (CMRB) being developed for the AH 64D Apache Longbow helicopter. This article describes the initial approach taken for the analysis of such a joint and its evolution into a failure initiation methodology to evaluate its static and fatigue strength. The methodology uses results of very detailed Finite Element Analysis (FEA) of the joint to evaluate the strain state in the adhesive and its initial failure. Residual strains from cure cycle and environmental effects are also to be taken into account in the analysis. The analysis is guided by extensive coupon test results (lap shear, film adhesive, and subcomponent) of different joint geometries. Tests were also required to characterize the mechanical properties of the adhesive for use in the analysis. The methodology has been used to direct the design the root end joint for the CMRB with enhanced fatigue strength over its initial design. The same methodology has also been used to evaluate other bonded joints in the blade structure.