ECCM15 - 15 TH EUROPEAN CONFERENCE ON COMPOSITE MATERIALS, Venice, Italy, 24-28 June 2012 1 INVESTIGATING THE DAMAGE-TOLERANT DESIGN OF STIFFENER RUN-OUTS S.Psarras 1* , S.T.Pinho 1 , B.G.Falzon 2 1 Dept. Aeronautics, Imperial College London, SW7 2AZ, United Kingdom 2 Dept. Mechanical and Aerospace Engineering, Monash University, Victoria, 3800, Australia * s.psarras07@imperial.ac.uk Keywords: A. Carbon fibre; B. Damage tolerance; C. Finite element analysis (FEA); D. Acoustic emission Abstract In this work, the use of a compliant web design for improved damage tolerance in stiffener run-outs is investigated. Three different configurations were compared to establish the merits of a compliant design: a baseline configuration, a configuration with optimised tapering and a compliant configuration. The performance of these configurations, in terms of strength and damage tolerance, was compared numerically using a parametric finite element analysis. The energy release rates for debonding and delamination, for different crack lengths across the specimen width, were used for this comparison. The three configurations were subsequently manufactured and tested. In order to monitor the failure process, Acoustic Emission (AE) equipment was used and proved valuable in the detection and analysis of failure. The predicted failure loads, based on the energy release rates, showed good accuracy, particularly when the distribution of the energy release rate across the width of the specimen was taken into account. As expected, the compliant configuration failed by debonding and showed improved damage tolerance compared to the baseline and tapered stiffener run-outs. 1 Introduction Modern aerostructures are predominantly of semi-monocoque construction characterized by a thin skin and stiffeners. The latest generation of large passenger aircraft also use mostly carbon-fibre composite material in their primary structure and there is a trend towards the utilization of bonding of subcomponents in preference of mechanical fastening. Current design philosophy requires that certain stiffeners are terminated, for example due to an intersecting structural feature or an inspection cut-out. In these circumstances, the loading in the stiffener must be diffused into the skin, leading to complex three-dimensional stress- states. The development and utilization of reliable virtual component testing, in the design of composite aerostructures, can potentially yield significant cost reductions. Such reliability requires a thorough understanding of the damage mechanisms and failure processes in realistic aerostructures, particularly in critical regions such as stiffener run- outs. When a stiffener is terminated, the loads which it carries must be transferred to the skin, making the design of the run-out region vital; hence, improved design methodologies are