Application of three-dimensional spline variational analysis for composite repair q B.M. Fredrickson a , G.A. Schoeppner b , D.H. Mollenhauer b , A.N. Palazotto a, * a Air Force Institute of Technology, 2950 Hobson Way, WPAFB, OH 45433-7765, United States b Air Force Research Laboratory, United States Available online 24 April 2007 Abstract The purpose of this research was to apply a non-commercialized spline-based (B-spline analysis method or BSAM) computer program to model and predict strain fields in two composite repairs, the scarf joint and the stepped-lap joint, subjected to static tensile loading. Test specimens with scarf and stepped-lap joints were fabricated using quasi-isotropic 16 ply panels made from IM6/3501-6 pregreg with a [+45 2 /0 2 /45 2 /90 2 ] S lay-up. Moire ´ interferometry, a high-resolution full-field optical technique used to measure displacements on a surface, was used to determine experimental strain data. Full-field strains along the free-edge were predicted and compared to experi- mental strain data. Line plots are generated from predicted and experimental data extracted at several points across the width of the repair to provide a quantitative comparison. Overall, both the full-field strain images and line plots show good agreement between the BSAM analysis and the experimental data. Ó 2007 Published by Elsevier Ltd. Keywords: Bonded joints; Three-dimensional analysis; Scarf joints; Step-lap joints 1. Introduction The maintenance and repair of composite aircraft structures has always been of great concern in both the private and public sectors. Damaged composite compo- nents on aircraft are often a result of maintenance mis- haps, accidental impact, bird strike, hailstones, or lightning strike. External patches can be used for perma- nent repair in lightly loaded and relatively thin structures however their use may not be viable for primary struc- tures because external patches create high bondline peel and shear stresses in highly loaded structures. Bolted and riveted repairs are also not a viable option in strength-critical structures as load transfer paths through mechanical fasteners are not efficient and can cause local overloads and damage in brittle composites. Instead, flush bonded repairs are preferred for permanent repairs of composite components that support these primary loads. While flush patch repairs, such as scarf and stepped-lap joints (see Fig. 1), are more highly refined and structurally efficient than external patches, they are also more expen- sive and time intensive to apply. Flush type repairs have become increasingly important as usage of composite materials has expanded from non-critical structures to strength-critical structures [1]. The use of flush repairs that do not change the outer mold-line of the aircraft for aerodynamic consideration has not been systemati- cally addressed in the literature. In particular, high fidelity analyses of flush repairs to account for variations in par- ent structure and patch material stacking sequences and to account for residual curing stresses due to co-cured and/or post bonded patches have not been adequately addressed. 0263-8223/$ - see front matter Ó 2007 Published by Elsevier Ltd. doi:10.1016/j.compstruct.2007.04.017 q The views expressed in this work are those of the authors and do not reflect the official policy or position of the United States Air Force, the Department of Defense, or the US Government. This material is declared a work of the US Government and is not subject to copyright protection in the United States. * Corresponding author. E-mail address: Anthony.palazotto@afit.edu (A.N. Palazotto). www.elsevier.com/locate/compstruct Available online at www.sciencedirect.com Composite Structures 83 (2008) 119–130