Spar Corner Radius Integrity for the A400M Wing

Abstract:

The paper focuses on the structural integrity of the corner radius of the carbon fibre composite, ‘C’-section spar for the Airbus A400M wing. The corner radius is subject to opening moments generated by internal wing box fuel pressures. The low inter-lamina strength of composites makes de-lamination of the corner of prime concern. The paper describes initial development of analytical techniques to calculate the through-thickness tensile stresses and inter-lamina shear stresses developed in a corner radius under applied bending moments and transverse shear forces. A test programme is also described, aimed at the determination of the failure moment of curved laminates under pure bending moments. Using the analytical expressions developed, a through-thickness failure stress is calculated from the failure moments. A variation of the failure stress with specimen thickness is indicated, showing that thicker specimens fail at higher inter-lamina stresses – a characteristic that must be exploited in the design of the spar. Using finite element analysis of the test configuration, in conjunction with virtual crack extension techniques, it is demonstrated that, at the failure load, a constant rate of strain energy release accompanies inter-lamina crack growth in the different test specimens. A critical energy release rate for uncontrolled crack growth is thus established, which is used, in conjunction with further finite element analysis, to predict the failure stress of specimens with different values of thickness and corner radius. It is concluded that this fracture mechanics approach to integrity can be applied to the A400M spar corner and to similar aircraft structures. Recommendations for further testing and correlation with analysis are proposed to strengthen the theoretical basis for such integrity assessments.