Application of constitutive modelling and advanced repair technology to F111C aircraft

Abstract To overcome cracking in F111 aircraft in service with the RAAF it was necessary to: (1) Provide an alternate load path so as to partially by pass the critical region; and (2) Change the geometry of the local region, thereby reducing the K t . To achieve the first a boron epoxy doubler (reinforcement) was developed. To meet the requirements for continued airworthiness it was necessary to determine the associated inspection intervals. To this end it was necessary to obtain the residual stress, after CPLT, and the stress “per g” both with and without doubler and with various grind out configurations. Since during CPLT the critical region undergoes gross plastic yielding to obtain this information requires a detailed elastic–plastic analysis. However, classical techniques for modelling this cyclic behaviour have inherent difficulties in representing the response to large cyclic inelastic strain excursions. Indeed, the use of classical analysis techniques resulted in an inspection interval, for the modified structure, of under 500 h. This contrasts with service experience which has shown that there is little further cracking. Indeed, for modified aircraft there has been no further cracking since 1985. To overcome this shortcoming an advanced “unified constitutive” model was used.