Simplified procedure for damage-oriented evaluation of a stiffened panel with skin-stringer de-bonding in preliminary design stage

In aeronautical design activities it is necessary to have reasonable indications on the characteristic dimensions of typical defects that may occur in structures under load in order to indicate their correct hazard/risk level as a function of size and to allocate time/ importance of any necessary corrective action. With reference to the behavior of the stiffened panels loaded in compression, which constitutes vast majority of thin-walled structures inside modern transport aircraft, it is of interest to evaluate when and under what conditions any skin-to-stringer de-bonding induces a local critical condition at a load lower than the global critical load related to the whole panel, compromising the theoretical bearing capacity of the panel itself. A simplified procedure is proposed to solve this problem, useful in the context of the preliminary aeronautical design stage: a defect size has been provided considering it as a significant dimension for the design and sizing of the structure. Dimensionless approach is also provided to allow extensive application of the procedure to several practical structural cases. The variation in damage size as a function of the geometrical parameters of the stiffened configuration and in function of the elastic modulus of the material constituting the panel itself is presented and investigated. The significant size tends to reduce if the stringer stiffness is higher than the skin one. This can be obtained both by means of geometrical parameters variation, as indicated, and by the introduction of a more performant stringer material at fixed geometry. The significant de-bonding dimension remains of the order of the stringer distance for practical application. The reported results allow a first approximate definition of design assumptions and maintenance indications. The generality of the presented approach makes it useful in evaluating all practical cases under similar operating conditions.