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An explicit 3D Finite Element (FE) model was developed in the LS-Dyna code to simulate the laser shock paint stripping on aircraft aluminum substrates. The main objective of the model is to explain the physical mechanisms of the laser shock stripping process in terms of shock wave propagation, stress and strain evolution and stripping shape and size and to evaluate the effects of laser and material parameters on the stripping pattern. To simulate the behavior of aluminum, the Johnson–Cook plasticity model and the Gruneisen equation of state were applied. To simulate stripping, the cohesive zone modeling method was applied. The FE model was compared successfully against experiments in terms of back-face velocity profiles. The parameters considered in the study are the aluminum thickness, the epoxy paint thickness, the laser spot diameter, the fracture toughness of the aluminum/epoxy interface and the maximum applied pressure. In all cases, a circular solid or hollow stripping pattern was predicted, which agrees with the experimental findings. All parameters were found to affect the stripping pattern. The numerical results could be used for the design of selective laser shock stripping tests.

In this paper, analytical and numerical models have been developed to compute the stress field and predict fracture of the aluminum/epoxy interface subjected to laser shock loading, in the frame of the investigation of the paint stripping process. An explicit finite element (FE) model combined with the cohesive zone modeling (CZM) method, an analytical stress analysis model, and a spall fracture model have been developed. The numerical model has been calibrated and validated against tests in terms of the stripping pattern, while the analytical models have been compared with the numerical model. The models were combined to generate computational tools for decreasing computational effort. The FE model with the CZM is the most accurate tool although it is the most computationally expensive. The spall fracture model gives trusted estimations of the spall strength of the interface which are very sensitive to the interface thickness and when incorporated into a continuum FE-based damage model can predict the stripping initiation faster than the FE model with the CZM. The analytical stress analysis model can be used to efficiently describe the shock wave propagation into the material system, but it can give only a rough estimation of the tensile stress at the epoxy, which when combined with the spall strength does not give reliable predictions of the stripping initiation. The three models require as input different material properties, some of which are very difficult to determine. Nevertheless, the availability of accurate material parameters and properties of the aluminum, the epoxy, and, especially, their interface can significantly improve the efficiency of the developed models.

Bird strike represents a critical loading scenario for aeronautical structures, especially for engine fan blades. It is, therefore, of great importance both to protect the leading edge of the fan blade from impact damage and to be able to detect impact damage directly. In the present study, an explicit finite element (FE) model was developed using the LS-DYNA software to simulate bird strike on the leading edge of a CFRP fan blade protected by an adhesively bonded Ti layer. The model accounts for damage on the CFRP blade through a progressive damage modeling scheme and for debonding through a cohesive zone modeling scheme but not for damage on the Ti layer. For the modeling of the bird, the smooth particle hydrodynamics (SPH) method was used, due to the large deformations that were expected. Using the model, a parametric study on the effects of bird mass and impact energy was performed. The numerical results show that impact damage depends more on impact velocity than the bird mass. In all cases, debonding of the tip of the leading edge was predicted, while for combinations of small bird mass and large impact velocity a more extensive debonding of the protective layer was predicted. Regarding damage in the CFRP, only matrix cracking on the leading edge has been predicted. Aiming to assess the effectiveness of FBGs to detect debonding of the Ti layer due to bird strike, an FBG network has been modeled into the bondline and a study was performed on the correlation of the measured strains with impact damage.