PSI - Issue 5

F. Romano et al. / Procedia Structural Integrity 5 (2017) 721–728 Author name / Structural Integrity Procedia 00 (2017) 000 – 000

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transferred to the MSC Nastran ® model, for the subsequent compression after impact analysis (PFA), Fig. 5. The entire panels is discretized by means of four-node layered shell element with a full integration scheme and the linkage between the skin and the stringers is modelled by contact elements with multi-point constraints option.

Fig. 5. 2-stringer panel with 50 joule impact damage. (a) FE model; (b) Damaged elements.

The above model has been object of similar works at CIRA, and the numerical results were correlated quite well with the experimental data. Fig. 6 a) compares the Reaction Force vs the Applied Strain; Fig. 6 b) the failure propagation at the time step in correspondence of the collapse load (TF).

Fig. 6. 2-stringer panel with 50 joule impact damage. (a) Reaction vs Applied Strain; (b) Collapse of the panel (without deformed shape).

The presence of the discrete damage triggers off the degradation of the material inducing a propagation of the damage from the initiation of the failure in a lamina, FPF load, up to the total failure/collapse of the panel, TF load; FPF load and TF load are not coincident. The panel after the FPF shows the capability to withstand additional load (i.e. residual strength) up to its final collapse load: the TF load is higher about 1.66 times the FPF load (traditional design). Consequently, after also other dedicated analyses, it emerges that the stiffened panel designed traditionally by using the reduced allowable for BVID determined at coupon level (150 mm x 100 mm, CAI tests), 3710  at 50 J/cut-off energy level), and applying FPF design criteria, has the following limits respect to the discrete damage model:  a lower FPF load, respect the case of the impact damage induced directly on the skin of the panel;  a load carrying capability even lower, if the analysis on the discrete model is not stopped at FPF, but it is allowed the propagation of the failure after the damage initiation up to the collapse load of the panel.