Compression after Impact Analysis of Composite Panels and Equivalent Hole Method

The necessity to exploit the full potential of the composite materials in aviation has stimulated in the last two decades the development of the Progressive Failure Analysis methodology. This kind of analysis foresees discrete damages that are introduced in the finite element model of the structure by preserving the full mechanical strength properties of the material. Since the numerical analysis of the impact event is very expensive, simplified damage models are commonly used in the preliminary design phase; the degradation of the strength due to a Barely Visible Impact Damage is considered equivalent to that one due to a 1/4 inch hole diameter. In this work, the above mentioned equivalence has been studied by experimental compression tests and numerical analyses on impacted and notched composite coupons. After the validation of the numerical models by the Progressive Failure Analysis, the hole diameter equivalent, in terms of residual strength, to the experimental impact damage, has been numerically evaluated considering different holes sizes up to 1 inch. The computed equivalent hole diameter, found at coupon level, has been considered in the subsequent numerical simulations concerning the skin of a 2-stringer panel, in order to verify the validity of the equivalence damage-hole also at panel level. The preliminary results of this work show that the possibility to simulate the Barely Visible Impact Damage with an equivalent hole, if completely demonstrated, could simplify the future numerical analyses of impact damaged laminates. The activity performed in this work is preparatory to the development of the design methodologies that are under investigation in the European project Airgreen 2 within the frame of Clean Sky 2.