EQUIVALENT MATERIAL IDENTIFICATION IN COMPOSITE SCALED HULLS VERTICAL IMPACT TESTS

Airworthiness regulations require that aircraft should be proved to ensure the survivability of the ditching for the passengers. In particular seaplane aircrafts must be designed for water loads developed during take-off and landing with the seaplane in any attitude likely to occur in normal operation at appropriate forward and sinking velocities under the most severe sea conditions [1]. In order to make a stress analysis of seaplane floats, and especially of the members connecting the floats with the fuselage, it is of great importance to determine the maximum pressure acting on the floats during landing [1,2,6]. The impact in water is a very complicated phenomenon due to the interaction between the hydrodynamic field and the structures so the possibility to investigate its characteristics by means of a scaled model seems straightforward. In general the similarity procedure requires the scaling of different properties (mass, length, speed, etc.) in order to properly represent such a phenomenon. In the present case the fluid-dynamic phenomena are scaled thanks to the Froude similarity while the definition of specific material and its scaling rules for a complete similarity is required in several cases particularly when composites play a predominant role on the test response. The introduction of TRACE parameter is proposed as a method orienting the designer for the right material selection in these situations [2,3,4,5]. Non-dimensional TRACE behaviour in function of shear-ratio coefficient is shown in figure 1a. Materials can be grouped in classes according to the value of the orthotropic ratio and each class can be scaled by TRACE. Non-dimensional deflection is determined as a function of load parameters and TRACE according to typical material classes definition. The procedure is extended to a typical drop-test impacted on water including structural flexibility [7,8]. Numerical simulation using the LS-DYNA software is used to validate the procedure in predicting the maximum pressure in the first phases of impact (figure 1b and figure 2b). The model shown in figure1b provides the adoption of SPHs, a particular tool adopted for the modelling of a fluid like water. Each SPH (Smoothed Particle Hydrodynamics) is described like a structural FEM node and by a EOS (Equation of State) that describes the mechanical behaviour and interaction with other SPH and the structure. LS-DYNA is an explicit software, so it can simulate dynamic phenomena. An experimental test is finally designed as shown in figure 2a to calibrate the physical and numerical parameters useful for the definition of SPH properties with the aim to define analyse and correctly describe the dynamics of fluid-structure interaction during a seaplane hull water impact (figure 2b).