Comparison of finite element analysis of impact on water and soil using Lagrangian, ALE, and SPH approaches and airframe impact applications

Abstract

According to Federal Aviation Administration (FAA) reports on aircraft accidents, 80 percent of all crashes occur on water and land. The research in this thesis used computational finite element modeling and analysis to study the effects of impacts on water and soft soil with respect to the structural integrity of an aircraft’s airframe during a crash. The effects of these impacts play a vital role in the design phase of an aircraft. The objective of this research was to compare the computational finite element technique for impact on water and soil, and to correlate it with the experiments. The nonlinear explicit finite element code together with the equation of state (EOS) and fluid-structure interaction (FSI) are used in the arbitrary Lagrangian-Eulerian (ALE) technique. A validation study of water and soft-soil properties on impact analyses was carried out in three different experimental phases using the following: a rigid spherical ball, a penetrometer, and a flask ball. Finally, an airframe structure study was conducted in water and soil. Initially, the impact simulations were carried out using Lagrangian analysis, followed by the ALE technique, and then the smoothed particle hydrodynamics (SPH) method. Acceleration was observed as an important parameter to validate in the analysis. Both ALE and SPH methods showed more accurate results than those obtained using Lagrangian analysis and were similar to that of the experimental data. Comparing the results from analyzing water and soil impacts with a rotorcraft indicated that water impact produces 33% less g-value and around 90% deformation compared to soil impact, thus indicating that impact on a water surface might be safer than impact on a soil surface. The impact study on water and soil was intended to evaluate the general behavior of the deformation, or g-value, for structural analyses only, and results show that the Lagrangian approach is recommended, if the soil and water is of small interest.