Crashworthiness analysis of a composite light fixed-wing aircraft including occupants using numerical modelling

Abstract

The development and validation of reliable numerical modelling approaches is important for higher levels of aircraft crashworthiness performance to meet the increasing demand for occupant safety. With the use of finite element analysis (FEA), development costs and certification tests may be reduced, whilst satisfying aircraft safety requirements. The primary aim of this study was the development and implementation of an explicit nonlinear dynamic finite element based methodology for investigating the crashworthiness of a small lightweight fibre reinforced composite aircraft with occupants. The aircraft was analysed as it crashed into soft soil and the FEA software MSC Dytran was selected for this purpose. The aircraft considered for the purposes of this study was based on a typical four-seater single engine fibre-reinforced plastic composite aircraft.

The definition of a survivable accident is given by Coltman [1] as: “an accident in which the forces transmitted to the occupant through his seat and restraint system do not exceed the limits of human tolerance to abrupt accelerations and in which the structure in the occupant’s immediate environment remains substantially intact to the extent that a liveable volume is provided for the occupants throughout the crash sequence”. From this definition, it was determined that the FEA models must primarily provide an assessment on the crashworthiness of the aircraft in terms of the structural integrity of the airframe to ensure a minimum safe occupant volume and the tolerance of humans to abrupt (de)accelerations. An assessment of other crashworthiness factors have been ignored in this study, such as post-crash hazards (e.g. fire) and safe egress for the occupants.

Stockwell [2] performed a dynamic crash analysis of an all-composite Lear Fan aircraft impacting into concrete with the explicit nonlinear dynamic finite element code MSC Dytran. The structural response of components was qualitatively verified by comparison to experimental data such as video and still camera images. The composite fuselage materials were represented with the use of simplified isotropic elastic-plastic material models, and therefore did not account for the anisotropic properties of composite materials and the associated failure mechanisms. The occupants were represented as lumped masses; therefore occupant response could not be investigated.

Malis and Splichal [3] performed a dynamic crash analysis of a composite glider impacting into a rigid surface with MSC Dytran; however further model verification was required. The 50th percentile adult male (occupant of average height and mass) Hybrid III anthropomorphic test device (ATD), also referred to as a crash test dummy, was represented in the analyses with the Articulated Total Body (ATB) model integrated within MSC Dytran. Various injury criteria of the ATB model were evaluated to determine the crashworthiness of the glider.

Bossak and Kaczkowski [4] performed global dynamic crash analyses of a composite light aircraft crash landing. Representative wet soil, concrete and rigid impact terrains were modelled using Lagrangian-based finite element techniques and only the vertical velocity component of the aircraft was considered to simplify analyses. It was assumed that the previous use of only a downward vertical velocity component was a result of possible numerical instabilities which commonly occur with the use of Lagrangian solvers when considering problems with large deformations, which is a characteristic of crash analyses (i.e. the addition of a horizontal velocity component may result in severe element deformation of the soft soil terrain, resulting in premature analysis termination). Analyses of the occupant were performed in separate local models, using accelerations derived from the global analyses results. The real-time interactions between the occupant and aircraft therefore could not be investigated, which is considered a major disadvantage.

Impact analyses of helicopters into water were performed by Clarke and Shen [5], and Wittlin et al. [6]. Both these papers showed promising results with the use of Eulerian-based finite element techniques to model the water. Additionally, combined horizontal and forward velocity components were assigned to the fuselages with success. It must be noted that the fuselages were modelled as rigid bodies; therefore the effect of structural failure on analyses could not be investigated. Fasanella et al. [7] performed drop tests of a composite energy absorbing fuselage section into water using Eulerian, Arbitrary Lagrange Eulerian (ALE) and Smooth Particle Hydrodynamics (SPH) meshless Lagrangian-based finite element techniques to represent water. Successful correlation between experimental and numerical data was achieved; however, structural failure could not be modelled with the Eulerian-based finite element technique due to analysis code limitations at the time.

A “building block” approach was used in this study to develop accurate numerical modelling techniques prior to the implementation of the full-scale crash analyses. Once the blocks produced satisfactory results in themselves, they were then integrated in order to achieve the abovementioned primary aim of this study. The sub-components (or blocks) were the occupant (viz, FEA of the human bodies’ response to impact), (FEA of) soft soil impact and (FEA of) fibre-reinforced plastic composite structures. This approach is intuitive and provides key understanding of how each sub-component contributes to the full-scale crash analyses. Published literature was reviewed, where possible, as a basis for the development and validation of the techniques employed for each sub-component.

The technique required to examine the dynamic response of an occupant with MSC Dytran, integrated with the ATB model, was demonstrated through the analysis of a sled test. The numerical results were found to be comparable to experimental results found in the literature. An Eulerian-based finite element technique was implemented for soft soil impact analyses, and its effectiveness was determined through correlation of experimental penetrometer drop test results found in the literature. An investigation into the performance of the Tsai-Wu failure criterion to capture the onset and progression of failure through the layers of fibre reinforced composite laminates was conducted for an impulsively loaded unidirectional laminate strip model. Based on the results obtained, the techniques implemented for each sub-component were deemed valid for crashworthiness applications (viz. to achieve the project aim).

Full-scale crash analyses of impacts into rigid and soft soil terrains with varying aircraft impact and pitch angles were investigated. Typical limitations encountered in previously published works were overcome with the techniques presented in this study. The aircrafts’ laminate layup schedule was explicitly defined in MSC Dytran, thereby eliminating the inherent inaccuracies of using isotropic models to approximate laminated composite materials. The aircraft was assigned both horizontal and vertical velocity components instead of only a vertical component, which increased the model accuracy. Numerical instabilities, due to element distortion of the terrain when using a Lagrangian approach, were eliminated with the use of an Eulerian soft soil model (Eulerian techniques are typically used to model fluids where large deformations occur, which is a characteristic of crash analyses). Structural failure was successfully implemented by coupling Lagrangian and Eulerian solvers. The ATB model allowed for the real-time interactions between the occupant and aircraft to be investigated, unlike previously where analyses of the occupant were performed in separate local models using accelerations derived from the global analyses results.

The results obtained from the crash analyses provide an indication of the forces transmitted to the occupant through the seat and restraint system, and the aircraft’s ability to provide a survivable volume throughout the crash event. The explicit nonlinear dynamic finite element based methodology was successfully implemented for investigating the crashworthiness of small lightweight composite aircraft, satisfying the primary aim of this study.

Chapter 1 provides a review of fibre reinforced composite materials, the finite element method (FEM), ATDs and associated analysis codes, human tolerance limits to abrupt (de)accelerations, and crash dynamics and environment. The review of the FEM initially focuses on the fundamentals of FEA and then on the features specific to MSC Dytran as it is used throughout this study. Chapter 2 discusses the development of suitable numerical modelling techniques at the sub-component level and the implementation of these techniques within the full-scale crash analyses. Chapter 3 presents and discusses the full-scale crash analyses results for three impacts into rigid terrain and three impacts into soft soil terrain with varying aircraft pitch and impact angles. The results obtained from the crash analyses provide an indication of the forces transmitted to the occupant through the seat and restraint system, and the aircraft’s ability to provide a survivable volume throughout the crash event. Chapter 4 provides a conclusion of the work performed in this study and highlights various areas for future work.