A finite element model of an aircraft seat subjected to static certification loads (Certification Specifications CS25.561) involves material, geometric and contact non-linearities. Implicit algorithms can model the physics of such problems appropriately but suffer from shortcomings such as significant finite element modelling efforts, high disk space and memory requirements and unconverged solutions. Explicit finite element schemes offer a more robust alternative for convergence for quasi-static loadcases but may come at an even higher computational cost as smaller solution time steps are required, in addition to unwanted inertial effects. A methodology to apply an explicit formulation for simulating static certification loading for an aircraft seat-structure is presented and validated in this article. The first part reviews the design novelties of the triple seat-structure considered, the safety regulations used in aircraft seat certification. The key theoretical aspects of an explicit solver are presented, together with the numerical challenges faced when applied to solving quasi-static problems. Time scaling, mass scaling and damping are common approaches to assist in artificially reducing the computational time but previous articles provide little insight into how to apply these techniques correctly and the level of checking that is required to ensure the quality of the results are unaffected by these modifications. The main focus of this article is to clearly define the procedure to establish appropriate factors for mass scaling, time scaling and damping. Quality checks, such as ratio of kinetic energy to internal energy and their time-histories have been investigated to ensure a quasi-static solution. finite element analysis results are validated against experimental testing for the 8.6 g downward loadcase. Parameters such as kinematic behaviour and deflections at key locations been used for comparison. An acceptable level of correlation between finite element analysis results and physical tests validates the proposed methodology, which will be extended in a future article (Part II) to consider additional contact complexities with the inclusion of body blocks.

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Loading an aerospace and automotive seat statically through lap or body blocks is a complex and highly non-linear problem, as the key numerical challenge is to replicate the contact and slipping kinematics between seat, lap block and belt. In addition, severe element distortions and unexpected contact between parts can occur due to the large deformations involved, which result in implicit solvers struggling to find a converged solution. This paper focuses on the use of an explicit Finite Element Analysis (FEA) solver (LS-DYNA3D) for an aircraft seat subject to Certification Specifications CS25.561, although the ideas presented are equally applicable to automotive seat designers. Explicit codes are better able to overcome contact convergence issues and are often used with appropriate damping to achieve a quasi-static solution. This paper reviews the methodology presented in Part I, whereby issues relating to damping, mass and time scaling are outlined in order to overcome the high computational time step costs (Courant-Friedrichs-Lewy (CFL) condition), together with the procedural and error checks required to ensure a quasi-static response. This paper extends the methodology by considering load cases that use lap blocks, such as 'forward 9g' and 'upward 3g' certification requirements. Alternative modelling approaches to represent the loading mechanism and effect of lap block mass on solution accuracy are discussed. This paper concludes with a verification framework that outlines the quality checks on various model energies and their ratios, where the numerical results are validated against test in terms of displacements and seat kinematics. Thus, 'Part I' and 'Part II' cover all elements related with the application of an explicit dynamic integration scheme to demonstrate static seat compliance, and together, form a clear framework to assist a Computer Aided Engineering (CAE) analyst involved in applying an explicit integration scheme to solve non-linear quasi-static analyses.

Shock recovery techniques, such as the flyer-plate impact test, are used to examine a material that has been subjected to a single well-defined shock, followed by a single release wave. One of the key requirements of this type of technique is that the process should be such that any change found in the sample after recovery, can only be attributed to the shock process alone. Therefore, the principal problem for a test specimen-fixture assembly is that it is designed such that the loading history of the recovered specimen is known. This has motivated this research through the analysis led design of a shock recovery experiment. The choice of Lagrangian Finite Element Analysis for this design work was driven by the method's ability to accurately track history variables (for plastic deformation) and treat contact interactions which are crucial in this problem. Starting from an initial configuration, LS-Dyna has been used to analyse in detail the resulting wave propagation to ensure the generation of a uniaxial strain state in the specimen through Lagrangian distance-time diagrams. These iso-maps enabled the identification of potential shortcomings with the initial design, in terms of the transmission of contact and the influence of radial release waves at the different boundaries between specimen and supporting fixture rings. The benefits of using Lagrangian Finite Element Analysis for this design work are its ability to track history variables (for plastic deformation) and contact treatment. Based on these findings, a new configuration was developed, which consists of an array of concentric rings that support the specimen. During shock formation in the specimen, these rings progressively transfer the loading in the impact direction and radially away from the specimen, acting as momentum traps and preventing unwanted release waves from affecting the strain state experienced by the specimen. Comparing distance time diagrams between original and proposed configurations, a design sensitivity analysis was performed, where the new geometry resulted in a decrease of both the residual velocity (−38%) and radial displacement (−27%) of the target when compared to the original setup