Kevin Hughes

Experimental observations of an 8 m/s drop test of a metallic helicopter underfloor structure onto water: Part 2

Abstract This is the second part of a two-part paper that describes the experimental observations for two similar sections of floor that were dropped onto both hard and water surfaces at 8 m/s, as part of one experimental campaign. The current paper provides an assessment of a simple box-beam under floor structure typically found in metallic helicopters and provides an overview of the failure modes and the collapse mechanism observed when dropped onto water at 8 m/s, as well as providing quantitative data for the skin deflections observed. The results demonstrate that the lack of frame and intersection joint collapse is a common feature, which is caused by the high failure strength of the existing construction, together with the inability of the skin to generate membrane loads that are sufficiently large to trigger progressive collapse within the structure. It is therefore recommended to reduce the collapse force of the structure through the use of geometry, material type, and inclusion of triggers. However, the caveat with this approach is that if the failure strength is optimized for a water impact, a poor crashworthy response may occur during a hard surface impact. The current paper discusses three main limitations with the design, which are heavily interrelated, as improvements in frame and joint collapse cannot be achieved without considering developing the ductile behaviour of the skin. However, maintaining skin integrity will be critical to maintain the floatation capabilities of the helicopter. The current paper recommends that a next generation design should encompass a passive dual role capability for both hard and soft surface impacts, by being able to degrade the localized strength depending upon the type of surface encountered. This will significantly improve the crashworthy response of a metallic under floor structure and have a significant impact on improving occupant survivability for an impact on water.

Explicit dynamic formulation to demonstrate compliance against quasi-static aircraft seat certification loads (CS25.561) – Part I: influence of time and mass scaling

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.

A study of the effect of projectile orientation on the results of ballistic impact tests as described in the EASA CS-25 regulations for fuel tank access covers

This paper presents the results of an investigation of the ballistic limits and failure modes of AA2024-T351 sheets impacted with cubical projectiles. The experiment/test setup was based on EASA CS-25 regulations for fuel tank access covers. The effect of cube orientation on the ballistic limit and failure modes was considered in detail. A 25% variation in ballistic limit was observed with the lowest ballistic limit (202 m/s) observed for the cubical projectile edge impacted on the target. In the cube face impacts, the ballistic limit was higher (223 m/s), and the highest ballistic limit (254 m/s) was observed for the corner impact. The observed differences in the ballistic limit were due to differences in failure mechanism, which resulted in different localised deformations near the projectile impact point, but also led to differences in global dishing deformation.

Prediction of the ballistic limit of an aluminium sandwich panel

This work was funded by the European Space Agency under a contract with Airbus Defence and Space.

Explicit dynamic formulation to demonstrate compliance against quasi-static aircraft seat certification loads (CS25.561) – Part II: Influence of body blocks

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.

SIMULATION OF A SHOCK RECOVERY EXPERIMENT

It is difficult to obtain experimental data for the behaviour of material under shock loading due to the dynamic nature of this process and the finite time available during which measurements can be taken. As a result the shock recovery technique was been developed to allow recovery of material after a single shock‐release loading. The main goal of this experimental technique is to examine material properties after a single, well‐defined shock wave followed by a single release wave have been introduced. The process should be such that any change found in the sample after recovery can be attributed to the shock process alone, the geometry and design of the target and the fixture play an important role in achieving this. In this work a shock recovery experimental configuration is proposed from simulations performed using the Lagrangian hydrocode DYNA. The investigation of the shock wave in the simulation entails examining the time histories for a single element, or block of elements of interest. A new desig...