Eric Deletombe

Assessment of multi-physics FE methods for bird strike modelling-Application to a metallic riveted airframe

Abstract The first part of the paper deals with the assessment of several methods using Lagrangian and Arbitrary Lagrangian Eulerian (ALE) formulations in Radioss to model bird impacts onto rigid panels. FE results are compared with experiments in terms of local pressure, including Hugoniot and Stagnation ones, and global load. Two methodologies are chosen: one using a Lagrangian method and an hydrodynamic viscous model for the bird, and another using ALE method and an hydrodynamic bi-phase liquid/gas medium for the air and bird model. With the Lagrangian method, results are in a good agreement compared with available experimental results but the computing cost is expensive. With the ALE method, the calculation cost is lower. The second part of the paper deals with bird impact modelling onto metallic riveted panels using both FE methods. Results are in a good agreement with experiment when the metallic panel IS dented only. For bird penetration cases. results highlight the problem of the modelling of the structural embrittlement due to the riveted joints (structural failure comes from a crack propagation between rivets).

An alternative numerical approach for full scale characterisation for riveted joint design

This paper describes a pure numerical methodology (FE database) to improve the representativeness of joint equivalent models for airframe crashworthiness. This method is based on material constitutive models and failure criterion in accurate 3D FE simulations. The interest of FE simulations is to define the dynamic strength of many types of riveted joints with a reduced cost compared to a pure experimental way. The FE database method is carried out on elementary riveted joints to predict and to analyse: first the post-riveting initial strain and stress state, and second several kinds of joint failure (e.g., crack propagation, rivet shearing or pull-out). The aim of the first step is to start the mechanical strength simulations with a correct deformed shape and post-riveting state. The responses of 3D riveted joint simulations can then be considered as reference and be used to optimise the mechanical properties of equivalent joint elements. A new equivalent joint element is developed to improve the representativeness of an airframe crash simulation.

Improvement of numerical methods for crash analysis in future composite aircraft design

Abstract Recent aircraft as well as rotorcraft design technologies include more and more composite materials. Their high mechanical characteristics and high mass specific energy absorption capability motivate their use in large primary structures as well as in sub-floor structural and crashworthy components in preference to metals. Due to the increased performance of computers and new explicit finite element (FE) software developments industry now considers using crash simulation technologies to study the crashworthiness of new aircraft design. In order to address the crash analysis of composite structures, which is much more difficult than the behaviour of ductile metallic structures, a German/French research co-operation was set up between ONERA and DLR. This paper summarises results from the first 3 years collaboration and some work performed within a European research project on composite fuselage structures. In the first part of the paper, ONERA presents its contribution to the characterisation of composite materials from 10−5 s−1 up to 100 s−1 on hydraulic machines. Simulations have been undertaken to model the tests and evaluate the FE codes. In the second part DLR studies are presented on the application of a commercial explicit FE code to simulate the behaviour of generic energy absorbing composite sub-floor elements, representative for helicopters and general aviation aircraft, under low velocity crash conditions (up to 15 m/s). This includes some comparisons between predicted structural response and failure modes with observed test results.

Full scale experimental characterisation for riveted joint design

This paper deals with experimental works the objective of which aims at improving the design of riveted joints for airframe crashworthiness purposes. Complex assemblies are considered at this stage as the sum of simpler ones constituted of 1 rivet and 2 plates, the behaviour of which is investigated under different points of view. Research is divided in two major parts. The first one investigates the consequence of rivet processing in term of possible material and structural embrittlement (residual stress and strain in rivets and plates). In the second part, the overall behaviour and strength of the considered basic assembly (1 rivet and 2 plates) is studied. The aim is here to characterise basic failure modes of assemblies linked to rivet failure or crack propagation in punched metal plates. The measurements of local variables enable to assess the influence or not of dynamics on the different failure mechanisms. Eventually, an original test procedure based on the ARCAN test rig is presented, the objective of which is to give access to multi-axial failure criterions for rivets.

Numerical approach for assessment of dynamic strength for riveted joints

Abstract Numerous rivets have to be modelled for aeronautical framework crashes. A numerical procedure based on FE modelling and characterisation of material failure constitutive models is proposed in order to limit the experimental procedure. Quasi-static and dynamic experiments are carried out on elementary tension (punched) and shear (riveted) specimens. No strain rate sensitivity has been measured on the riveted joint assemblies failure. The experiments are used to identify, by an inverse method, the Gurson damage parameters of each material (2024-T351 and 7050 aluminium alloys for the sheet metal plate and the rivet). The characterisation gives rise to a satisfactory correlation between FE models and experiments. Optimised parameters are validated for each material by means of a uniaxial tension test for the sheet metal plate and an ARCAN type specimen in pure tension for the rivet. Results can then be used to identify macroscopic failure criterion to model the rivet behaviour in aeronautical framework crashes. FE tools can also resolve problems linked to limit-design or the design of new riveted joint assemblies more rapidly and cost effectively than experiments.

A measurement study of a pressure transducer subjected to water drop impact

Numerous numerical methods exist that can be used to solve fluid/structure interactions during hydrodynamic impacts on deformable structures. In order to validate these methods, experimental references are needed and the use of pressure transducers appears to be quite sensible and natural. But the objectivity of such a pressure measurement may sometimes be questioned (bias due to the presence of the transducer itself, which has its own dynamic natural response). Such a systematic error is difficult to notice when structural tests are concerned, other uncertainties concerning the experimental configuration being possibly called for as an obvious explanation of any differences between test and simulation results. The purpose of the present paper is to propose a methodology of exploitation of dynamic tests responses, dedicated to the elimination of such measurement errors if needed, in order to get the true contact pressure that deformable structures have to support during hydrodynamic impacts. For that purpose, the dynamic calibration of a pressure transducer is performed using a shock tube, and a dynamic correction function is established, first theoretically, then practically. The obtained function is then applied on existing test results coming from water drop impacts onto the studied pressure transducer, in order to calculate the corresponding contact pressure on a flat panel, and to compare these latter data to different FE simulation solutions.

Outils de conception au choc : un panorama

The increase in the number of individual and collective means of transport leads to a potential raise in the amount of dangerous collisions for the human beings. Following this remark, and also due to government pressure, the automobile industry has since the 1970s conducted extensive research in the aim of reducing these risks and if necessary to limit the effects by improving passenger safety. To reach these objectives, two main actions have been driven through the development of preventive and palliative technologies, namely, active and passive safety. Even if these advances have led to considerable improvement, the actual situation remains worrying, so that this particular set of problems still represents a major challenge to our society. In terms of passive safety, two different important phenomena are in competition with each other; on the one hand, energy dissipation must be maximised, whereas on the other hand, deceleration levels sustained by the passengers must be minimised. A compromise must therefore be reached so as to make a good crashworthiness design. To meet these objectives, two main methods of calculation strategies are put forward, which are the global and local strategy studies. The global strategy method can be solved by using elementary approaches. The goal is to bring solutions to problems encountered in complex structural modelisation (or structural assemblies) by reproducing only the global phenomena. The main interest of this approach is to study rapidly the dimensionning and optimisation of the structure. Multibody articulated systems and beam modelisation coupled to upper bound methods are particularly adapted to this problem. The local strategy study which is meant to reproduce as accurately as possible the behaviour of structures to choc, uses the finite element method. This method allows one to tackle and to solve with the help of powerful computers the problems which have up to now been left without solutions. Nevertheless, the mesh which corresponds to the back bone of finite element analysis still demands considerable time during the structural modelisation process. This unfortunately does not permit using this approach in an iterative dimensionning process during a planning stage. Furthermore, even if geometric non-linearities have been well implemented in nowadays calculation codes, a lack of information on material non-linearities is present due to a poor knowledge of dynamic material behaviour. This article tends to give an overview on these various methods as well as their industrial applications.

Tôles perforées et fragilisation structurale liée à la mise en oeuvre des techniques d'assemblage par rivetage

In the field of aircraft design, a recent problem deals with the improving of aircraft behaviour during survivable crash events. Two different approaches may be taken. The first one, on an experimental stage, uses replica scale crash models. The second one is based on calculation tools such as FE codes. In spite of many advantages (e.g. cost, parametric studies possibilities), these tools still have drawbacks linked to the representativeness of material and geometrical non-linear behaviours and to mesh sensitivity. According to that point of view, an important problem still concerns the modelling of the plastic strain location in the plates, especially near the holes due to the riveting process. The plastic strain and damage concentration embrittles the structural strength of frameworks and pilots their mechanical ruin. The concepts of the structural embrittlement and if it measurement are introduced by an analytical analysis in the linear and nonlinear field. The measurement of embrittlement is characterised by experimental and numerical ways in order to make up for the limits of analytic methods for the large strain field. Finally a phenomenological model for structural embrittlement is presented and discussed.