Bertrand Langrand

Riveted joint modeling for numerical analysis of airframe crashworthiness

Abstract Numerous rivets have to be taken into account to simulate the behavior of aeronautical frames under crash loading conditions. Until now this type of bonding modeling has not been judged satisfactory. The influence of structural embrittlement due to the riveting process, the strength of a riveted joint under dynamic loading and the characterization of a simplified rivet element, in particular, were sources of questions. For each topic, we did solid finite element modeling and carried out experiments to measure the influence of strain rates under quasi-static and dynamic loading conditions. We also determined the elastic–plastic and damage mechanical properties for sheet metal plates and rivet materials. Our results showed that it was essential to develop a new kind of rivet element taking material non-linearities into account. Experiments and solid finite element modeling of an adapted Arcan test procedure were then conducted and pure shear and tensile non-linear responses as well as parameters of a macroscopic criterion were identified. The use of this new rivet element was found to improve prediction of the dynamic behavior for a frame assembled with 700 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.

Identification technique of constitutive model parameters for crashworthiness modelling

Abstract This paper deals with a parametric identification technique, based on the conjugate gradient methods. As part of this research, compression tests are performed on a XC48 steel (AFNOR norm 1048AISI), which is strain rate sensitive. Two kinds of specimens, tubular and cylindrical “dumbbell”, are tested at different strain rates, from 10 −3 to 130 s −1 , and for impact velocities ranging from 5 mm/min to 6 m/s. The parameters of the Johnson-Cook constitutive model are evaluated directly from the XC48 steel compression stress-strain diagrams expressed in terms of true stress and strain measurements. These parameters are next used as input data for a FE crash simulation of the tubular specimen. Comparison with experimental results prove the validity of the identified parameters.

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.

Détermination expérimentale du comportement mécanique et de critère de rupture d'assemblages sous chargements mixtes. Application aux rivets et points soudés

The paper deals with a new experimental procedure for joint characterisation. New experimental set-ups are based on the Arcan protocol which was first developed for composite delamination studies. This specific test enables one to mix and control the distribution of tensile and shear loads in the specimen. In the first part of the paper, a specific set-up is developed and the Arcan procedure is used and applied for riveted joints. The non-linear behaviour and a failure criterion are defined and identified. In the second part, this procedure is used and applied to spotwelds. For this kind of joints, the main problem is to attach the specimen to the set-up properly (without stamping or bending the metal plates of the specimen). The solution is based on the use of the soldering joint. However, it was first necessary to measure the influence of the melting temperature of the silver/tin alloy regarding the material properties, the strength and the microhardness of the spotweld. Arcan test procedure is then applied to spotwelds. Finally, results prove the ability of this new experimental protocol to define non-linear behaviour of rivets and spotwelds until failure. The interest of these experiments is also to evaluate and eventually improve the representativeness of equivalent joint models used for structural analysis in terms of behaviour and failure criterion.

Optimization of an Image-Based Experimental Setup for the Dynamic Behaviour Characterization of Materials

The present work aims at identifying an elastic-viscoplastic material constitutive model over a wide strain and strain-rate range (up to 0.1 and 1000 s−1 respectively), using the so-called Virtual Fields Method. To define the experimental campaign, a design process has been set. It relies on the numerical optimization of the setup – notably the specimen shape, the impact conditions and the measurement resolution (time and space) – with respects to user-defined criteria. Finally, the selected configuration ensures an accurate and robust identification.

Macro-modeling of spot weld strength and failure: Formulation and identification procedure based on pure and mixed modes of loading

Purpose This paper aims to propose a macro modeling approach to simulate the mechanical behavior and the failure of spot welded joints in structural crashworthiness computations. Design/methodology/approach A connector element is proposed to simulate the behavior and failure of spot weld joints. An elastic-plastic damageable model is used to describe the non-linear response and rupture. The connector model involves several parameters that have to be defined. Some are directly identified based on mechanical interpretations and experimental tests characteristics. The remaining parameters are identified through a finite element model updating approach using Arcan tests. Resulting from a sensitivity analysis, an original two steps optimization methodology, using the Modes I and II Arcan tests results sequentially, has been implemented to identify the remaining model parameters. Findings The numerical results for Arcan tests in mixed Modes I/II are in a good agreement with the experimental ones. The model is also validated on tensile pull-out, single lap shear and coach-peel tests. Originality/value By comparison with previous published results, the proposed model brings a significant improvement. The main innovative aspects of this work are as follows: the proposed formulation, a reduced number of parameters to optimize, an original sequential optimization methodology based on physical and mechanical analyses and a mesh size independent connector element.

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.