Elsevier

Composite Structures

Volume 94, Issue 2, January 2012, Pages 423-430
Composite Structures

Lightweight design and crash analysis of composite frontal impact energy absorbing structures

https://doi.org/10.1016/j.compstruct.2011.08.005Get rights and content

Abstract

Carbon fibre composites have shown to be able to perform extremely well in the case of a crash and are being used to manufacture dedicated energy-absorbing components, both in the motor sport world and in constructions of aerospace engineering. While in metallic structures the energy absorption is achieved by plastic deformation, in composite ones it relies on the material diffuse fracture. The design of composite parts should provide stable, regular and controlled dissipation of kinetic energy in order to keep the deceleration level as least as possible. That is possible only after detailed analytical, experimental and numerical analysis of the structural crashworthiness.

This paper is presenting the steps to follow in order to design specific lightweight impact attenuators. Only after having characterised the composite material to use, it is possible to model and realise simple CFRP tubular structures through mathematical formulation and explicit FE code LS-DYNA. Also, experimental dynamic tests are performed by use of a drop weight test machine.

Achieving a good agreement of the results in previously mentioned analyses, follows to the design of impact attenuator with a more complex geometry, as a composite nose cone of the Formula SAE racing car. In particular, the quasi-static test is performed and reported together with numerical simulation of dynamic stroke. In order to initialize the collapse in a stable way, the design of the composite impact attenuator has been completed with a trigger which is consisted of a very simple smoothing (progressive reduction) of the wall thickness. Initial requirements were set in accordance with the 2008 Formula SAE rules and they were satisfied with the final configuration both in experimental and numerical crash analysis.

Highlights

► Analytical, numerical and experimental research on composite impact attenuators. ► Simple tubular shape and more complex geometry for a Formula SAE car. ► Performed CFRP material characterisation tests to set values of LS-DYNA models. ► Good agreement between results achieved in terms of deceleration and deformation.

Introduction

In order to ensure the driver’s safety in case of high-speed crashes, special impact structures are designed to absorb the race car’s kinetic energy and limit the deceleration acting on the human body. In current automotive development, in order to improve their crashworthiness and increase stiffness to weight ratio, composite material is introduced with the scope of optimisation of car body components. In fact, composites have a greater capacity to absorb energy compared to metals, mainly due to the different modes of failure that govern energy absorption.

Crash investigations on composite structures reported in the literature are mainly based on experimental test analysis of small plates submitted to bending impact and on simple bars, of circular or rectangular cross section, of prismatic or tapered shape, submitted to axial impact [1], [2], [3], [4], [5], [6], [7]. Also, a couple of analytical models have been proposed to predict the energy absorption characteristics of thin-walled tubular structures [11], [12], [13], [14]. Furthermore, some studies can be found in the literature concerning composite crash-boxes for automotive applications, but they are still few and do not cover all aspects of composite structure modelling [8], [9], [10], [16], [17], [18], [19].

An important aspect of crashworthiness research is the validation of analytical and numerical models for accurate simulation of structural response to crash impacts. Indeed, they constitute the necessary tools for the designer to study the response of the specific structures to dynamic crash loads, to predict global response to impact, to estimate probability of injury and to evaluate numerous crash scenarios, not economically feasible with full scale crash testing.

This study covers the steps to follow during the design of a specific impact attenuator. After the mechanical characterisation of the CFRP material, it is possible to calibrate the numerical material model, to properly design and to perform experimental tests on thin-walled tubular structures. The good correlation between experimental and numerical test represents efficient modelling of composite laminates. Also, the numerical simulation has been coupled with a simplified analytical model, able to predict the energy absorption of thin-walled composite structures with circular cross section. The thin-walled tubular structures made of composite material, are used as frontal impact attenuator, applied for urban vehicle’s body-in-white. Instead, in the case of racing cars, it is usually used the conical absorbing structure [20].

Therefore, it is also presented the development, quasi static testing and numerical simulation of impact event for the lightweight frontal safety structure of Formula SAE vehicle, designed by the Politecnico di Torino team (Fig. 1). The production strategy of this car consists in a concurrent analytical and experimental development, from the initial conceptual design and coupon testing, through the stages of element and subcomponent engineering, to final component manufacturing.

Section snippets

Material characterisation tests

The used carbon fibre type material to manufacture thin-walled cylindrical specimens and the impact attenuator is plain weave prepreg GG203PIMP530R-42. The matrix type is resin epoxy and it is developed for automotive sector, in particular for sport application. It is characterised by good impact resistance properties and quality surface finish and is adapted for high speed cold stamping. The resin content is 42 ± 3%.

To obtain appropriate input data for the simulation of the composite components

Definition of energy absorbing structures

After the characterisation of the material, it is possible to constitute three complementary phases: the first one is the definition of a simplified analytical model that reproduces, as faithfully as possible, the crash phenomenon by an energetic point of view, the second is the implementation of a numerical model able to discretize the structure and impose testing conditions using finite elements and the last one is the execution of experimental tests in order to verify the effective brittle

Analytical model of cylindrical tubes

It is seen that the crushing mechanism of composite shells is quite complex. Little work has been done on the analytical formulation of their deformation behaviour in progressive collapse [11], [12], [13], [14].

In this section is reported the analytical model for a generic conical shell subjected to impact; the expression obtained can be reduced to get that for the cylindrical shells, simply by modifying the cone angle. In particular the model works from the energetic point of view, considering

Experimental dynamic tests on cylindrical tubes

All the dynamic experimental tests reported in this paper were performed at the Picchio S.p.A. plant in Ancarano (TE) using a drop weight test machine with a 6 m free-fall height and a maximum mass of 413 kg. For the experimental tests on cylindrical tubes was used an impact mass of 294 kg and an initial velocity of about 4 m/s. During the tests every tube was supported at the bottom edge on a metallic base with air holes. The acceleration of the mass and the velocity at impact were measured using

Cylindrical tubes

After having performed a mesh sensitivity analysis, a uniform mesh with elements of about 2 mm per side has been chosen. A sequence of the deformed shapes of a fabric tube (diameter 50 mm and thickness 2 mm) at different simulation times, is shown in Fig. 9.

The numerical and experimental deceleration vs. shortening curves for the tube with diameter 50 mm and thickness 2 mm are compared in Fig. 10, while some crash parameters obtained from experimental tests are reported together with the analytical

Conclusions

The present paper developed an analytical, numerical and experimental research on the energy absorbing composite structures, initially of simple tubular shape and then of more complex geometry.

In order to set the appropriate values of the numerical LS-DYNA models, composite material characterisation tests and tube crushing experiments were performed. The crash-tests were performed using a drop test machine, measuring the deceleration-time diagram and after integration processes load-shortening

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