IMPACT ATTENUATOR

Design and Development of an Impact Attenuator

by

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.

Table of Contents

ABSTRACTii

CHAPTER 1: INTRODUCTION1

Problem Statement3

CHAPTER 2: LITERATURE REVIEW4

Attenuators and Highway Safety4

Putting the Places Together7

Safety through Design8

Impact Attenuator Design8

Material Characterisation Tests10

Definition of Energy Absorbing Structures11

CHAPTER 3: ANALYTICAL AND NUMERICAL MODELLING14

Analytical Model Of Cylindrical Tubes14

Finite Element Analysis16

CHAPTER 4: EXPERIMENTAL QUASI-STATIC AND DYNAMIC TESTS17

Experimental Dynamic Tests On Cylindrical Tubes17

Experimental Quasi-Static Tests On Impact Attenuator20

CHAPTER 5: ANALYTICAL, NUMERICAL AND EXPERIMENTAL RESULTS22

Cylindrical Tubes22

CHAPTER 6: CONCLUSION27

CHAPTER 1: 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. Also, a couple of analytical models have been proposed to predict the energy absorption characteristics of thin-walled tubular structures. Furthermore, some studies can be found in the literature concerning composite crash-boxes ...