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