G.A. Demosthenous

Crashworthy capability of composite material structures

Abstract Considerable research interest has been directed towards the use of composite materials for crashworthiness applications, because they can be designed to provide impact energy absorption capabilities which are superior to those of metals when compared on a weight basis. This review draws together information from a variety of sources to compare the findings of researchers in this field. The anisotropy of composite materials means that there are a great number of variables controlling mechanical behaviour and much of the investigative experimental work conducted in this area has concentrated on composite tubular specimens. The material, geometrical and experimental factors which have been shown to affect the energy absorption capability of such samples are related and a comparison is made of some of the specific energy absorption values which have been quoted in the literature. A selection of methods for predicting composite material energy absorption capability is presented and consideration is given to some of the more practical aspects of employing composite materials for crashworthiness purposes.

The static and dynamic axial crumbling of thin-walled fibreglass composite square tubes

In the present paper we report on the behaviour and crashworthiness characteristics of square composite tubes subjected to static and dynamic axial compression exerted by a hydraulic press and a drop-hammer, respectively. The effect of specimen geometry, i.e. of thickness and axial length, and of the loading rate on the energy absorbing capability are studied in detail. Attention is directed towards the mechanics of the axial crumbling process from macroscopic and microscopic point of view for facilitating engineering design calculations of the amount of energy dissipated and for a somewhat more complete aspect on the actual fracture mechanism during the failure of the composite material tested. A theoretical analysis of the collapse mechanism of the components tested under axial compression is proposed, leading to a good approximation of the energy absorbed during crushing.

Analysis of failure mechanisms observed in axial collapse of thin-walled circular fibreglass composite tubes

Theoretical analysis of the failure mechanism of the stable mode of collapse of thin-walled fibreglass composite tubes under static axial compression, based on experimental observations and taking into account all possible energy absorbing mechanisms developed during the process, is reported. Crushing loads and the energy absorbed are theoretically predicted. The proposed theoretical model was experimentally verified for various composite materials and tube geometries and proved to be very efficient for theoretically predicting the energy absorbing capacity of the shell.

The static and dynamic axial collapse of fibreglass composite automotive frame rails

Abstract An automotive frame rail of hourglass cross-section, made of a glass fibre/ vinylester composite, was designed for use in the apron construction of the car in order to obtain a high degree of crashworthiness at this location of the car body. The crashworthy behaviour of this structural component in axial compression at various strain rates (head-on collision) was studied experimentally. The modes of collapse at macroscopic scale, the microscopic fracture patterns and the energy absorbing capability of such rail beams were examined and discussed. A theoretical analysis of the collapse mechanism of the components tested under axial compression is proposed leading to a good approximation of the energy absorbed during crushing.

Analytical modelling of the static and dynamic axial collapse of thin-walled fibreglass composite conical shells

A theoretical analysis of the observed stable collapse mechanism of thin-walled circular frusta and tubes, crushed under axial static and/or dynamic loading, for calculating crushing loads and the energy absorbed during collapse, is reported. The analysis is based on experimental observations regarding the energy-absorbing collapse mechanisms developed during the crushing process. The proposed theoretical model was experimentally verified and proved to be very efficient for theoretically predicting the energy-absorbing capability of the conical shells.

Axial plastic collapse of thin bi-material tubes as energy dissipating systems

Abstract The deformation characteristics, crumpling mechanisms and energy absorption efficiency of bi-material circular tubes subjected to axial compression are investigated and analysed both experimentally and theoretically. Theoretical models describing extensible and inextensional type of collapse are proposed and corresponding results are found to be in good agreement with experimental ones.

Energy absorption capability of fibreglass composite square frusta subjected to static and dynamic axial collapse

The crashworthy behaviour of square frusta of fibreglass composite material subjected to axial compression at various strain rates is reported. The effect of specimen geometry and the loading rate on the energy absorbing capability was experimentally studied. The mechanics of the axial crumbling process from macroscopic and microscopic points of view were also investigated theoretically and experimentally. The collapse modes at macroscopic and microscopic scale during the failure process were observed and analysed. A theoretical analysis of the observed stable collapse mechanism of the components crushed under axial compression, for calculating crushing loads and energy absorbed during collapse, is proposed. A good agreement between theoretical and experimental results was obtained indicating the efficiency of the theoretical model in predicting the energy absorbing capacity of the collapsed shell.

Axial collapse of thin-walled fibreglass composite tubular components at elevated strain rates

Abstract In the present paper, we deal with the crush behaviour of axisymmetric composite structures consisting of circular tubes and frusta made from a chopped-strand glass mat and polyester resin and subjected to dynamic axial compression exerted by a drop-hammer. The effect of specimen geometry and loading rate on the energy-absorbing efficiency is studied in detail. Attention is also directed towards the mechanics of the axial crumpling process from the macroscopic point of view, and the systematic study of the microfailure process and understanding of the crack propagation mechanism during the stable collapse of the shell. This gives a somewhat more complete aspect on the actual fracture mechanics during the failure of the composite material tested. Finally, the dynamically obtained experimental results are compared with the static ones for shells of similar geometry, and useful semi-empirical relations concerning the amount of energy absorbed and the post-crushing loading are derived for the material tested.

Experimental determination of splitting in axially collapsed thick-walled fibre-reinforced composite frusta

Abstract The failure mechanisms of thick-walled circular frusta, made of glassfibre-reinforced composite material, when subjected to axial compression are reported. Depending on wall thickness and the semi-apical angle, a conical shell may fail by four distinct deformation modes: progressive crushing, splitting, mid-length failure and progressive folding. Progressive crushing was found to offer the highest energy-absorption efficiency, the two latter belonging to catastrophic deformation modes with low energy-absorbing characteristics. Splitting usually follows after a short distance of progressive crushing for specimens with semi-apical angles greater than 25°, constituting a transition mode from stable collapse to catastrophic collapse, which should be avoided in the loading of crashworthy structures. The present paper deals mainly with the splitting mechanism. On the basis of experimental observations pertaining to axially collapsed, relatively thick-walled composite frusta of large semi-apical angles, a criterion for transition from stable collapse to this catastrophic deformation pattern is proposed. Strain gauges were mounted on the tube wall all over its axial length at highly strained positions and useful conclusions on the mechanical response of the composite material over the critical region are drawn from the strain data obtained.

The deformation mechanism of thin-walled non-circular composite tubes subjected to bending

Abstract The present work is dealing with the experimental investigation of the bending of thin-walled composite tubes of non-circular cross-section, designed for the construction of various parts of the car body. Simultaneously much effort is given to explore the fracture mechanism governing the phenomenon examining as well as the influence of many of the factors associated with the energy absorbing efficiency and crashworthiness characteristics of the structures tested. Moreover, a theoretical analysis for the prediction of the ultimate bending strength for tubes of various composite materials and cross-sections subjected to bending, is presented. Theoretical results are compared with experimetal measurements and are found to be in good agreement.