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Abstract
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Extensive studies have been reported on the behaviour of systems which absorb external energy through the irreversible inelastic response of a material. These dedicated energy absorbers are employed in many practical applications, but, in severe accidents, the actual structural member (e.g., a car framework) could be designed to deform in a specified manner and absorb additional dynamic energy. It turns out that a significant proportion of the studies undertaken have been reported for mild steel tubes having various cross-sectional shapes and struck axially by rigid masses travelling with relatively low velocities up to about 10–15 m/s. These energy absorbers are efficient, reliable, relatively inexpensive and are readily available in many structural geometries.
A study is reported on the energy-absorbing effectiveness factor which was introduced recently. The factor is defined as the quotient of the total energy, which can be absorbed in a system, to the maximum energy up to failure in a normal tensile specimen, which is made from the same volume of material. This dimensionless parameter allows comparisons to be made of the effectiveness of various geometrical shapes and of energy absorbers made from different materials. The influence of material properties and various geometrical parameters on the value of the dimensionless parameter has been examined for the static and dynamic axial crushing behaviours of thin-walled sections. The influence of foam fillings and the stiffening of circular and square tubes is examined.
It transpires that, according to the energy-absorbing effectiveness factor, an axially crushed circular tube is the most effective structural shape. Moreover multi-cellular cross-sections, and axial stiffening, increases the effectiveness of thin-walled sections. In these latter two cases, the energy absorbed by the additional material in a tensile test is included in the denominator of the energy-absorbing effectiveness factor. The influence of foam filling was found to increase the energy-absorbing effectiveness factor even though the additional energy absorbed by the foam is retained in the denominator. It was also noted that a circular tube, crushed axially either statically or dynamically, and made from an aluminium alloy, had a larger energy-absorbing effectiveness factor than a similar one made from a stainless steel, because the steel had a larger rupture strain which was not required during the deformation of the particular geometry examined. |