Inertia effects in impact energy absorbing materials and structures
Prof. S.R. Reid
UMIST
The literature concerning materials and structural components used as
impact energy absorbers is extensive. In addition to absorbing the
kinetic energy associated with an impact, the absorber is also often
required to limit the loads transmitted to the system being protected.
However there have been relatively few studies which have examined
the variation in these loads as impact conditions change.
In low velocity impacts (impact velocity <10 ms-1) the response of
many energy absorbers is quasi-static in the sense that the modes of
deformation of the energy absorber are very similar to those produced
in compression tests in a static testing machine and the peak loads
generally occur at the same stage of the deformation. Consequently
much of the early work was concerned with measuring and modelling the
large deformation response of energy absorbers under static loading.
These results were then adapted to impact conditions by introducing
dynamic enhancement factors usually associated with the effects of
strain-rate on the yield stress of the material.
This procedure gives satisfactory estimates for the energy absorption
capacities of a wide range of components. However, as noted by
Calladine and English in 1984 and subsequently by other workers in the
field, a certain class of structures (so-called Type II structures)
exhibit changes in behaviour under dynamic loading conditions, even at
low impact velocities, which are inertial in origin. These changes
are particularly significant in introducing high peak forces which
need to be quantified for designers. In a similar vein, work on the
crushing of a range of cellular materials used in impact energy
absorption at high impact velocities has revealed large increases in
their crushing strengths. These too are believed to be essentially
inertial in origin.
Recent work at UMIST has been directed towards the accurate
measurement of force pulses generated during the deformation of impact
energy absorbers, especially at high impact velocities. Experimental
procedures have been developed which allow such pulses to be measured
reliably using signal deconvolution techniques. The behaviour of
metal tubes subjected to internal inversion and axial buckling and the
dynamic compression of wood, honeycombs and aluminium foams have been
examined. An overview of this work will be presented together with
comparisons between the experimental data and corresponding
theoretical/computational models where these are available.
Particular attention will be given to assessments of the maximum
forces produced during the deformation and to the differences between
the forces generated at the end of the absorber where contact is first
made (the proximal end) and at the supported end of the device (the
distal end). The practical significance of the results will be
discussed.
