The effects of various schemes of end constraints, heat-treatment, filler density and impact velocity on the behaviour of axially crushed tubes of different L/D and D/t ratios are investigated. It is evident that these factors, individually or collectively, influence the collapse mechanism and hence determine the collapse mode. It follows that the total energy absorption capacity. The concertina mode represents the low energy mode whereas the multi-lobe mode represents the high-Icnergy mode. The former is attributed to instability and/or geometric and loading imperfections. The latter mode, however, occurs as a result of stiffness enhancement.