Flat vs Curved Crush Stress Results

As well as material choice and layup, the geometry of the crushing section can influence crush performance. The best performing cross-sections are circular but, in general, design constraints lead to geometries which are combinations of flat and curved regions.

If we consider this rectangular section <inset graphic> the four curved corner zones (coloured red) give higher crush stresses than the flat regions. The elevated performance of curved regions can be illustrated by comparison of this GFRP material crush tested in two geometry configurations. The flat material crushes, exhibiting a partial delamination mode and absorbs 50 SEA kJ/kg. The same material crushes to much finer debris with more thorough fragmentation of the fibres, as the curved geometry resists delamination between the layers of material. The energy absorption of the curved specimen is 110 kJ/kg, 120% higher than the flat material.

In order to properly measure and characterise the crush performance of a material it is necessary to determine both the flat crush performance and the curved or ‘delamination-suppressed’ crush performance. We have developed a special design of pin stabilized crush plate <inset graphic> to mimic the curved region crush performance from flat coupons, by suppressing the free delamination between layers. This enables us to measure all of the crush data needed for a full material characterisation from low-cost flat sheets of material. The pin stabilized crush plate is easily installed in place of the standard flat crush plates within the Engenuity Mk3 crush test fixture.