Abstract:
A comprehensive numerical and experimental study was performed to investigate
the energy absorbing capabilities of glass/epoxy and carbon/epoxy members that
could serve as stanchions in the subfloor structure of aircraft or rotorcraft. Circular
cross sections with chamfered-ends failure trigger mechanism were investigated
under axial and off-axis loading conditions. The optimal configuration that
resulted in the highest possible specific energy absorption (SEA) was identified,
which was at axial loading. The parameters in off-axis loading conditions that
affected energy absorption capability were identified. Several cases were
experimentally studied to cancel off-axis (oblique) loading effect.
To increase interlaminar fracture toughness, stitching through the thickness was
considered. Single, multi and pattern-stitching were studied to increase energy
absorption capability of GFRP composite sections. The failure mechanisms,
crushing process and force-displacement curve diagram of each case was
studied to establish the effect of stitching on energy absorption capability. A
correlation between stitching location and localised and global increase of energy
absorption was established. It was identified, that the closer the stitching locations
are, the higher the localised peak load becomes, and it influences the Mode-I
crack propagation (main central crack) resistance, bending of fronds and friction,
consequently, pattern-stitching resulted in a 15% increase in specific energy
absorption capability (SEA) under quasi-static loading. Similarly, this stitching
pattern resulted in a 14% increase in SEA using CFRP sections. Under impact
loading, it was identified that pattern-stitching through the thickness resulted into
17% and 18% increase in SEA using GFRP and CFRP sections, respectively.
Finite element models were also developed to simulate the crushing behaviour
of the CFRP and GFRP sections observed experimentally under axial, off-axis,
quasi-static and impact loading conditions. A multi-layer modelling methodology
was developed by determining the most effective element size, number of shells,
formulation, contact definitions, delamination interface, material model, friction
and trigger mechanism. This approach captured the failure process, predicted the
SEA and sustained crush load quite accurately within 5% error. Stitching through
the thickness was modelled using an energy-based contact card to implement
stitched and non-stitched Mode-I and Mode-II energy release rate parameters.
This method accurately predicted stitched composite sections with 3% error
compared with experimental data. Such modelling could thus support the future
design of aircraft stitched and non-stitched stanchions within reasonable
computer efficiency and accuracy.
