Title:
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On delamination migration in composite laminates
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Carbon fibre/epoxy composite laminates are well known to be susceptible to delamination.
Delamination usually grows at interfaces between plies with dissimilar fibre orientation
and may migrate from one interface to another, as in low-velocity impact damage or
skin-stringer debonding. Much of the modelling work in the literature focuses on delamination
contained within a single ply interface, using data from characterization tests in
which delamination grows at interfaces between plies of the same fibre orientation, mainly
because of a lack of understanding of the fundamental causes of delamination migration.
However, to obtain representative results, delamination migration should be accounted for
by finite element methods aimed at accurately simulating delamination growth.
The objective of this work was to investigate and understand delamination migration at
interfaces between plies with dissimilar fibre orientation in composite laminates. Initially,
delamination migration was observed experimentally in a Double Cantilever Beam (DCB)
specimen containing ±θ ply interfaces. Finite element analyses of the DCB specimen were
then performed, to simulate delamination migration using a cohesive zone model-based
approach. Comparison of experimental and numerical results highlighted the need for a
more detailed experimental understanding of the fundamental driving forces for delamination
migration, before methods to simulate delamination migration could be developed
and validated. To this end, further experimental tests were conducted, using the Delamination
Migration test method, recently developed at NASA Langley Research Center,
which allows the isolation of a single migration event that can then be studied in detail.
A novel delamination migration specimen was employed, to investigate delamination
migration at a generic 0/θ° interface. Tests were performed in the laboratories of the
Durability, Damage Tolerance and Reliability Branch at NASA Langley Research Center.
Damage was characterised in detail using C-scan and X-ray Computed Tomography
techniques. During the test, delamination initially propagated along a 0°/θ ply interface,
by growing closer to one of the bounding plies, turning into it and subsequently arresting.
Eventually, delamination migrated to a different 0/θ° ply interface. Linear elastic
finite element analyses and the Virtual Crack Closure Technique were employed to interpret
experimental results. Results suggested that delamination migration is governed by
the shear stress sign and the strain energy release rate along the delamination front, which
both vary across the specimen width at a 0/θ° ply interface.
Results of this study represent a step forward in the understanding of the key mechanism
of delamination propagation and migration at ply interfaces with dissimilar fibre orientation,
and help in the understanding of complex damage patterns such as low-velocity
impact. Understanding delamination migration can inform design of damage tolerant
composite structures and provide guidelines and benchmark data to develop and validate
modelling methods aimed at simulating and predicting delamination migration.
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