Abstract
Composite laminate tapering is an essential route to realising highly weight efficient structures.In order to achieve a tapered profile, individual lamina must be terminated which leads to the
introduction of local material discontinuities within the structure – commonly referred to in the
available literature as the ‘resin pocket’, based on the small pocket of resin that forms ahead of
the terminated ply. This material discontinuity has been well established to act as an initiation
site for cracks and in turn delamination – a critical failure mode. Many attempts have been
made to develop techniques for improving the delamination resistance of composites in general,
and are applicable to tapered laminates, such as careful design that minimises the structural
impact of ply drops, or interlaminar toughness enhancement by interleaving. Most methods
studied however are subject to some clear negative trade-offs. Design based solutions to the
problem of ply drop initiated delamination typically impose rules on laminate design that result
in restricted efficiency owing to the incorporation of additional material or large ply drop spacings.
Manufacturing based approaches will typically involve the use of additional expensive or exotic
materials, and thus in turn additional material costs and manufacturing steps.
In this work a novel and simple ply termination method for tapering the dropped ply end, and
thus minimising the severity of the material discontinuity is investigated. The method – dubbed
‘ply scarfing’, is proposed specifically for implementation within an AFP machine, and has the
benefit of directly addressing the source of delamination owing to ply drops, without necessitating
the inclusion of new materials or complex processes. The aim of this work was to investigate
the feasibility of the concept, along with the effect the method has on laminate performance in
different loading regimes and with respect to different parameters relevant to laminate design.
This thesis is separated into 4 sections. The first section presents the development of an initial
mechanism, followed by its subsequent use to demonstrate the feasibility of the concept, prove the
achievable cutting quality and measure some of the key parameters related to cutter actuation.
The second part investigates the performance of scarfed ply drops in tension as compared to
conventional, along with parametric studies into the optimal scarf ratio and spacing between
adjacent ply drops. In the third section numerical and analytical methods are used to further
develop a mechanistic understanding of the failure mechanics of scarfed ply drops. Finally, the
performance of scarfed ply drops is tested in low velocity impact in order to investigate their
effect on damage tolerance.
This work demonstrated that ply scarfing is a highly effective technique for improving the
delamination resistance of tapered composite laminates, as experimentally observed in both
tension and impact. Further to this, scarfed plies may also allow for a relaxation of current design
‘rules of thumb’, for example a reduction in the required spacing between adjacent ply drops
which could result in more efficient structures. Finally the method also has shown promise for
minimising the computational cost of predicting delamination onset stresses, with very simple
stress based predictions providing reasonably accurate failure estimates.
Date of Award | 28 Sept 2021 |
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Original language | English |
Awarding Institution |
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Supervisor | Byung Chul (Eric) Kim (Supervisor), Stephen R Hallett (Supervisor) & Michael R Wisnom (Supervisor) |