5 research outputs found
Hygro-thermal residual stresses in unsymmetrical multi-stable composite laminates
In this study, an approach to predict and analyse the effects of moisture ingress on
residual stresses in multi-stable composite laminates is developed. Residual stresses are
a common consequence of the manufacturing process of composite laminates (e.g.
formed thermally, following cool-down from manufacture). Imbalance in these stresses
about the mid-plane can lead to warping, and so composite laminates are usually
restricted to symmetrical lay-ups. In certain cases, unbalanced residual stresses can be
used advantageously, such as in novel morphing structures by use of multi-stable parts.
These are parts which feature two or more stable shape configurations, which are
obtainable through a force application. With energy only being required to alternate
between shapes, multi-stable laminates have been proposed as morphing aerodynamic
surfaces for aerospace and wind-energy applications. In these cases (and others in which
the laminates are sensitive to residual stresses such as thin plates, ply drop off and
bonded repairs) a thorough understanding of the residual stresses (both as-manufactured
and in-service) is required.
The residual stresses in fibre-resin composites are known to be sensitive to
environmental effects, which can be encountered under in-service conditions. One such
effect is moisture absorption, which alters the residual stress state of a laminate through
matrix swelling and plasticisation. These changes may lead to a change in the laminate’s
shape, and in the case of multi-stable laminates, a change in the multi-stable behaviour.
Applications based upon these unsymmetrical laminates therefore require consideration
to moisture effects at a design stage.
In this work, a combined numerical/experimental approach is presented whereby the
macro-scale through-thickness residual stresses of dry and saturated unsymmetrical
composite laminates can be predicted and analysed. A range of unsymmetrical
laminates were manufactured from carbon-fibre reinforced plastic (unidirectional
continuous fibres pre-impregnated in a polymer-resin matrix), featuring both square
cross-ply and tailored (i.e. featuring local variations in lay-up and/or thickness –
representative of laminates that would be used in complex applications) laminate
configurations. Following manufacture, the dry laminate shapes were measured, with, in
the case of the tailored laminates, laser scanning – a full-field, non-contact surface
measuring technique. Three-dimensional continuum based finite element models were
created (using the software Abaqus) to simulate the thermal deformation of the
laminates. The models were benchmarked using analytical approaches, and
subsequently calibrated to match the experimentally measured laminate shapes by
means of equivalent orthotropic thermal expansion coefficients, negating the need to
account for individual residual stress contributors. Subsequently, laminates were
immersed in water until saturation, and the change in shape due to matrix swelling was
measured. The numerical models were then adapted to take into account moisture
induced matrix swelling by use of the analogy between thermal expansion and moisture
induced swelling. Subsequently, the variation in shape and residual stress distribution in
the laminates following moisture saturation could be analysed. Using laser scanning to
measure the tailored laminate shapes allowed for a detailed analysis of the full-field
variation between numerically-predicted and experimentally-measured laminate shapes.
Using this analysis technique, macro-scale through-thickness residual stress profiles
were extracted for each of the cross-ply and tailored laminate configurations. It was
found that peak residual stresses can drop by over 70% following moisture saturation
resulting in a significant loss of curvature. Likewise, laminate potential energy can drop
by over 90%, impacting upon the laminates multi-stable behaviour
Morphing lattice boom for space applications
Structures used in space applications demand the highest levels of stiffness for their mass whilst also performing in a hostile environment. To partly address these requirements and so as to also pack efficiently for stowage during launch we propose a new type of compact telescopic morphing lattice space boom. This boom stows within a 1U CubeSat volume and is lightweight being only 0.4 kg. The boom has a total length of 2 m in its deployed state which is 20 times its stowed height. The device comprises two multi-stable cylindrical composite lattices that are joined telescopically. These lattices nest inside one another in the stowed configuration, with the objective of improving packaging efficiency. Notably, prestress and lamina orientation are used to smoothly change shape from being compact when stowed to being extended when deployed. The lattices in the boom have
been designed to maximise deployment force and to be self-deploying by tuning manufacturing parameters. As a result, only a small, lightweight mechanism is required to regulate deployment speed of the lattice boom. By reversing its direction, this mechanism can be used to retract the lattice boom to its stowed configuration, thereby enabling two-way reconfigurability
Experimental and numerical study of bending-induced buckling of stiffened composite plate assemblies
Despite their importance in benchmarking numerical simulations, buckling tests still feature compromises between component-level and high-fidelity large-scale tests. For example, compression-induced buckling tests
cannot capture the through-thickness or span-wise stress gradients in wing skins. Consequently, the results obtained often require careful interpretation and conservative considerations before applying to a structure.
Alternatively, a system-level large-scale test can be used, yet at considerably increased time and expense. There
has been little progression towards capturing system-level behaviour in a simplified test.
Herein, for buckling behaviour assessment, a three-point bending test is used, which is quick, simple to
implement, and cost-effective compared to existing conventional methods. The proposed method relies on
subjecting a panel with auxiliary stiffeners to bending to introduce compression-induced buckling in the skin.
The three-point bend test is used, because it provides readily controllable loading and boundary conditions. The
location of the neutral plane can be tailored via design of the stiffeners, thus allowing for control of the through thickness stress gradient induced in the skin. This method is applicable to buckling of stiffened structures subject
to bending (e.g., aircraft wingboxes). Numerical models are used to explore the limits of the proposed method
and comparing it against traditional coupon and full-scale structural level tests. The test method is experimentally demonstrated for capturing the buckling behaviour of a thermoplastic composite panel made via automated
fibre placement. The proposed approach is shown to reliably capture the buckling behaviour of a large-scale test
using a simpler and more time and cost-efficient setup than conventional methods
Enhanced buckling performance of a stiffened, variable angle tow thermoplastic composite panel
Variable stiffness composites are exciting emerging structures capable of improving structural performance through tailored load redistribution. This technology is particularly relevant to aerospace structures, such as aircraft wings, which rely on stressed skins to resist compressive, buckling loads. Variable Angle Tow (VAT) composite laminates manufactured via tow steering can increase buckling capacity of composite structures, leading to reduce material weight and costs. Numerical models have progressed to the point whereby this technology can be explored for complex aerospace structures. Further progress can be made through incorporating the latest manufacturing methods with simple and representative testing techniques to analyze buckling performance and benchmark numerical models.
This work aims to analyze the buckling performance of a stiffened VAT panel using a novel test method. Laser assisted tape placement is used to manufacture the panel using thermoplastic composite tape, improving manufacturing accuracy and speed. The buckling response of this component is then tested using a newly developed three-point bending test method. The test method was designed using finite element models, experimentally validated, and the results were compared against a numerical model (based on the Ritz approach). It was found that the developed test can produce buckling in the skin, with the buckling mode matching that of the numerical model
Static test of a thermoplastic composite wingbox under shear and bending moment
The proof of an aircraft’s structural integrity and safety is typically provided by analysis
and supported by structural test evidence. The experimental test of a wingbox, which is the
main structural component of a wing, can provide useful data on assessment of its structural
performance in an actual airplane for the design objectives. To this end, a testing fixture was
designed and manufactured to introduce a prescribed shear force and bending moment at one
end of a variable stiffness thermoplastic composite wingbox and react the load at the other end.
We report experimental results and compare them with detailed finite element data