How to test thick 3D-reinforced composite materials?
Michael Gower (NPL) and Steve Ogin/Paul Smith/David Jesson (University of Surrey)
Summary: This proposal fits within the Advanced Manufacturing and Materials Science priority area and will provide
experimental and theoretical support to a current National Measurement System (NMS) project (Theme 2, Structural
Elements) within NPL. The project combines mechanical testing, state-of-the-art characterisation (Digital Image
Correlation, DIC; micro computed X-ray tomography, µCT) and finite-element (FE) modelling, and will provide an exciting
and challenging mixture for a PhD student. The project addresses the need to provide the scientific underpinning for the
metrology of thick, three-dimensionally reinforced composite materials, combining the expertise of NPL, Surrey and our
(UGPN) international partner, North Carolina State University (NCSU). An example of a cross-section of a 3D non-crimp
orthogonal woven (3DNCOW) composite, a type of material pioneered at NCSU, is shown in Figure 1.
Figure 1. From Bogdanovich and
Mohamed, SAMPE Journal, 2009
The material shown in Figure 1 is about 4 mm thick, but commercially available
fabrics for composites are now up to 19 mm thick. The advantage of these materials
is that the through-thickness reinforcing yarns provide an extraordinary resistance to
delamination. These exciting new materials are beginning to be used in applications
where resistance to delamination between the reinforcing plies is paramount (e.g. in
marine tidal turbines), but there are two major metrology difficulties limiting the use
of thick 3D composites. First, there is the problem of how to grip the materials to
measure their mechanical properties when design data are required, since standard
gripping methods, suitable for thin 2D composites are unsuitable (there is a danger
of crushing the through-thickness reinforcement). Second, there is an absence of
appropriate experimental and modelling work available to support the development
of test standards for testing 3D composites – this project addresses both areas.
1. Case for support
1.1 Project aims/scientific/technological case
The scientific challenges centre around the following aims of the project: (a) to identify the limitations of current
conventional gripping systems when used to test thick 3DNCOW composites and to design an appropriate gripping
system; (b) to understand issues of scale in testing 3DNCOW specimens, where this relates to the size of the unit cell of
the material in relation to specimen dimensions and the dimensions of any stress concentrations (holes, cut-outs).
In general, these are complex materials and very little is currently
available in the literature on identifying the relationship between
the structure of the materials and their performance. Figure 2
shows some recent work carried out in the MES Department, in
collaboration with NCSU and Manchester University, on a thin
3DNCOW composite using micro computed X-ray tomography
(µCT). This state-of the-art technique will be used in this PhD
project, both to investigate the complex structure of the
fabricated 3DNCOW composites and to determine the
relationship between stress concentrations (due to grips and
machined in holes) and the development of damage. Figure 2
shows the development of fibre fractures (circled) as a
consequence of fatigue damage.
Figure 2. Virtual cross-section obtained using µCT
(from Topal et al, submitted to Composites Part A)
1.2 Specific objectives: These are as follows:
• To survey the literature to identify current understanding of the effects of specimen geometry and fibre architecture on the
measured mechanical response of thick composites, and thick 3DNCOW composites in particular.
• To develop an improved understanding of the behaviour of a 3DNCOW composite under a number of critically important
quasi-static test conditions, of varying complexity, which link directly to industry requirements.
• To use Digital Image Correlation (DIC) to determine strain fields in the specimens and hence determine the effectiveness
of gripping arrangements in tensile coupons and the influence of a stress raiser in a notched coupon.
• To use microscopy and µCT to provide information relating to the sub-critical damage within a specimen prior to failure.
• To develop analytical and Finite Element (FE) based analyses to inform the selection and modification of suitable test
geometries and gripping conditions which will support the subsequent development of appropriate test protocols.
• To explore the possibility of using the optimised protocol for quasi-static testing in cyclic (i.e. fatigue) loading.
It is envisaged that the student will work at the University and NPL, possibly making several trips to NCSU to
assist in the manufacture of any 3DNCOW fabrics required for the project.
1.3 Outline work programme and project plan
1.3.1 Focussed literature review; scoping the project; sourcing the material (6 months): The main materials will be
provided by NPL as part of the related NMS project. We will also make use of our excellent working relationship with the
College of Textiles at NCSU to source other fabrics. One basic fabric architecture with three thicknesses will be required;
a thin fabric (probably 2 mm thick), a fabric with intermediate thickness, and a thick fabric, (15 - 20 mm thick).
MILESTONE 1: 6 months: Definition of project scope, specimen configuration, sourcing the materials
1.3.2 Materials characterisation; quasi-static testing on plain coupons and coupons with holes: Current work funded by
Dstl, on the behaviour of thin (2 mm – 4mm thick) 3DNCOW hybrid composites in fatigue, has shown that the architecture
of 3D composites can vary unexpectedly with thickness. Consequently, the materials will be characterised using a
combination of micro computed X-ray tomography (µCT) and optical and scanning electron microscopy. The key features
to investigate with regard to mechanical properties are the effect of specimen dimensions (i.e. specimen scale effects)
and the relationship between test configuration and damage onset. Initial testing will be based on current standards.
With regard to notched strength, a systematic investigation of the effect of hole size in relation to
thickness and unit cell size is required. Figure 3 illustrates the complexity of the damage at an
early stage of loading for a thin, transparent specimen with a hole size of 2.5 mm, which is smaller
than the unit cell size (approximately 8 mm). For thick specimens, particularly when they are not
transparent, µCT will be required to investigate damage development and hence facilitate
directions for modelling. For all tests, the displacement distributions will be investigated using
recently acquired state-of-the-art DIC equipment. Features of the displacement/strain distributions
Figure 3.
relating to the limitations of current test standards will inform the redesign of test jigs.
Figure 4 shows recent results obtained using DIC to measure the non-uniformity of the
longitudinal displacements for a thin 3DNCOW composite tested off-axis, using standard end
tabs and gripping. Figure 4(a) shows an experimental visualisation of the displacements - for a
correctly loaded specimen, the colours (grey stripes in black and white) should be horizontal
stripes across the specimen. The lower set of results in Figure 4(b) shows the quantification of
these displacements, indicating that the displacements vary linearly across the specimen, when
for correct loading the displacements should be constant; the upper set of results show almost
constant displacements obtained with new, improved end tabs, designed to take into account the
(a)
(b)
anisotropy of the material. DIC will thus provide a powerful tool for the student to compare
Figure 4
experimental results with FE predictions of displacements for thick 3DNCOW composites.
MILESTONE 2: 12 months: Initial test data and submission of confirmation document.
MILESTONE 3: 24 months: Recommendations for improved test methods (plain and open-hole tension).
1.3.4 Development of a new test jig: Based on the recommendations for improved test methods, proposals will be made,
with the aid of FE modelling, for the design of at least one improved loading configuration for thick 3DNCOW composites;
the loading jig will be manufactured and evaluated using a new set of tests for comparison with previous results. The new
loading protocol will also be evaluated, to a more limited extent, for fatigue loading in a comparison with results obtained
using a conventional loading jig.
MILESTONE 4: 30 months: Validation of at least one new loading jig for testing of thick 3DNCOW composites.
MILESTONE 5: 36 months: Submission of PhD dissertation.
2.
The contribution of University and NPL researchers to the programme: Colleagues at NPL and the University will
jointly supervise all aspects of the project. The University brings extensive expertise in 3D composites, damage
characterisation and modelling (SLO/PAS) and testing/DIC (DAJ). MG has particular expertise in damage monitoring and
standardisation. Other colleagues at NPL who can potentially advise include Dr Louise Crocker (expertise in FE).
3. The likely outputs and consequent added value to the NPL/Surrey Partnership: In addition to contributing to Theme
2 (Structural Elements) of the NMS programme, there will be an opportunity to bid for further work in this area, to the
benefit of the partnership, as a consequence of the industrial importance of the topic. The student and University
supervisors will be available to make contributions to any NPL Industrial Advisory Group (IAG) or other meetings for
which the PhD project is relevant. Within the wider context, the University and NPL are looking at ways of working
together with other organisations, including McLaren and the National Composites Centre in Bristol. Support for the
present project will contribute to this wider agenda in an area of NPL and University strength.
4. Details of any relevant previous NPL/University collaboration: SLO, PAS and DAJ have all collaborated with NPL
successfully for a number of years, notably under the EngD schemes (MiNMaT and SEES) but also through PhD projects.
SLO worked with MG as an invited member of the TSB “IMAJINE” project (from 2009), including co-supervision of Dr
Andrew Sanderson’s PhD. SLO was an invited member of NPL’s IAG on Structural Health Monitoring (from 2007). These
collaborations have let to joint publications in high calibre journals (Comp. part A, Comp. Sci. & Tech., Carbon). Currently
SLO and PAS have two EngDs and a funded PhD with NPL, in research areas that complement the current proposal.
5. Additional Funding: If the fees and stipend are funded, then the most effective way of accessing funds to support
additional costs of materials, consumables and travel would be to run the project as an EngD within the Centre for
Doctoral Training (MiNMat). The Director of the Centre (Professor Julie Yeomans) is supportive of looking at ways to do
this. In the event that this approach is not acceptable to the Panel, or not achievable for some other reason, then the
project costs would be associated with other relevant projects (e.g. an ongoing Dstl-funded project on fatigue of hybrid 3D
composites, where our collaboration with the University of Manchester would facilitate supply of material).