JEC COMPOSITES MAGAZINE - Issue #109 - November/December 2016 - 61

Non-destructive characterisation of composite microstructures

ed pending the availability of adequate NDT data from mechanical
test specimens.
Fig. 6: Relationship between
failure stress knock-down
(colour scale in %) and maximum angle (vertical axis),
as a function of the wrinkle
amplitude and wavelength.
Black 'x' locations represent
the modelled values before
interpolation. The extent of
the wrinkled zone was kept
the same throughout [6]

Ultrasonic 3D characterisation
It has been shown that it is possible to map the actual material properties of manufactured components, including the 3D fibre-tow orientation at each location in the microstructure [2, 3]. This way, any
wrinkles or waviness that could reduce the performance can be characterised, the component modelled, and the strength determined.
This enables feedback to process control before the variation in properties becomes serious and provides information potentially justifying
a concession for the detected deviation from design. This methodology was developed and demonstrated using relatively standard ultrasonic scanning as is currently performed on all aerospace composite
components. The data capture has to be optimised for microstructural
imaging; full-waveform data should be captured and stored and then
the data should be processed using an inversion algorithm to determine exactly what microstructure must exist to give the measured
ultrasonic response. Various characteristics of ply drops, wrinkles
and delaminations can be detected in the instantaneous amplitude,
phase and frequency within the ultrasonic analytic signal. Using this
information, detailed ply tracking through ply drops has been demonstrated [2]. Validation is being achieved using X-ray micro-CT of small
samples and micro-sectioning (Figure 1). Measurement of out-ofplane ply angles is possible using a Radon-transform method on the
instantaneous phase (see Figure 2) [3]. This can then be extended to
in-plane fibre-tow orientation using instantaneous amplitude, resulting in a complete vector map of the 3D fibre orientation (Figure 3) [3].

NDT-based performance modelling
A transfer process was created to automatically generate a finite-element (FE) model of the structure based on the inverted NDT information; this model can then predict the strength of the as-manufactured structure [4].
Validation of the transfer process has so far been achieved by comparisons between experimental data and predicted results. Coupons
were manufactured and tested by Mukhopadhyay et al [5] and models were created based on simulated material-property maps using the
transfer process. One aspect of the comparison between predicted and
experimental results is the damage distribution (Figure 5). During the
experimental compression test, delamination failure was observed to
initiate near the highest-angle region, and the same distributions were
displayed in the models. Agreement with the model was also achieved
when comparing the distributions of matrix-crack and fibre-kink failures. Hence, at this stage the transfer process has been partially validat-

Finally, the FE models are being used for a parametric study to determine exactly which metrics about wrinkles are important for determining remanent strength. The conclusions are that the maximum
angle of deviation of any ply from the load direction is the most important metric to determine compression strength (see Figure 6). The
extent of the wrinkled region also has an effect - the wrinkled proportion of the sample size perpendicular to the load being the important
information [6].

Conclusion
In response to future environmental targets in aerospace and automotive manufacturing, the use of composite materials will increase
and the drive for more efficient designs will require more information
about the as-manufactured components. The UK NDT community
has mobilised to develop methods providing detailed maps of microstructural information for composite laminates, confirming conformance to design and informing the concession process. From the NDT
information, finite-element models can be automatically created to
predict the strength and performance of the manufactured component including any deviations from design. These models were used
to investigate the effect of out-of-plane wrinkles, concluding that the
most important metrics are the maximum angle of deviation of a ply
from the load direction and the extent of the wrinkled zone perpendicular to the load direction.
More information:
www.ndtatbristol.com
robert.smith@bristol.ac.uk
References
[1] "Report from the Workshop on NDT and SHM Requirements
for Aerospace Composites", British Institute of Non-destructive
Testing, Ed. R.A. Smith, ISBN: 978 0 903132 61 3, DOI: 10.1784/
book.2016.001, pp. 1-19, May 2016. Download:
www.bindt.org/events/Aerospace-Workshop-February-2016.
[2] R.A. Smith, L.J. Nelson, M.J. Mienczakowski and P.D. Wilcox, "Ultrasonic Tracking of Ply Drops in Composite Laminates", Proc. Review
of Quantitative NDE, Minneapolis, July 2015, published in AIP Conference Proceedings 1706, 050006 (2016); doi: 10.1063/1.4940505,
2016.
[3] R.A. Smith, L.J. Nelson, N. Xie, C. Fraij and S.R. Hallett, "Progress
in 3D characterisation and modelling of monolithic carbon-fibre
composites", Insight - The Journal of The British Institute of NDT,
Vol. 57, No 3, pp. 131-139, March 2015.
[4] N. Xie, R.A. Smith, S. Mukhopadhyay and S.R. Hallett, "Modelling
the Mechanical Properties of Wrinkled Composites from NDT Data",
Proc. ICCM20, Copenhagen, Denmark, July 2015.
[5] S. Mukhopadhyay, M.I. Jones and S.R. Hallett, "Compressive
failure of laminates containing an embedded wrinkle; experimental
and numerical study", Composites Part A. 73 (2015) 132-142.
[6] N. Xie, R. A. Smith and S. R. Hallett, "Parametric study of the
effect of out-of-plane wrinkles on compressive strength of composites." To be published, 2016/17.

No109 November-December 2016 /

jec composites magazine

http://www.ndtatbristol.com http://www.bindt.org/events/Aerospace-Workshop-February-2016

Table of Contents for the Digital Edition of JEC COMPOSITES MAGAZINE - Issue #109 - November/December 2016