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7/28/2019 CN56_Modelling_NDT_Aircraft_Ind.pdf
1/2
SOFTWARE SOLUTIONS
8
Flux Solutions & Mechatronics Products - N 56 - May 2008
Modelling NDT for Aircraft industries...Fabrice Foucher (CEDRAT SA), Sbastien Lonn (CEA LIST) .
The increasing use of composite
m a t e r i a l s i n a i r c r a f t ,
particularly carbon-fiber
reinforced epoxy composites (CFRP),
presents new challenges for Non-
Destructive Testing operations to ensure
reliable control of these parts and the
detection of potential delaminations.
The aircraft industry is showing a
growing interest in the simulation of
NDT techniques adapted to composite
structures, particularly using ultrasound,
as simulation provides powerful tools
to analyze experimental results andoptimize control configurations. The
CIVAnde modelling platform, which
now addresses the three major NDT
techniques (ultrasonics, eddy current,
radiography), includes models to
accurately analyze phenomena involved
in the propagation of ultrasonic waves
in a composite body.
Wave propagationin a typical composite
structureFigure 1 is a macrograph of a typical
RTM (Resin Transfer Molding) composite
structure used in aeronautics. Parts or
RTMs are made of plies of carbon fibers in
a mould giving overall geometry, before
resin is injected at high pressure.
Ultrasonic attenuation is possibly high
and strongly variable (up to 15dB) in
these parts which posses an irregular
inner structure. To be able to simulate
this kind of structure accurately,
modelling tools need to account for the
multiple scattering by fibers coupled
with viscoelastic losses that occur in UT
wave propagation.
Composites underCIVAnde
Simulation codes developed at CEA
and included in the CIVAnde software
platform aim at providing cost-
effective tools to predict the results of
inspection techniques. It includes beam
propagation and flaw interactions models
[1]. CIVAnde has been continuously
extended through the development
of simulation models to incorporate
realistic testing configurations in terms
of probes (monolithic, phased arrays),
flaws, specimen geometries (canonical
shapes, parametrically defined or 2D/3D
CAD defined) and structures. More
particularly, the models are used and
validated in order to simulate ultrasonic
bulk wave propagation in anisotropicmaterials, defined in homogeneous or
heterogeneous media (set of different
homogeneous media), see for instance
[2].
In order to efficiently simulate multilayer
composite structures, the strategy has
been to implement homogenization
algorithms which consider the different
phenomena involved in composite
structures and at the same time, is able
to represent the composite using one
equivalent homogeneous anisotropic
material.
Thus, once the structure is homogenized,
it is possible to run a CIVAnde simulation
using existing tools (beam or defect
response computation).
For unidirectional fiber composite
materials, the model developed under
CIVAnde uses the acoustic and mechanical
properties of the structures component
materials to calculate the elastic
constants of an equivalent orthotropic
structure as well as related attenuation
coefficient values (accounting for multiple
diffusion and viscoelastic losses). Then,a second homogenization process,
using a so-called Ray Theory Based
Homogenization method (RBH) seeks to
predict an anisotropic equivalent material
representative of an arrangement of
(b)
Figure1: Micrograph of a 2mm thick RTM plate
showing four plies of three unidirectional layers
- courtesy of Dassault Aviation.
Figure 2:
a) Individual layer.
b) ply arrangement defined in CIVAnde
Figure 2:
c) Equivalent homogenized orthotropicmaterial calculated by CIVAnde (stiffness
constants of the anisotropic matrix).
b)
a)
(see continued on page 9)
7/28/2019 CN56_Modelling_NDT_Aircraft_Ind.pdf
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SOFTWARE SOLUTIONS
9
Flux Solutions & Mechatronics Products - N 56 - May 2008
several unidirectional fiber sub-layersof arbitrary thicknesses and orientation
(typically 0/45/90/135/180
combinations). This second one can
be applied to a heterogeneous medium
when the geometry of the component
means that fiber layers are not exactly
parallel (see fig. 3a).
For more details on these homogenization
techniques, readers can refer to the
following papers: [3], [4], [5]. The
integration of these 2 methods under
CIVAnde allows NDT processes in a
composite structure to be simulated with
very reasonable calculation times.
Users can select the individual fiber and
matrix acoustic properties (velocities,
density, etc) from the materials
database then define the fiber diameter
and the percentage of fiber in the
composite alloy (fig 2a).
Finally, by defining the number of plies
in the whole assembly and their relative
orientation (fig 2b), CIVAndewill compute
the equivalent orthotropic structure to
account for in the simulation (fig 2c).Besides giving equivalent isotropic
or anisotropic structures, these two
homogenization models also provide
attenuation coefficient values.
Experimentalvalidation
Below is an experimental carbon-epoxy
mock-up which has been used for the
validation of the models implemented
under CIVAnde (as part of a joint project
with EADS Innovation Works and theCEA LIST). It consists of two horizontal
areas with two different thicknesses,
separated by a tapered region in which
the number of fiber layers increases
progressively. An immersion transducer
is used at vertical incidence (diameter
12.7mm, frequency 5Mhz). Four
transducer positions were studied
and one compared the measured and
simulated ultrasonic beam by sizing
the -6dB beam spot width and its
deflection. For modelling purpose,
the tapered part was divided into 4
sub-assemblies in order to accountfor different orientations for the plies
planes, which are clearly non parallel
in this case.
As you can see in the figure below,
a really good correlation between
measurements and computations was
obtained.
Conclusion
The particular nature of composite
materials, which are increasingly
present in aircraft structures, presented
new challenges for non-destructive
techniques for precise detection of any
delaminations. The implementation of
homogenization algorithms in the CIVAnde
software combined with accurate beam
computation predictions and defect
responses in anisotropic heterogeneous
structures in the tool have made the
simulation of this type of structure
possible and relatively easy.
References
[1] P. Calmon, A. Lhmery, I. Lecoeur-
Tabi, R. Raillon, L. Paradis, Models for
the computation of ultrasonic fields and
their interaction with defects in realistic
NDT configurations, Nucl. Eng. and
Design 180, 271, 1998.
[2] Simulation tools for predicting non
destructive testing of heterogneousand anisotropic structures, S. Mahaut,
S. Chatillon, N. Leymarie, F. Jenson
and P. Calmon, To be published in
Modelling NDT for Aircraft industries...(continued)Fabrice Foucher (CEDRAT SA), Sbastien Lonn (CEA LIST) .
the proceedings of The International
Congress of Ultrasonics, Vienna, April
9-12 2007, paper 1652
[3] Lonn S., Lhmery A., Calmon P.,
Biwa S. and Thvenot F., Modelling of
ultrasonic attenuation in uni-directional
fiber reinforced composites combining
multiple-scattering and viscoelastic
losses, in Review of Progress in QNDE,
Vol.23, eds. D. O. Thompson and D. E.
Chimenti, AIP Conference Proceedings,
Melville, New-York, 2004.
[4] A simulation study to explain the
variability of ultrasonic attenuation
measurement in RTM composites, S.
Lonn, A. Lhmery and F. Thvenot,
Review of Progress in QNDE 23, ed. by
D. O. Thompson and D. E. Chimenti (AIP
Conference Proceedings 700, Melville,
2004), pp. 898-905.
[5] Ultrasonic field computation into
multilayered composite materials using
a homogenization method based on
ray theory, S. Deydier, N. Gengembre,
P. Calmon, V. Mengeling, O. Ptillon,
Review of Progress in QNDE 24, ed. by
D. O. Thompson and D. E. Chimenti (AIPConference Proceedings 760, Melville,
2005), pp. 1057-1064.
Figure 3:
a) Experimental set up: Carbon-epoxy piece
with parallel and non parallel layers (slope: 8)-
courtesy of EADS IW.
b) UT Beam in a horizontal plane, comparison
measurements/calculations.
b)
a)