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SOFTWARE SOLUTIONS

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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)

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SOFTWARE SOLUTIONS

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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)