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Materials Science and Engineering A 412 (2005) 9396

Measuring elastic properties and anisotropy of microstructural units oflaminate composite materials by microacoustical technique

Yu.S. Petronyuka,, V.M. Levin a, Songping Liu b, Qianlin Zhang c

a Laboratory of Acoustic Microscopy, Institute of Biochemical Physics of Russian Academy of Sciences,

4 Kosygin St., 119991 Moscow, Russian FederationbNDT&E Center for Composites, Beijing Aeronautical Manufacturing Technology Research Institute,

P.O. Box 863, Beijing 100024, Peoples Republic of Chinac School of Information Science & Engineering, The Graduate School of CAS, 100039, 19 Yu Quan Road, Beijing, China

Abstract

The paper is devoted to application of focused ultrasonic beams for measuring bulk elastic properties in advanced fiber composite materials, such

as carbon fiber-reinforced composite (CFRC) laminates. Long-focus convergent beam of 50 MHz ultrasound frequencies provides the exceptional

means of measuring elastic propertiesof microstructural unitsin compositematerials with intricatemicrostructure as wellas theirintegral properties.

The microacoustical measurements reveal high anisotropy of CFR-laminate layers: sonic velocity across plies was found to be equal 3.1 km/s; in

ply plane across fiber packing7.0 km/s and along fibers9.09.8 km/s.

2005 Elsevier B.V. All rights reserved.

Keywords: Fiber-reinforced composites; Laminate composite acoustic microscopy; Ultrasonic measuring

1. Introduction

Reinforced composites form a wide class of materials thatdiffer in componentsmaterials of matrix and reinforcing ele-

ments, as well as in principles of spatial arrangement of rein-

forcing elements. Reinforced composites find ever-widening

application in advanced technologies, especially as critical con-

struction elements due to combination of unique elastic and

strength properties with small weight, high corrosion resistance

and outstanding heat conductivity characteristics [1]. A wide

set of research methods is in use to study structure and proper-

ties of composite materials. Ultrasonic methods are of special

interest because of their non-destructive character. The meth-

ods are highly informative and applicable as for measuring

elastic properties so for bulk visualization of internal struc-

ture and studying defects in the body of reinforced composites

[25].

Traditionally, to measure sonic velocities and elastic modules

of reinforced composites the relatively low-frequency ultrasonic

techniques (115 MHz) is widespread[2,3]. Such experiments

provide studying integral values of elastic characteristics since

Corresponding author.

E-mail address: [email protected] (Yu.S. Petronyuk).

the ultrasonic wavelength substantially exceeds sizes of struc-

tural elements. The reinforced composites are treated by sonic

waves as a continuous medium. Low-frequency ultrasonic char-acterization does not enable to get information on distribution of

the elements over the composite medium, their perfection and

properties. Acoustic microscopy may be applied to study local

elastic properties of reinforced composites with microscopically

ordering structure and their individual components, to investi-

gate character of elastic parameter distribution over the material

bulk. These data can be the base for following design of com-

posite materials with prescribed properties.

In the paper, results of microacoustical measuring for carbon

fiber-reinforced laminates composites (CFRC) are presented.

The CFR composites are manufactured as laminate packages

of ordered carbon fiber layers (prepreg plies) embedded into a

polymer binder (epoxy, bismeleimide or other types of poly-

mer resin)[1]. Thickness of fiber layers in CFRC structure is

100150m. The layers usually are arranged as unidirectional

or cross-ply stacks with or without the resin layers between

them. Short pulse of focused ultrasound (50 MHz) is sensitive

to the interlayer boundaries of such a composite structure. Due

to big difference in the pure fiber and matrix elastic property

it becomes possible to observe obviously the interlayer reflec-

tions, to measure thickness of the binding (matrix) layers and

to determine binder distribution in the matter. Microacoustical

0921-5093/$ see front matter 2005 Elsevier B.V. All rights reserved.

doi:10.1016/j.msea.2005.08.038


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94 Yu.S. Petronyuk et al. / Materials Science and Engineering A 412 (2005) 9396

technique admits of measuring elastic anisotropy in individual

layers of CFR composites.

2. Experimental method and facilities

All experimental work has been performed with the wide-

field pulse acoustic microscope (WFPAM) designed in Labora-

tory of Acoustic Microscopy, Institute of Biochemical Physics,RAS. The microscope employs short pulses (11.5 period of

oscillations) at frequencies within the 25100 MHz range. The

digital output of the microscope is connected with a computer to

display echo signal oscillograms or acoustic images, produced

by one- (B-scans) or two-coordinate (C-scans) mechanical scan-

ning of the acoustical objective. A set of acoustical objectives

with diverse operation frequencies and aperture angles lets us to

implement diverse regimes of measuring and visualization with

the microscope. In our experiments the low aperture (11 of

half-angular aperture) long-focused probe ultrasonic beam have

been in use for the bulk elastic property measurements. The

experimental setup has provided 60m resolution of measure-ments and 2.5 mm depth penetration at the operation ultrasonic

frequency of 50 MHz.

The ultra-short pulse of focused ultrasound (duration 40 ns)

penetrates from the coupling liquid (pure water) into a plane-

parallel specimen as convergent beams of longitudinal (L) and

transverse (T) waves (Fig. 1a). The pulse is reflected from the

specimen face and bottom as well as from structural elements

within the specimen body. Reflected echoes are separated in

time. Typical echo pattern (Fig. 1b) involves the reference signal

F (reflection from the specimen face), the L, T and LT sig-

nals (reflected from the specimen bottom). The signals L and

T are caused by round-trip of longitudinal and transverse waves

through the specimen; the signal LT results from mode conver-sion (LT) while wave reflecting at the specimen back side.

Time intervals L, LT and T between F and L, LT, T signals

are used to find the longitudinal and transverse sound velocities

(dis the specimen thickness):

cL =2d

L; (1)

cT =2d

T; (2)

cT =d

LT 0.5T. (3)

3. Specimens

Specimens were prepared in Beijing Aeronautical Manufac-

turing Technology Research Institute. The style of the fibers is

T300/3k, bismeleimide resin QY8911 has been used as a poly-

mer binder. The microacoustical technique has been employed

to study elastic properties of resin binder (resin plate 3.18 mm

thick), a single prepreg layer and individual layers in a cross-

ply CFR-laminate composite. Two samples of a unidirectional

fiber prepreg have been used to measure sonic velocity across

the prepreg ply. One of them was the free prepreg ply 120 m

thick. The other was the same ply embedded into resin; full

thickness of the composition was 380m. Measuring sonic

velocities of individual prepreg layers in a ply stacking has been

performed with the sample of CFRC laminates composed of

15 layers 200300m thick. The individual layer consists of

two or three prepreg plies with the same orientation of carbonfibers 120m thick each. Neighbour layers differ in fiber orien-

tation by 90 (combined cross-ply packing of CFR laminates).

We arranged ultrasonic measurements through the specimen in

two directionsacross and inside prepreg plies. The measuring

inside the composite plies provides two opportunitiesto get

value of sonic velocity along fiber bundles and across them in

the plane of their parallel arrangement.

4. Results and discussions

Summary of experimental results is presented in Table 1.

Individual specimens of the resin, free single prepreg layer andsingle prepreg layer embedded into resin have been studied to

get information about elastic properties of the main components

of CFR-laminate matter.

The echo pattern for the solidified bismeleimide plate

(d= 3.18 mm) involves the F, L and weak LT signals (Fig. 2a).

The measured values of elastic wave velocities in the poly-

mer binder are: cL= 2.71 km/s for longitudinal waves and

cT= 1.67 km/s for shear waves. Together with the measured

magnitude of density ( = 1.24 g/cm3), the data resulted in find-

ing values of elastic module of bismaleimide resin (bulk mod-

Fig. 1. Echo-pulse technique: (a) principal diagram of reflections and (b) echo pattern of reflected pulses for isotropic plate: (F) echo from the specimen face, (L)

echo signal from the specimen bottom formed by longitudinal waves, (T) signal resulted from reflection of transverse waves from the bottom, (LT) signal received

from the bottom after mode conversion and (2L) signal from the twice bottom reflection.


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Yu.S. Petronyuk et al. / Materials Science and Engineering A 412 (2005) 9396 95

Fig. 2. Echo pattern of microacoustical measuring for individual elements of CFR composite structure: (a) bismeleimide resin matrix, (b) free prepreg ply and (c)

sandwich structure of single prepreg ply in matrix;Ltime of flight; Fsurface echo; Lbottom echo formed by longitudinal waves; Dinternal defects;PFand

PBechoes from surface and bottom of ply within the sandwich system.

Table 1

Longitudinal elastic wave velocitycL in CFR-laminate composite and its indi-

vidual structural elements

Sample Direction of measuring Longitudinal elastic

wave velocitycL(km/s)

Polymer binder Any direction 2.71

Free layer of fiber threads Across composite ply 3.20

Sandwich system (fiber

ply+ resin)

Across composite ply 3.07

Cross-ply CFR-laminate

composite

Across plies 3.10

Inside composite ply

Across fiber thread 6.927.08

Along fiber thread 9.009.80

Accuracy of measurement, 2.5%.

ulus K= 6.68 GPa, shear modulusG = 3.46 GPa or for compo-nent of the elasticity matrices: C11= 11.29 GPa, C12= 4.37 GPa,

C44= 3.46 GPa and Poisson ratio = 0.28).

By means of microacoustical technique the unique measure-

ments for basic low-dimensional components of carbon fiber-

reinforced systemfree prepreg plies, have been performed.

The echo pattern for a single free ply contains the F, L signals

and 2L, 3L, etc. echoes formed by repeatedly reflected longi-

tudinal waves (Fig. 2b). Across the reinforced ply of 120 m

thick the sonic velocity value cL equal to 3.2 km/s has been

obtained. Difference between sonic velocities of binding resin

matrix and reinforcing prepreg ply appears essential. This dis-

tinction is enough to employ measuring velocity of elastic waves

to estimate resin content in the body of composite. The feasibil-

ity of estimations has been investigated with a single prepreg ply

(120130m thick) embedded into resin tape. Total thicknessof the sandwich system was 380m; measured value of lon-

gitudinal wave velocitycL= 3.07 km/s (Fig. 2c) is intermediate

between values of two pure structural componentsthe resin

and a free prepreg ply. Increasing resin content in the sandwich

systemcomparedto thefree prepreg plycauses thereduced value

of cL. So microacoustical measurements can be employed as

non-destructive method to estimate local resin content in CFRC

laminates to find resin distribution over the specimen bulk. A

tested scale for resin content in the composite structure can be

obtained by means sonic velocity measuring.

The main idea of the studies has been to elucidate poten-

tialities of the microacoustical technique for measuring sonic

velocities in individual strata of an ordered system. The tech-

nique has been applied to a CFR-laminate specimen with cross-

ply stacking of prepreg layers to get data on longitudinal wave

velocities of a single layer along different directions with respect

to the fiber orientation (Fig. 3). Longitudinal elastic wave veloc-

ity that has been measured across individual prepreg ply should

be equivalent to the value obtained across the whole laminate

specimen. Despite the cross-ply package of neighbour layers the

fiber orientation in all layers of the stacking is the same, respec-

tively, to the elastic wave polarization and direction of beam

Fig. 3. Sketch of CFR-laminate sample orientation to measure sonic velocity: (a) across composite plies and (b) inside composite plies. In the last case, scanning of

an acoustic lens allows to measure the longitudinal elastic wave velocity cL along fiber package as well as across it (by shifting the lens position).


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96 Yu.S. Petronyuk et al. / Materials Science and Engineering A 412 (2005) 9396

Fig.4. Study ofthe0 and90 layers distributionin cross-ply composite structure by means of microacoustical technique: (a) echopattern nearby the strata boundary:

L1signal from bottom of a 0-oriented layer and L2echo from bottom of a 90

-oriented layer; (b) acoustic imaging of the sample bottom, acoustical contrast is

results in difference in elastic property for neighbour layers.

propagation (Fig. 3a). The value of sonic velocity cL= 3.10 km/s

obtained in the case of beam propagation across composite plies

is close to the value that was as a result of measuring across a

ply, embedded into the resin.

To measure elastic wave velocities inside individual compos-

ite layers the thin plates of cross-ply CFR laminate (2.11mm)

have been cut off as shown inFig. 3b. The focal waist length(lF 23 mm) was compared with a thickness of the cut sample;

the diameterdF of the focal spot (dF 80m) was sufficiently

smaller than the width (200300m) of an individual stratum

in the ply stack. So the probe beam was able to reach the bottom

of the cut sample being inside the individual composite stra-

tum. In cut sample the neighbour plies have a different fiber

orientationnormal to the sample side surface or parallel to it.

Different position of the focal spot on the surface provides mea-

suring sonic velocity along andacross fibers within an individual

stratum of the stack. Echo patterns obtained for neighbour lay-

ers demonstrate markedly different values of delay times and,

respectively, different magnitudes of sonic velocities along car-

bon fibers (cL= 9.69.8 km/s) and across them in the plane of

fiber plies (cL= 6.937.05 km/s). The longitudinal wave velocity

for along fiber orientation is significantly higher than the veloc-

ity for across fiber orientation in the same ply. Both of these

values are essentially larger than sonic velocity across plies.

Elastic and acoustic properties of particular components of

CFRC laminatespolymer binder, single plies and their combi-

nations; are of special interest for investigating mechanisms of

acoustic contrast in CFRC imaging, for developing principles

for acoustic images (C- and B-scans) interpretation in CFRC

laminates and for interpretation of results of measuring sonic

velocities and elastic properties of such materials. The received

data are in good agreement with results of measuring integralvalues of sonic velocities and elastic module for different orien-

tations of unidirectional CFRC laminate specimens performed

by low-frequency ultrasonic methods (see, for instance, papers

[2,3]).

Different delay times L and, respectively, different posi-

tions of echo pulses reflected from specimen backside can be

employed to display distribution of layers with distinct orien-

tation of fiber packing over the specimen body. In Fig. 4, we

present the acoustic image (C-scan) of the specimen bottom

for different positions of an electronic gate. The electronic gate

allows picking from the received echoes only part, which can be

taken for visualizing the level of the signal in depth (Fig. 4a).

White colour in acoustical images corresponds to the high level

of reflection pulse.The C-scanin Fig. 4b has been done when the

electronic gate involves only the distant pulse L1resulted from

reflection at the bottom of layers with fiber orientation parallelto the sample surface.

5. Conclusion

Acoustic microscopy is a powerful non-destructive method

for quantitative characterization of CFR-laminate composites.

It provides measuring the local elastic properties and visualiza-

tion of their distribution over the material body. The method

can be applied also to estimate topological characteristics of

laminatesthickness of layers, etc. Penetrating ability of high

frequency ultrasound make it possible to apply of this technique

to fairly thick (28 mm) specimens of CFRC laminates. The

work demonstrates the acoustic microscopy, besides of poten-

tialities of imaging internal microstructure of non-transparent

advanced fiber materials, gives an effective technique for quan-

titative characterization of ordered composites.

Acknowledgements

The work has been supported by Grant OXHM PAH No.

04-PAH-07 Development of methods and facilities for microa-

coustical investigation of structure and properties of advanced

materials of Russian Academy of Sciences and by the program

KJCXZ-N12 of Chinese Academy of Sciences.

References

[1] D.D. Edie, J.J. McHugh, High performance carbon fibers, in: T.D.

Burchell (Ed.), Carbon Materials for Advanced Technologies, Pergamon,

1999, pp. 183221.

[2] Y.C. Chu, S.I. Rokhlin, J. Acoust. Soc. Am. 95 (1994) 3204

3212.

[3] S.I. Rokhlin, W. Huang, Y.C. Chu, Ultrasonics 33 (1995) 351364.

[4] F. Aymerich, S. Meili, Composites: Part B 31 (2000) 16.

[5] L. Wang, Ultrasonics 37 (1999) 283289.

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