ACTA MECHANICA SINICA, Vol.3, No.3, August, 1987 Science Press, Beijing, China Allerton Press, INC.New York.U.S.A.

ISSPO Oti~-F/lll

IN-PLANE DISPLACEMENT MEASUREMENT OF COMPOSITE MATERIAL BY OBJECTIVE SPECKLE

Mao Tienxiang, H~m Jinhu, Li Chunxiu, Zou Rongda

(Institute of Mechanics, Academia Sinica)

ABSTRACT: The objective speckle was used to measure the in-plane displacement around the hole in a coupon specimen of composite material under tensile loading. Mirror transplantation method was adopted to ensure the high reflectivity of specimen surface and to obtain high quality double exposure specklegram. The adjustable spatial frequency of the whole-field pattern of the displacement made it possible to measure a broad range of strain, from elastic to plastic. The results obtained by objective speckle agree well with those by Moire method.

KEY WORDS: objective speckle, composite material, displacement measurement, Moire method.

I. INTRODUCTION

It is necessary to measure the in-plane strain distribution around the hole or at the crack tip,

when fracture and damage of composite materials are concerned. Optical method, such as Moire or

speckle method, provides an efficient means to measure the whole-field displacement distribution.

Moire method can be used to obtain the displacement field and its change with load, but its

sensitivity is limited by the pitch of Moire grids used in the investigation. In the last two decades,

speckle technique has been extensively used in displacement measurement for its advantage of

adjustable sensitivity. The position of filter aperture is usually chosen to obtain appropriate density

of fringes, which fits both elastic and plastic strain measurement. Besides, it has the advantages of

nondestruction, remote control and whole-field information, etc.

In this paper, the in-plane displacement has been measured and analysed by using objective

laser speckle. Mirror transplantation was used to increase the reflectivity of the surface of opaque

specimen. Fine furrows running along the direction perpendicular to the loading were carefully

created with abrasive paper to produce more information on the specklegram.

Laser beam was used to obtain the distribution of the speckle pattern (Fig. 1). The rigid

displacement of the measured surface with respect to the recording system has been compensated

by clamping a photographic plate to the specimen urder study at one point. In objective speckle-

gram, the spatial frequency spectrum is much broader than that of subjective ones, because it is

recorded directly on photosensitive materials without using a lens. Therefore the whole-field

information can be easily obtained and sensitivities can be adjusted within a large range.

The displacement and strain field of glass fibre reinforced polyester specimen with a central

hole has been investigated by using the method above. The results obtained agree well with those

by Moire method.

H. TESTING PROCEDURE

1. Double exposure recording The recording setup is shown in Fig.1. A photographic plate is clamped to one point of the

Received 8 Jan. 1966.

292 ACTA MECHANICA SINICA 1987

specimen under study. The objective was illuminated by collimated light with propagation direc-

tion parallel with the plate normal. The photographic plate used in this study is holographic with

the resolution high enough to record the fine detail of the speckle pattern. Since the specimen is

opaque, illumination takes place through the plate, and the useful signal should be almost the

same, a uniform blackening of the plate disturbs or even impedes the data extraction. A high

reflectance specimen surface can be achieved by mirror transplantation method described in detail

later. The specklegram is obtained by making two exposures and recording the speckle pattern of

specimen surface before and after the specimen deformed on one plate.

Clamping

ilium iti:~i~ e ~ ' ~

Hologram Emulsion

Specimen

Fig. 1 Setup for recording

Double exposure hologram p

Coll nation 0

Emulsion / I . .~ Lens '1 ~ / ~

T•Soeck i e

Vo Correlation L Image

Frequency /Plan mask

Fig. 2 Setup for reconstruction

2. Reconstruction of the speeklegrtun No fringe can be seen on the specklegram since various intensity gratings are all superposed.

The whole-field fringe pattern with a certain sensitivity can be unveiled, provided the specklegram

is put in a Fourier transform system shown in Fig.2, because it can extract necessary information

with an assigned spatial frequency. Different whole-field fringe pattern is reconstructed by

selecting the different position of the aperture in the frequency plan mask. If the off-axis angle at

point P is 0, the sensitivity is expressed as follows

sin0 _ Vo/f= V o 2 2 2 s

where 2 =6328 ]k is the wavelength of the laser beam;

0 is the off-axis angle shown in Fig.2;

V 0 is the height of point P measured from laser beam axis;

f is the focal length of the lens.

3. Treatment of specimen surface

To enhance the reflectivity of the opaque specimen surface is very important. In the case of

glass fibre reinforced composite specimen, the mirror transplantation method was used to gain a

high reflectance surface. The procedure is a~ follows: At first, a very thin adhesive layer ,ff epoxy

was put onto the specimen surface. Then a piece of glass mirror was put onto the adhesive layer

with the silver coated side against it. A certain pressure provided by a weight was kept on the

specimen while curing. As a result the silver layer as a mirror surface was transplanted to the

specimen surface. Finally, fine score,~ perpendicular to the loading direction were "created" on the mirror surface with fine abrasive paper.

Vol. 3, No. 3 Mao T.H. et al: In-plane Displacement Measurement of Composite Material 293

4. Spoeimen, laser power and testing machine The specimen used in this study is glass fibre reinforced polyester coupon specimen. (Fig.3)

The initial Young's modulus along the load direction is 18.32 GN/M 2. The light source is He-Ne laser with 3my output and wavelength of 6328 /~.

Testing was conducted on a 100 KN MTS 810 Material Test System, with the aid of hydraulic grips. The machine was cGntrolled manually in order to carry out the measurement of specklegram.

Ill. TESTING RES10LTS The specimen was monitored with the optical arrangement shown in Fig.1 and the load

applied between exposures were 0.25 KN, 0.5 KN, 1 KN, 1.5 KN, 2.5 KN and 4 KN respectively, to obtain six specklegrams.

Five different positions were chosen when the specklegram was reconstructed in the setup shown in Fig.2, i.e. five different values of V o were selected, from which the whole-field fringe patterns for vertical displacement at the equivalent moir6 resolution of 248 line pairs/mm, 267 line

pairs/mm, 439 line pairs/mm, 575 line pairs/mm and 694 line pairs/mm were obtained respec- tively. To examine the specklegram obtained at the loads higher than 1.5KN, only the whole-field fringe pattern for vertical displacement, at an equivalent moire resolution of 267 line pairs / ram, was given. Similarly for specklegram taken at load of 1 KN, the whole-field fringe patterns for vertical displacement, at resolution of 267 line pairs / mm and 439 line pairs / ram, were given and SO o n .

P P

I _a I ~ 2 0 0 r a m - - - I

Fig.3 Specimen

0.4

.~" 0.3

0.2

0.I

" ,/ram' �9 439 l/mm

�9 575 I / ram

P=0 .5KN

j , t I I t . .1 I | t t ~ _ 1.0 1.5 2.0

X/a

Fig.4 Strain distribution along X-axis measured by

objective speckle ( P = 0.5KN)

r

Z_

09

4KN ,, Speckle m e thod

3 - - ~ Moire t~ method

2.5KN ~

1 . 5 K ~ ~ ~ - x . . . . x

3-25KNI , , , ~ i , ~ I C I 1.0 1.5 2,0

X / a

Fig.5 Strain distribution along X-axis

294 ACTA MECHANICA SINICA 1987

Fig.4 shows the strain distribution along X-axis (Y = 0), which was deduced from three

whole-field fringe patterns for vertical displacement at resolution of 267 line pairs / mm, 439 line

pairs / mm and 575 line pairs / mm, respectively, but all of them were from the same specklegram

taken at load of 0.5 KN between exposures. The results shown in Fig.4 indicate that for the same

specklegram, the deduced strain distributions along X-axis were coincident with each other,

although they were obtained from the whole-field fringe patterns at different resolution. The strain

distribution along the X-axis at different load levels measured by objective speckle is shown in

Fig.5. For compariso,n, the strain distribution at load of 4 KN measured by Moir4 method is also

Fig.6 575 l/mm 0.25KN Fig.7 267 l/ram 0,5KN Fig.8 4391/mm 0.5KN

Fig.9 5751/mm 0.5KN Fig.10 2671/rnm t.5KN

Fig.ll 2481/mm 4KN Fig.12 1001/ram 4KN

Vol. 3, No. 3 Mao T.H. et al: In-plane Displacement Measurement of Composite Material 295

shown in Fig.5, which agrees we]] with the resu]t obtained by objective speckle method F igs .6 - - ] ] disp]ay some who]e-fidd fringe patterns.

"['he whole-field fringe patterns shown in Figs.6 and 7 are similar, since the latler was got at twice toad but half resolution of the former. The patterns shown in F igs.7--9 were all obtained at load of 0.5 KN, but corresponding to different resolutions, respectively. Pattern in Fig.7 shows I wo black fringes at both sides of the hole. However pattertls it] Fig.8 and Fig.9 show three and four black fringes respectively. The fringe density of the pattern in Fig.10 is about three times higher

than the one in Fig.7 due to same order higher load. Finally, the' pattern in Fig.12 was obtained by

Moir6 method with resolution o'f 100 line pairs/mm, whereas the fringe density in Fig. t 1 is about

2.5 times higher than that in Fig.12, for the corresponding resolution of 248 line p urs 'mm.

All the results shown in this paper indicate that the objective speckle can provide an efficient

means for measuring the displacement and strain field of composite material specimen, especially

when the displacement is small at lower load level. The mirror transplantation method can provide

a mirror-like surface to composite specimen with good reflectivity. The carefully created furrows

along X-axis would increase the frequency of Fourier components and broaden the spectrum iu I,,.

direction, so that even more sensitive measurement for vertical displacement could be achieved.

REFERENCES [1] Tu, Meirong, ,'tcta Mechanica Sinica, 5, 5(1983), 515--531.

[2] Han, J.H., Research Report of Institute of Mechanics. Academia Sinica,

[3] Boone, P.M., Optical Engineering, 21, 3(1982), 407--410.

1984 (in Chinese)