A FRACTURE STUDY IN GLASS FIBER COMPOSITES IN DYNAMIC TENSION BY MECHANICAL LUMINESCENCE METHOD

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A FRACTURE STUDY IN GLASS FIBER

COMPOSITES IN DYNAMIC TENSION BY

MECHANICAL LUMINESCENCE METHOD

V. Tamuzh, U. Krauja, P. Kalninsh

To cite this version:

V. Tamuzh, U. Krauja, P. Kalninsh. A FRACTURE STUDY IN GLASS FIBER COMPOSITES IN DYNAMIC TENSION BY MECHANICAL LUMINESCENCE METHOD. Journal de Physique Colloques, 1985, 46 (C5), pp.C5-661-C5-666. �10.1051/jphyscol:1985586�. �jpa-00224821�

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A FRACTURE STUDY IN GLASS FIBER COMPOSITES IN DYNAMIC TENSION BY MECHANICAL LUMINESCENCE METHOD

V.P. Tamuzh, U.E. Krauja and P.P. Kalninsh

I n s t i t u t e o f PoZyrner Mechanics, L a t v i a n SSR Academy of S c i e n c e s , 23, A i z k r a u k Z e s s t r e e t , R i g a 226006, L a t v i a n SSR, U.S.S.R.

Resume

-

Le processus de destruction de materiaux composites

renforces de fibres de verre soumis 2 une extension dynamique

est analyse. Le caractitre Btage de la destruction du materiau est montrg. L'apparition des premisres destructions dans le do- maine du comportement lineaire du milieu est constatee. Le debut de la destruction des mat6riaux composites soumis 2 une extension dynamique depend du tissage des fibres et de la vitesse de deformation des echantillons.

Abstract

-

Some dependencies of the fracture development in glasa fiber composites under dynamic tension have been estab- lished,

A

stageful character of the fracture of an angle-ply glass fiber composite has been shown, The onset of the very initial fracture events within the range of the linear material behavior has been detected. The fracture development in glass fiber composites under dynamic tension depends both on the reinforcement layup and strain rate of specimens,

I

-

INTRODUCTION

It was found in early 60ies that failure of polymers ie accompanied by the generation of some photoemission /I,2/ which was later term- ed as a mechanical luminescence (NIL). Application of mechanoluminescence in the study of composites has opened new ways for the exami- nation of failure regularities /3,4/. Since the glow c9n be detected without lag and light pulse count rate reaches 3.10 pulses/sec, the mechanoluminescence method can also be used for the investigation of the fracture mechanisms in composites under dynamic loading, I1

-

EXPERIMENTAL PROCEDURE

The experimental study of angle-ply glass fiber specimens in high- speed uniaxial tension was carried out in a specially darkened rot- ational impact testing machine. The angles of reinforcement layup in the composite were as follows: + w O ,

-wO,

90'9 -30°9 +30° (periodically repeated) along the

Y

axis (Fig.

I

)

.

Thickness of a lamina was 0.19

m ,

thickness in the gage section of specimens cut along the X axis was

2

mm, along the Y axis

-

8 mm. Specimen8 of variable cross section were cut from the sheets. In tot- al,

35

such specimens were tested. In the course of the experiment

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JOURNAL DE PHYSIQUE

the applied force as a function of

,

X

time was measured by a pie tric dynamometer and the mechano- zo elec- luminescence as a function of time by a photoelectronic multiplier FEU-79 at a fixed speed of movable grips 0.5 m/s, I m/s, 2 m/s. The

4 I

signal from the dynamometers was

applied to the input of high-volt- age (1011

$2

) preamplifier and that of the display 4102 /4/. The -

Y

output of FEU-'753 was directly con-

nected to another display 4102 through a stabilizing network. The dieplags were turned on quiescent

i conditions and then put into opera-

tion br external sync pulse. which had been generated-in %he loading device shortly before the applica- Fig. I

-

A reinforcement layup tion of a load to the specimen.

in a glass fiber composite The pulses in digital form through

1024 points were stored in the memory of displays and controlled visually on the screen of a cathode- ray tube incorporated in the device, Further the signal was trans- mitted to a communication line and stored in the memory of the elec-

tronic computer,

I11

-

ANALYSIS OF RESULTS

The mechanism of mechanoluminescence excitation in high-speed tension of a glass fiber composite along the X axis indicates that fracture of the material is a stageful process (Fig. 2).

Fig. 2

-

ML intensity N (2) and tensile stress 6

(I)

vereue time for glass fiber composite specimens cut along the X axis at impact velocity of 0.5 m/sec.

After the load application at about the 240th l(r sec mechanoluminescence is generated which can be associated with rupture in laminae

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tension have lower breaking stress. Upon further tension of the material powerful peaks in the mechanoluminescence activity appear which indicate rupture of the laminae laid in the loading directi0n.A sim- ilar character of fracture of glass fiber composite specimens under static tension was observed by

a

mechanoluminescence method in

/ 3 , 5 / .

As can be seen, the high pulse count rate in mechanoluminescence and persistence of operation of mechanoluminescence method make it posa- ible to detect the fracture development in composites when loading duration is being measured in microseconds. Rupture o* laminae laid in the direction of tension proceeds very fast. It can be seen from Fig. 2 that the laminae have completely failed in about 5 Thi a has been confirmed by jumps in mechanoluminescence intens&:BC60th the number of mechanoluminescence jumps and peaks in P

-

C curve (where P is force,

t

is time) indicated by the piezoelectric trans ducer coincides with the number of failed laminae laid in the loading direction. Therefore, in the case of the above-mentioned reinforcement layup one can judge about rupture of laminae laid in the loading direction from mechanoluminescence bursts, Attention must also be paid to precise coincidence of mechanoluminescence jumps and peaks in P

-

8

curve

(Fig. 2) which can be attributed to high-speed of response of the piezodynamometer. The above described shape of P

-

Z

curve can only be observed under certain conditions (in our case, at strain rate of 0.5 m/s). Upon changing of the reinforcement layup or loading direction relative to the type of reinforcement and strain rate the modes of fracture in laminae overlap with time (Fig.

3 )

.

Fig.

3 -

NIL intensity (2) and stress

6 (I)

versus time for glass fiber composite specimens cut along the Y axis at impact velocity of

I

m/sec.

In static testing of specimens (Fig. 4) when the force is measured by a mechanical dynamometer with strain gages individual fracture events in laminae have not been observed.

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Fig, 4

-

MI, intensity

N

(in arbitrarg units)/5/ (2) and stress

6

(I) versus time for glass fiber composite specimens cut along the X axis under static loading at the rate of 2 mm/ min.

Mechanoluxinescence reflects instantaneously the fracture development in a composite through light quanta over a wide range of loading rates.

In

the case $f $ension of specimens along the

Y

axis with laminae laid at angle -30 to the direction of tension a stageful change in mechanoluminescence has also been observed. The first mechanoluminescence bursts were registered at about the 300th fisec and at about 6OOthpsec more intensive glow appeared (see Fig.

3).

At the 1050th

fk

sec very high bursts of mechanoluminescence wer$ epited which corresponded to rupture of laminae laid at angles

-30

to the direction of tension. Various stagee in mechanoluminescence intensity were observed in all tests of the specimens at loading rates of 0.5 and

I

m/sec. At impact velocity of 2 m/sec the various modes of fracture in a laminated composite merged together and only a gradual increase in mechanoluminescence intensity could be observed (Fig.

5 )

.

Fig.

5 -

I L intensity N (2) and stress 6

(I)

versus time for glass fiber composite epecimens cut along the

Y

axis at impact velocity of 2 m/sec.

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specimens stretched along the

X

axis. This can be attributed to the fact that a large fracture surface is formed in tension of laminae laid in the loading direction which, in turn, gives rise to high en- ergy photon flux. At high strain rates (just like in static tests) two stages can be diatinguiehed:

I) the process before the occurrence of damage;

2) the process associated with damage sequence and fracture development,

It is possible to ascertain the boundaries of the firat stage from P

-

t

curve only in rare cases, namely, when after elastic behavior of the material the stage of damage accumulation changes the shape o f P

-

t

curve. It has been stressed in /6/ that even in the straight section of

P

-

t

curve microcracks can be formed, This has been confirmed by our experiments when in a practically linear section of

P

-

t

curve the mechanoluminescence was excited indicat- ing the onset of damages (see Figs. 2,3). That is why the mechanoluminescence indicates exactly the onset of damages in laminae of a laminated composite both in static loading and high-speed tension, This permits one to predict the onset of damages in the material, which is of importance in practical applications of structural composites. The study of th gene~atiof of mechanoluminescence as a function of loading rate (

f

=I0 -10- I/ssc; Pig. 6) is of great interest.

Fig. 6

-

Dependence of stress level6Y/6& (

6~

is the stress oorres- pondin

to

the appearance of glow, 6~ is the breaking .treesf at which the mechanoluminescence is excited on the loading rate of C/ for specimens stretched along the

X

(I) and Y (2) axes.

It can be seen that both in. etatic and impact loading

( v

= 2 m/eec) the stress level of initial fracture

fu/+

differs slightly for specimens of both reinforcement lagups.

It

can also be assumed that the

state of stress in a specimen differ6 slightly in etatic and high- epeed tension ( V = 2 m/sec).

I. A

methodology for the investigation of fracture development in composites under dynamic loading in real time scale has been

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C5-666 JOURNAL DE PHYSIQUE

evolved by employing a mechanoluminescence method.

2, Some peculiarities of.fracture 3rocess in glass fiber composites in dynamic tension

( 6

= 20+IO I/sec) have been stated, The onset of initial fracture events in the section of the linear material behavior ( in accordance with P

-

t curve) has been ascert- ained ,

3.

!Phe occurrence of damages in a composite under dynamic tension depends both on the reinforcement layup and strain rate of the specimens.

/I/ Butyagin, P,Yu,, Erofeeev, V.S., Musaelyan, I.M., Pstrikeev, G,A,, Streletskii, A.N, and Shulyak, A.D., Vysdkrm~l, Soed. Ser.A (1970) 290

/2/

**h-*,**

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13 CQ

4

L. and Elsts, M.A,, Pribori, tekhnika eksperimenta

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(1981)

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,

(1982)

537

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