fill direction of GRP laminates

Composite Structures 21 ( 1992 ) 101-106

I

Acoustic response in warp/fill direction of GRP laminates V. K. Srivastava, ~ O. M. Kannan b "Department of Mechanical Engineering, bSchool of Materials Science and Technology, Institute of Technology, B.H.U. Varanasi221 005, India

T. C. Koshy Fibre Reinforced Plastic Division, Composites Group, Vikram Sarabhai Space Centre, Trivandrum, India The objective of the present work is to study the acousto-ultrasonic response in the warp and fill directions of glass fire reinforced plastic (GRP) laminate composites with the effect of gain, threshold voltage and to predict the relationship of stress wave factor with flexural strength and flexural modulus. The results reveal that a nonlinear (exponential) relationship exists between stress wave factor and flexural strength as well as flexural modulus.

INTRODUCTION Composite materials have potential apphcations ranging from aerospace to ordinary transport vehicles. It is extremely important to evaluate nondestructively the components or structures for their reliability in use. In the method o f nondestructive evaluation, the interaction between the defect and the microstructural environment surrounding that defect is also considered. Therefore, it is possible to predict the severity of the flaw and to make an assessment of the ability of the component or structure to perform a mission in the presence of the flaw. Acousto-ultrasonics (AU) is a newly developed nondestructive evaluation method that combines the features of both acoustic emission and ultrasonics. The acousto-ultrasonic measurement is used to quantify mechanical properties like strength and fracture resistance in terms of stress wave factor (SWF). The SWF will indicate regions where strain energy is likely to concentrate and result in crack nucleation and fracture; SWF mainly depends on the dampening effect of the material medium of ultrasonic pulses, such as microstructure, morphology, porosity, bond quality, cure state and microcracks.J,2 A good quality material dampens the stress waves to a lesser extent as compared to material which has grossly distributed defects. Higher values of SWF will indicate the regions where the strength is greater. In the case of unidirectional

GRP composite the stress wave can easily transmit along the direction of the fibre due to the wave guide effect and hence higher values of SWF are obtained). 4 However, in the transverse direction the attenuation of stress wave will be greater due to the fibre-matrix interface; hence, lower values of SWF are obtained. SWF is very sensitive to the orientation of fibre and probe pressure, etc. 5-7 The main objective of the present paper is to describe the effect of gain, threshold voltage and inter-transducer separation on SWF along the warp and fill directions of GRP laminates, and also to predict the relationship of flexural modulus and flexural strength with the stress wave factor.

2 EXPERIMENTATION 2.1 Laminate preparation GRP laminates were moulded with a fibre volume fraction (37-135), using hand-lay-up technique. The laminates were prepared using 1543-style E-glass cloth and resin system LY-556; HT-972 hardener and resin were used in the weight ratio of 27:100; in each laminate, the number of layers of glass cloth was kept at 10 (Fig. 1). Also, the number of fibres along the fill direction was kept less than that in the warp direction. Curing of the laminate was carded out in an oven at 100°C for 4h.

101 Composite Structures 0263-8223/92/S05.00 © 1992 Elsevier Science Publishers Ltd, England. Printed in Great Britain

V. K. Srivastava, O. M. Kannan, T. C. Koshy

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2.2 Effect of auto-threshold, gain and intertransducer separation on SWF The scanning of the laminate was carried out using the grid method as shown in Fig. 2. The grid was formed with the help of a glass marker. The AU-instrument was set on the following modes before measurements of stress wave factor along the warp direction of laminates were taken:

Acoustic emission (AE) module Threshold = 0"8 V (auto) Gain = 60 dB Rate = ~ s Scale = 1 Count/events = counts

The frequency, duration knobs and gate width were kept totally anticlockwise and clockwise, respectively. The effect of gain, threshold voltage and inter-transducer separation on stress wave factor were recorded in the warp direction of GRP laminates. Similarly, these effects were measured along the fill direction of the laminate in the following modes of the AU-instrument: Threshold = 0"65 V (auto) Gain = 85 dB Scale = 10

2.3 Relation between SWF and flexurai properties 2.3.1 Measurements of SWF Specimens were prepared according to ASTMD790 and specimens were cut along the warp and fill directions to find out the flexural properties. Fibre specimens in both warp and fill directions were cut from each laminate. The geometry of the specimens was: width 25 mm, thickness 3 m m and

Pulse-module Trigger rate = 1 K Energy = E 2 Sweep = 12.5 V/division Trigger mode = pulse synchronous Pulser/burst = pulser

ATE A SPA ATE B

f-

/ LAMINATE

POLYESTER RELEASE FILM

Fig. 1.

The mould.

WARP A

B

C

D

E

F

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H

A FILL

1 2

2

3

3

d

L, 5 6

6 dh

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7

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7

Fig. 2. Laminate scanning.

B

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F

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H

103

Acoustic response in warp/fill direction of GRP laminates

span length 120 mm. The values of SWF were measured before the flexural test on the initial setting of the AU-instrument (Fig. 3), as discussed in Section 2.2. In each specimen, exactly at the centre, six readings were taken and the average values were taken for evaluation of the relation between SWF and flexural properties. 2. 3.2 Flexural test Flexural tests were conducted on these specimens to measure the flexural strength and flexural modulus as per test standard of ASTM-D790, method I. The universal testing machine used for this purpose was Tinius-Olsen 12.5-T strain rate controlled machine. The tests were conducted with a three-point loading system, utilizing centre loading on a simply-supported beam as shown in

Fig. 3.

Measurement of SWE

Fig. 4. A dial gauge was fixed at the centre of the specimen to record the deflection during tests. The deflections were noted down at different intervals of load until fracture of the specimen. The flexural strength and flexural modulus were calculated on the basis of recorded data of flexural tests.

3 RESULTS AND DISCUSSION In Fig. 5, the results indicate that the autothreshold voltage has wide advantages over the fixed threshold voltage. When the gain is increased, the auto-threshold voltage continues to increase while the fixed threshold voltage remains constant, due to increase of amplitude of the noise. Therefore, the number of waves crossing the fixed threshold voltage (counts) is increased due to noise. In order to avoid the counts wrongly made due to noise, the auto-threshold voltage increases by itself above the noise level? The statistical quality control (SQC) chart indicates the variations of stress wave factor within the limits as shown in Fig. 6. Seven columns were marked on the laminate and its SWF readings were noted. Each point in the SQC chart is the average of seven SWF readings. It has been found that the variations of SWF are within three levels and the control limits can be narrowed down to two levels. This indicates that the fabricated laminate possesses better properties and can be used for further study. Figure 7 illustrates the effect of threshold voltage on the stress wave factor. The results show

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Set-up of flexural test.

r

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Gain versus threshold when the probes are in contact with the specimen.

104

V. K. Srivastava, O. M. Kannan, T. C. Koshy

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t LCL=1457

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Fig. 6. Statistical quality control chart for laminate (UCL, upper control limit; CL, control limit; LCL, lower control limit).

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Energy = E2 Gain : 60 dB

Warp

of waves crossing the fixed threshold voltage is decreased (Fig. 9), and this gives a decrease in the value of SWF. In other words, the smaller the inter-transducer separation, the greater will be the value of SWE After recording and characterizing the effect of acoustic response on SWF of all laminates, load and deflection curves were plotted as shown in Fig. 10. The differences between these two curves show that the rate of deflection in the fill direction is greater than in the warp direction, due to the greater resin-rich area. The specimens in the warp direction failed in an irregular fashion, whereas in the fill direction they failed in a regular way because less number of fibres are in the fill direction, just to withstand the fibres, which are in the warp direction. How-

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Threshold versus wave factor.

4001 60

that the values of threshold voltage increase as the stress wave factor (SWF) decreases in steps in both warp and fill directions. The steps indicate that even though there is slight change in autothreshold voltage, SWF remains constant within the step width. SWF becomes virtually zero in both warp and fill directions at a particular value of threshold. This means that the threshold voltage level has been raised to peak amplitude of the pulse. Similarly, the gain versus stress wave factor curve (Fig. 8) shows that the SWF increases with gain in both warp and fill directions in a nonlinear way. At lower gain ( < 71 dB), the increase in SWF is almost constant for both directions. It shows that the value of SWF will be reproducible below 71 dB gain. -~ At higher gain (>71 dB), noises other than minimal noise created by deformation of materials will be picked up which cannot be avoided. Even if the distance between the pulser probe and sensor probe is increased, the number

I 64

Fig. 8.

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I 72 Gain (dB)

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Gain versus stress wave factor.

1600 --

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separation factor.

versus stress wave

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Acoustic response in warp/fill direction of GRP laminates 120

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Load versus deflection curves.

Fig. 11.

ever, the flexural strength and flexural modulus of the warp direction laminate specimens are greater than those of the fill direction, as shown in Figs 11 and 12. The results show that the stress wave factor increases with increase of flexural properties. Also, the values of SWF in the warp direction are higher than in the fill direction. The reason for higher SWF readings for the warp direction is that more acousto-ultrasonic waves run parallel to the fibre directions. These results are also in agreement with those of Vary and Lark) Hence, the stress wave factor based on oscillation counting is a good way of measuring the flexural strength and flexural modulus in the warp and fill directions of laminate composites.

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14 18 22 26 30 Flexural strength ( kg mr~ 2)

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Stress wave factor versus flexural strength.

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Stress wave factor versus flexural modulus.

4 CONCLUSIONS

ACKNOWLEDGEMENT

Based on the experimental results, the following conclusions can be drawn:

The authors are grateful to the Department of Mechanical Engineering and School of Materials Science and Technology at the Institute of Technology, B.H.U., and also to the Composites Group, Vikram Sarabhai Space Research Centre, Trivandrum, India, for providing research facilities.

(i) Stress wave factor decreases in steps with varying step width, as the value of autothreshold voltage is increased and it becomes virtually zero when the fixed threshold voltage crosses the peak amplitude of the ultrasonic pulse. (ii) More threshold voltage is required to transfer the energy along the warp direction than the fill direction; SWF may be considered to be free of extraneous noise and reproducibility is enhanced by working at less than 71 dB gain. (iii) Finally, the flexural strength and flexural modulus along the warp and fill directions of GRP laminate can be predicted by stress wave factor.

REFERENCES 1. Vary, A., Ultrasonic measurement of material properties. In Research Techniques in Nondestructive Testing IV,, ed. R. S. Sharp. Academic Press, London, 1980, p. 159-65. 2. Duke, J. C., Jr, Nondestructive evaluation of composite materials: a philosophy and approach to processing. In

Composite Materials Quality Assurance and Processing, ASTM-STP 797, ed. C. E. Browning. American Society for Testing and Materials, Philadelphia, PA, 1983, p. 75.

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V. K. Srivastava, O. M. Kannan, T. C Koshy

3. Vary, A. & Lark, R. F., Correlation of fibre composite tensile strength with the ultrasonic stress wave factor. J.

Testing &Evaluation, 7(4) ( 1979) 185. 4. Srivastava, V. K. & Prakash, R., Prediction of material property parameter of FRP composites using ultrasonic and aeousto-ultrasonic techniques. Composite Structures, 8(1987) 311. 5. Bhatt, M. & Hogg, P. J., Test conditions in stress wave

factor measurements for fibre reinforced composites and laminates. NDTInt. 21 (1988) 3. 6. Vary, A., Acousto-ultrasonic characterisation of FRP. Materials Evaluation, 40 (1982) 650. 7. Duke, J. C., Jr, Henneke, E. G., II, Kiernan, M. T. & Gross Kopf, P. P., A study of the stress wave factor technique for evaluation of composite materials. NASA contractor report 4195, NAG3-172, Jan. 1989, p. 1.