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NON-DESTRUCTIVE TESTING OF FIBRE

REINFORCED COMPOSITES AND HONEYCOMB

STRUCTURES

Brian Stephen Wong, Chua Fong Ming RonNanyang Technological University

Singapore

Ow Wing Yoong

Defence Science Organisation

National Laboratories

Singapore

Tui Chen Guan

Republic of Singapore Airforce

ABSTRACT : This paper describes experiments conducted using various non-destructive testingtechniques on carbon and glass fibre reinforced composites (FRC) and honeycomb structures. The

advantages and disadvantages of each technique are described and all the techniques are compared. A

radiograph of an impact damage on an FRC using an enhancing fluid of high radiographic absorption

showed clearly the detailed delamination characteristics, this technique would be the best procedure for

characterisation of defects. However the defect must be open to the surface to allow penetration of theenhancing fluid which is also toxic. The radiation is also dangerous. Mechanical impedance tests proved to

be the most sensitive one sided test of delaminations between the skin and core of a honeycomb structure.

Through transmission ultrasonic tests can also had equivalent sensitivity with this procedure. A and

immersion C scan ultrasonic tests, the most widely used procedures, were conducted on on fibre

reinforced composites. It was found possible to detect a one mm defect 0.3 from the specimen surface up to

3 mm below the surface. C scan enabled reasonably accurate determination of defect size. Test

procedures for fibre reinforced composites have also been developed. Because immersion ultrasonic C

scan cannot be used in situ, the lock-in thermography procedure has been compared with it. The sensitivityof the thermography which was found to be slightly less, was still adequate to recommend its use over

ultrasonics because of its portability, remote inspection capability and speed of interrogation.

1. INTRODUCTION

Fibre reinforced composite materials are being more and more used in numerous products. These products

vary from consumer and sporting goods to aircraft and spacecraft components. In the latter components,

honeycomb structures are also widely used and these may be fabricated from polymeric or metallic

materials. Their increased use is due to that the fact that their properties can be tailored to satisfy a numberof structural functions such as stiffness, toughness and strength which can be provided where they are

needed. However until recently a deterrent to their use has been the concern that some difficult to detect

defects may be present in the materials. Defects may result from the raw product such as fibres, matrix and

prepregs, as well as due to poor bonding between the fibres and matrices and between individual lamina.

Defects may also occur from in-service use such as low velocity impacts in aircraft structures. This paperfocusses on the ability of non-destructive testing (NDT) techniques to be able to detect defects in

composites.

Table 1 summarises the capabilities of the major NDT techniques for defect detection. With reference to

this table Yes indicates that the defects can be detected with the NDT technique and Some indicates

lower sensitivity and less application than Yes. Fortunately the most serious defects such as

delaminations, impact damage, debonds and resin cracks can be detected using several techniques.


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Ultrasonics can be seen to be the powerful technique because of its sensitivity to all the defects shown.

However other techniques need to be considered because of others advantages they may have. For example

thermography is complete remote technique requiring no contact with the inspection surface and is also a

fast testing procedure.


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2. RADIOGRAPHY

Radiography is based on differential absorption of penetrating radiation by the specimen being inspected.

Defects or variations in the specimen composition will cause these absorption differences which can be

recorded on film. Because the polmer matrix and some fibres such as carbon have low absorption toradiation good contrast is difficult to obtain and low kilovoltage (or low energy) radiation must be used.

With radiography a better image of defects is produced than ultrasonics since visual two dimensional

images are produced and it is nearly always possible to determine what has produced the image.

To detect air-filled defects these need to have a dimension parallel to the radiation beam approximately

equal to or greater than two per cent of the adjacent surrounding material. Using conventional radiographytherefore delaminations are not detectable.

Defects such as voids are detectable if large enough and certain fibre such as glass which have a much

higher radiation absorption coefficient than the surrounding matrix are detectable. Therefore features such

as volume fraction and fibre allignment can be determined.

Much better contrast can be achieved by using radiographically opaque penetrants, for example

tetrabormoethane (TBE), which has a much higher absorption coefficient than the composite materials. The

defect, for example impact damage, must be open to the surface. This fluid is applied to the surface of the

specimen before radiography. It is allowed to penetrate into the damaged regions for about 30 minutesbefore the excess penetrant is removed with absorbent cloth. The theory is that the radiation passing

through the defect area is now absorbed much more than the surrounding areas and therefore the defectcontrast is considerably enhanced. The technique even operates effectively on the normally undetectable

delamination defects.

Fig. 1 shows impact delamination and crack damage of a composite using this technique. The great detail

and accurate delineation of the damage extremities can be clearly seen. In conclusion it can be deduced thatradiography is the best non-destructive testing technique for characterisation of defects.


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Fig. 1. A TBE Radiograph Of Impact Damage Of A Woven Glass Fibre Reinforced Composite

3 MECHANICAL IMPEDANCE INSPECTION

The mechanical impedance instrument utilizes an electronic transmitter and receiver, which acquires data

from the material under test, analyses the data and presents it in graphic form on a cathode ray tube. To

perform these functions the instrument uses a probe containing two piezo-electric crystals, one of which

converts electrical signals into vibrations and, by the reverse procedure, the modified vibrations on the

other crystal are converted back into electrical signals.

The probe is utilized to apply a localized forced oscillation to the area under investigation. A defective areacan be recognized from the different response it produces when compared with a non-defective area. One

difference would be a reduced resonant frequency due to the reduced stiffness of the defective area. Theseresponses can be deduced from the frequency spectrum displays produced by the instrument.

The mechanical impedance technique is most sensitive to defects which are parallel to the structures

surface e.g. skin-core disbonds in honeycomb structures.

The ability of the technique to detect near surface defects in various specimens has been described (Adams

and Cawley, 1988).

In a structure, the layer of material above a disbond or delamination may be regarded as a plate that is being

restrained around its edges. If the plate is being excited, it can resonate with the first mode being the


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membrane resonance. From the vibration of a plate the first mode of the membrane resonant frequency for

a plate fixed around its boundary is given by

( )F

h

r

E

r=

0 47

12 2

..

(1)

where h = depth of the defect (in this case the skin thickness)

r = defect radius

E = Youngs modulus of the layer above the defect

= density of the material

= Poisson's ratio of the material above the defect

The above equation represents the resonant frequency at the centre of the thin (membrane) circular plate.

As a general guide, the defect diameter must be at least twenty times the skin thickness for this equation to

be valid. The constant, 0.47, is the result of using a fixed boundary condition. Hence it is can be seen thatfor a given structure the membrane resonant frequency, Fr, at the centre of the defect is a function of the

depth and size of the defect and its boundary conditions.

Fig. 2 (Wong et al. 1996) shows resonant frequencies plotted against defect or discontinuity sizes. Thespecimens had aluminium skins and nomex cores. The defects were deliberately fabricated delaminations

between the top skin and core. The delaminations were made from plastic inserts. The probe was placed

over the top skin for the tests. The theoretical resonant frequencies derived from equation (1) agree well

with the experimental values. The good agreement implies that for the autoclave cured aluminium skin

specimens used in this paper (which is homogeneous and relatively stiff) it is possible to determine the

diameter of the defects in the range of 40 mm to 90 mm in diameter from the resonant frequency recorded

and the use of equation (1). This assumes that the defect would be circular or near circular in shape. Thelimitation exists because smaller defects have resonant frequencies above the 8 kHz maximum of the

instrument used. For larger defects i.e. above 90 mm (3.6 in.) in diameter the rate of change of frequency

with diameter then becomes too small for accurate assessment.


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Fig. 2. Resonant Frequency Versus Discontinuity Size For Mechanical Impedance Tests

An advantage of the technique over ultrasonics is that it does not require a couplant between the probe and

specimen.

4 ULTRASONICS

Ultrasonic testing is the most widely used and most powerful procedure for inspecting fibre reinforced

composites for internal defects. Fundamentally a probe with a piezoelectric crystal transmits ultrasonic

pulses into the specimen and whenever a change in material acoustic impedance occurs the pulses are

reflected back and received by the same or another crystal. Acoustic impedance is the material densitymultiplied by the ultrasonic velocity in the material. Appropriate instrumentation can display the

information in various ways. The A scan display is similar to an oscilloscope display giving time of flight

and reflection amplitude data. The C scan display requires the use of a rectilinear robot system and

displays a pictorial plan view of the defects detected. A common technique is immersion testing where the

transduced is coupled to the specimen with water. Contact testing is also possible where the probe is placedon the specimen with a viscous couplant being used between the probe and specimen.

Fig. 2 shows an immersion C scan image of an impact damage defect on a honeycomb specimen. Nodamage is visible on the surface of the specimen. The specimen has a skin made of ten layers of carbon

fibre prepreg material. In the figure image G1, G2 etc represents a defect between the first and second,

second and third, etc prepreg layers. The delineation of the defect features and extremities can be

observed. Also the interface between the core and surface skin of the honeycomb can be clearly observed

showing the good adhesion between the two.


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Fig. 3. C Scan Ultrasonic Images Of A Honeycomb Specimen With Image Damage In The Upper Skin

Which Is Made Of Ten Layers Of Carbon Fibre Prepreg.

Disadvantages of ultrasonics are it is slow testing procedure and the probe requires intimate contact with

the test specimen. The sensitivity of ultrasonics to small defects is summarised in the next section.

5 THERMOGRAPHY

In Lock-in thermographic evaluation of materials, such as fibre reinforced composites, a sinusoidal

thermal wave is directed at the surface of a specimen. Part of the wave penetrates into the specimen andwill reflect from internal defects. The reflected wave will interfere with the surface wave. Changes in phase

and amplitude of the surface interference pattern will enable defect characteristics to be determined. Fig. 4

(left hand image) shows the Lock-in thermographic phase image of the impact specimen in Fig. 3. The

thermographic image can still show the general shape of the defect but its characteristics are not as clearly

defined as by ultrasonic C scan.


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Fig. 4 Lock-in image of a low impact damaged specimens (the left hand specimen was compared with an

ultrasonic investigation)

Fig. 5 shows that defects will produce positive or negative changes to the surface phase (value 0). Either

can be used for defect detection. The defects are round delaminations artificially produced by inserting

plastic sheets into the specimens. Small defects down to 1mm in size still can produce sufficient phasedifference to produce good contrast for detectability. There are certain frequencies such as 0.025 Hz in the

chart where no phase difference is produced because the returning wave is in phase with the incident wave.This blind frequency zone is narrow but should be avoided by experimental evaluation tests or testing at

several frequencies. Frequencies giving maximum contrast should be selected for testing e.g 0.12 Hz in Fig.

4.


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Fig. 5. Phase Differences Caused By Defects

Fig. 6 shows that thermographic sensitivity is critically dependent on the defects depth with sensitivity

decreasing linearly with defect depth. Lock-in phase thermography is shown to be more sensitive thanthe conventional reflection and through transmission thermography techniques. However all thermographictechniques are considerably less sensitive than ultrasonics which thermography is intended to replace.

However thermography does have the advantages of remote testing of a large area quickly.

-15

-20

-28

-14

7

12

15

-15

-20

-10

0

8

10 10

-35

-30

-25

-20

-15

-10

-5

0

5

10

15

20

0.234 0.12 0.058 0.029 0.015 0.01 0.007

Frequency (Hz)

Defect

PhaseDifference

Large defects20 to 60 mm

Small defects 1to 10 mm


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LOCK IN METHOD

TRANSMISSION METHOD

REFLECTION METHOD

ULTRASONIC DETECTABILITY LIMIT

Fig. 6. Comparison Of Various Testing Techniques

0

1

2

3

4

5

6

7

8

9

10

11

12

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 1.1 1.2 1.3 1.4 1.5 1.6 1.7 1.8

Depth, mm

Diamete

r,mm

DETECTABLE

REGION

THERMOGRAGH-ICALLY

NOT DETECTABLE

REGION


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6 CONCLUSIONS

Radiography can show defect characteristics and extremities clearly but it is limited to certain techniques

such as the TBE enhancement technique where the defect is open to the surface.

The mechanical impedance is the most suitable technique for a one sided test of delaminations between the

skin and core of a honeycomb specimen specimen. It is less sensitive than other techniques for other

defects.

The following table attempts a comparison between Lock-in Thermography and ultrasonic C scan for the

detection of defects in fibre reinforced composites.

Table 2 Summary Of A Comparison Between Thermography And Ultrasonic C Scan

Near surfacedefects < 1 mm

deep

Defect sizing Smallest size

detectable

(mm)

Accuracy of

depth measure-

ment

Detailed

features of

defect

Noisiness of

image

Lock-in

thermography

Size quite

accurate

1 - 2 Not clear Noisier than

ultrasonics

Ultrasonic C

scan

Sizing accurate 1 Can determine

ply layer( 0.1mm)

Clear Clear

Deep defects >

1 mm deep

Defect sizing Smallest size

detectable

(mm)

Accuracy of

depth measure-

ment

Detailed

features of

defect

Noisiness of

image

Lock-inthermo

graphy

Size quiteaccurate

8 Not clear Noisier thanultrasonics

Ultrasonic C

scan

Sizing accurate 1 Can determine

ply layer

( 0.1mm)

Clear Clear

7 REFERENCES

1. Adams, R. D. and Cawly, P. D. A Review of Defect Types and Non-Destructive TestingTechniques for Composites and Bonded Joints, NDT International 21 (1988) pp 208-222.

2. Wong, B. S., Guo, N., Tui, C. G. and Teng K. H. Mechanical impedance


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inspection of aluminium honeycomb structures, Materials Evaluation, USA, (1996)

Vol. 54, No. 12, pp. 1390-1396.

3. Wong, B. S. and Tui, C. G. Thermographic Evaluation of Aerospace Materials, Aerospace

Technology Seminar, Singapore, 1998.

4. Wong, B. S., Tui, C. G., Low Bah Soon, Tan Peck Hui and Tan Kha Sheng Thermographic andUltrasonic Evaluation of Composite Materials, Proceedings of Non-Destructive Testing 98,

UK, 1998.

AUTHOR - BRIAN STEPHEN WONG

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