MIA Evaluation of Material Defects Detected using ESPI and Shearography

D M Findeis and J Gryzagoridis Mech. Eng. Dept

University of Cape Town

ABSTRACT

Most manufactured components have to undergo routine maintenance inspections. This

paper presents two non-contacting NDE techniques known as Electronic Speckle Pattern

Interferometry (ESPI) and Shearography which, through rapid inspection, produce real

time qualitative results. A third technique known as Mechanical Impedance Analysis

(MIA), which evaluates an object’s vibrational response to an audible signal, is also

outlined. The authors show how MIA can be used to supplement ESPI and

Shearography by quantifying flaws initially detected by the optical interference

techniques. The principles of the optical NDE methods are described as applied using a

portable NDT unit. Samples are inspected and defects are located in the fringe patterns

obtained. MIA is applied to the samples in the vicinity of the suspected flaws and a clear

outline of the boundary of the defect in obtained. Discussion of the advantages and

limitations of ESPI and Shearography as well as the benefits of MIA conclude this paper.

Keywords: Interferometry, Shearography, composite pipes and vessels, non-destructive

evaluation, mechanical impedance analysis, defects, flaws

NOMENCLATURE A , amplitude of the laser light I , image intensity θ , phase of the laser light ∆φ , change in phase of the laser light Subscripts: a , first image b , second image r , resultant image x , adjacent points of interest δx , magnitude of object shear

INTRODUCTION

Almost all of today’s manufactured components, which are used in industrial and domestic

applications, undergo some form of inspection. This mostly is in the form of prescribed quality

control inspections during the manufacturing process and routine maintenance inspections

[3,4,5]. In industrial applications such as piping networks where on site assembly is required, it is

good practice to conduct a final inspection prior to commissioning of the equipment. Non

destructive Evaluation (NDE) methods such as dye penetrants, X-Ray, ultrasound, and acoustic

emission [6] are well established methods which are widely used. Modern materials such as

glass reinforced plastics (GRP) and fibreglass and aluminium composites are a lot more difficult

to inspect using these conventional NDE methods and are only capable of detecting certain

defect aspects [6]. This is mainly due to the structure of the material or makeup of the

compostite.

INSPECTION THECHNIQUES

Two non-contacting optical interference techniques which have been used for non destructive

evaluation of composites are Electronic Speckle Pattern Interferometry (ESPI)[1,2], sometimes

referred to as Digital Interferometry, and Shearography. Although derived from the principles of

holographic interferometry, ESPI differs in that the interference fringes are produced digitally

through image addition and subtraction routines.

A typical ESPI set-up as outlined in Figure 1

utilises a laser beam which is split into a

reference and object beam. The object beam

is directed onto the object and expanded to

illuminate the required inspection area. The

object is then viewed through a CCD video

camera on a personal computer (PC), which is

equipped with video digitisation hardware. By

placing a partially reflecting mirror directly in

front of the CCD camera, the reference beam is

superimposed onto the image of object under

inspection. If the object and reference beam

paths are equal in length, the interference of the

two beams produces a speckled image. Any

movement of the object causes the object beam pat

modulation in the speckle image.

Partial Mirror

Mirror

Beam Splitter

Mirror

Beam Expander

Object

Laser

MirrorCamera

Computer+

Digitiser

Video Monitor

p

Shearography on the other hand utilises only a sing

object. The object is then viewed through a shearin

beamsplitter splits the single incoming image into tw

Figure 1. Typical ESPI set-u

h to be altered which generates a phase

le laser beam which is used to illuminate the

g device as outlined in Figure 2. The

o separate images which are directed onto

and reflected off mirrors 1 and 2. The two

images are then recombined at the beamsplitter

and viewed via the CCD camera on a personal

computer. By adjusting the tilt of mirror 1 and

thereby the angle of reflection, one of the

combined images can be sheared with respect

to the other. Due to the temporal coherence

length of the laser, the area of overlap of the

combined images will interfere and produce a

speckled image. When the object is displaced,

both image path lengths are altered. Phase

modulation will only occur when there is a

change in the relative object displacement from

point to point. This also implies that the set-up

is only sensitive to the displacement gradient in the d

displacement gradient which is at 90º to the direction

Camera

Mirror

Beam Expander

Object

Partial Mirror

Mirror1

Laser

Mirror2

Computer+

Digitiser

Video Monitor

(I A A A Aa = + + −12

22

12

22 1 22 cos θ θ )

( )(I A A A Ab = + + − +12

22

12

22 1 22 cos θ θ φ∆ )

( )( )I I I A Ar a b= − = − +4 0512

22 1 2sin . sin .θ θ φ∆ 0 5 φ∆

))x x

))φ

Due to the different methods of generating the speck

object surface displacement and shearography the ra

Mathematically this is presented in equations (1) – (3

[8]. The method used to produce an interferogram is

speckle image of the object is digitised and stored in

mathematical model of the phase interference which

then stressed using either vibrational, thermal of mec

of the object to displace, the magnitude of which is in

second speckle image is then digitised and subtracte

The intensity of the resultant interferogram is as a res

equations (3) and (6). Regions of phase correlation p

decorrelation produce bright bands which are commo

((I Aa x= + −+2 12 cos θ θδ

( )((I Ab x x x= + − ++2 12 cos θ θδ ∆

( )( )I I I Ar a b x x x= − = − ++4 052 sin . sin .θ θ φδ ∆ 05 φ∆

Figure 2. Typical Shearography set-up

irection of image shearing and ignores the

of image shearing.

(1)

(2)

(3)

le images, ESPI is capable of detecting

te of change in object surface displacement.

) for ESPI [7] and (4) – (6) for Shearohraphy

the same for both methods. First a

the PC. Equations (1) and (4) provide a

governs the speckle intensity. The object is

hanical methods. This causes the surface

troduced as ∆φ in equations (2) and (5). A

d from or added to the first stored image.

ult of the magnitude of sin 0.5∆φ in

roduce dark bands and regions of

nly referred to as fringes.

(4)

(5)

(6)

The presence of a defect will alter the surface displacement due to localised weakening of the

structure in that region. ESPI and shearography are able to detect the area in which the flaw is

located as fringe irregularities in the generated interferograms.

Mechanical Impedance Analysis (MIA) employs mechanical vibration, which is applied to the

object surface via a probe, to analyse the local stiffness of a structure. The probe consists of two

piezo-electric crystals. One of the crystals converts electrical signals into vibrations. By resting

the probe on the object surface the vibrations are transferred to the structure under investigation.

The other crystal picks up the objects response to the applied vibration, converts it back an

electrical signal which is then compared with the source signal. Any change in the structural

rigidity due to the presence of a flaw will affect the objects response to the mechanical vibration,

which can be detected when compared with the response of an unflawed section. By using low

frequencies, the need for coupling fluid at the probe structure interface is eliminated.

APPLICATION

ESPI and Shearography have the ability to rapidly inspect large surface areas qualitatively. MIA,

on the other hand can provide quantitative results, but is a very laborious inspection method, as

the whole object has to be scanned with the probe. By combining the two inspection methods,

quantitative results should be attainable much more rapidly. The authors demonstrated this in

the following manner.

Two samples were selected and put to the test. The first was a cross-section of a composite

aircraft wing. The composite consisted of a fibreglass honeycomb core which was covered on

both sides with a fibreglass skin. This construction made the wing section extremely light, but at

the same time rigid. A sharp knife was used to delaminate a 15 mm * 40 mm section of the

inner skin from the honeycomb filler. As the flaw was created on the inner side of the wing

structure and the wing was painted a matt grey, no outer surface deformation was visible. The

second sample, which was made from a GRP, was a 90º elbow tube with an internal diameter of

100 mm and an average wall thickness of 6 mm. The tube was flawed in that a star shaped

internal impact crack, which penetrated approximately 10% of the wall thickness, had been

created. As a grey pigment had been added to the resin, the flaw was not visible from the

outside.

RESULTS

The two samples were first inspected using ESPI and Shearography. The inspection was carried

out at the University of Cape Town’s NDE laboratory. Both the ESPI and Shearography set-ups

were located on a vibration isolation table. A Helium Neon laser was used as the light source.

The two samples were simply propped on a separate wooden bench, with no special vibration

isolation procedures. A Matrox digitiser, which was operated via custom written software, was

used to capture the incoming video signal. The testing procedure was conducted as follows.

Firstly an image of the sample was digitised and stored. The software was then set into real time

mode and the object was stressed thermally. In this mode, the incoming video stream is instantly

subtracted from the stored initial image and the resultant fringe pattern displayed on the video

monitor. The process could be stopped at any time and the frozen image then stored on the

system’s hard drive.

Figure 3a. Elbow result using ESPI 3b. Elbow result using Shearography

Figure 3 a & b above list the results obtained from the ESPI and Shearography investigation of

the GRP elbow. Note the circular fringe which is formed in the ESPI interferogram, which

indicates a localised deformation peak and is indicative of the presence of a defect. This is

supported in the result from the Sherography investigation, where the location is indicated at the

fringe junction in the centre of the image. This junction is as a result of a localised change in the

rate of displacement. For this investigation the images were sheared horizontally and as the flaw

was detected with this set-up, there was no need to perform vertical shearing.

Figure 4a. Wing result using ESPI 4b. Wing result using Shearography

The results obtained from the inspection of the wing cross-section are presented in Figure 4 a &

b. The location of the flaw is clearly visible as an irregular spike in the otherwise circular central

fringe. This irregularity is carried through to the next fringe immediately above the spike. The

otherwise symmetrical fringe spacing across the rest of the wing section indicates that the

remaining sub-structure is free of flaws. The second interferogram, which was obtained using

Shearography, also reveals the flawed region in the top centre. The close fringe spacing, which

is flanked by two circular fringes, is as a result of a large localised displacement gradient.

After the two flawed sections had been marked, MIA was applied to the two samples. The unit

used was a Staveley NDT Tchnologies MIA2500 Portable Bond Tester. In order to calibrate the

instrument, the probe is applied to a flawless section of the sample. The frequency range, which

is from 1 kHz to 8kHz, is scanned and the amplitude and phase of the returned spectra recorded.

The probe is then applied to a flawed section and the scan repeated. The two spectra are then

compared to determine at which frequency the amplitude and phase are most sensitive to a

change in the structural stiffness of the test piece.

For the GRP elbow it was determined that

the reading of the amplitude was too erratic

to produce any reliable results and was

ignored. This was due to the poor contact

between the probe and the irregular

surface. The phase response however

was insensitive to the rough surface, but

showed very little variation in magnitude

between a good and bad section. A

maximum variation of 10 units on a scale of

100 units occurred at 2600Hz. Using the

optimum frequency, the unit was set to

single frequency test mode the flaw threshold magnitudes entered and the marked area scanned.

The boundary of the detected flaw is reproduced in Figure 5. The marked area falls over the

internal star crack, but does not follow the

exact outline of the star crack. There was

no evident step in phase magnitude at the

flaw boundary, but rather a gradual

change. This is due to the small variation

in phase as well as the shallow nature of

the crack.

Figure 5. Flaw outline in elbow using MIA

The same inspection routine was applied

to the wing section. Here the phase

response proved to very erratic and had to

be ignored. The amplitudes response to Figure 6. Flaw outline in wing section using MIA

structural integrity was good and the variation in reading was 35 units on a scale of 100. The

most suited frequency was found to be 1500 Hz. The change in magnitude of the amplitude

provided a clear separation at the flaw boundary. The result of the investigation is given in

Figure 6. The outline of the flaw has been clearly identified and follows the internal flaw

geometry. The knife used to create the flaw has been inserted to note the location. Note how

the defect area widens at the edge of the wing section, which was only noted after MIA had been

applied to the marked section. The closely spaced circular fringes in the interferogram obtained

using shearography supports this finding.

CONCLUSIONS

The authors have successfully demonstrated that MIA, in conjunction with ESPI and

Shearography, can be used to quantify the location of flaws in GRP and composite materials. By

using ESPI and Shearography to perform the initial investigation, the time consuming process of

scanning the entire object surface with a probe can be reduced to only those areas containing a

flaw. This process also reduces the possibility of missing a flaw, as flawed areas are located

prior to the scanning procedure.

REFERENCES

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damage in Fibre-reinforced Composites, Meas. Sci. Technol. 5,1994, pp 100-104. 2. Findeis D, Gryzagoridis J, Inspecting Glassfibre Reinforced Plastic Piping using Portable ESPI

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