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Nondestructive Assessment of Glass Fibre Compositesby Mid-Wave and Near Infrared Vision

Clemente Ibarra-Castanedo1, Abdelhakim Bendada1, Nicolas P. Avdelidis2 and Xavier P. V. Maldague1

1Computer vision and systems laboratory, Electrical and Computing Engineering Department, Université Laval,Quebec City (Quebec) G1V 0A6, Canada2Materials Science and Engineering Section, School of Chemical Engineering, National Technical University of Athens,Athens 157 80, Greece

The nondestructive testing (NDT) of composites is a critical step for the early detection and repair of industrial components. Infrared (IR)vision has demonstrated to be an attractive inspection technique, allowing a fast and straightforward examination of the integrity of materials andparts in a non-invasive and non-contact manner. In this study, mid-wave infrared (MWIR) thermography and near infrared (NIR) vision areinvestigated for the NDT inspection of a glass fibre sample with fabricated subsurface defects of different types. The main advantages andlimitations of each technique are discussed and some comparative experimental results are provided. [doi:10.2320/matertrans.I-M2011856]

(Received November 20, 2009; Accepted March 11, 2011; Published February 29, 2012)

Keywords: infrared vision, infrared thermography, mid-wave infrared, near infrared, reflectography, glass fibre composites, nondestructivetesting

1. Infrared (IR) Thermography

IR thermography, working in the mid-wave (MWIR) (3­5µm) and long-wave (LWIR) (7.5­14 µm) portions of theinfrared spectrum, provides thermal maps, i.e. thermograms,which are the result of thermal emissions from the specimensurface. IR thermography is gaining popularity in many areassuch as aerospace where large surfaces need to be inspectedin situ in a fast and safe manner.1,2) Figure 1 depicts thetypical setup configuration in IR thermography.

As can be seeing in this illustration, the specimen can beinspected in reflection (camera and heat source on the sameside) or in transmission (the specimen is stimulated from oneside and the camera and heat source on the same side) modesdepending on the application. In the case of transparent andsemi-transparent materials such as glass fibre, near infrared(NIR) vision constitutes an interesting alternative worthy oftaking into consideration as discussed next.

2. Near Infrared (NIR) Vision

NIR vision recovers the reflected or transmitted (non-thermal) radiation from or through the specimen in the nearportion of the infrared spectrum (0.9­2.5 µm). It can be usedto reconstruct complete images of the specimen, which inmany cases provide an enhanced contrast of the eventualdefects inside the components (as long as these features are atleast partially opaque to NIR radiation).

The experimental setup is similar to the one used for IRthermography and is illustrated in Fig. 2, with the differencethat in this case an illumination source (and not a heat source)is required. This technique, commonly referred as reflecto-graphy (in reflection mode), is extensively employed in theexamination of artworks where underdrawings (opaque toNIR radiation) can be detected through the painting layers(semi-transparent to NIR radiation) providing informationabout the integrity of the piece, intentional and unintentionalalterations and artists’ motifs.3) Nevertheless, to our knowl-

edge, NIR vision has seldom been exploited for theassessment of industrial parts.

In the next section, the experimental results obtained froma glass fibre specimen inspected by both, MWIR thermo-graphy and NIR vision, are presented and discussed.

3. Experimental Results and Discussion

A glass fibre plate (30 cm © 30 cm) shown in Fig. 3(a) wasinvestigated. This specimen contains different types offabricated defects as depicted and detailed in Fig. 3(b).An IR camera (Santa Barbara Focalplane InSb, 3­5µm,320 © 256 pixel resolution) was used for the IR visioninspection, and a NIR camera (Goodrich InGaAs, 0.9­1.7 µm, 640 © 512 pixel resolution) for NIR vision testing.

Figure 4 presents a result obtained by IR thermographyand processed using the pulsed phase thermography (PPT)technique.4) The specimen’s surface was black-painted forthis experiment and it was inspected in transmission modeusing two photographic flashes (Balcar, 5ms pulse, 3.2kJ/flash). Some of the defects can be seen and they areidentified in Fig. 4(a). Figure 4(b) shows the specimen’sdefects locations for reference. For instance, three of the fourdelaminations (D2, D3, and D4), two of the three impacts(I1 and I2) and two of the four non-classified defects (O1

and O2) are detected. The countersink defects (C1 to C3) andthe burned drill holes (B1 to B3) of different sizes can beperfectly seen of course since they are holes. However,although these defects were created to simulate different realconditions on defects of the same type (countersink andburned drill), no distinctions between them can be made fromFig. 4. Further testing with increased spatial resolution wouldbe required for this manner.

Figure 5(a) shows a NIR image obtained using anincandescent light in transmission mode as an illuminationsource. Figure 5(b) shows the defects’ locations for refer-ence. As can be seen from this figure, the same defectsdetected by IR thermography can be clearly identified by NIR

Materials Transactions, Vol. 53, No. 4 (2012) pp. 601 to 603Special Issue on APCNDT 2009©2012 The Japanese Society for Non-Destructive Inspection

vision. In addition, there are three other defects that can bedetected in Fig. 5(a): D1, I3 and O4. Furthermore, defectcontrast is in general better in the NIR result when comparedto the IR thermography result, which could be explained inpart because of the superior spatial resolution of the NIRcamera (640 © 512 vs. 320 © 256 pixel resolution). Forinstance, evidences of the loading differences in the impactdefects (type “I”) are better resolved in the NIR image.Additionally, judging from Fig. 5(a), the non-classified defectO1 appears to have a triangular shape and delamination D4 adiamond. Defect O4 has a rectangular shape and it probably

corresponds to a very thin and/or deep semi-transparentinsert since is not seen in the thermogram of Fig. 4(a).

Although defect O2 can be detected by both NIR andMWIR inspection, its shape is not the same in these tworesults. Defect O2 appears rounded in the thermogram ofFig. 4(a) (very similar to defect O1 in this same figure), whilstit appears with reduced contrast in an apparently non-circularshape (quite different to defect O1) in the NIR result ofFig. 5. Moreover, the result presented in Fig. 4(a) corre-sponds to a phase image obtained after processing a sequenceof 500 thermograms by pulsed phase thermography, whilst

Reflection

NIR Camera

Display and acquisition

SpecimenTransmission

Fig. 2 Experimental setup for near infrared vision.

D1

D2

D3

D4

I3

I2

I1

C1

C2

C3

B1

B2

B3

O1

50 mm

O4

O3

O2

50 mm

30 cm

30cm

D: Delamination D1: 1 mm x 1 mmD2: 2.5 mm x 2.5 mmD3: 5 mm x 5 mmD4: 10 mm x 10 mm

I: ImpactI1: Load 1I2: Load 2I3: Load 3

C: Countersink

B: Burned drill hole

O: Other defects

(a) (b)

Fig. 3 Glass fibre specimen: (a) photograph, and (b) drawing showing defects’ nature and approximate locations.

Reflection

Transmission

IR Camera Processing

Synchronization

Specimen

Fig. 1 Typical experimental setup for infrared thermography.

C. Ibarra-Castanedo, A. Bendada, N. P. Avdelidis and X. P. V. Maldague602

only one image was required and no advance processing wasnecessary in order to obtain the NIR result of Fig. 5(a).

These results suggest that NIR vision could be aninteresting approach for the assessment of glass fibrecomponents, which could provide single images showinginternal defects with enhanced contrast. It should be notedhowever, that in order to obtain this enhanced defect contrastby NIR vision, access to both sides of the specimen isrequired, which could be difficult to carried out in practice.

Acknowledgements

This work has been supported by the grant from the

Canada Research Program (CRC): Multipolar Infrared VisionCanada Research Chair (MIVIM).

REFERENCES

1) X. P. V. Maldague: Theory and Practice of Infrared Technology forNonDestructive Testing, (John Wiley-Interscience, 2001) p. 684.

2) R. L. Crane: Nondestructive Handbook, Third edition, Infrared andThermal Testing, Volume 3, ed. by P. O. Moore, (Columbus, Ohio,ASNT Press, 2001) p. 718.

3) R. Fontana, D. Bencini, P. Carcagni, M. Greco, M. Mastroianni, M.Materazzi, E. Pampaloni and L. Pezzati: Optics for Arts, Architecture,and Archeology, Proc. SPIE, 6618 (2007) 661813.

4) C. Ibarra-Castanedo and X. Maldague: QIRT J. 1 (2004) 47­70.

(a) (b)

Fig. 5 Results by NIR vision: (a) image obtained in transmission mode with a NIR camera (0.9 to 1.7mm) using a wide-spectrum lightsource and a narrow spectrum filter (1300 nm), and (b) specimen drawing highlighting the detected defect locations for reference.

(a) (b)

Fig. 4 Results by IR thermography processed using pulsed phase thermography (PPT):4) (a) PPT phasegram at f = 0.12Hz, and(b) specimen drawing highlighting the detected defect locations for reference.

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