Proceedings of the National Seminar & Exhibitionon Non-Destructive Evaluation

NDE 2011, December 8-10, 2011

EXPERIMENTAL SETUP

Thermography in pulsed mode with reflection method wasused, where both the thermal heating source and IR camera(Model ThrmaCAM SC3000) are located on the same side ofthe test specimen. New heating system was configured andinstalled to work with existing IR thermography camera.Heating system produces a short-duration high peak powerexcitation heating pulse (flash energy of 6100 Joules), which

INTRODUCTION

Thermography is quick, real time, non-contact and can examineover a relatively large area (1 Square meter) with the IR cameraat a distance of 1 meter from object in one shot. It was possibleto detect near-surface defects using a simple excitation with aheat gun. Deeper surface defects are not detectable by lowheat energy heat gun. In such cases, higher energy flash lampswere used and the entire sequence of images captured duringthermal excitation and analyzed. Typically within few seconds,captured data was processed and it provides an image ofsubsurface defects that exist in the calibration samples.Calibration samples were examined prior to implementing thetechnique on actual components.

Pulsed Thermography (PT) finds many applications [1,2] asnon-destructive evaluation (NDE) technique and can beapplied for honeycomb sandwich deck plates, shear web panelsand central cylinder made of aluminum core and compositeface sheets. Detection of debonds between the face sheet andhoneycomb core and the edge insert areas are important toassess the integrity of the spacecraft structure.

Thermography is a technique of producing a live thermalpicture of an object based on the infrared radiation receivedfrom it (emitted by an object). Thermography can be eitherpassive or active in nature.

IMPLEMENTATION OF THERMOGRAPHY TECHNIQUE FOR SPACECRAFTSTRUCTURAL COMPONENTS

A.Ananthan, Harsh Kumar, H.A.Venkatesh Prasad, D.Venkateshappa,Krushna Chandra Dakua and T.S.Sriranga

Structures Group, ISRO Satellite Centre, Old Airport road,Vimanapura, Bangalore.

ABSTRACT

Infrared Thermography technique has been used for inspection of spacecraft structural components like carbon-fiber-reinforced polymer (CFRP) face sheet with Aluminum core panels, CFRP face sheet with CFRP core panel, compositecylinder and composite shear web etc. In this paper inspection requirements, speed and reliability of the technique areaddressed. Infrared (IR) Thermography is useful, as it is quick, almost real time and can examine over relatively largearea (1 square meter) with the IR camera at a distance of 1 meter from object in one shot. Details of specimens,simulated defects, case studies and efforts made in applying appropriate heat input to test specimen are explained.These components posed several inspection challenges such as thick face sheet, emissivity range, small field of view,curvature etc. This paper presents efforts in overcoming such challenges. Instrumented Coin Tapping and Shearographywere also used for validating the IR Thermography results.

Keywords: Thermography, Reinforced Polymers.

PACS: 87.63 Hg and 81.05 QK

Fig. 1 : Pulsed Thermography Set-up with flash lamps

was applied to the test specimen and thermal response wasrecorded using Infrared camera. The thermal energypropagates, by diffusion, under the surface and also sufferslosses because of radiation and convection. The thermalresponse is analyzed using image-processing techniques. Thepresence of a defect reduces the diffusion rate so that defectsappear as areas of different temperatures (around 1 to 2°C)with respect to surrounding good areas. Also deeper defectswill be observed later and with a reduced contrast. A photographof experimental set-up with flash lamps was shown in Fig. 1.

TEST SPECIMEN PREPARATION

Implementing the Pulse Heating Technique to achieve thedetection sensitivity of the infrared Thermography system forNDE of honeycomb sandwich panel (size 200x 200 x 15 mmthick) made of CFRP face skin varying thickness from0.3 mm to 2.4 mm. Defects were simulated by Insertion ofTeflon sheet and wax papers between the face skin layup.Debonds were simulated by removing the adhesive componentbetween the face skin and honeycomb core. The measuredemissivity of the panel was around 0.90. Curved panel size230 x230 x 15 mm thick covered with Poly Tetra FloraEthylene (PTFE) cloth was taken and defects were simulatedby Insertion of Teflon sheet between the face skin layup.Specimens were made with different thermal conductivitymaterials i.e. CFRP face skins (1.7 W/m °C) with honeycombcore panels embedded with solid metal (250 W/m °C) hold-downs of 100 mm thick and tested.

VARIOUS CASE STUDIES

1. Case study of thicker face skin

1.1 Calibration studies were done on CFRP face skin (0.8mm thick) with metallic honeycomb sandwich panel withsimulated defects of 3 numbers were introduced by Teflon& wax paper insertion in the 4th layer of the face skinand one more debond was introduced between the faceskin and core and tested using pulsed Thermography. Heatenergy pulse was given with two flash lamps were placedat 45o angle to the panel center. Results of theThermography image were shown in figure 2.

1.2 CFRP face skin (0.3 mm thick) with metallic honeycombsandwich panel (size: 200x 200 x 15 mm thick) with

Fig. 2 : CFRP Sandwich panel with Simulated Defects

simulated debonds were introduced between the face skinand core. Panel was tested using reflection method pulsedthermography with heating system (two flash lamps) at45o angle to the panel center and result was shown infigure no: 3A. Later panel was validated usingshearography and instrumented coin tapping techniques.Shearography was conformed all defects, which wereidentified by Thermography. A shearography test resultwas shown in figure no: 3B. Instrumented coin tapping(ICT) technique was used on the same panel. It measuresthe reference value as 35 milli seconds and measuredvalue on defects area was 52 milliseconds. ICT test resultswere also conformed the defects.

1.3 CFRP Laminate of 100 x 100 x 2.4 mm thick withsimulated delaminations by insertion of Teflon piecebetween the layers was tested using pulsed thermography.One flash lamp was placed above the IR camera inlinewith object center. The thermography image was shownin figure 4.

1.4 New developmental work of CFRP core with CFRP faceskin panel was developed. It was tested using pulsed

�thermography with flash lamps placed at 45 angle tothe object center. Thermography identified debonds inthe panel. Thermal image was shown in the figure 5.

2. CASE STUDY OF DIFFERENCE IN THERMALCONDUCTIVITY

2.1 Pulsed Thermography was implemented on CFRP (K=1.7W/m °C) doublers bonded with Epoxy adhesives onaluminum (K=250 W/m °C) honeycomb panel. After

Fig. 3 : (a) Thermography image of Composite Honeycombsandwich Panel with Simulated Debonds (b)Shearography image of Composite HoneycombSandwich panel with Simulated Debonds

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422 Ananthanet.al : Proceedings of the National Seminar & Exhibition on Non-Destructive Evaluation

conducting flexural test on panel was subjected to PT,where it was able to identify debond in between thedoubler and the aluminum face sheet of the panel.Thermal image was shown in the figure 6. Later CFRPdoubler was subjected to Shearography inspection. It wasalso conformed debond in between the doubler and thealuminum face sheet of the panel. Shearography imagewas shown in the figure 7.

2.2 Application of pulsed thermography on 2.4 mm thickCFRP (emissivity of 0.90) face skin with 100 mm thickaluminum core panel. CFRP Panel was embedded withsolid metal (emissivity of 0.03) hold-downs, which hadthe different thermal emissivity. PT was able to find outdebond between thick CFRP face skin and aluminumcore. Thermal image shows debond in corner of the panelas shown in figure 8. PT was also identified debondbetween the CFRP face skin and the metal hold down ofhoneycomb panel.

3. CASE STUDY OF CURVED COMPONENTS

3.1 Thermography was implemented on Curved CFRPComponent (Interface ring). Debond between inner

Fig. 5 : CFRP core with CFRP face skin, debonds were shownas Circled Areas

Fig. 4 : Delaminations in thick (2.4 mm) Composite laminate

aluminum ring and outer CFRP laminate was identifiedby pulsed Thermography. Thermal image of the Interfacering with debonded areas were shown in the figure 9.

3.2 Study to implement PT on curved structural components,a Curved coupon (230 x230 x 15 mm thick) with PolyTetra Flora Ethylene (PTFE) cloth and 9 numbers ofsimulated defects was tested. 8 defects were identifiedout of 9 simulated defects. Identified defects were 3Inclusions at 4th Layer varying in size 8x8, 5x5 and 3x3mm, Inclusions (size: 5x5 mm) made between the layers2/3 &4/5 were seen, except at layer 6/7, which is notseen. Debonds between core to face sheet varying in size:3 x3, 5x 5 and 8x8 mm were seen. Thermal image wasshown in figure 10.

4. CASE STUDY OF SMALLER FIELD OF VIEW

Weld joint (1 square millimeter) between solar cell junctionswas tested using pulsed thermography. In this test 20° angleinfrared lens was used for small field of view covering 10square millimeter areas and one flash lamp was placed abovethe IR camera inline with object center. The thermographyimage was shown in figure 11.

Fig. 7 : Shearography image of CFRP Doubler on aluminumpanel.

Fig. 6 : Thermographic images of CFRP Doubler on aluminumpanel.

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Fig. 9 : Thermal images of CFRP Interface ring (Debonds wereshown in circled area)

Fig. 8 : Thermal images of Honeycomb Frame with cornerdebond

CONCLUSIONS

Pulsed Thermography technique was successfullyimplementing for Spacecraft Structural Components likeantenna panels, CFRP Interface ring, Shear webs and CFRPdoublers bonded on aluminum honeycomb panels. Thesecomponents posed several inspection challenges such asthicker face sheet, different thermal conductivity, small fieldof view, curvature of the component etc. They were over comeby configuring and installing a proper heating system to theexisting IR thermography camera. IR Thermography resultswere validated using Instrumented Coin Tapping andShearography.

ACKNOWLEDGEMENTS

The authors are grateful to Dr. T.K. Alex, Director ISROSatellite Centre, R.K.Srinivasan, Deputy Director, Mechanical

Systems Area, Dr. K. Renji Group Director, Structures andS.ShankarNarayan, Division Head Experimental Structure,S. Dasgupta, Visiting Scientist for their encouragement andsupport. Prof.Krishnan Balasubramanyam, IIT Chennai, fortechnical input in configuring a suitable heating technique.

REFERENCES

1. N.P. Avdelidis – “Pulsed thermography: philosophy,qualitative & quantitative analysis on aircraft materials& applications” proceedings V International Workshop,Advances in Signal Processing for Non DestructiveEvaluation of Materials, Quebec City (Canada), 2-4 Aug.2005.

2. MALDAGUE (X.). – “Applications of InfraredThermography in Non Destructive Evaluation” Rastogi(P.K.), Inaudi (D.) eds. Trends in optical nondestructivetesting, Elsevier Science, 2000.

Fig. 11 : Weld joint of 1 square millimeter areaFig. 10 : Curved composite coupon with PTFE cloth 8 defectswere Identified out of 9 simulated defects