Digital Radiography Systems Techniques and PerformanceEvaluation for Space ApplicationsV.N. Misale, S. Ravi and R. NarayanLiquid Propulsion Systems Centre (LPSC) Indian Space Research Organization (ISRO) Bangalore

AbstractThe x-ray radiography is a mandatory NDT method for qualification of propulsion system components and assemblies to ensure structuralintegrity with zero defects. The major development in conventional film radiography is digitization of x-ray films, followed by radiographicimage processing and quantitative evaluation to enable accept/ reject decisions. An LS85 Kodak film digitizer is used for digitization and analysis.Performance evaluation of this system with reference to current standards and a case study for estimating depth of defect are discussed. Thenext major development is film less amorphous Silicon based flat panel system with pulsed x ray source The Fox Rayzor flat panel systemis being used for various space applications. This performance evaluation of this system and two case studies to measure the gap betweenthe vanes and hemisphere and Electron beam impingement analysis are discussed in this paper.

VIDISCO Fox-Rayzor flat panel imaging system for near realtime image capture, image processing, archiving and loss-less image transfer for tele-interpretation. In this paper wediscuss performance evaluation of these systems and fewcase studies as applied to our propellant tanks.

2. X ray Film digitization system

The purpose of x-ray film digitization is to capture thefilm radiographic image and convert it into digital image.Digitization helps in digital archiving, quantitative evaluation,image processing, automatic image evaluation, (remote) imagetransfer and production of reference catalogues for flaw

1. Introduction

Liquid propulsion systems are the main entities of ourspace program. The propulsion systems consists ofpropellant tanks, pressurant tanks, various components likefilter assemblies, pyro valves, latch valves, venturi, plumbline systems with transition joints, reducers, welds, engines,thrusters, etc. The propulsion system elements are realized inall welded condition. And X ray radiography is a majormethod to ensure that welds are free from internal defects.Other NDT techniques like ultrasonic testing, penetranttesting, eddy current testing, holography, are also used assupplementary methods.

Propulsion systems with zero defects are used in bothlaunch vehicles and satellites. Once launched space systemsare not amenable for any repair or rework. Further the effectsof defective components can be catastrophic leading tomission failure. Hence it is necessary to ensure zero defectsat various stages of realization of the systems. X rayradiographic NDT plays a vital role in this direction and it isone of the mandatory tests for qualification of the systems.Using X ray radiography, the discontinuities are characterizedand analyzed to determine if the observed discontinuities areacceptable. If they are not acceptable, they are repaired andrechecked. If they are beyond repair, the components arerejected.

Traditionally x-ray film radiography with fine grain x-rayfilms and mini focus x-ray machines were good enough todetect the unacceptable defects in propellant tanks. Recentlythe film radiography is being supplemented with moderndigital radiography methods for defect characterization, bettercoverage, improved detection capabilities, faster inspection,better archiving and loss-less image transfer for tele-interpretation. These new methods being used are x-ray-filmdigitization and image processing techniques using KodakLS85 film digitizer and amorphous silicon detector based

Fig. 1 : Principle of a laser scanner (Kodak LS85) andphotograph

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evaluation. The principle of point-by-point digitization isemployed in our system. Fig.1 shows the principle of ourlaser scanner.

2.1 Digitization process

The x-ray film is moved in front of a collection tube. Alaser beam (wavelength ~680 nm, red) with a fixed diameter(50 m) passes through the film. The collection tubeintegrates the diffuse transmitted light through the film anda photo multiplier (PMT) registers the same with voltageproportional to the light intensity behind the film. During thescan the folding mirror moves the laser beam along ahorizontal line on the film. The output after logarithmicamplification and digitization with 12 bits yield grey valuesthat are proportional to the optical density of the film. Thescanning speed is 75 lines per second.

The laser scanner illuminates the film with focused lightand measures the diffuse light intensity behind the film.Whereas the other methods of digitization namely line byline digitization using CCD scanners and array digitization byCMOS cameras illuminate film with diffuse light and measurethe light intensity that passes the film in one direction. Laserscanner principle has two main advantages:

l The quantity of collected light is considerably higher.The laser focuses the whole light intensity on the pointto measure. The radiation passing the film is collectedover a spatial angle as great as possible in the detector.

l Scattered light from regions with low optical densitypassing to regions of high optical density, therebydistorting the measurement, is nearly avoided by pointwise illumination.

2.2 Film digitizer experiments

An EPRI reference radiograph (8"x10") as per ASTM E1936-03 (1) shown in Fig. 2 was scanned for the evaluationof our digitization systems to evaluate basic parameters ofour film digitization system. The reference radiograph hasfollowing six types of targets.

1. Spatial resolution targets, consisting of converging linepairs in a range of 1... 20lp/mm.

2. Density contrast sensitivity targets, block targets of 1cm with D =0.05 at D=2 and D=0.10 at a darkerbackground of density D=3.5,

3. Stepped density targets, a series of 13 Nos of 1cm2blocks with density between D=0.5 and 4.5: 4.5, 4.02, 4.0,3.5, 3.02, 3.0, 2.5, 2.02, 2.0, 1.5, 1.02, 1.0 and 0.5,

4. Sharp edge targets to ensure unsharpness < 10 micron

5. Linearity targets, these targets provide a geometricalmeasurement scale in horizontal and vertical direction of25.4 mm units,

6. Parallel line pair target: a parallel line pair gage with aresolution between 0.5 and 20 lp/mm.

2.3 Results

1. Spatial resolution achieved 4.0 lp/mm2. Density contrast sensitivity achieved D =0.02 at D=2

and D=0.1 at a darker background of D=3.5,3. Linear response of [Grey values (GV)] to optical densities

(OD) is evident in the range 0.0 to 3.5 D. and from 3.5 4.0D the response needs correction factor nearlyproportional to optical density. See table 1

4. Linearity is uniform and equal in both horizontal andvertical directions.

5. Working ranges of optical densities: 0-4;6. When the optical densities are more than 4.0 clipping

occurs. And digitizer cannot be used.

2.4 Performance of digitizer

The European standard CEN TC 138 (2)Non-destructivetesting - Qualification of radiographic film digitization systemshas 2 parts namely - Part 1: Definitions, quantitativemeasurements of image quality parameters, standard referencefilm and qualitative control. And Part 2: Minimumrequirements. In this section we discus performance of ourLS85 Kodak Digitizer with reference to the standard.

1. Standard requires that digitizer must be able to reach thedensity of 4 or 5, without increase the image noise byits own detector noise. Our scanner is able to cover thedensity range up to 4.0D with linear response is evidentup to density level 3.5D and from 3.5 D to 4.0 D minorcorrection is needed. The correction factor is as follows

Fig. 2 : EPRI Reference radiograph for digitizer Evaluation (notto scale)

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True Grey value = Observed grey value0

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By scanning the x-ray film, digital image was obtainedby LS85 Kodak digitizer. This digital image is shown in figure4. Magnified view shows the presence of the under beadundercut. The size in terms of breadth and width could bemeasured easily from magnified image.

To estimate depth dimension, we made a Ti6AL4V stepwedge of known step thick nesses and its radiograph alongwith the discontinuity is shown in Fig. 5. Calibrated Greyvalue line profiles of thickness steps were compared with thedefect line profile and an estimate of depth of defect wasmade. The defect depth (0.3mm) was well within operatingmargins and the tank was cleared for flight and flownsuccessfully.

3. Digital flat panel radiographic imaging system

Digital flat panel radiography is a form of x-ray imaging,where digital flat panel x ray sensors are used instead of xray film. to record the X-ray image and make it available asa digital file that can be presented for interpretation andsaved as an x-ray record. The advantages of DR over filminclude immediate image preview and availability, a widerdynamic range and ability to apply special image processingtechniques that enhance overall display of the image. In thispaper we discus Amorphous Silicon (a-Si) based FPD imagingsysstem perforamance and case studies on NDE of Propellanttanks.

3.1 Amorphous Silicon

(a-Si) type of FPD have a-Si detectors, positioned justbelow Gadolinium Oxysulfide (Gd2O2S) scintillator thatconverts X-ray to light. The light is then channeled throughthe a-Si photodiode layer where it is converted to a digitaloutput signal. The digital signal is then read out by ThinFilm Transistors (TFTs) The image data file is sent to acomputer for display, processing and interpretation. Figure 5shows pixel layout individual pixel and circuit

Fig. 4 : Electron beam weld of a propellant tank

Fig. 5 : Magnified view of under bead undercut and its grey valueline profile

Amorphous silicon FPD Pixel Layout, single pixel & circuit.

Fig. 6 : Vidisco Fox Rayzor FPD Imaging

Fig. 7 : Mechanism of amorphous System silicon x ray detectionand Gadox Scintillator

representation(3). Figure 6 shows a photograph of VidiscoFox Rayzor FPD Imaging system and figure 7 shows themechanism of x ray detection in amorphous silicon detector.

3.2 Case Study of Propellant tank burn through and repair

During the realization of a titanium propellant tank,electron beam weld was abruptly stopped due toinstantaneous variation in power supply. The weld showedpresence a deep open cavity. Possibility of burn throughcould not be ruled out, even though depth of open cavitywas visually measured to be ~ 2mm against wall thickness of5.5 mm. The defect was rewelded to fill the cavity. And the

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while the gap was uniform in 3 vanes (~3 mm) and the othervane was touching the hemisphere. Both are shown in Fig. 9.

4. Conclusions

The digital radiography has supplimented x ray filmradiography to a greater extent. The X ray film digitizers havegiven a new dimension for information extraction from x rayfilm. They help in archiving the image, they provide greyvalue levels which are linearly proportional to opticaldensities to enable quantitative characterization ofdiscontinuities and depth evaluation. A case study ofunderbead undercut proves the point. They do notdeteriorate the image quality and maintain acceptable qualitylevels. Image processing functions do helpininterpretation.Our LS85 Kodak digitizer function is inaccordance with current standards. One of the limitation wasnon linearity in film densitiesgreater than 3.5 and clippingeffect at 4.0 D.

The Vidisco Fox rayzor amorphous silicon flat panelimaging system is very versatile, amazingly quicker, andprovides acceptable radiography quality and two case studieshave given a new insight into the way we can donondestructive evaluation of Propulsion systems.

5. Acknowledgements

Authors are greatful to LPSC Management, DD SRQA,DD LPSC(B) and Director LPSC for constant encouragementin preparing this paper and kind permission to present thispaper in National Seminar NDE 2009 at Tiruchirapalli Dec2009.

References

1. ASTM, Annual Book of ASTM standards section 3 Vol 3.03 Oct2007.

2. www.ndt.net U Zscherpel and BAM Berlin May 2000 Vol 05,No05

3. P R Vaidya Flat panel detectors in Indusrrial radiography BARCMumbai

4. Operating Manual Kodak LS85 X-Ray Film Digitizer5. Operating Manaual Vidisco Fox Rayzor system

weld surface appeared to be satisfactory. However Normalradiographic exposure was as shown in figure 8. The defectappeared as burn through and was not in acceptable zone.This called for serious investigation and Vidisco Fox RayzorFPD system was put to use.

The digital radiography scanning with FPD was carriedout at small angle and the defect began to resolve into 2overlapping pores one darker image and other as lighterimage. And when the angle was nearly tangential, totalseparation of images was seen and it was found that theweld was free from defect but the EB impingement has causedholes in guide plate of 0.6mm thickness and intermediatebottom of 2.0mm thickness. The hole sizes were characterizedand effect of the holes on the function of the tank wereinvestigated. The FPD system was found to be very effective.

3.3 Case study II: Propellant tank vane Assembly

The surface tension type propellant tanks have vaneassembly as apart of propellant management system. Thevane assemblies are made of thin commercial pure titaniumsheets. Normally a template is used during assembly toensure certain minimum gap between vane and hemisphere.In one of the propellant tanks, during vibration testingvariation in natural frequency of the internals was noticedand further investigations were called for to verify that gapexists. Vidisco Flat panel digital X ray system was put to use.Minimum gap between vane and hemispheres were imagedas a tangential projection all along the length of vanes andthe same was repeated for all 4 vanes. It was observed that

Fig. 9 : Left image shows vane gap and right image shows nogap.

Fig. 8 : EB weld defect in propellant tank, which was resolved as holes in guide cone and intermediate part, and weld is free from defects.