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Radiography of Thin Sections of Aluminium and Composites with Americium-241 and X-RAYSG.C. Das, V.K. Sharma, P.Sreeramkrishnan and Gursharan Singh
Isotope Applications Division,Bhabha Atomic Research Centre,Mumbai-400085,INDIA.
Radiography testing of thin sections of aluminium components used in aerospace industry and of various metallic and non-metallic composites used in automobile industry has been of considerable interest in NDT applications. Use of low energy X-ray radiography for such applications has been reported in literature. The composite materials have various types of built in flaws such as porosites, voids, fiber mis-orientation, thermal or shrinkage cracks etc. It is reported that these defects can be detected by radiography using low energy X-rays from 10 kV to 150 kV. However there is hardly any literature available on the use of low energy gamma sources for such applications. The authors have attempted to use gamma rays (59.5 KeV) from Americium-241 and compared the results with X-rays (40 to 80 kV) for radiography testing of thin sections of aluminium and composite specimen such as brake liners and clutch facings of automobiles. As these composites used in the study had similar density as aluminium, a comparative study of characteristics and sensitivity for aluminium sections in the range 2-18 mm thick was carried out using Am-241 source and X-radiations (40 to 80 kV). Radiographs of a few samples of composites are shown in the paper.
Keywords: Composites, radiography, Americium, gamma, x-rays
A composite material is defined as a material containing two or more distinct phases combined in such a way that each remain distinct. The traditional light weight aluminium alloy has been used since ages for manufacture of aircraft structure and other components in aerospace industry. However, with the advent of advanced fibre-reinforced composites the metallic aluminium alloy has been replaced by the new material in the industry. Fibre-reinforced composites have high mechanical strength-to-weight and stiffness-to-weight ratios and hence becoming important in manufacture of various types of components in aircraft and automotive industries. There are various types of composites material (4) namely carbon-fibre reinforced composites (cfc), glass-fibre reinforced composites (gfc) and asbestos fibre based composites. The former are used for construction of primary and secondary structures, such as decorative panels of civil aircraft, helicopter blades in aerospace industry while the latter are used for manufacture of brake-liners and clutch-facings etc. in automobile industry. Rubber fibre reinforced composites find extensive use in manufacture of brake liners for two/ three wheelers.
The composite materials develop various kinds of flaws (1), built into them during their manufacturing process. These include porosities, voids, fibre misorientation, thermal or shrinkage cracks etc. Non-destructive testing of composites differs from that of metals as these materials are heterogeneous, anisotropic and multi-layered structures and defects occur usually at their interfaces. These defects are amenable to detection by low energy X- or gamma rays. Hence, the authors have used low energy gamma rays ( 59.5 KeV ) from Am-241 source and X-rays ( 40 KV to 80 KV ) for radiography of composite specimen such as brake liners and clutch facings of automobiles and thin sections of aluminium. As the composite specimen under study had similar density as aluminium, a comparative study of radiographic characteristic curve and sensitivity for aluminium in the range 2- 18 mm thickness was carried out using Am-241 and low energy X-rays. Few sample radiographs of specimen are cited in the paper.
As the composite specimen used in the study has similar density as aluminium exposure charts of aluminium were made using Am-241 source  and low energy X - rays. Agfa-Gaevert D-7 film, was used for this work. These charts ( Figure- 7 & 8) were used for calculation of exposure time to obtain an optical density of 2.0. Gamma rays from a source of 370 GBq of Am-241-Be source were used for radiographic exposures, keeping the SFD( source to film distance) of 25 cms. The effective diameter of the source as determined by pin-hole radiography method, was about 4 mm. The radiation intensity at 25 cms was 75 mR/hr. The exposure time for the test specimen ranged from 2 - 60 hours. No intensifying screen was used as the incident beam was low energy gamma rays.
X- ray source with beryllium window having an effective focal spot of diameter 1.5 mm, operated at 40 - 80 kV, at 4 - 8 mA was used for X- radiography. The exposure times ranged from 0.5 to 1.3 minutes at FFD ( focus to film distance) of 75 cms. As the energy of radiation was below 100 kV, no intensifying screen was used for exposures. The exposed films were manually processed under standard processing conditions.
Figure-1,2 shows gamma and X- radiographs of car brake liners. Figure-3,4 shows radiograph of a clutch facing of a car. Figure-5,6 shows gamma and X-ray radiographs of truck brake liners. These radiographs clearly show porosities and metal inclusions. The contrasts of radiographs obtained with X-rays and gamma rays are comparable.
The radiographic sensitivity of various thickness of aluminium, obtained with DIN type IQI for various X-ray energies and Am-241 gamma rays, is plotted in figure-9. The best sensitivity of 2.7 % is attainable with Am-241 gamma rays at 9 - 10 mm of aluminium, while with X-rays 2 -2.5 % sensitivity is achievable. Lower sensitivity with Am-241 could be due to the fact that the effective size of Am-241 source used was 4.0 mm in diameter, whereas X- ray source has focal spot of 1.5 mm. Also the FFD used for X -rays was 75 cms in comparison to SFD of 25 cms with Am -241. It was not feasible to increase the SFD in case of Am-241 source as the exposure time would have become unaffordable.
From the above studies it is clear that Am-241 source can produce radiographs comparable with those obtained with low energy X - rays. However, a source of high activity is needed to reduce the exposure time and improve the sensitivity by increasing SFD.
Authors are thankful to Dr. J.P. Mittal, Director, Chemistry & Isotope Group, Bhabha Atomic Research Centre for encouragement in pursuing this work
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