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Non-destructive examination of adhesive bonds with neutron radiography
Non-destructive examination of adhesive bonds with neutron radiography E. Sancaktar
The neutron radiography technique is a powerful addition to non-destructive testing methods. Its advantages over X-radiography is the capability of differentiating materials with varying neutron absorption cross sections even in the absence of significant changes in the density. Thus it is possible to identify the presence of two neighbouring elements (eg boron and carbon) or inclusions of light elements (eg hydrogen, lithium, boron) in heavy metals. The transmission of a neutron flux decreases with increases in the neutron absorption cross section, material thickness and density. Such a change in the flux can produce varying grey tones on an X-ray film and reveal the presence of flaws or different materials.
Experimental The neutron radiography method was recently applied in the non-destructive examination of aluminium single lap specimens bonded with Metlbond-1113 adhesive. The resolution obtained on the neutron radiographs (photographs) was excellent. All of the bonding flaws visible on the radiographs were easily detectable on the ~ 0.152 mm thick adhesive post-fracture surfaces. A direct exposure method can be applied to bonded elements with metal, polymeric or composite adherends. The element is placed in between the source and the X-ray film which is placed in front of a screen capable of emitting
radiation. It is necessary to place the film and the screen in a light-tight container (cassette). The cassette is made of 1100 alumimum which has a short half-life. The size of the cassette is determined according to the size of the film (and specimen) to be used. A 100 x 130 mm (4 in x 5 in) film is sufficient for most applicati6ns. During exposure the neutron beam enters the cassette directly behind the specimen through a thin aluminium plate "-0.127 mm thick. It then travels through a "" 7.6 mm air space, passes through the X-ray fdrn and hits the converter screen which emits radiation and exposes the film. The converter screen is made of ~ 2.54 mm glass which is painted with ~ 0.025 mm thick layer (two coats) of gadolinium spray paint. It is necessary to have firm contact between the film and the gadolinium screen. This can be achieved simply by drawing air out
of the cassette with the aid of a pump. The air suction hole should be placed in the comer of the cassette where the fLim is in contact with the screen. The exposure procedure is very much like taking a picture. The strength of the flux, time of exposure, and the distance from the source affect the quality of the picture. For best results the neutron flux is maintained at l OSn/cm2sec- l O6n/cm2sec and the specimen is exposed for "" 5 minutes. The collimator length vs diameter ratio for the neutron beam should be as large as possible (LID ~ 267).
Fig. 1 Neutron radiography and fracture surface photographs of Metlbond-1113 specimen A-12. The large inherent void (arrow) is also visible on the fracture surface
0143-7496/81/060329-02 $02.00 © IPC Business Press Limited 1981 INT.J.ADHESION AND ADHESIVES OCTOBER 1981
329
During the experiments, the specimen (which was attached to the cassette wi~h a rubber band) was placed 80 mm away from the exit port (to avoid scatter in the beam) and the beam diameter was "~ 50 mm. The exposed t'tlm can be developed immediately after the testing procedure and most specimens can be handled a few days later. Results
The neutron radiography and post-fracture surface photographs shown in Figs. 1-3 indicate the resolution obtained with the method. Many of the inherent flaws visible on the X-ray photographs can be matched with the void spots on the corresponding post-fracture surfaces. The specimen was a 1.59 mm thick, 25.4 mm wide aluminium single
identified on the neutron radiographs. ,0:Fig. 4 shows the stress/strain behaviour of five aluminium/Metlbond specimens which contained inherent flaws of various sizes. It should be noted that the same strain rate was used for all five specimens. Three of these specimens are the ones shown in Figs 1-3 and are marked accordingly in Fig. 4. Apparently i 2o every one of the specimens exhibited different ultimate stress and rupture strain properties; and such variation should be attributed to the presence of inherent flaws. The shear stress/strain ~°
I
!
(A-r1
#
'
4,
,~
,-~
o
Fig. 4 Symmetric single lap adhesive stress/strain response of various Metlbond1 1 13 specimens with inherent voids, compared with the predicted curve obtained from bulk shear data for = 8.6 X 10-3%/s
behaviour predicted for the Metlbond1113 adhesive with the application of the modified Bingham visco-elasticplastic model is also shown by broken lines in Fig. 4. Conclusions
Fig. 2 Neutron radiography and fracture surface photographs of Metlbond-l113 specimen A-7 with various inherent voids
lap joint bonded with ~ 0.152 mm thick structural epoxy adhesive (Methlbond-1113), which also contains a carrier cloth. Experiments have shown that 0.1 mm thick advance wire embedded in the adhesive layer of the above mentioned specimens could be
The importance of determining the presence of inherent flaws through an accurate nondestructive testing method becomes obvious when the stress/strain curves of Fig. 4 are compared with the theoretical prediction. Examination of the ultimate values indicates an 85% difference in strains and the predicted ultimate strength for the joints, at the rate used is about 40% higher than that for specimen A-10. Therefore, it is necessary to use a non-destructive testing technique, preferably neutron radiography - due to its good resolution - to either discard the joints with flaws, or account for the associated strength reduction for design considerations. Due to its capability of producing high resolution X-ray pictures of modem lightweight materials, the neutron radiography technique is considered an important addition to the methods of non-destructive testing.
Author
Fig. 3 Neutron radiography and fracture surface photographs of Metlbond-1 113 specimen A-10 with connected inherent void and interfacial regions
330
I N T . J . A D H E S l O N A N D A D H E S I V E S O C T O B E R 1981
Professor Sancaktar is with The Mechanical and Industrial Engineering Department, Clark.son College of Technology, Potsdam, NY 13676, USA.
The neutron radiography technique is a powerful addition to non-destructive testing methods. Its advantages over X-radiography is the capability of differentiating materials with varying neutron absorption cross sections even in the absence of significant changes in the density. Thus it is possible to identify the presence of two neighbouring elements (eg boron and carbon) or inclusions of light elements (eg hydrogen, lithium, boron) in heavy metals. The transmission of a neutron flux decreases with increases in the neutron absorption cross section, material thickness and density. Such a change in the flux can produce varying grey tones on an X-ray film and reveal the presence of flaws or different materials.
Experimental The neutron radiography method was recently applied in the non-destructive examination of aluminium single lap specimens bonded with Metlbond-1113 adhesive. The resolution obtained on the neutron radiographs (photographs) was excellent. All of the bonding flaws visible on the radiographs were easily detectable on the ~ 0.152 mm thick adhesive post-fracture surfaces. A direct exposure method can be applied to bonded elements with metal, polymeric or composite adherends. The element is placed in between the source and the X-ray film which is placed in front of a screen capable of emitting
radiation. It is necessary to place the film and the screen in a light-tight container (cassette). The cassette is made of 1100 alumimum which has a short half-life. The size of the cassette is determined according to the size of the film (and specimen) to be used. A 100 x 130 mm (4 in x 5 in) film is sufficient for most applicati6ns. During exposure the neutron beam enters the cassette directly behind the specimen through a thin aluminium plate "-0.127 mm thick. It then travels through a "" 7.6 mm air space, passes through the X-ray fdrn and hits the converter screen which emits radiation and exposes the film. The converter screen is made of ~ 2.54 mm glass which is painted with ~ 0.025 mm thick layer (two coats) of gadolinium spray paint. It is necessary to have firm contact between the film and the gadolinium screen. This can be achieved simply by drawing air out
of the cassette with the aid of a pump. The air suction hole should be placed in the comer of the cassette where the fLim is in contact with the screen. The exposure procedure is very much like taking a picture. The strength of the flux, time of exposure, and the distance from the source affect the quality of the picture. For best results the neutron flux is maintained at l OSn/cm2sec- l O6n/cm2sec and the specimen is exposed for "" 5 minutes. The collimator length vs diameter ratio for the neutron beam should be as large as possible (LID ~ 267).
Fig. 1 Neutron radiography and fracture surface photographs of Metlbond-1113 specimen A-12. The large inherent void (arrow) is also visible on the fracture surface
0143-7496/81/060329-02 $02.00 © IPC Business Press Limited 1981 INT.J.ADHESION AND ADHESIVES OCTOBER 1981
329
During the experiments, the specimen (which was attached to the cassette wi~h a rubber band) was placed 80 mm away from the exit port (to avoid scatter in the beam) and the beam diameter was "~ 50 mm. The exposed t'tlm can be developed immediately after the testing procedure and most specimens can be handled a few days later. Results
The neutron radiography and post-fracture surface photographs shown in Figs. 1-3 indicate the resolution obtained with the method. Many of the inherent flaws visible on the X-ray photographs can be matched with the void spots on the corresponding post-fracture surfaces. The specimen was a 1.59 mm thick, 25.4 mm wide aluminium single
identified on the neutron radiographs. ,0:Fig. 4 shows the stress/strain behaviour of five aluminium/Metlbond specimens which contained inherent flaws of various sizes. It should be noted that the same strain rate was used for all five specimens. Three of these specimens are the ones shown in Figs 1-3 and are marked accordingly in Fig. 4. Apparently i 2o every one of the specimens exhibited different ultimate stress and rupture strain properties; and such variation should be attributed to the presence of inherent flaws. The shear stress/strain ~°
I
!
(A-r1
#
'
4,
,~
,-~
o
Fig. 4 Symmetric single lap adhesive stress/strain response of various Metlbond1 1 13 specimens with inherent voids, compared with the predicted curve obtained from bulk shear data for = 8.6 X 10-3%/s
behaviour predicted for the Metlbond1113 adhesive with the application of the modified Bingham visco-elasticplastic model is also shown by broken lines in Fig. 4. Conclusions
Fig. 2 Neutron radiography and fracture surface photographs of Metlbond-l113 specimen A-7 with various inherent voids
lap joint bonded with ~ 0.152 mm thick structural epoxy adhesive (Methlbond-1113), which also contains a carrier cloth. Experiments have shown that 0.1 mm thick advance wire embedded in the adhesive layer of the above mentioned specimens could be
The importance of determining the presence of inherent flaws through an accurate nondestructive testing method becomes obvious when the stress/strain curves of Fig. 4 are compared with the theoretical prediction. Examination of the ultimate values indicates an 85% difference in strains and the predicted ultimate strength for the joints, at the rate used is about 40% higher than that for specimen A-10. Therefore, it is necessary to use a non-destructive testing technique, preferably neutron radiography - due to its good resolution - to either discard the joints with flaws, or account for the associated strength reduction for design considerations. Due to its capability of producing high resolution X-ray pictures of modem lightweight materials, the neutron radiography technique is considered an important addition to the methods of non-destructive testing.
Author
Fig. 3 Neutron radiography and fracture surface photographs of Metlbond-1 113 specimen A-10 with connected inherent void and interfacial regions
330
I N T . J . A D H E S l O N A N D A D H E S I V E S O C T O B E R 1981
Professor Sancaktar is with The Mechanical and Industrial Engineering Department, Clark.son College of Technology, Potsdam, NY 13676, USA.