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Gel electrode imaging of fatigue cracks in aluminium alloys
Gel electrode imaging of fatigue cracks in aluminium alloys W. J. Baxter Previous papers have described a gel electrode technique recently devised for detecting and imaging fatigue cracks in aluminium tested in simple bending. In this study, the technique is shown to be applicable to testing in both bending and torsion and to high strength aluminium alloys 7075-7"6, 2 0 2 4 - T 3 and 2024-T4. Fatigue cracks as short as lO pm in length are consistently detected and located. The flow o f charge during image formation under standard conditions provides a quantitative measure of crack length, which is independent o f a]/oy composition. A crack 1 O0 pxn long can be reliably detected by charge flow measurement; thus, this approach is not as sensitive as the information contained in the actual images.
Key words: fatigue; crack measurement; aluminium alloys; gel electrode imaging
A reliabletechnique for detecting very small fatigue cracks in metal structures would make it possible to evaluate fatigue performance after a short-term test,instead of the customary procedure of waiting for the eventual development of a large crack or total fracture. In the firstof this seriesof reports,1 a new and simple electrochemical method of locating and imaging fatigue cracks in 6061-T6 aluminium was described. It isbased upon two factors:a modified version of a printing technique developed by Klein 2 for the study of defective sitesin anodic oxide films; and adaptation of this technique to detection of defective sites,in the form of microcracks, in the oxide film created during fatigue of the underlying metal. The procedure consists simply of contacting the specimen with a dried surface film of a gel-electrolytecontaining potassium iodide (KI) and starch, and applying a voltage pulse to stimulate the flow of corrosion current which flows preferentially to the microcracks. This current anodicaliy oxidizes the KI to release iodine ions, which react with the starch to form a black complex. These iodine complexes are retained in the skin of the gel and provide an accurate spatialrepresentation of the sitesof current flow. In the initialstudy, this technique was confined to the imaging of fatigue cracks in 6061-T6 aluminium, t It was shown that cracks as short as 30 pxn could be detected, while the charge flow during the formation of the image provided an alternativemeasure of crack length. In a second seriesof experiments, 3 the technique was extended to the detection of fatigue deformation in 1100-0 aluminium. This report demonstrates that the technique is equally applicable to other higher strength aluminium alloys. The effect is illustratedby examples of images of fatigue cracks in 7075-T6, 2024-T4, and 2024-T3 aluminium. In addition, a simple modification of the technique is described for simultaneous imaging of large areas with pronounced curvature. The imaging conditions were selected to provide good spatialresolution at the expense of sensitivity.Nevertheless, fatigue cracks only 10pro long were detected, while the gel electrode image recorded some features of the fatigue damage which were difficult to discern with a scanning electron microscope. Finally, the flow of charge during the formation of the image is shown to be proportional to the length of the fatigue crack, and independent of the alloy composition. This charge flow provides an alternativemeasure of the
crack length, but it is not as sensitiveas direct observation of the image itself.
Experimental procedure The procedure, which has previously been described in detail,I consistsof three steps: specimen preparation including anodization to produce a surface oxide film 14 n m thick; fatigue cycling of the specimen; and gel electrode printing of the microcracks in the oxide film.
Specimen preparation The specimens of 7075-T6 alurninium (of composition 5.5%Zn, 2.5%Mg, 1.7%Cu, 0.3%Fe, and 0.2%Cr) and 2024-T3 aluminium (of composition 4.5% Cu, 1.5% Mg, and 0.6% Mn) were machined from sheet material 1.5 m m thick to produce a conventional tapered bending fatigue specimen, as used previously for the experiments on 6061-T6 aluminium (composition 0.6% Si, I% Mg, 0.25% Cu and 0.2% Cr). A small notch (radius ~0.5 m m ) was filed in one edge to accelerate the process of crack initiation.The specimens of 2024-T4 aluminium (of same composition as 2024-T3 aluminium) were machined from I0 m m diameter rod to form a gauge section of 6.5 turn diameter and 30 m m long. Some of the 7075-T6 specimens were mechanically polished to aid subsequent examination by optical and scanning electron microscopy, but polishing is not necessary for successful gel electrode imaging. The other specimens were cleaned in chromic acid at 70°C. All specimens were then anodized in a 3% solution of tartaricacid at a potential of 10 V to form a surface oxide film 1 4 n m thick. 4
Fatigue cycling The specimens of 7075-T6 and 2024-T3 aluminium were fatigued by reverse bending, and required about 105 cycles to initiatea fatigue crack at the notch. The cylindrical specimens of 2024-T4 aluminium were fatigued in torsion to produce a surface cyclic strainof ±6.5 xl0 -3.
Gel electrode printing The printing electrolyte consists of an agar gel containing 0.2 M potassium iodide, 0.05 M borax and 30 g/l of starch. For most of the experiments this warm fluid mixture was
0142-1123/83/010037--6 $03.00© 1983 Butterworth & Co (Publishers) Ltd
INT. J. FATIGUE January 1983
37
also provided the basis for independent measurements of the length of the fatigue cracks. The flow of current during image formation was recorded on a Nicolet digital oscilloscope and stored on magnetic discs. This information was later displayed on an X - Y recorder, and the total charge flow during printing was obtained by measuring the area under the curve.
Images of fatigue c r a c k s
Fig. 1 Photograph of flexible gel electrode being held in contact with a torsional fatigue specimen. The gel coating on the polyurethane foam wraps around half of the specimen
As in the earlier experiments on 6061-T6 aluminium, it was found that the sensitivity and spatial resolution attainable are determined to some extent by the charge flow, and that this can be controlled by the duration of the vokage pulse. For a 10V pulse, a duration of 100ms consistently produced an image which was clearly visible to the unaided eye, but subsequent examination of the gel with an optical microscope revealed that the image was overexposed, so that some of the more subtle features were not resolvable. A pulse of I ms duration produced an image with good spatial resolution, but the image was so faint that it was difficultto photograph with an optical microscope. A pulse of i0 ms duration was in general the best compromise, providing an image with sufficient spatial resolution and contrast to permit direct correlation with scanning electron micrographs of the specimen itself.Unless specified to the contrary, all the images presented below were formed with 1 0 V pulses of 10ms duration.
2024 Aluminium
Fig. 2 Photograph of gel electrode sandwich being held to contact a torsional fatigue specimen while a voltage pulse is applied to the cathode
dispensed into small lengths (~30 nun) of plastic tube 6 m m in diameter. The plastic tube was over-filled so that, upon cooling, the liquid formed a smooth hemispherical gel tip at one end of the tube. A piece of aluminium wire was sealed into the other end of the tube to serve as a cathode. A flexible skin formed on the gel tip in approximately 5 rain. This tip was then pressed gently against the surface of the specimen and a pulse of negative potential (I0 V) was applied to the cathode. In the case of the torsion fatigue specimens, a fatigue crack could develop anywhere along the uniform gauge section, so it was necessary to contact the entire surface with a gel electrode. A simple and effective large-area mapping technique is illustrated in Figs 1 and 2. T w o pieces of white polyurethane foam were coated with a thin layer of the gel and then dried for 5 min. These flexible gel electrodes were held with the specimen sandwiched between them (Fig. 2), the flexible gel wrapping around and contacting the entire gauge section. A n aluminium wire contacted the gel at the edge of the sandwich to provide a cathode. Again a negative voltage pulse (I0 V) was applied to the cathode. In this way an image of the total surface was formed without removing the specimen from the fatigue machine. The images formed in the skin of the gel were viewed and photographed with an optical microscope. These images were then correlated with observations of the specimens in a scanning electron microscope. The latter
38
I N T . J. F A T I G U E January 1983
The photograph in Fig. 3 shows a mirror image of a fatigue crack in a torsional fatigue specimen. This image was obtained by the large area mapping technique, with a pulse duration of 100ms to provide high contrast. In this case the crack is quite long (~11 mm), but the image is noteworthy because (as detailed comparisons with scanning electron micrographs of the specimen showed) the image is a faithful and clearly visible reproduction of the many irregular features of the crack, including many secondary branches. The complete absence of any other features such as roughness on the surface of the torsion specimen should also be noted. One example of the correlation between an image and scanning electron micrographs of the actual fatigue
Fig. 3 Photograph of a gel electrode image of a fatigue crack in a torsion specimen of 2024--T4 aluminium. Image formed by large area mapping technique with a 10 V, 100 ms pulse
crack is illustrated in Figs 4 - 8 . A magnified view of a portion of a gel electrode image, formed with a 10 ms pulse, is shown in Fig. 4. The total crack length in this case was ~0.8 ram, but the portion of the image shown here contains several features that are very distinctive and illustrates the sensitivity attainable. The scanning electron micro*graph in Fig. 5 shows the cracks corresponding to just the central portion of the gel electrode image, as can be readily identified by the 'island' structure. Note that the two images are left-right mirror images. Of particular interest are two small features indicated by arrows (A and B) in Fig. 4. At A, at the top of the 'island', there is a small secondary crack, only ~20/~m long, which is clearly visible in Fig. 4 but not discernible in the scanning electron micrograph in Fig. 5. However, as shown in Fig. 6, at higher magnification this crack can be seen, but only because its location is known. The feature at B, pinpointed in Fig. 4 but again not visible in Fig. 5, is due to a small side-branch on a secondary crack. A magnified portion of the secondary crack is shown in the
scanning electron micrograph in Fig. 7, the small side branch being only ~10/zm long. One other feature of the image shown in Fig. 4 that is of particular interest is the large spot (C) with adjacent gaps in the gel electrode image. This was caused by the extruded material shown in the scanning electron micrograph, Fig. 8, which provided a localized but large area of contact with the gel, thereby producing the large spot, but apparently preventing the gel from contacting the adjacent regions of the crack.
7 0 7 5 - 7"6 A luminium Similar images may also be formed of fatigue cracks in 7075-T6 aluminium. A magnified view of an image formed by a 10ms pulse is shown in Fig. 9. In this case two main cracks have grown from a notch; one crack is 0.9ram long, the other is 0.1 mm long. The image of the longer crack is of interest because of the unusual non-uniformities. Examination of the specimen in a scanning electron microscope
Fig. 4 Magnified view of a portion of a gel electrode image of a crack in 2024--T4 aluminium. Image formed with a 10 V, 10 ms pulse
Figl 5 Scanning electron micrograph of fatigue cracks in 2024--T4 aluminium corresponding to central portion of image shown in Fig. 4. This figure is a mirror image of Fig. 4
INT. J. FATIGUE January 1983
39
Fig. 6 Scanning electron micrograph showing small secondary crack responsible for the feature labelled 'A' in Fig. 4 Fig. 8 Scanning electron micrograph showing pronounced extrusion of material out of a fatigue crack at location corresponding to the large spot labelled "C' in Fig. 4
Fig. 7 Scanning electron micrograph showing a small side branch on a secondary crack, responsible for the feature labelled 'B' in Fig. 4
revealed that these could he correlated with irregularities of the crack itself. For example, the gap in the image at location A (Fig. 9) corresponds to a region where two sections of the crack have not quite linked together, as shown by the scanning electron micrograph in Fig. 10. Similarly, the very broad portion of the image just below the gap is due to the combined and overlapping contributions of the main crack and a short, almost parallel, secondary crack (Fig. 10). As another example, the large spot and branch at location B in Fig. 9 was produced by the combination of an extrusion emanating from the crack and some fine secondary cracks nearby. These features are clearly visible in the scanning electron micrograph in Fig. 11. The gel tip image obtained from the opposite surface of this same specimen is shown in the optical micrograph in Fig. 12. The main feature of this image was formed by a fatigue crack 160 ~ long, which is clearly defined in the scanning electron micrograph in Fig. 13. More interesting, however, is the small triangular feature in Fig. 12, which is an image of the two much smaller cracks at the edge of the specimen (Fig. 13). One of these cracks is ~25 pm in length, while the other is only ~14/n'n. If a fatigue crack is only ~10/lrn in length, the image formed by a 10ms pulse is often simply a spot rather than a linear feature. This represents the limit of resolution (but
40
I N T . J. F A T I G U E J a n u a r y 1983
Fig. 9 Magnified image of fatigue cracks in a polished specimen of 7075--T6 aluminium. Image printed with a 10 V, 10 ms pulse
not sensitivity) of an image formed in this way. Finally it is worth noting that on several occasions the fatigue damage associated with such spots in the gel image could not be identified using a scanning electron microscope. This aspect, namely the ultimate sensitivity of the gel electrode technique for detecting fatigue damage, will be the subject of a future report.
"E J OF :ClMEN
Fig. 12 Magnified image of fatigue cracks in polished 7075--T6 aluminium, printed with a 10 V, 10 ms pulse
Fig. 10 Scanning electron micrograph showing a discontinuity in a fatigue crack in 7075--T6 aluminium, corresponding to the 'gap' in the image at location A in Fig. 9. This is a mirror image of A in Fig. 9
EXTRUS I ON
SECONDARY CRACK
Fig. 13 Scanning electron micrograph showing fatigue cracks in 7075--T6 aluminium corresponding to image in Fig. 12
Charge f l o w and crack l e n g t h
i
!
100 Frn Fig. 11 Scanning electron micrograph of a portion of fatigue crack in 7075--T6 aluminium corresponding to location B in Fig. 9
The total flow of charge during the formation of an image of a fatigue crack is proportional to the total length of the crack being imaged. This is illustrated in Fig. 14 using the measurements from this study on the 7075-T6, 2024-T3 and 2 0 2 4 - T 4 alloys, as well as the earlier results with 6 0 6 1 - T 6 aluminium. The line drawn through the data is given by: Q = 1.25 x 10-51 coulombs
I N T . J. F A T I G U E January 1983
41
o]
, # 1
10-4
10v Oxide
IOV IOms Pulse --
i
o
0 2024-T4
_~
~
7075-T6
/ 0/
I
I,
1
I'
I
flow does not create a visible image. Thus, as demonstrated in the previous section, image inspection by optical microscopy can definitely reveal the presence of fatigue cracks as short as 10#m. Summary
g 16:
• I0
-2
I0
'
-t
I0
I
I0
Crock length (mm) Fig. 14 Effect of crack length on the charge flow during the formation of an image with a I 0 V, I 0 ms pulse
where I is the length of the crack in mm. Within the limits of the experimental scatter evident in Fig. 14, this result is independent of the alloy composition. Many sources of the scatter in Fig. 14 can be understood in terms of the information contained in the images themselves. For example, it was shown above that irregularities in the density of the image, which simply manifest irregularities in the current density, can arise from: variations in the width of a crack (Figs 5 and 10); the presence of extrusions (Figs 8 and 11); and the extent of secondary cracking (Figs 4 and 9). If the latter is at all extensive, as imaged in Figs 4 and 9, then this also introduces errors in the measurement of total crack length. The apparent deviation from linearity in Fig. 14 for cracks shorter than ~200/nn, is due primarily to the flow of current to locations other than the crack itself. For example, the conductive gel tip typically contacts an area of 13 mm 2 of the oxide coated surface of the metal, forming a capacitance of ~0.1/~F. Thus even in the absence of any fatigue cracks, when a 10V pulse is applied, 10.6 C will flow to charge up this capacitance. This factor alone can account for most of the above mentioned deviations from linearity. Finally it should be noted that the recorded values of charge flow to regions without fatigue cracks can vary from specimen to specimen, as indicated by the bracket (lower left) in Fig. 14. This variation is believed to arise from imperfections in the oxide film, which permit small leakage currents to flow (the original problem investigated by Klein). 2 The presence of these unpredictable currents limits the sensitivity of charge flow measurements for the detection of fatigue cracks. Experience indicates that cracks of length >/100/.an can be reliably detected by charge flow measurements. On the other hand, the images themselves do not suffer from this limitation, since a small uniform charge
42
INT. J. FATIGUE January 1983
The gel electrode method for detecting and imaging fatigue cracks is shown to be applicable to a range of aluminium alloys. Images of fatigue cracks are formed in a gel without removing the test-piece from the fatigue machine. Although the imaging conditions were selected to provide good spatial resolution at the expense of sensitivity, fatigue cracks as short as 10pro in length were consistently detected and located. The images in the gel may be conveniently viewed with a pocket magnifier or an optical microscope. In this way, the gel imaging method endows an optical microscope with the sensitivity of a scanning electron microscope, insofar as fatigue cracks are concerned. The flow of charge during the formation of an image is proportional to crack length, and is independent of the composition of all alloys studied so far. Thus the charge flow provides an alternative measure of crack length, and can detect cracks as short as 100 p_m. However, charge flow measurements are not as sensitive as the gel electrode images themselves, which reveal cracks only 10/~m long. The ultimate sensitivity of the technique for detecting fatigue damage in high strength aluminium alloys will be the subject of a future report.
Acknowledgments
The author is grateful for the assistance of D. W. Gorkiewicz in performing these experiments and to W. Lange for the scanning electron micrographs. References 1.
Baxter, W. J. 'Gel electrode imaging of metal fatigue: I. Cracks in 6061--T6 aluminum' Metallurgical Transactions 13A (1982) pp 1413--1419
2.
Klein, G. P. 'A redox printing technique for the study of the electronic conductivity of anodic oxide films on valve metals' J Electrochem Soc 113 (1966) pp 345--348
3.
Baxter, W. J. 'Gel electrode imaging of metal fatigue: I1. Deformation in 1100 aluminum' Metallurgical Transactions
4.
13A (1982) pp 1421--1427 Gro~kreutz, J. C. 'Mechanical properties of metal oxide films' J Electrochern Soc 116 (1969) pp 1232-1237
Author
The author is with the Physics Department, General Motors Research Laboratories. Inquiries should be addressed to Dr W. J. Baxter, General Motors Research Laboratories, Warren, Michigan 48090-9055, USA.
Key words: fatigue; crack measurement; aluminium alloys; gel electrode imaging
A reliabletechnique for detecting very small fatigue cracks in metal structures would make it possible to evaluate fatigue performance after a short-term test,instead of the customary procedure of waiting for the eventual development of a large crack or total fracture. In the firstof this seriesof reports,1 a new and simple electrochemical method of locating and imaging fatigue cracks in 6061-T6 aluminium was described. It isbased upon two factors:a modified version of a printing technique developed by Klein 2 for the study of defective sitesin anodic oxide films; and adaptation of this technique to detection of defective sites,in the form of microcracks, in the oxide film created during fatigue of the underlying metal. The procedure consists simply of contacting the specimen with a dried surface film of a gel-electrolytecontaining potassium iodide (KI) and starch, and applying a voltage pulse to stimulate the flow of corrosion current which flows preferentially to the microcracks. This current anodicaliy oxidizes the KI to release iodine ions, which react with the starch to form a black complex. These iodine complexes are retained in the skin of the gel and provide an accurate spatialrepresentation of the sitesof current flow. In the initialstudy, this technique was confined to the imaging of fatigue cracks in 6061-T6 aluminium, t It was shown that cracks as short as 30 pxn could be detected, while the charge flow during the formation of the image provided an alternativemeasure of crack length. In a second seriesof experiments, 3 the technique was extended to the detection of fatigue deformation in 1100-0 aluminium. This report demonstrates that the technique is equally applicable to other higher strength aluminium alloys. The effect is illustratedby examples of images of fatigue cracks in 7075-T6, 2024-T4, and 2024-T3 aluminium. In addition, a simple modification of the technique is described for simultaneous imaging of large areas with pronounced curvature. The imaging conditions were selected to provide good spatialresolution at the expense of sensitivity.Nevertheless, fatigue cracks only 10pro long were detected, while the gel electrode image recorded some features of the fatigue damage which were difficult to discern with a scanning electron microscope. Finally, the flow of charge during the formation of the image is shown to be proportional to the length of the fatigue crack, and independent of the alloy composition. This charge flow provides an alternativemeasure of the
crack length, but it is not as sensitiveas direct observation of the image itself.
Experimental procedure The procedure, which has previously been described in detail,I consistsof three steps: specimen preparation including anodization to produce a surface oxide film 14 n m thick; fatigue cycling of the specimen; and gel electrode printing of the microcracks in the oxide film.
Specimen preparation The specimens of 7075-T6 alurninium (of composition 5.5%Zn, 2.5%Mg, 1.7%Cu, 0.3%Fe, and 0.2%Cr) and 2024-T3 aluminium (of composition 4.5% Cu, 1.5% Mg, and 0.6% Mn) were machined from sheet material 1.5 m m thick to produce a conventional tapered bending fatigue specimen, as used previously for the experiments on 6061-T6 aluminium (composition 0.6% Si, I% Mg, 0.25% Cu and 0.2% Cr). A small notch (radius ~0.5 m m ) was filed in one edge to accelerate the process of crack initiation.The specimens of 2024-T4 aluminium (of same composition as 2024-T3 aluminium) were machined from I0 m m diameter rod to form a gauge section of 6.5 turn diameter and 30 m m long. Some of the 7075-T6 specimens were mechanically polished to aid subsequent examination by optical and scanning electron microscopy, but polishing is not necessary for successful gel electrode imaging. The other specimens were cleaned in chromic acid at 70°C. All specimens were then anodized in a 3% solution of tartaricacid at a potential of 10 V to form a surface oxide film 1 4 n m thick. 4
Fatigue cycling The specimens of 7075-T6 and 2024-T3 aluminium were fatigued by reverse bending, and required about 105 cycles to initiatea fatigue crack at the notch. The cylindrical specimens of 2024-T4 aluminium were fatigued in torsion to produce a surface cyclic strainof ±6.5 xl0 -3.
Gel electrode printing The printing electrolyte consists of an agar gel containing 0.2 M potassium iodide, 0.05 M borax and 30 g/l of starch. For most of the experiments this warm fluid mixture was
0142-1123/83/010037--6 $03.00© 1983 Butterworth & Co (Publishers) Ltd
INT. J. FATIGUE January 1983
37
also provided the basis for independent measurements of the length of the fatigue cracks. The flow of current during image formation was recorded on a Nicolet digital oscilloscope and stored on magnetic discs. This information was later displayed on an X - Y recorder, and the total charge flow during printing was obtained by measuring the area under the curve.
Images of fatigue c r a c k s
Fig. 1 Photograph of flexible gel electrode being held in contact with a torsional fatigue specimen. The gel coating on the polyurethane foam wraps around half of the specimen
As in the earlier experiments on 6061-T6 aluminium, it was found that the sensitivity and spatial resolution attainable are determined to some extent by the charge flow, and that this can be controlled by the duration of the vokage pulse. For a 10V pulse, a duration of 100ms consistently produced an image which was clearly visible to the unaided eye, but subsequent examination of the gel with an optical microscope revealed that the image was overexposed, so that some of the more subtle features were not resolvable. A pulse of I ms duration produced an image with good spatial resolution, but the image was so faint that it was difficultto photograph with an optical microscope. A pulse of i0 ms duration was in general the best compromise, providing an image with sufficient spatial resolution and contrast to permit direct correlation with scanning electron micrographs of the specimen itself.Unless specified to the contrary, all the images presented below were formed with 1 0 V pulses of 10ms duration.
2024 Aluminium
Fig. 2 Photograph of gel electrode sandwich being held to contact a torsional fatigue specimen while a voltage pulse is applied to the cathode
dispensed into small lengths (~30 nun) of plastic tube 6 m m in diameter. The plastic tube was over-filled so that, upon cooling, the liquid formed a smooth hemispherical gel tip at one end of the tube. A piece of aluminium wire was sealed into the other end of the tube to serve as a cathode. A flexible skin formed on the gel tip in approximately 5 rain. This tip was then pressed gently against the surface of the specimen and a pulse of negative potential (I0 V) was applied to the cathode. In the case of the torsion fatigue specimens, a fatigue crack could develop anywhere along the uniform gauge section, so it was necessary to contact the entire surface with a gel electrode. A simple and effective large-area mapping technique is illustrated in Figs 1 and 2. T w o pieces of white polyurethane foam were coated with a thin layer of the gel and then dried for 5 min. These flexible gel electrodes were held with the specimen sandwiched between them (Fig. 2), the flexible gel wrapping around and contacting the entire gauge section. A n aluminium wire contacted the gel at the edge of the sandwich to provide a cathode. Again a negative voltage pulse (I0 V) was applied to the cathode. In this way an image of the total surface was formed without removing the specimen from the fatigue machine. The images formed in the skin of the gel were viewed and photographed with an optical microscope. These images were then correlated with observations of the specimens in a scanning electron microscope. The latter
38
I N T . J. F A T I G U E January 1983
The photograph in Fig. 3 shows a mirror image of a fatigue crack in a torsional fatigue specimen. This image was obtained by the large area mapping technique, with a pulse duration of 100ms to provide high contrast. In this case the crack is quite long (~11 mm), but the image is noteworthy because (as detailed comparisons with scanning electron micrographs of the specimen showed) the image is a faithful and clearly visible reproduction of the many irregular features of the crack, including many secondary branches. The complete absence of any other features such as roughness on the surface of the torsion specimen should also be noted. One example of the correlation between an image and scanning electron micrographs of the actual fatigue
Fig. 3 Photograph of a gel electrode image of a fatigue crack in a torsion specimen of 2024--T4 aluminium. Image formed by large area mapping technique with a 10 V, 100 ms pulse
crack is illustrated in Figs 4 - 8 . A magnified view of a portion of a gel electrode image, formed with a 10 ms pulse, is shown in Fig. 4. The total crack length in this case was ~0.8 ram, but the portion of the image shown here contains several features that are very distinctive and illustrates the sensitivity attainable. The scanning electron micro*graph in Fig. 5 shows the cracks corresponding to just the central portion of the gel electrode image, as can be readily identified by the 'island' structure. Note that the two images are left-right mirror images. Of particular interest are two small features indicated by arrows (A and B) in Fig. 4. At A, at the top of the 'island', there is a small secondary crack, only ~20/~m long, which is clearly visible in Fig. 4 but not discernible in the scanning electron micrograph in Fig. 5. However, as shown in Fig. 6, at higher magnification this crack can be seen, but only because its location is known. The feature at B, pinpointed in Fig. 4 but again not visible in Fig. 5, is due to a small side-branch on a secondary crack. A magnified portion of the secondary crack is shown in the
scanning electron micrograph in Fig. 7, the small side branch being only ~10/zm long. One other feature of the image shown in Fig. 4 that is of particular interest is the large spot (C) with adjacent gaps in the gel electrode image. This was caused by the extruded material shown in the scanning electron micrograph, Fig. 8, which provided a localized but large area of contact with the gel, thereby producing the large spot, but apparently preventing the gel from contacting the adjacent regions of the crack.
7 0 7 5 - 7"6 A luminium Similar images may also be formed of fatigue cracks in 7075-T6 aluminium. A magnified view of an image formed by a 10ms pulse is shown in Fig. 9. In this case two main cracks have grown from a notch; one crack is 0.9ram long, the other is 0.1 mm long. The image of the longer crack is of interest because of the unusual non-uniformities. Examination of the specimen in a scanning electron microscope
Fig. 4 Magnified view of a portion of a gel electrode image of a crack in 2024--T4 aluminium. Image formed with a 10 V, 10 ms pulse
Figl 5 Scanning electron micrograph of fatigue cracks in 2024--T4 aluminium corresponding to central portion of image shown in Fig. 4. This figure is a mirror image of Fig. 4
INT. J. FATIGUE January 1983
39
Fig. 6 Scanning electron micrograph showing small secondary crack responsible for the feature labelled 'A' in Fig. 4 Fig. 8 Scanning electron micrograph showing pronounced extrusion of material out of a fatigue crack at location corresponding to the large spot labelled "C' in Fig. 4
Fig. 7 Scanning electron micrograph showing a small side branch on a secondary crack, responsible for the feature labelled 'B' in Fig. 4
revealed that these could he correlated with irregularities of the crack itself. For example, the gap in the image at location A (Fig. 9) corresponds to a region where two sections of the crack have not quite linked together, as shown by the scanning electron micrograph in Fig. 10. Similarly, the very broad portion of the image just below the gap is due to the combined and overlapping contributions of the main crack and a short, almost parallel, secondary crack (Fig. 10). As another example, the large spot and branch at location B in Fig. 9 was produced by the combination of an extrusion emanating from the crack and some fine secondary cracks nearby. These features are clearly visible in the scanning electron micrograph in Fig. 11. The gel tip image obtained from the opposite surface of this same specimen is shown in the optical micrograph in Fig. 12. The main feature of this image was formed by a fatigue crack 160 ~ long, which is clearly defined in the scanning electron micrograph in Fig. 13. More interesting, however, is the small triangular feature in Fig. 12, which is an image of the two much smaller cracks at the edge of the specimen (Fig. 13). One of these cracks is ~25 pm in length, while the other is only ~14/n'n. If a fatigue crack is only ~10/lrn in length, the image formed by a 10ms pulse is often simply a spot rather than a linear feature. This represents the limit of resolution (but
40
I N T . J. F A T I G U E J a n u a r y 1983
Fig. 9 Magnified image of fatigue cracks in a polished specimen of 7075--T6 aluminium. Image printed with a 10 V, 10 ms pulse
not sensitivity) of an image formed in this way. Finally it is worth noting that on several occasions the fatigue damage associated with such spots in the gel image could not be identified using a scanning electron microscope. This aspect, namely the ultimate sensitivity of the gel electrode technique for detecting fatigue damage, will be the subject of a future report.
"E J OF :ClMEN
Fig. 12 Magnified image of fatigue cracks in polished 7075--T6 aluminium, printed with a 10 V, 10 ms pulse
Fig. 10 Scanning electron micrograph showing a discontinuity in a fatigue crack in 7075--T6 aluminium, corresponding to the 'gap' in the image at location A in Fig. 9. This is a mirror image of A in Fig. 9
EXTRUS I ON
SECONDARY CRACK
Fig. 13 Scanning electron micrograph showing fatigue cracks in 7075--T6 aluminium corresponding to image in Fig. 12
Charge f l o w and crack l e n g t h
i
!
100 Frn Fig. 11 Scanning electron micrograph of a portion of fatigue crack in 7075--T6 aluminium corresponding to location B in Fig. 9
The total flow of charge during the formation of an image of a fatigue crack is proportional to the total length of the crack being imaged. This is illustrated in Fig. 14 using the measurements from this study on the 7075-T6, 2024-T3 and 2 0 2 4 - T 4 alloys, as well as the earlier results with 6 0 6 1 - T 6 aluminium. The line drawn through the data is given by: Q = 1.25 x 10-51 coulombs
I N T . J. F A T I G U E January 1983
41
o]
, # 1
10-4
10v Oxide
IOV IOms Pulse --
i
o
0 2024-T4
_~
~
7075-T6
/ 0/
I
I,
1
I'
I
flow does not create a visible image. Thus, as demonstrated in the previous section, image inspection by optical microscopy can definitely reveal the presence of fatigue cracks as short as 10#m. Summary
g 16:
• I0
-2
I0
'
-t
I0
I
I0
Crock length (mm) Fig. 14 Effect of crack length on the charge flow during the formation of an image with a I 0 V, I 0 ms pulse
where I is the length of the crack in mm. Within the limits of the experimental scatter evident in Fig. 14, this result is independent of the alloy composition. Many sources of the scatter in Fig. 14 can be understood in terms of the information contained in the images themselves. For example, it was shown above that irregularities in the density of the image, which simply manifest irregularities in the current density, can arise from: variations in the width of a crack (Figs 5 and 10); the presence of extrusions (Figs 8 and 11); and the extent of secondary cracking (Figs 4 and 9). If the latter is at all extensive, as imaged in Figs 4 and 9, then this also introduces errors in the measurement of total crack length. The apparent deviation from linearity in Fig. 14 for cracks shorter than ~200/nn, is due primarily to the flow of current to locations other than the crack itself. For example, the conductive gel tip typically contacts an area of 13 mm 2 of the oxide coated surface of the metal, forming a capacitance of ~0.1/~F. Thus even in the absence of any fatigue cracks, when a 10V pulse is applied, 10.6 C will flow to charge up this capacitance. This factor alone can account for most of the above mentioned deviations from linearity. Finally it should be noted that the recorded values of charge flow to regions without fatigue cracks can vary from specimen to specimen, as indicated by the bracket (lower left) in Fig. 14. This variation is believed to arise from imperfections in the oxide film, which permit small leakage currents to flow (the original problem investigated by Klein). 2 The presence of these unpredictable currents limits the sensitivity of charge flow measurements for the detection of fatigue cracks. Experience indicates that cracks of length >/100/.an can be reliably detected by charge flow measurements. On the other hand, the images themselves do not suffer from this limitation, since a small uniform charge
42
INT. J. FATIGUE January 1983
The gel electrode method for detecting and imaging fatigue cracks is shown to be applicable to a range of aluminium alloys. Images of fatigue cracks are formed in a gel without removing the test-piece from the fatigue machine. Although the imaging conditions were selected to provide good spatial resolution at the expense of sensitivity, fatigue cracks as short as 10pro in length were consistently detected and located. The images in the gel may be conveniently viewed with a pocket magnifier or an optical microscope. In this way, the gel imaging method endows an optical microscope with the sensitivity of a scanning electron microscope, insofar as fatigue cracks are concerned. The flow of charge during the formation of an image is proportional to crack length, and is independent of the composition of all alloys studied so far. Thus the charge flow provides an alternative measure of crack length, and can detect cracks as short as 100 p_m. However, charge flow measurements are not as sensitive as the gel electrode images themselves, which reveal cracks only 10/~m long. The ultimate sensitivity of the technique for detecting fatigue damage in high strength aluminium alloys will be the subject of a future report.
Acknowledgments
The author is grateful for the assistance of D. W. Gorkiewicz in performing these experiments and to W. Lange for the scanning electron micrographs. References 1.
Baxter, W. J. 'Gel electrode imaging of metal fatigue: I. Cracks in 6061--T6 aluminum' Metallurgical Transactions 13A (1982) pp 1413--1419
2.
Klein, G. P. 'A redox printing technique for the study of the electronic conductivity of anodic oxide films on valve metals' J Electrochem Soc 113 (1966) pp 345--348
3.
Baxter, W. J. 'Gel electrode imaging of metal fatigue: I1. Deformation in 1100 aluminum' Metallurgical Transactions
4.
13A (1982) pp 1421--1427 Gro~kreutz, J. C. 'Mechanical properties of metal oxide films' J Electrochern Soc 116 (1969) pp 1232-1237
Author
The author is with the Physics Department, General Motors Research Laboratories. Inquiries should be addressed to Dr W. J. Baxter, General Motors Research Laboratories, Warren, Michigan 48090-9055, USA.