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Repeatability of gel electrode measurements of fatigue deformation in 6061-T6 aluminium
InrJFarigue 6 No 4 (1984) pp 243--252
Repeatability of gel electrode measurements of fatigue deformation in 6061-T6 aluminium W. J. B a x t e r In a previous report, using 5 V, 5 s pulses, the gel electrode method was shown to detect fatigue deformation in 6061-T6 aluminium, but each specimen was only measured once. In this study the technique is refined so that the measurements can be repeated many times to monitor the distribution and accumulation of deformation at intervals during a fatigue test. The current from the gel electrode produces both passivation (film formation) and corrosion at the sites of deformation on the metal surface. These two influences are balanced if the gel electrode measurement is performed with a 5 V, 350 ms pulse, and the charge flow during subsequent measurements is then repeatable to within +15%. Although this pulse duration is an order of magnitude shorter than that employed in the previous study, excellent sensitivity is retained. Fatigue deformation is detected in 6061-T6 aluminium as early as 0.1% of the fatigue life. The charge flow measurements and gel electrode images show that the deformation accumulates at many sites during the early stages (~5%) of fatigue life, but by about 50% of life clusters of sites of fatigue damage are well established, and provide a path for final fracture. Key words: fatigue; deformation; 6061-T6 aluminium; gel electrode method A method of detecting and measuring the development of the early stages of metal fatigue could provide the basis of an abbreviated testing technique for the prediction of fatigue life. In a recent series of reports, 1-s a new and simple technique was introduced which detects and images fatigue damage in aluminium alloys with an electrochemical gel electrode. This new technique is based upon two factors: (i) the creation of microcracks in a 14 nm surface anodic oxide film during fatigue of the underlying metal, and (ii) the detection of these microcracks by contacting the surface with a gel electrolyte containing KI and starch and applying a voltage pulse. Current flows preferentially through the microcracks in the oxide film and releases iodine ions which react with the starch to form a black complex in the surface of the gel. In this way an image is formed of the sites of current flow, ie the sites of fatigue damage in the underlying metal. Further, the total charge that flows to form the image is a quantitative measure of the extent of the fatigue damage. The emphasis in this paper is on the quantitative capability of the technique, particularly the repeatability of the current flow through the microcracks. The previously reported experiments on aluminium alloys may be classified in terms of the duration of the voltage pulse applied to the gel electrode. In general, shorter pulses produce images with better spatial resolution: fatigue cracks as short as 10~um have been imaged with a 10 ms pulse, 1'3 while a 1 ms pulse can even image individual slip bands. 4 On the other hand, longer pulses (which overexpose the image) have the advantage of increasing the charge flow during imaging, so that the sensitivity for detecting and quantitatively measuring deformation is greatly enhanced. The ultimate sensitivity is limited by the eventual breakdown of the 14 nm oxide film during the application of a long voltage pulse. In a series of experiments on 1100-0 and 6061-T6 aluminium, l's a 5 V, 5 s pulse was found to be just within this limit, and the resulting current flow could easily detect the fatigue deformation
that precedes the appearance of a fatigue crack. The gel electrode images are greatly overexposed under these conditions but the presence of black spots on the gel, as well as measurements of the charge flow during imaging, detected the deformation at <(1% of the fatigue life. In the above experiments each specimen was measured only once with the gel electrode. This report is concerned with repetitive measurements at the same location on 6061-T6 aluminium, as is required to monitor the continuous development of surface deformation during the early stages of metal fatigue. The current flow during gel electrode imaging is shown to promote both passivation and corrosion at the fatigue-induced microcracks in the 14 nm oxide, and both of these processes can exert a substantial influence on the current flow during a subsequent repeat measurement. However, it will be shown that the passivation and corrosion processes may be balanced by the selection of a 5 V, 350 ms pulse, so that very good repeatability can be achieved, while retaining excellent sensitivity. Experimental details The experiments described in this study can be divided into two categories: tensile tests and fatigue tests. Since experiments aimed at defining the effects of pulse duration on subsequent measurements involved many comparative measurements, they were performed on tensile specimens where the amount and uniformity of the deformation could be easily controlled. Subsequently, these results were applied to the accurate, repetitive measurement of deformation in fatigue specimens. The experimental procedure was the same in both sets of experiments and consisted of three steps. Specimen preparation The specimens were machined from sheet material 1.5 mm thick, with an average grain size of ~ 3 0 # m . The tensile
0142-1123/84/040243--10 $3.00 © 1984 Butterworth & Co (Publishers) Ltd Int J Fatigue October 1984
243
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This charge flow always correlated with the density of spots that developed on the gel tip, which could be viewed with a pocket magnifier or an optical microscope.
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E f f e c t o f pulse d u r a t i o n
Fig. 1 G e o m e t r y o f fatigue specimens. A l l d i m e n s i o n s in m m
specimens had a gauge section with parallel sides 60 mm long and 13 mm wide (ASTM Specification No A370). The fatigue specimens had the conventional tapered cantilever beam geometry as shown in Fig. 1. All the specimens were degreased with acetone and cleaned by immersion in chromic acid at 75°C for 5 min. After rinsing with water and alcohol, they were anodized in a 3% solution of tartaric acid at a potential of 10 V to form a surface oxide film 14 nm thick. Specimen
The previously reported measurements of deformation in 6061-T6 aluminium s with the gel electrode were performed with a 5 V, 5 s pulse, but each location on the specimen was only measured once. In the present experiments, it was found that if such measurements are repeated at the same location, the flow of charge is much greater than that observed initially. This observation warranted a more complete investigation of the effect of repeated measurements at the same location with pulses of different duration. As shown in Fig. 2, the charge flow during successive measurements on a specimen deformed to a tensile strain of 3 × 10 -2 could be either greater or less than the initial charge flow, depending on the duration of the pulse. For a 5 V, 500 ms pulse the charge increases each time, but for a 5 V, 50 ms pulse the charge decreases quite rapidly before stabilizing at a much lower value. A 5 V, 250 ms pulse offers a compromise between these two opposing effects, the charge decreasing only slightly during a series of six measurements. These results demonstrate that
deformation
The tensile specimens were deformed in an Instron Universal Test System, and the elongation determined by the cross head movement (previously calibrated for similar specimens with an extensometer). The fatigue specimens were cycled by reverse bending to produce surface, maximum cyclic strains of +3.0 × 10 -3. This resulted in an average fatigue life of (2.1 + 0.2) × l 0 s cycles. The tensile specimens were removed from the Instron for gel electrode measurements, but the fatigue specimens were measured in situ at intervals during a fatigue test. Gel e l e c t r o d e
l0-z
imaging
The electrolyte consists of an agar gel containing 0.2 M potassium iodide, 0.05 M borax and 30 g of starch per litre. This warm fluid mixture was dispensed into small lengths ( ~ 3 0 mm) of plastic tube 6 mm 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, sealed into the other end of the tube, served as a cathode. After approximately 5 min a flexible skin formed on the gel tip, which could then be pressed gently against the surface of the specimen. A strip of mylar drawing film with a linear array of holes, 4 mm in diameter and 6.5 m m separation between centres, was taped to the specimen. Flexible gel tips were placed, in turn, within each of these holes to contact the surface of the specimen. In this way the area of contact was defined, and exactly the same area could be contacted repeatedly with a series of gel tips. The true area of gel/ metal contact (7.1 mm 2) was determined in a series of preliminary tests, by measuring the diameter of the gel electrode image with a travelling microscope. A pulse of negative potential (5 V) was applied to the cathode, and the current flow was recorded on a Nicolet digital oscilloscope and stored on magnetic discs. This information was transferred to a Hewlett Packard Model 85 computer, and the total charge flow obtained by numerical integration.
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Sequence Fig. 2 T h e d e n s i t y o f charge w h i c h f l o w e d t o gel electrodes during t h r e e sequences o f measurements at three l o c a t i o n s on a 6 0 6 1 - T 6 specimen d e f o r m e d t o a t o t a l tensile strain o f 3 × 10 -=. D u r a t i o n o f 5 V pulse at each l o c a t i o n is as indicated
Int J Fatigue October 1984
4
the application of the gel electrode can modify the surface of the specimen, the nature of this modification being a function of the duration of the voltage pulse. On the other hand, the relative reproducibility of the charge density during repeated application of the 250 ms pulse does not imply that the surface is not being modified. In fact, examination of the shape of the current transient reveals that changes are taking place. This is illustrated by the two oscilloscope recordings shown in Fig. 3, which correspond to the first and fifth measurements plotted in Fig. 2. The initial charging transient is the same for both measurements, but thereafter the time dependence of the current flow differs considerably. This change in the shape of the current transient curve occurs primarily during the first three pulses, and indicates that the current flow itself changes the conditions at the base of the microcracks in the 14 nm oxide film.
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E f f e c t o f e l e c t r o c h e m i c a l reactions
Previous measurements with the gel electrode on deformed 1100-0 aluminium 2 have shown that the current to the microcracks in the 14 nm oxide film can be separated into three stages: (1) the initial charging of the oxide film, followed by (2) passivation (partial reoxidation) of the surface at the base of the microcracks, and finally (3) corrosion. Stage (1) is an electrical effect and remains the same in all of these experiments. The important influence of the processes of passivation and corrosion during repetitive measurements on 6061-1"6 aluminium is illustrated below.
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During plastic deformation, the 14 nm oxide film develops microcracks exposing fresh metal surfaces. Under normal atmospheric conditions, these surfaces reoxidize very rapidly, but only a very thin layer of 'natural' oxide is formed. During the first application of a gel electrode, it is this layer which controls the initial current flow. But within the first 10-3s this layer is itself modified by a passivation process. This is illustrated in Fig. 4 by the oscilloscope recordings of the current flow produced by four successive 1 ms pulses. (The specimen had been deformed to a tensile strain of 3 x 10-2.) All four pulses exhibit a common initial charging transient, but after ~0.1 ms there is a marked reduction in the rate of decay
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I n t J Fatigue O c t o b e r 1984
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Fig. 5 Effect of duration of prior 5 V pulse on the charge density that flowed during subsequent imaging with a 5 V, 250 ms pulse of a 6061-T6 specimen with 14 nm oxide deformed to a tensile strain of 6 X 10 -=
of the current due to the onset of repassivation. This is particularly pronounced during the first pulse, but the third pulse effectively completes this passivation process, so that the last two pulses are almost indistinguishable. Although this passivation process occurs within only ~ 1 0 -3 s, it exerts an unexpectedly strong influence on the gel electrode imaging process. This is illustrated in Figs 5 and 6, which show the effect of the duration of a preliminary pulse on the charge flow (see Fig. 5) and the appearance of the image (see Fig. 6) produced during a subsequent measurement with a 250 ms pulse. (The specimen in this case had been deformed to a tensile strain of 6 × 10-2.) A basis for comparison is provided by the two data points on the ordinate axis in Fig. 5, and the gel electrode image in Fig. 6a, which were obtained from a location with no prior contact by a gel electrode. The striking result is that a prior pulse of 10 -3 s, which is sufficient for repassivation, has the largest effect, and reduces the subsequent charge flow by 90%, with a corresponding reduction of spot density in the image (see Fig. 6b). Thus the effect of a prior pulse on a subsequent measurement with the gel electrode can be divided into
245
I mm Fig. 6 Enlarged views of gel electrode images formed with a 5 V, 250 ms pulse, corresponding to data points indicated in Fig. 5: (a) no prior contact of surface, (b) after prior imaging with a 5 V, 1 ms pulse, (c) after prior imaging with a 5 V, 10 ms pulse
two regimes, defined by the duration (r) of that pulse. (i) If r < 10 -3 s, an increase of 7- increases the degree of passivation, which in turn decreases the charge flow during the subsequent imaging. (ii) If r > 10 -3 s, a second process is initiated with the opposite effect on subsequent gel electrode measurements (see Figs 5 and 6c). If r ~, 2 5 0 m s , this second process negates the passivation effect, so that the charge flow during subsequent imaging is the same as the initial value (see Fig. 5 and Fig. 2).
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Corrosion The second process, which opposes the effect of passivation, is electrochemical corrosion. This may be conveniently demonstrated by replacing 10% of the KI in the gel electrode with KC1, thereby providing C1- ions which are renowned for their aggressive, corrosive effects on aluminium alloys. Such a substitution substantially increases the current flow, as illustrated by the oscilloscope traces shown in Fig. 7, which compares the current transients produced by a 50 ms pulse with and without the KC1 in the gel. The presence of the CI- ions deters the passivation process and quickly stimulates a rapidly increasing current as the integrity of the thin oxide film at the base of the microcracks is undermined. A beneficial effect of the chloride substitution is a large increase in charge flow and image development. In the case of a 250 ms pulse a~plied to a specimen deformed to a tensile strain of 4 x 10- , the charge flow is increased by a factor of 10. This is accompanied by a dramatic increase in the density of the spots in the gel electrode image (see Fig. 8). Thus it appears that the role of the C1- ions is to promote breakdown of the thin oxide film at the base of the microcracks. This permits more current to flow and increases the deposition of the I- ions at the gel/specimen interface. Unfortunately, if repetitive measurements are to be performed, this enhanced current flow is of little or no benefit. For example, if a 250 ms pulse is applied, the
246
5 0 ms Fig. 7 Oscilloscope recordings of the current flow to gel electrodes with and without the addition of KCI (5 V, 50 ms pulse)
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Fig. 8 Enlarged views of gel electrode images formed with a 5 V, 250 ms pulse: (a) with gel containing 0.2 M KI, (b) with gel containi n g O . 1 9 M K l a n d 0 . 0 1 M KCI
I nt J F a t i g u e O c t o b e r 1 9 8 4
corrosion produced dominates any passivation effect, so that when the measurement is repeated the charge flow increases substantially each time. This is illustrated in Fig. 9, where the results for a chloride-containing gel electrode are compared with the quite repeatable results obtained with the usual KI gel. Both series of measurements were performed on the same specimen, which had been deformed to a tensile strain of 4 x 10 -2. For ease of comparison, the measured values of charge flow are normalized with respect to the first measurement in each series. To obtain repeatable values of charge flow with the chloride-containing gel electrode, it is necessary to shorten the pulse duration to 30 ms, thereby drastically reducing the corrosive component of the current flow. Thus while the addition of KC1 increases the sensitivity of a single measurement (see Fig. 8), no net advantage is realized for repetitive measurements.
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A quantitative model
The above experiments demonstrated that, to obtain repeatable values of the charge flow from the gel electrode to a deformed specimen, it is necessary to balance the effects of the processes of passivation and corrosion. If the pulse duration is too short, the initial passivation process dominates, and subsequent values of charge flow are reduced. On the other hand if the pulse duration is too long, the corrosion proceeds too far, overcompensates for the passivation effect, and subsequent values of the charge flow are increased. A remarkable feature of this behaviour is that, although the passivation process occurs during the initial 1 ms, it is necessary to maintain the voltage pulse and stimulate a corrosion current for at least another 2 5 0 m s to achieve a balance. This important influence of the repassivation process on the subsequent flow of current is now considered in more detail.
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I n t J Fatigue O c t o b e r 1984
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Fig. 10 The effect of pulse voltage on the charge density produced by a 3 5 0 ms pulse, and the current at the end of the pulse. 6 0 6 1 - T 6 specimen, strain = 3 X 10 -2
The physical basis of the proposed model is that the current flow from the gel electrode is controlled by the thickness of the thin oxide film at the base of the microcracks in the 14 nm oxide. Prior to the application of the gel electrode, this film will be the naturally formed oxide of thickness X0. The gel electrode current changes the thickness of this film: a short pulse will increase the thickness by an amount +X, so that less current will flow during a second measurement, while a long pulse will cause film breakdown so that more current flows during a second measurement. Note that since the conductivity of the passivating film may differ from that of the naturally formed oxide, X is an equivalent thickness. The effect of the addition of this passivating film is amenable to analysis as outlined below. During the application of the gel electrode very high electric fields ( E ~ 107 V/cm) are impressed across the thin oxide film at the base of the microcracks. It is well known 6 that under these conditions the current flow (!) through aluminium oxide is carried by the field-induced flow of ions, and is of the form I = A exp (~]E). This type of relationship also applies to the currents induced by the gel electrode. This is illustrated in Fig. 10, which shows the effect of pulse voltage (V) on the charge density (Q) produced by a 350 ms pulse. Each data point was obtained from a different location on the gauge section of a specimen deformed to a tensile strain of 3 x 10 -u. Also plotted in Fig. 10 are the corresponding values of the final current (If) flowing at the end of each pulse, and these obey the
247
same exponential relationship. Thus we can write:
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(1)
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fit to the data shown in Fig. 11 yields a value of X/Xo = 0.5 -+ 0.03 for a 1 ms pulse. Thus the charge flow produced by a 5 V, 250 ms pulse Q(250) will be reduced by the prior application of a 5 V, 1 ms pulse to a value Q(1;250) given by: Q(250)
X0 + a X where the term ooX accounts for the thickness of the film increasing during the measurement. For a 5 V pulse with a duration in the range of 250 ms to 350 ms, there is no change in the value of Q during successive measurements, so that in this case effectively X "~ X0. Thus the exponents in Equations (1) and (2) are equal. The data in Fig. 10 indicates that this holds true within the range from 3 V to 7 V, and yields the value:
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(3)
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In the case of a series of repetitive measurements with a short pulse, let us assume that the same thickness of passivating film is deposited each time. Then the current flow at the end of the nth measurement will be:
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(5)
Substituting the experimentally derived values of /3/Xo = 1.54 V -1 and X/Xo = 0.5 yields:
Q(250)/Q(1;250) = 13 -+ 2 in good agreement with the result shown in Fig. 5. The absolute value of X for a 1 ms pulse can be estimated from the measured value of the charge density (Q). Integration of the first current trace in Fig. 4, and subtraction of the contribution of the initial charging transient, yielded a value of 2 × 10 -s C/cm 2. From a microscopic viewpoint the density of charge flow is much larger than this, because the current is confined to the small fraction of the surface area (AA/A) exposed by the microcracks, namely,7
AA/A = 0.57 (e -- % )
~v If(n) = I0 exp - X0 + nx
(4)
This relationship can only be expected to hold true for small values of n, because the passivation process is probably analogous to anodization and will terminate after the addition of a finite film thickness, dependent on the applied voltage. However, it does apply very well to the case of a series of three 5 V, 1 m s pulses applied to a specimen deformed to a tensile strain of 3 × 10 -2. The best
where e is the tensile strain of 3 × 10 -2, and e0, the strain required for the onset of oxide rupture, is ~1 x 10 -2. To estimate the upper limit of the film thickness (X), assume that the charge flow is all associated with the production of A1203. It is known s that during anodization of aluminium, 10 s C produces 4.55 cm 3 of oxide. If this conversion factor is applied to the passivation process, then the oxide thickness X is:
4.55 × 10 -s X-
X
/
0.57 (e - % )
Q = 0.8 nm.
Thus a 5 V, 1 ms pulse is estimated to deposit a film of 0.8 nm on top of the already present film of oxide (X0) formed immediately after deformation under normal atmospheric conditions. The value of X0 = 2X = 1.6 nm is in good agreement with previously reported values of this naturally formed thin film on aluminium.9
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Measurements of fatigue deformation 7
S e l e c t i o n o f pulse d u r a t i o n
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Fig. 11 The final current at the end of eachof a series of three 5 V, 1 ms pulses, compared with Equation (4). 6061-T6 specimen, strain = 3 X 10 -=
248
Since the data (see Fig. 2) obtained from tensile specimens showed that a pulse duration of 250 ms almost achieves a balance between the effects of passivation and corrosion, a series of preliminary experiments was performed on partially fatigued specimens, with a pulse duration encompassing the range of 250 ms to 400 ms. On the basis of these measurements, it was concluded that a pulse duration of 350 ms provided the most consistently repeatable values of charge flow. Typical sequences of the charge flow produced by a series of 350 ms pulses are shown in Fig. 12, for specimens fatigued by different amounts. 3"he charge flow produced by a 5 V, 350 ms pulse is shown as a function of position along the centreline of a fatigued specimen in Fig. 13. The data points correspond to three successive surveys of the specimen, and show that the charge flow at each location is repeatable to within -+15%. This accuracy is sufficient to define the overall distribution of fatigue deformation, and each time to identify the location of the most severe deformation.
I nt J Fatigue October 1984
While a 5 V, 350 ms pulse does provide repeatable values of the charge flow to the gel electrode, it should be noted that the absolute magnitude of the charge (Q) is only about 3% of that produced by a 5 V, 5 s pulse, as applied in the earlier experiments. (In this regime the corrosion current predominates and increases linearly with time (see Fig. 3), so that Q c= ¢2.) Fortunately this resuhs in only a small loss of sensitivity because the background current to the gel electrode from an undeformed specimen (Q(O)) also increases rapidly with ¢. This is illustrated in Fig. 14, where the sensitivity, defined as Q/Q(O), is shown as a function of the pulse duration. The two sets of data represent the time dependence of the charge flow during the application of a 5 s pulse to two specimens, before and after fatigue cycling. The values of Q and Q(O) were obtained as a function of time by partial integration of the current transients with the computer. These results show that the maximum sensitivity is reached with a pulse duration of about 1 s, and that the selection of a 350 ms pulse incurs only a 30-40% loss of sensitivity.
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of fatigue
cycles
In this series of experiments, the fatigue cycling was interrupted at intervals to permit surveys of the fatigue deformation with the gel electrode using the new standard pulse (5 V, 350ms). Each specimen was measured along the centreline at the locations of the series of holes in the attached mylar drawing film, so that each location was measured many times during the complete fatigue test. While such surveys obviously omitted the regions between the holes, they nevertheless sufficed to map out the overall distribution of the fatigue deformation. An example of a series of surveys is shown in Fig. 15, where the density of charge flow to the gel electrode is shown as a function of position. The background level of charge flow measured prior to fatigue cycling is indicated by the dashed line. After only 200 fatigue cycles, or 0.09% of the fatigue life, there is already a definite increase of charge
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Fig. 13 Three surveys along the centreline of a 6061-T6 specimen fatigued for 12 000 cycles showing the charge density as a function of location of the gel electrode (5 V, 350 ms pulse)
I n t J Fatigue O c t o b e r 1984
Q
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Fig. 1 4 E f f e c t o f p u l s e d u r a t i o n o n t h e s e n s i t i v i t y o f t h e gel elect r o d e : 6061-T6 specimen, 5 V pulse. Sensitivity defined as ratio of
charge density after (0) and before (0(0)) fatigue deformation
249
flow as the first microcracks are initiated in the 14 nm oxide film. This process develops quite rapidly with continued fatigue cycling, resulting in substantial increases of charge flow. For example, after 2000 cycles, or 0.9% of fatigue life, the charge density is already 10x the initial background value. Thus the 5 V, 3 5 0 m s pulse clearly provides more than adequate sensitivity for detecting the early stages of fatigue deformation. The results in Fig. 15 also illustrate that the accumulation of fatigue deformation at different locations of the specimen proceeds in an apparently competitive manner. In this case, the deformation developed primarily in two distinct regions during the first 2000 cycles, but thereafter the left-hand one (see Fig. 15) began to dominate. Such behaviour seemed to be a characteristic feature of the early development of the fatigue deformation in these specimens, with the deformation eventually concentrating at the location of eventual failure. Thus in general, during the early portion of these tests, ie the first 10% of life, the gel electrode provided measurements of the distribution of the fatigue deformation, but did not necessarily pinpoint the site of ultimate fracture. This is not regarded as a deficiency in the gel electrode technique, but rather as representative of the statistical nature of the fatigue process in commercial alloys. The charge flow to the gel electrode during a series of similar surveys on another specimen is plotted in Fig. 16, where again the distribution of deformation develops
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Fig. 16 The charge density during a series of surveys of a 6061-T6 specimen showing the effect of a large number o f fatigue cycles. Fatigue life = 1.84 X 10 s cycles (5 V, 350 ms pulse)
in an unpredictable manner during the first 25 000 cycles (13.6% of life), but thereafter becomes more systematic and dominant in the region of final fracture. This series of curves spans a larger number of fatigue cycles than in the previous example (see Fig. 15), and illustrates that the charge flow eventually increases one thousandfold. This specimen failed after 1.84 x lO s cycles, with a crack propagating from one edge of the specimen and passing across the specimen at the location indicated by the arrow in Fig. 16. In this experiment the gel electrode images obtained at the location of the third data point (at the 19 mm position in Fig. 16) could be easily correlated throughout the test. The optical micrographs in Fig. 17 show enlarged views of a portion of the images obtained after 10 3, 6 x 103, 2.5 x 104 and 10 s cycles. All the images are overexposed and consist of rather confusing arrays of spots, but the unusual feature at A provides a convenient reference point for a detailed correlation of the entire sequence. (Before fatigue cycling the images are virtually free of spots.) It is noteworthy that the spots obtained after the first 1000 cycles (see Fig. 17a) do not enlarge very much during the early stages (cf Fig. 17b), the increase of charge flow being associated primarily with the emergence of more spots. However, after 2.5 X 10 a cycles, there is not only an increase in the number of spots, but many of the former spots are also becoming more pronounced (see Fig. 17c). At 105 cycles (see Fig. 17d), some closely spaced groups of spots have developed and linked together. Thus the statistical nature of the early stages of the development of the fatigue deformation is again evident when viewed on this finer scale. It is noteworthy that despite the overexposed nature of the gel electrode images, the diameter of the individual spots does not exceed 100gtm. Thus
Int J Fatigue October 1984
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b
~
~
Fig. 17 Optical micrographs showing enlarged view of gel electrode images obtained from the same location (at 19 mm in Fig. 14) at intervals during a fatigue test: (a) 10 3 cycles, (b} 6 X 10 3 cycles, (c) 2.5 X 10 4 cycles, (d) 10 S cycles
each spot is considered to correspond to microcracks in the 14 nm oxide film caused by clusters of persistent slip bands in a particular grain of the material. With continued cycling this occurs in more and more grains until clusters of grains accumulate deformation, thereby setting the stage for a cooperative linking-up process and providing a path for eventual crack propagation. These details of the fatigue process, recorded by the gel electrode images, are not distinguished by measurements of the charge flow. The latter simply provide a summation within the total area of contact of the gel, and the quoted values of charge density are several orders of magnitude smaller than the actual density of charge flowing through the microcracks in the oxide film. Nevertheless, such measurements do provide an overall assessment of the total amount of deformation within the contact area, including not only the primary sites of most severe deformation, but also the nearby sites of less severe damage. However, since these sites must eventually cooperate (see, for example, Fig. 17d) for final failure to occur, such an assessment can still provide useful information. This is illustrated in Fig. 18 where the largest value of charge density (obtained from plots such as in Figs 15 and 16) is plotted as a function of the number of fatigue cycles. The
Int J Fatigue October 1984
data points correspond to eight specimens, five of which were fatigued to failure, the average fatigue life, Nf (2.1 + 0.2 × 10 s cycles), being indicated by the arrow. The reproducibility of the data from specimen to specimen is very good over the entire range of fatigue cycles. Deformation was detectable as early as 0.1% of the fatigue life, the maximum charge density being approximately twice that measured prior to fatigue cycling (see Figs 15 and 18). Thereafter the charge density increased systematically with continued cycling, as more and more grains otf the metal accumulated deformation. Thus in this series of experiments the flow of charge provides a quantitative assessment of the total extent of fatigue damage within the contact area of the gel electrode. It is noteworthy that two of the s[~ecimens were simply fatigued for just 6 × 10 3 and 1.2 x 10 ~ cycles respectively, yet yielded values of charge density in excellent agreement with those obtained from the other specimens, which had been measured many times prior to reaching that stage in the fatigue test. Thus the subtle passivation and corrosion events produced during the gel electrode measurement have little or no influence on the fatigue process itself. In this regard, a further conclusion to be drawn is that the emergence, rather than development,
251
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Advantage was taken of this repeatability to measure the accumulation of deformation at intervals during fatigue tests. Measurements of charge flow correlated with the density of spots developed on the gel electrode image, and both provided information on the distribution and extent of the fatigue deformation. The application of the 5 V, 350 ms pulse provided good repeatability, reproducibility between specimens and excellent sensitivity. Fatigue deformation was detected as early as 0.1% of the fatigue life, and observed to increase systematically with increasing number of fatigue cycles. During the early stages (ie up to "~5% of life), deformation appears to accumulate at similar rates at several locations, but thereafter tends to concentrate at the location of eventual failure. The gel electrode image provides additional information on a finer scale: each spot in the image is considered to result from the presence of clusters of slip band extrusions. In general terms, the initial 10% of fatigue life is concerned with accumulating deformation at many locations (ie, grains of the material), thereby setting the stage for a linking-up process which is well established by ~50% of life and provides a path for final crack propagation.
I~
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Fig. 18 E f f e c t o f fatigue cycling on the m a x i m u m value of charge density that flowed t o a gel electrode. 5 V, 350 ms pulse, 6061-T6 specimen, 14 nm oxide. Fatigue life, N f = 2.1 -+ 0.2 X 10 s cycles
Acknowledgements The author is grateful for the assistance of D. Gorkiewicz in performing these experiments, and for programming the computer to integrate the current transients.
References of spots in the gel electrode image during the early stages of fatigue (see Figs 17a and 17b) is a manifestation of the fatigue deformation process, and is not due to any inhibiting effects of the gel electrode technique.
1.
Baxter, W. J. 'Gel electrode imaging of metal fatigue: I. Cracks in 6061-T6 aluminum' Metallurgical Transactions 1 3 A (1982) top 1413--1420
2.
Baxter, W. J. 'Gel electrode imaging of metal fatigue: II. Deformation in 1100 aluminum' Metallurgical Transactions 13A (1982) pp 1421--1427
3.
Baxter, W. J. 'Gel electrode imaging of fatigue cracks in aluminium alloys' I n t J Fatigue 5 No 1 (1983) pp 37--42
4.
Baxter, W. J. and McKinney, T. R, 'Gel electrode imaging of deformation in aluminum alloys' Scripta Metallurgica 17 (1983) pp 1371--1375
5.
Baxter, W. J. 'Oxide films: Quantitative sensors of metal fatigue' in J. Lankford, D. L. Davidson, W. L. Morris and R. P. Wei (eds) Fatigue Mechanisms: Advances in Quantitative Measurement o f Physical Damage, ASTM STP 811 (American Society for Testing and Materials, 1983) pp 115--136
6.
Young, L. Anodic Oxide Films (Academic Press, London, 1961)
7.
Arnott, D. R., Baxter, W. J. and Rouze, S. R. 'The role of
Summary
The gel electrode technique provides a quantitative assessment of the extent of deformation in aluminium, by stimulating an electrical current to pass through deformation-induced microcracks in a previously formed 14 nm surface oxide film. As metal is exposed by these microcracks it is immediately reoxidized by the ambient atmosphere, but only a thin (~1.6 nm) oxide reforms. The current from the gel electrode passes through these regions of thin oxide in the microcracks, producing both passivation and corrosion thereat. The passivation occurs very rapidly (~10 -3 s), but is immediately followed by a steadily increasing corrosion reaction. The process of passivation deposits an additional, very thin (~0.8 nm) film on the 1.6 nm oxide already present, but this is sufficient to inhibit the current flow during a subsequent repeat measurement. On the other hand, the corrosion reaction apparently removes oxide so that a larger current flows during subsequent measurements. These two processes can be balanced by controlling the duration of the voltage pulse applied to the gel electrode. In these experiments on 6061-T6 aluminium, it is shown that very repeatable measurements can be obtained with a 5 V, 350 ms pulse.
252
defects in the fracture of oxide films on metals' J o f the Electrochem Soc 28 (1981) pp 843--847 8.
Tajima, S. 'Anodic oxidation of aluminum' Advances in Corrosion Sci and Tech 1 (1970) pp 2 2 9 - 3 6 2
9.
Kubaschewski, O. and Hopkins, B. E. Oxidation o f Metals and Alloys, second edition (Academic Press, New York, 1962) p 38
Author The author is with the Physics Department, General Motors Research Laboratories, Warren, MI 48090-9055, USA.
Int J Fatigue October 1984
Repeatability of gel electrode measurements of fatigue deformation in 6061-T6 aluminium W. J. B a x t e r In a previous report, using 5 V, 5 s pulses, the gel electrode method was shown to detect fatigue deformation in 6061-T6 aluminium, but each specimen was only measured once. In this study the technique is refined so that the measurements can be repeated many times to monitor the distribution and accumulation of deformation at intervals during a fatigue test. The current from the gel electrode produces both passivation (film formation) and corrosion at the sites of deformation on the metal surface. These two influences are balanced if the gel electrode measurement is performed with a 5 V, 350 ms pulse, and the charge flow during subsequent measurements is then repeatable to within +15%. Although this pulse duration is an order of magnitude shorter than that employed in the previous study, excellent sensitivity is retained. Fatigue deformation is detected in 6061-T6 aluminium as early as 0.1% of the fatigue life. The charge flow measurements and gel electrode images show that the deformation accumulates at many sites during the early stages (~5%) of fatigue life, but by about 50% of life clusters of sites of fatigue damage are well established, and provide a path for final fracture. Key words: fatigue; deformation; 6061-T6 aluminium; gel electrode method A method of detecting and measuring the development of the early stages of metal fatigue could provide the basis of an abbreviated testing technique for the prediction of fatigue life. In a recent series of reports, 1-s a new and simple technique was introduced which detects and images fatigue damage in aluminium alloys with an electrochemical gel electrode. This new technique is based upon two factors: (i) the creation of microcracks in a 14 nm surface anodic oxide film during fatigue of the underlying metal, and (ii) the detection of these microcracks by contacting the surface with a gel electrolyte containing KI and starch and applying a voltage pulse. Current flows preferentially through the microcracks in the oxide film and releases iodine ions which react with the starch to form a black complex in the surface of the gel. In this way an image is formed of the sites of current flow, ie the sites of fatigue damage in the underlying metal. Further, the total charge that flows to form the image is a quantitative measure of the extent of the fatigue damage. The emphasis in this paper is on the quantitative capability of the technique, particularly the repeatability of the current flow through the microcracks. The previously reported experiments on aluminium alloys may be classified in terms of the duration of the voltage pulse applied to the gel electrode. In general, shorter pulses produce images with better spatial resolution: fatigue cracks as short as 10~um have been imaged with a 10 ms pulse, 1'3 while a 1 ms pulse can even image individual slip bands. 4 On the other hand, longer pulses (which overexpose the image) have the advantage of increasing the charge flow during imaging, so that the sensitivity for detecting and quantitatively measuring deformation is greatly enhanced. The ultimate sensitivity is limited by the eventual breakdown of the 14 nm oxide film during the application of a long voltage pulse. In a series of experiments on 1100-0 and 6061-T6 aluminium, l's a 5 V, 5 s pulse was found to be just within this limit, and the resulting current flow could easily detect the fatigue deformation
that precedes the appearance of a fatigue crack. The gel electrode images are greatly overexposed under these conditions but the presence of black spots on the gel, as well as measurements of the charge flow during imaging, detected the deformation at <(1% of the fatigue life. In the above experiments each specimen was measured only once with the gel electrode. This report is concerned with repetitive measurements at the same location on 6061-T6 aluminium, as is required to monitor the continuous development of surface deformation during the early stages of metal fatigue. The current flow during gel electrode imaging is shown to promote both passivation and corrosion at the fatigue-induced microcracks in the 14 nm oxide, and both of these processes can exert a substantial influence on the current flow during a subsequent repeat measurement. However, it will be shown that the passivation and corrosion processes may be balanced by the selection of a 5 V, 350 ms pulse, so that very good repeatability can be achieved, while retaining excellent sensitivity. Experimental details The experiments described in this study can be divided into two categories: tensile tests and fatigue tests. Since experiments aimed at defining the effects of pulse duration on subsequent measurements involved many comparative measurements, they were performed on tensile specimens where the amount and uniformity of the deformation could be easily controlled. Subsequently, these results were applied to the accurate, repetitive measurement of deformation in fatigue specimens. The experimental procedure was the same in both sets of experiments and consisted of three steps. Specimen preparation The specimens were machined from sheet material 1.5 mm thick, with an average grain size of ~ 3 0 # m . The tensile
0142-1123/84/040243--10 $3.00 © 1984 Butterworth & Co (Publishers) Ltd Int J Fatigue October 1984
243
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This charge flow always correlated with the density of spots that developed on the gel tip, which could be viewed with a pocket magnifier or an optical microscope.
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Repetitive measurements of tensile deformation
I
E f f e c t o f pulse d u r a t i o n
Fig. 1 G e o m e t r y o f fatigue specimens. A l l d i m e n s i o n s in m m
specimens had a gauge section with parallel sides 60 mm long and 13 mm wide (ASTM Specification No A370). The fatigue specimens had the conventional tapered cantilever beam geometry as shown in Fig. 1. All the specimens were degreased with acetone and cleaned by immersion in chromic acid at 75°C for 5 min. After rinsing with water and alcohol, they were anodized in a 3% solution of tartaric acid at a potential of 10 V to form a surface oxide film 14 nm thick. Specimen
The previously reported measurements of deformation in 6061-T6 aluminium s with the gel electrode were performed with a 5 V, 5 s pulse, but each location on the specimen was only measured once. In the present experiments, it was found that if such measurements are repeated at the same location, the flow of charge is much greater than that observed initially. This observation warranted a more complete investigation of the effect of repeated measurements at the same location with pulses of different duration. As shown in Fig. 2, the charge flow during successive measurements on a specimen deformed to a tensile strain of 3 × 10 -2 could be either greater or less than the initial charge flow, depending on the duration of the pulse. For a 5 V, 500 ms pulse the charge increases each time, but for a 5 V, 50 ms pulse the charge decreases quite rapidly before stabilizing at a much lower value. A 5 V, 250 ms pulse offers a compromise between these two opposing effects, the charge decreasing only slightly during a series of six measurements. These results demonstrate that
deformation
The tensile specimens were deformed in an Instron Universal Test System, and the elongation determined by the cross head movement (previously calibrated for similar specimens with an extensometer). The fatigue specimens were cycled by reverse bending to produce surface, maximum cyclic strains of +3.0 × 10 -3. This resulted in an average fatigue life of (2.1 + 0.2) × l 0 s cycles. The tensile specimens were removed from the Instron for gel electrode measurements, but the fatigue specimens were measured in situ at intervals during a fatigue test. Gel e l e c t r o d e
l0-z
imaging
The electrolyte consists of an agar gel containing 0.2 M potassium iodide, 0.05 M borax and 30 g of starch per litre. This warm fluid mixture was dispensed into small lengths ( ~ 3 0 mm) of plastic tube 6 mm 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, sealed into the other end of the tube, served as a cathode. After approximately 5 min a flexible skin formed on the gel tip, which could then be pressed gently against the surface of the specimen. A strip of mylar drawing film with a linear array of holes, 4 mm in diameter and 6.5 m m separation between centres, was taped to the specimen. Flexible gel tips were placed, in turn, within each of these holes to contact the surface of the specimen. In this way the area of contact was defined, and exactly the same area could be contacted repeatedly with a series of gel tips. The true area of gel/ metal contact (7.1 mm 2) was determined in a series of preliminary tests, by measuring the diameter of the gel electrode image with a travelling microscope. A pulse of negative potential (5 V) was applied to the cathode, and the current flow was recorded on a Nicolet digital oscilloscope and stored on magnetic discs. This information was transferred to a Hewlett Packard Model 85 computer, and the total charge flow obtained by numerical integration.
244
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Sequence Fig. 2 T h e d e n s i t y o f charge w h i c h f l o w e d t o gel electrodes during t h r e e sequences o f measurements at three l o c a t i o n s on a 6 0 6 1 - T 6 specimen d e f o r m e d t o a t o t a l tensile strain o f 3 × 10 -=. D u r a t i o n o f 5 V pulse at each l o c a t i o n is as indicated
Int J Fatigue October 1984
4
the application of the gel electrode can modify the surface of the specimen, the nature of this modification being a function of the duration of the voltage pulse. On the other hand, the relative reproducibility of the charge density during repeated application of the 250 ms pulse does not imply that the surface is not being modified. In fact, examination of the shape of the current transient reveals that changes are taking place. This is illustrated by the two oscilloscope recordings shown in Fig. 3, which correspond to the first and fifth measurements plotted in Fig. 2. The initial charging transient is the same for both measurements, but thereafter the time dependence of the current flow differs considerably. This change in the shape of the current transient curve occurs primarily during the first three pulses, and indicates that the current flow itself changes the conditions at the base of the microcracks in the 14 nm oxide film.
-0
v I I ms Fig. 4 Oscilloscope recordings of the current flow to the gel electrode produced by four successive 5 V, I ms pulses. Specimen deformed to a tensile strain of 3 X 10 -2
E f f e c t o f e l e c t r o c h e m i c a l reactions
Previous measurements with the gel electrode on deformed 1100-0 aluminium 2 have shown that the current to the microcracks in the 14 nm oxide film can be separated into three stages: (1) the initial charging of the oxide film, followed by (2) passivation (partial reoxidation) of the surface at the base of the microcracks, and finally (3) corrosion. Stage (1) is an electrical effect and remains the same in all of these experiments. The important influence of the processes of passivation and corrosion during repetitive measurements on 6061-1"6 aluminium is illustrated below.
12 No prior contoct
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During plastic deformation, the 14 nm oxide film develops microcracks exposing fresh metal surfaces. Under normal atmospheric conditions, these surfaces reoxidize very rapidly, but only a very thin layer of 'natural' oxide is formed. During the first application of a gel electrode, it is this layer which controls the initial current flow. But within the first 10-3s this layer is itself modified by a passivation process. This is illustrated in Fig. 4 by the oscilloscope recordings of the current flow produced by four successive 1 ms pulses. (The specimen had been deformed to a tensile strain of 3 x 10-2.) All four pulses exhibit a common initial charging transient, but after ~0.1 ms there is a marked reduction in the rate of decay
<~ E C
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0 2 5 0 ms Fig, 3 Oscilloscope recordings of the current flow to the gel electrode produced by a 5 V, 250 ms pulse. The two traces correspond to the first and fifth measurement in the sequence plotted in Fig. 2
I n t J Fatigue O c t o b e r 1984
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Fig. 5 Effect of duration of prior 5 V pulse on the charge density that flowed during subsequent imaging with a 5 V, 250 ms pulse of a 6061-T6 specimen with 14 nm oxide deformed to a tensile strain of 6 X 10 -=
of the current due to the onset of repassivation. This is particularly pronounced during the first pulse, but the third pulse effectively completes this passivation process, so that the last two pulses are almost indistinguishable. Although this passivation process occurs within only ~ 1 0 -3 s, it exerts an unexpectedly strong influence on the gel electrode imaging process. This is illustrated in Figs 5 and 6, which show the effect of the duration of a preliminary pulse on the charge flow (see Fig. 5) and the appearance of the image (see Fig. 6) produced during a subsequent measurement with a 250 ms pulse. (The specimen in this case had been deformed to a tensile strain of 6 × 10-2.) A basis for comparison is provided by the two data points on the ordinate axis in Fig. 5, and the gel electrode image in Fig. 6a, which were obtained from a location with no prior contact by a gel electrode. The striking result is that a prior pulse of 10 -3 s, which is sufficient for repassivation, has the largest effect, and reduces the subsequent charge flow by 90%, with a corresponding reduction of spot density in the image (see Fig. 6b). Thus the effect of a prior pulse on a subsequent measurement with the gel electrode can be divided into
245
I mm Fig. 6 Enlarged views of gel electrode images formed with a 5 V, 250 ms pulse, corresponding to data points indicated in Fig. 5: (a) no prior contact of surface, (b) after prior imaging with a 5 V, 1 ms pulse, (c) after prior imaging with a 5 V, 10 ms pulse
two regimes, defined by the duration (r) of that pulse. (i) If r < 10 -3 s, an increase of 7- increases the degree of passivation, which in turn decreases the charge flow during the subsequent imaging. (ii) If r > 10 -3 s, a second process is initiated with the opposite effect on subsequent gel electrode measurements (see Figs 5 and 6c). If r ~, 2 5 0 m s , this second process negates the passivation effect, so that the charge flow during subsequent imaging is the same as the initial value (see Fig. 5 and Fig. 2).
8
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Corrosion The second process, which opposes the effect of passivation, is electrochemical corrosion. This may be conveniently demonstrated by replacing 10% of the KI in the gel electrode with KC1, thereby providing C1- ions which are renowned for their aggressive, corrosive effects on aluminium alloys. Such a substitution substantially increases the current flow, as illustrated by the oscilloscope traces shown in Fig. 7, which compares the current transients produced by a 50 ms pulse with and without the KC1 in the gel. The presence of the CI- ions deters the passivation process and quickly stimulates a rapidly increasing current as the integrity of the thin oxide film at the base of the microcracks is undermined. A beneficial effect of the chloride substitution is a large increase in charge flow and image development. In the case of a 250 ms pulse a~plied to a specimen deformed to a tensile strain of 4 x 10- , the charge flow is increased by a factor of 10. This is accompanied by a dramatic increase in the density of the spots in the gel electrode image (see Fig. 8). Thus it appears that the role of the C1- ions is to promote breakdown of the thin oxide film at the base of the microcracks. This permits more current to flow and increases the deposition of the I- ions at the gel/specimen interface. Unfortunately, if repetitive measurements are to be performed, this enhanced current flow is of little or no benefit. For example, if a 250 ms pulse is applied, the
246
5 0 ms Fig. 7 Oscilloscope recordings of the current flow to gel electrodes with and without the addition of KCI (5 V, 50 ms pulse)
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Fig. 8 Enlarged views of gel electrode images formed with a 5 V, 250 ms pulse: (a) with gel containing 0.2 M KI, (b) with gel containi n g O . 1 9 M K l a n d 0 . 0 1 M KCI
I nt J F a t i g u e O c t o b e r 1 9 8 4
corrosion produced dominates any passivation effect, so that when the measurement is repeated the charge flow increases substantially each time. This is illustrated in Fig. 9, where the results for a chloride-containing gel electrode are compared with the quite repeatable results obtained with the usual KI gel. Both series of measurements were performed on the same specimen, which had been deformed to a tensile strain of 4 x 10 -2. For ease of comparison, the measured values of charge flow are normalized with respect to the first measurement in each series. To obtain repeatable values of charge flow with the chloride-containing gel electrode, it is necessary to shorten the pulse duration to 30 ms, thereby drastically reducing the corrosive component of the current flow. Thus while the addition of KC1 increases the sensitivity of a single measurement (see Fig. 8), no net advantage is realized for repetitive measurements.
io-=
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A quantitative model
The above experiments demonstrated that, to obtain repeatable values of the charge flow from the gel electrode to a deformed specimen, it is necessary to balance the effects of the processes of passivation and corrosion. If the pulse duration is too short, the initial passivation process dominates, and subsequent values of charge flow are reduced. On the other hand if the pulse duration is too long, the corrosion proceeds too far, overcompensates for the passivation effect, and subsequent values of the charge flow are increased. A remarkable feature of this behaviour is that, although the passivation process occurs during the initial 1 ms, it is necessary to maintain the voltage pulse and stimulate a corrosion current for at least another 2 5 0 m s to achieve a balance. This important influence of the repassivation process on the subsequent flow of current is now considered in more detail.
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I n t J Fatigue O c t o b e r 1984
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Fig. 10 The effect of pulse voltage on the charge density produced by a 3 5 0 ms pulse, and the current at the end of the pulse. 6 0 6 1 - T 6 specimen, strain = 3 X 10 -2
The physical basis of the proposed model is that the current flow from the gel electrode is controlled by the thickness of the thin oxide film at the base of the microcracks in the 14 nm oxide. Prior to the application of the gel electrode, this film will be the naturally formed oxide of thickness X0. The gel electrode current changes the thickness of this film: a short pulse will increase the thickness by an amount +X, so that less current will flow during a second measurement, while a long pulse will cause film breakdown so that more current flows during a second measurement. Note that since the conductivity of the passivating film may differ from that of the naturally formed oxide, X is an equivalent thickness. The effect of the addition of this passivating film is amenable to analysis as outlined below. During the application of the gel electrode very high electric fields ( E ~ 107 V/cm) are impressed across the thin oxide film at the base of the microcracks. It is well known 6 that under these conditions the current flow (!) through aluminium oxide is carried by the field-induced flow of ions, and is of the form I = A exp (~]E). This type of relationship also applies to the currents induced by the gel electrode. This is illustrated in Fig. 10, which shows the effect of pulse voltage (V) on the charge density (Q) produced by a 350 ms pulse. Each data point was obtained from a different location on the gauge section of a specimen deformed to a tensile strain of 3 x 10 -u. Also plotted in Fig. 10 are the corresponding values of the final current (If) flowing at the end of each pulse, and these obey the
247
same exponential relationship. Thus we can write:
t3v
If = Io exp - Xo+X
(1)
Q = Q0 e x p - -
(2)
and
fit to the data shown in Fig. 11 yields a value of X/Xo = 0.5 -+ 0.03 for a 1 ms pulse. Thus the charge flow produced by a 5 V, 250 ms pulse Q(250) will be reduced by the prior application of a 5 V, 1 ms pulse to a value Q(1;250) given by: Q(250)
X0 + a X where the term ooX accounts for the thickness of the film increasing during the measurement. For a 5 V pulse with a duration in the range of 250 ms to 350 ms, there is no change in the value of Q during successive measurements, so that in this case effectively X "~ X0. Thus the exponents in Equations (1) and (2) are equal. The data in Fig. 10 indicates that this holds true within the range from 3 V to 7 V, and yields the value:
f3/Xo = 1.54 +- 0.05
(3)
V -~
In the case of a series of repetitive measurements with a short pulse, let us assume that the same thickness of passivating film is deposited each time. Then the current flow at the end of the nth measurement will be:
l~VX
Q(1;250) = exp Xo(X0 + X)
(5)
Substituting the experimentally derived values of /3/Xo = 1.54 V -1 and X/Xo = 0.5 yields:
Q(250)/Q(1;250) = 13 -+ 2 in good agreement with the result shown in Fig. 5. The absolute value of X for a 1 ms pulse can be estimated from the measured value of the charge density (Q). Integration of the first current trace in Fig. 4, and subtraction of the contribution of the initial charging transient, yielded a value of 2 × 10 -s C/cm 2. From a microscopic viewpoint the density of charge flow is much larger than this, because the current is confined to the small fraction of the surface area (AA/A) exposed by the microcracks, namely,7
AA/A = 0.57 (e -- % )
~v If(n) = I0 exp - X0 + nx
(4)
This relationship can only be expected to hold true for small values of n, because the passivation process is probably analogous to anodization and will terminate after the addition of a finite film thickness, dependent on the applied voltage. However, it does apply very well to the case of a series of three 5 V, 1 m s pulses applied to a specimen deformed to a tensile strain of 3 × 10 -2. The best
where e is the tensile strain of 3 × 10 -2, and e0, the strain required for the onset of oxide rupture, is ~1 x 10 -2. To estimate the upper limit of the film thickness (X), assume that the charge flow is all associated with the production of A1203. It is known s that during anodization of aluminium, 10 s C produces 4.55 cm 3 of oxide. If this conversion factor is applied to the passivation process, then the oxide thickness X is:
4.55 × 10 -s X-
X
/
0.57 (e - % )
Q = 0.8 nm.
Thus a 5 V, 1 ms pulse is estimated to deposit a film of 0.8 nm on top of the already present film of oxide (X0) formed immediately after deformation under normal atmospheric conditions. The value of X0 = 2X = 1.6 nm is in good agreement with previously reported values of this naturally formed thin film on aluminium.9
n=l
:o.5 /
Measurements of fatigue deformation 7
S e l e c t i o n o f pulse d u r a t i o n
0
t~ ta_
I 0.2
I
I
I
0.4
I 0.6
l 0.8
( I + nX/Xo )-t
Fig. 11 The final current at the end of eachof a series of three 5 V, 1 ms pulses, compared with Equation (4). 6061-T6 specimen, strain = 3 X 10 -=
248
Since the data (see Fig. 2) obtained from tensile specimens showed that a pulse duration of 250 ms almost achieves a balance between the effects of passivation and corrosion, a series of preliminary experiments was performed on partially fatigued specimens, with a pulse duration encompassing the range of 250 ms to 400 ms. On the basis of these measurements, it was concluded that a pulse duration of 350 ms provided the most consistently repeatable values of charge flow. Typical sequences of the charge flow produced by a series of 350 ms pulses are shown in Fig. 12, for specimens fatigued by different amounts. 3"he charge flow produced by a 5 V, 350 ms pulse is shown as a function of position along the centreline of a fatigued specimen in Fig. 13. The data points correspond to three successive surveys of the specimen, and show that the charge flow at each location is repeatable to within -+15%. This accuracy is sufficient to define the overall distribution of fatigue deformation, and each time to identify the location of the most severe deformation.
I nt J Fatigue October 1984
While a 5 V, 350 ms pulse does provide repeatable values of the charge flow to the gel electrode, it should be noted that the absolute magnitude of the charge (Q) is only about 3% of that produced by a 5 V, 5 s pulse, as applied in the earlier experiments. (In this regime the corrosion current predominates and increases linearly with time (see Fig. 3), so that Q c= ¢2.) Fortunately this resuhs in only a small loss of sensitivity because the background current to the gel electrode from an undeformed specimen (Q(O)) also increases rapidly with ¢. This is illustrated in Fig. 14, where the sensitivity, defined as Q/Q(O), is shown as a function of the pulse duration. The two sets of data represent the time dependence of the charge flow during the application of a 5 s pulse to two specimens, before and after fatigue cycling. The values of Q and Q(O) were obtained as a function of time by partial integration of the current transients with the computer. These results show that the maximum sensitivity is reached with a pulse duration of about 1 s, and that the selection of a 350 ms pulse incurs only a 30-40% loss of sensitivity.
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In this series of experiments, the fatigue cycling was interrupted at intervals to permit surveys of the fatigue deformation with the gel electrode using the new standard pulse (5 V, 350ms). Each specimen was measured along the centreline at the locations of the series of holes in the attached mylar drawing film, so that each location was measured many times during the complete fatigue test. While such surveys obviously omitted the regions between the holes, they nevertheless sufficed to map out the overall distribution of the fatigue deformation. An example of a series of surveys is shown in Fig. 15, where the density of charge flow to the gel electrode is shown as a function of position. The background level of charge flow measured prior to fatigue cycling is indicated by the dashed line. After only 200 fatigue cycles, or 0.09% of the fatigue life, there is already a definite increase of charge
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I n t J Fatigue O c t o b e r 1984
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charge density after (0) and before (0(0)) fatigue deformation
249
flow as the first microcracks are initiated in the 14 nm oxide film. This process develops quite rapidly with continued fatigue cycling, resulting in substantial increases of charge flow. For example, after 2000 cycles, or 0.9% of fatigue life, the charge density is already 10x the initial background value. Thus the 5 V, 3 5 0 m s pulse clearly provides more than adequate sensitivity for detecting the early stages of fatigue deformation. The results in Fig. 15 also illustrate that the accumulation of fatigue deformation at different locations of the specimen proceeds in an apparently competitive manner. In this case, the deformation developed primarily in two distinct regions during the first 2000 cycles, but thereafter the left-hand one (see Fig. 15) began to dominate. Such behaviour seemed to be a characteristic feature of the early development of the fatigue deformation in these specimens, with the deformation eventually concentrating at the location of eventual failure. Thus in general, during the early portion of these tests, ie the first 10% of life, the gel electrode provided measurements of the distribution of the fatigue deformation, but did not necessarily pinpoint the site of ultimate fracture. This is not regarded as a deficiency in the gel electrode technique, but rather as representative of the statistical nature of the fatigue process in commercial alloys. The charge flow to the gel electrode during a series of similar surveys on another specimen is plotted in Fig. 16, where again the distribution of deformation develops
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in an unpredictable manner during the first 25 000 cycles (13.6% of life), but thereafter becomes more systematic and dominant in the region of final fracture. This series of curves spans a larger number of fatigue cycles than in the previous example (see Fig. 15), and illustrates that the charge flow eventually increases one thousandfold. This specimen failed after 1.84 x lO s cycles, with a crack propagating from one edge of the specimen and passing across the specimen at the location indicated by the arrow in Fig. 16. In this experiment the gel electrode images obtained at the location of the third data point (at the 19 mm position in Fig. 16) could be easily correlated throughout the test. The optical micrographs in Fig. 17 show enlarged views of a portion of the images obtained after 10 3, 6 x 103, 2.5 x 104 and 10 s cycles. All the images are overexposed and consist of rather confusing arrays of spots, but the unusual feature at A provides a convenient reference point for a detailed correlation of the entire sequence. (Before fatigue cycling the images are virtually free of spots.) It is noteworthy that the spots obtained after the first 1000 cycles (see Fig. 17a) do not enlarge very much during the early stages (cf Fig. 17b), the increase of charge flow being associated primarily with the emergence of more spots. However, after 2.5 X 10 a cycles, there is not only an increase in the number of spots, but many of the former spots are also becoming more pronounced (see Fig. 17c). At 105 cycles (see Fig. 17d), some closely spaced groups of spots have developed and linked together. Thus the statistical nature of the early stages of the development of the fatigue deformation is again evident when viewed on this finer scale. It is noteworthy that despite the overexposed nature of the gel electrode images, the diameter of the individual spots does not exceed 100gtm. Thus
Int J Fatigue October 1984
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Fig. 17 Optical micrographs showing enlarged view of gel electrode images obtained from the same location (at 19 mm in Fig. 14) at intervals during a fatigue test: (a) 10 3 cycles, (b} 6 X 10 3 cycles, (c) 2.5 X 10 4 cycles, (d) 10 S cycles
each spot is considered to correspond to microcracks in the 14 nm oxide film caused by clusters of persistent slip bands in a particular grain of the material. With continued cycling this occurs in more and more grains until clusters of grains accumulate deformation, thereby setting the stage for a cooperative linking-up process and providing a path for eventual crack propagation. These details of the fatigue process, recorded by the gel electrode images, are not distinguished by measurements of the charge flow. The latter simply provide a summation within the total area of contact of the gel, and the quoted values of charge density are several orders of magnitude smaller than the actual density of charge flowing through the microcracks in the oxide film. Nevertheless, such measurements do provide an overall assessment of the total amount of deformation within the contact area, including not only the primary sites of most severe deformation, but also the nearby sites of less severe damage. However, since these sites must eventually cooperate (see, for example, Fig. 17d) for final failure to occur, such an assessment can still provide useful information. This is illustrated in Fig. 18 where the largest value of charge density (obtained from plots such as in Figs 15 and 16) is plotted as a function of the number of fatigue cycles. The
Int J Fatigue October 1984
data points correspond to eight specimens, five of which were fatigued to failure, the average fatigue life, Nf (2.1 + 0.2 × 10 s cycles), being indicated by the arrow. The reproducibility of the data from specimen to specimen is very good over the entire range of fatigue cycles. Deformation was detectable as early as 0.1% of the fatigue life, the maximum charge density being approximately twice that measured prior to fatigue cycling (see Figs 15 and 18). Thereafter the charge density increased systematically with continued cycling, as more and more grains otf the metal accumulated deformation. Thus in this series of experiments the flow of charge provides a quantitative assessment of the total extent of fatigue damage within the contact area of the gel electrode. It is noteworthy that two of the s[~ecimens were simply fatigued for just 6 × 10 3 and 1.2 x 10 ~ cycles respectively, yet yielded values of charge density in excellent agreement with those obtained from the other specimens, which had been measured many times prior to reaching that stage in the fatigue test. Thus the subtle passivation and corrosion events produced during the gel electrode measurement have little or no influence on the fatigue process itself. In this regard, a further conclusion to be drawn is that the emergence, rather than development,
251
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Advantage was taken of this repeatability to measure the accumulation of deformation at intervals during fatigue tests. Measurements of charge flow correlated with the density of spots developed on the gel electrode image, and both provided information on the distribution and extent of the fatigue deformation. The application of the 5 V, 350 ms pulse provided good repeatability, reproducibility between specimens and excellent sensitivity. Fatigue deformation was detected as early as 0.1% of the fatigue life, and observed to increase systematically with increasing number of fatigue cycles. During the early stages (ie up to "~5% of life), deformation appears to accumulate at similar rates at several locations, but thereafter tends to concentrate at the location of eventual failure. The gel electrode image provides additional information on a finer scale: each spot in the image is considered to result from the presence of clusters of slip band extrusions. In general terms, the initial 10% of fatigue life is concerned with accumulating deformation at many locations (ie, grains of the material), thereby setting the stage for a linking-up process which is well established by ~50% of life and provides a path for final crack propagation.
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Fig. 18 E f f e c t o f fatigue cycling on the m a x i m u m value of charge density that flowed t o a gel electrode. 5 V, 350 ms pulse, 6061-T6 specimen, 14 nm oxide. Fatigue life, N f = 2.1 -+ 0.2 X 10 s cycles
Acknowledgements The author is grateful for the assistance of D. Gorkiewicz in performing these experiments, and for programming the computer to integrate the current transients.
References of spots in the gel electrode image during the early stages of fatigue (see Figs 17a and 17b) is a manifestation of the fatigue deformation process, and is not due to any inhibiting effects of the gel electrode technique.
1.
Baxter, W. J. 'Gel electrode imaging of metal fatigue: I. Cracks in 6061-T6 aluminum' Metallurgical Transactions 1 3 A (1982) top 1413--1420
2.
Baxter, W. J. 'Gel electrode imaging of metal fatigue: II. Deformation in 1100 aluminum' Metallurgical Transactions 13A (1982) pp 1421--1427
3.
Baxter, W. J. 'Gel electrode imaging of fatigue cracks in aluminium alloys' I n t J Fatigue 5 No 1 (1983) pp 37--42
4.
Baxter, W. J. and McKinney, T. R, 'Gel electrode imaging of deformation in aluminum alloys' Scripta Metallurgica 17 (1983) pp 1371--1375
5.
Baxter, W. J. 'Oxide films: Quantitative sensors of metal fatigue' in J. Lankford, D. L. Davidson, W. L. Morris and R. P. Wei (eds) Fatigue Mechanisms: Advances in Quantitative Measurement o f Physical Damage, ASTM STP 811 (American Society for Testing and Materials, 1983) pp 115--136
6.
Young, L. Anodic Oxide Films (Academic Press, London, 1961)
7.
Arnott, D. R., Baxter, W. J. and Rouze, S. R. 'The role of
Summary
The gel electrode technique provides a quantitative assessment of the extent of deformation in aluminium, by stimulating an electrical current to pass through deformation-induced microcracks in a previously formed 14 nm surface oxide film. As metal is exposed by these microcracks it is immediately reoxidized by the ambient atmosphere, but only a thin (~1.6 nm) oxide reforms. The current from the gel electrode passes through these regions of thin oxide in the microcracks, producing both passivation and corrosion thereat. The passivation occurs very rapidly (~10 -3 s), but is immediately followed by a steadily increasing corrosion reaction. The process of passivation deposits an additional, very thin (~0.8 nm) film on the 1.6 nm oxide already present, but this is sufficient to inhibit the current flow during a subsequent repeat measurement. On the other hand, the corrosion reaction apparently removes oxide so that a larger current flows during subsequent measurements. These two processes can be balanced by controlling the duration of the voltage pulse applied to the gel electrode. In these experiments on 6061-T6 aluminium, it is shown that very repeatable measurements can be obtained with a 5 V, 350 ms pulse.
252
defects in the fracture of oxide films on metals' J o f the Electrochem Soc 28 (1981) pp 843--847 8.
Tajima, S. 'Anodic oxidation of aluminum' Advances in Corrosion Sci and Tech 1 (1970) pp 2 2 9 - 3 6 2
9.
Kubaschewski, O. and Hopkins, B. E. Oxidation o f Metals and Alloys, second edition (Academic Press, New York, 1962) p 38
Author The author is with the Physics Department, General Motors Research Laboratories, Warren, MI 48090-9055, USA.
Int J Fatigue October 1984