Methods and devices for electrochemically determining metal fatigue status

Devices and methods for determining the fatigue status of a metallic specimen are provided. The methods include contacting the specimen with-a cell containing an electrode and an electrolyte in contact with the specimen. A voltage is then applied, or galvanically created, between the specimen and the electrode, and the current is measured passing through the electrolyte during a period in which the specimen is subjected to cyclic stresses. A signature is then prepared which is representative of a measured current at the electrode, and the signature is analyzed to determine the fatigue status of the specimen.

FIELD OF THE INVENTION 
This invention relates to methods and devices for determining the degree to 
which metallic materials suffer fatigue damage, and more particularly, to 
procedures for evaluating the electrochemical interactions in metallic 
substrates exposed to plastic and elastic deformation. 
BACKGROUND OF THE INVENTION 
Many of today's modern metallic structures, such as steel bridges and 
aluminum airplanes, are exposed to cyclical compressive and tensile forces 
over their useful life. These structures have a plastic component, in 
which the metal undergoes deformation above its yield point, and an 
elastic component, in which the metal is stressed at a level below its 
yield point. The degree to which the metal performs over the years that it 
is in service is largely affected by the nature of these forces, and the 
corrosive environment that surrounds the metal. These environments can 
contain atmospheric conditions, such as acid rain and salt water, as well 
as man-made corrodants, such as gasoline and acids. The combination of a 
corrosive environment and cyclic forces creates a damage mechanism 
commonly referred to as "corrosion fatigue". 
There are a number of techniques for measuring fatigue damage of a metal 
structure. Non-destructive testing, such as dye penetrant inspection, 
ultrasonic testing, and magnetic particle detection are just some of the 
more traditional techniques that have been used to determine the presence 
of cracks in components which have undergone fatigue damage. Although 
these methods are useful in forewarning catastrophic failure, they rely 
upon the existence of crack-like defects which are large enough to detect, 
and cannot perceive any other type of damage caused by cyclic stresses. 
Fatigue strain gauges and fuses have also been used to predict fatigue 
life. Fatigue gauges rely upon monotonic changes in resistance for 
determining the degree of fatigue. Fatigue fuses are essentially miniature 
fatigue specimens attached to a structure, which undergo the same cyclical 
stresses as the structure and provide advanced warning of the development 
of fatigue damage. Although these devices have practical utility, they 
require advanced knowledge of an existing fatigue problem and merely 
provide a cumulative assessment of the damage from the onset of service 
life. They have little or no value in detecting the current state of 
damage if they were not previously affixed to the structure prior to 
service. 
Accordingly, there is a need for a procedure for measuring fatigue damage 
which can be used at any point during the service life of the metallic 
structure, from the day it is placed in service, through the point at 
which it is no longer useful for its intended purpose. 
SUMMARY OF THE INVENTION 
This invention provides a method of determining the fatigue status of the 
metallic specimen at just about any point during its service life. The 
method includes contacting the specimen with a cell containing an 
electrolyte and an electrode. A voltage is then generated between the 
specimen and the electrode, either by applying it externally or by relying 
upon the specimen and electrode to generate a current galvanically, and 
the electrical current is measured as it passes through the cell during a 
period of cyclic deformation. This measured current is thereafter used to 
prepare a signature which is representative of a fatigue status of the 
specimen. 
The methods and devices of this invention can be used for analyzing the 
extent of fatigue damage in a metallic specimen. These procedures can be 
used to determine if the specimen is in the rapid hardening, saturation, 
crack nucleation, or crack propagation stages of its fatigue life. When 
compared to non-destructive testing techniques, the procedures of this 
invention provide a means for detecting fatigue damage long before a crack 
initiates. This invention also represents a significant improvement over 
fatigue strain gauges and fuses which, by their very nature, must be used 
with new metallic structures. Accordingly, the methods and devices of this 
invention can be used for determining the remaining fatigue life of 
various metal-containing structures, including aluminum airplane 
components, turbine shafts, and structural bridge components. 
In further aspects of this invention, devices are provided for determining 
a fatigue status of a metal or alloy specimen. The devices include a 
module containing an electrode and an electrolyte disposed in contact with 
a surface of the specimen or structure. A voltage source is also provided 
for creating an electrical potential between the electrode and the 
specimen surface. This device could be adapted for detecting cracks or 
similar flaws in new structures, or for evaluating the substrate cracking 
behavior or integrity of conductive and non-conductive coatings, paints, 
or oxide layers.

DETAILED DESCRIPTION OF THE INVENTION 
With reference to FIG. 1, there is shown a preferred fatigue gauge 100 of 
this invention having a module 10 and a plurality of electrical contacts, 
including a counter electrode 12, a reference electrode 13, and a working 
electrode 15. The module 10 is equipped with a solution inlet 14 and an 
air outlet 11 for introducing an electrolytic solution, such as liquid 
electrolyte 17. In order to provide a seal against a specimen surface, an 
o-ring 16 is provided in an annular cavity along the mouth of the module 
10. 
During use, the gauge 100 operates by directing a current along the working 
electrode 15, which in turn, feeds through the specimen surface and into 
the electrolyte 17. When the specimen is stressed, such as when it 
undergoes elastic or plastic tensile and compressive stresses, the metal 
will undergo enhanced dissolution of atoms into the electrolyte. Upon 
plastic deformation of the specimen surface, additional areas are created 
from which dissolution of atoms occurs more readily than for the 
relatively unstrained surface. It is this phenomenon that is controlled 
and advantageously employed for determining the fatigue status of various 
metallic substrates in accordance with this invention. 
The fatigue gauge 100 preferably includes a three-electrode arrangement to 
maintain accurate potential control. A working electrode 15 is provided in 
contact with the specimen. This electrode is preferably connected with a 
control unit 18. The end of the working electrode 15 can include 
connection means, such as an alligator clip, for mechanically joining the 
electrode 15 to the surface of the specimen. The counter electrode 12, on 
the other hand, is located within the module 10 and preferably includes a 
large surface area having a disk-like appearance, the counter electrode 12 
is preferably positioned near the rear of the module 10 and is 
electrically connected to the control unit 18. Finally, the reference 
electrode 13 can be used to provide a constant anodic potential preferably 
selected from the four distinct regions of the polarization curve: 
cathodic, active, passive, and transpassive regions respectively, as shown 
in FIG. 5. 
The control unit 18 preferably includes means, such as a potentiostat, for 
determining variations in the corrosion current caused by applied strains 
during the fatigue life of the specimen. This current is referred to as 
the "transient current" of the system. The control unit operates with a 
current monitor 19 which preferably provides electricity, checks the 
continuity of the system, and provides data storage means for recording 
the measured transient currents. The stored currents can be plotted, using 
an X-Y recorder and a chart recorder, to provide a signature of the 
fatigue status of the specimen. Although the preferred "signature" is a 
curve generated from indicia of a measured transient current, it could 
also be any recorded data that can be subsequently converted or analyzed 
mechanically, electronically, or visually to provide a measure of fatigue 
status. For example, the current could be measured so that when the 
measured amplitudes reach a certain minimum amount, or a crack spike is 
detected, a visual or audible alarm would be activated. 
With reference to FIGS. 2 and 3, there is shown alternative fatigue gauges 
200 and 300 of this invention. Fatigue gauge 200 is a simpler, 
two-electrode variant which could be employed when preliminary tests have 
already identified the optimal measuring conditions, such as the optimum 
potential. This embodiment includes a module 20 having an air outlet 21 
and a solution inlet 24. A counter electrode 22 is also provided, but 
since the optimum potential has already been identified, a reference 
electrode is not necessary. As with the fatigue gauge 100, this gauge 200 
includes an electrolyte 27 and an o-ring seal 26. 
In a further embodiment of this invention, a larger fatigue gauge 300 is 
provided. This gauge also includes an electrolyte 37 contained within a 
module 30 having a solution inlet 34 and an air outlet 31. An additional 
annular supporting lip 38 and an o-ring seal 36 are provided to assist in 
sealing the module 30 against a specimen. As can be readily observed, this 
gauge 300 accommodates a larger sensing area. 
The specimens of this invention can be both experimental and commercial 
metal-containing substrates. Examples include test specimens made from raw 
stock, or specimens taken from existing structures during their useful 
service life. Alternatively, the commercial structures themselves can be 
the basis for the test specimen, for example, an exposed surface of an 
airplane skin or structural bridge member. The specimen should, 
nevertheless, have a conductive surface thereon. It can consist of a 
metal-containing polymer or composite, as well as an assortment of metals, 
such as, carbon steel, stainless steel, copper, aluminum, titanium, and 
their alloys. These polymers and metals can also contain a coating, such 
as a paint, resin, oxide, or passivation layer. 
The modules 10, 20, and 30 of this invention preferably are "cup-shaped", 
and include a large opening at one end for Contacting the metallic 
specimen with the contained electrolyte 17, 27, and 37. The shape of the 
module is arbitrary, and can range from a variety of shaped rigid or 
flexible members adapted to seal against a flat or irregularly-shaped 
metallic article. It is further anticipated that the article itself may 
contain a cavity suitable for housing an electrolyte, in which case, the 
module may be nothing more than a flat plate sufficient to contain and 
seal the electrolyte in the metal cavity. The module is preferably made of 
a non-conductive ceramic or polymeric material, and preferably is 
corrosion-resistant. Preferred materials include polyethylene, 
polycarbonate, PTFE, polyimide, and polyamide, for example. 
The electrolyte 17, 27, and 37 can be any electrolytic substance that is 
capable of conducting current, and preferably is capable of dissociating 
into ions in solution to become electrically conductive. Liquids, such as 
aqueous salt solutions and acids, and gels are preferred, with the latter 
being especially adapted for high temperature service. The preferred pH 
depends on the metal, but for corrosion-resistant metals, is less that 
about 4, with a pH below 2 being more conducive to ions releasing from the 
metal specimen preferred. However, depending on the sensitivity of the 
specimen to corrosion, pH values equal to or greater than 7 could be used, 
and even a natural aqueous corroding medium in applications, for example, 
where a structure is submerged, or contains a corroding medium. 
EXAMPLE 
The material used in this study was commercial, polycrystalline AISI 316L 
stainless steel. 
The configuration of the test specimens was a 3.5 mm diameter, threaded 
test bar 47 as shown in FIG. 4. In some of the specimens, a planar region 
was spark cut on the gauge length, in order to produce a flat surface for 
good SEM observation. The specimens were annealed in vacuum at 
1050.degree. C. for one hour, then quenched in water. Then the surfaces of 
the specimens were polished mechanically and electrolytically. 
The electrolytic environment chosen was 1N H.sub.2 SO.sub.4 solution with a 
pH value of 0.4 (.+-.0.01). The reason for choosing this solution is that 
distinct reactions can be identified easily by controlling the applied 
potentials. The low pH value employed ensured that an active region 
appeared in the polarization curve. In order to maintain rigorous control 
of the electrochemical conditions, dissolved oxygen was removed before 
each fatigue test by deaerating the solution. The electrolytic solution 
and purified argon gas were kept flowing through the environment chamber 
during the experiment. The argon inlets 46 and 54 and outlet 56, and the 
solution inlet 51 were used for this purpose. 
Corrosion fatigue tests were performed by controlling the mechanical and 
electrochemical parameters, i.e., the plastic strain amplitude and the 
polarization potentials. Correspondingly, a special testing apparatus 
shown in FIG. 4 was designed to keep the specimen under well controlled 
corrosion fatigue conditions. 
The testing apparatus 400 is one element of four main parts, which included 
a mechanical testing machine; solution deaeration and argon purification 
system;-potential control electronics; and data recording system. 
Cyclic deformation of the specimens was carried out by an MTS 
servo-controlled electrohydraulic fatigue machine. In order to protect the 
extensometer 45 from damage by the corrosive solution, as explained below, 
a horizontal arrangement was adopted. 
A carefully designed environmental chamber was used both to immerse the 
specimen 47 in the aqueous environment and to maintain the specimen and 
solution in an argon atmosphere. The body of the chamber consisted of two 
side walls 52 made of polytetrafluoroethylene ("PTFE"), and one 
cylindrical shell 43 made of PLEXIGLASS.RTM. which allowed direct 
observation of the specimen-electrode assembly during a test. 
On the wall of the chamber's shell, inlets and outlets were provided for 
leading both the deaerated solution and purified argon gas through the 
chamber. A dam-was constructed near the solution outlet to maintain the 
solution at such a level that the entire sample was immersed in the 
solution, whereas the body of the extensometer 45 was kept outside the 
corrosive solution. The two knives of the extensometer, which were 
attached directly to the gauge section of the specimen were made of 
ceramic to prevent interference with both the potentiostat control system 
and the MTS control system. An argon inlet, provided at the bottom of the 
chamber was used to bubble argon gas through the solution during cathodic 
treatment before a fatigue test, in order to minimize the effect of the 
hydrogen gas produced as a by-product during cathodic treatment. 
The specimen 47 was mounted by locking nuts into two grips 49 made of AISI 
304 stainless steel and sealed with o-rings 48. Two pieces of PTFE 
insulation 42 were inserted between the metal parts of the grips 49 to 
ensure that the testing cell was electrically isolated from the testing 
machine. The heads of the grips and locking nuts were covered by PTFE 
shells and caps in order to keep the specimen 47 electrochemically 
separated from any of the other metals of the testing equipment. The whole 
set of grips 49 and specimen 47 was mounted in the MTS machine by locking 
nuts using a Wood's alloy pot for stress free insertion of the specimen. 
Deaeration was accomplished by bubbling purified argon gas through the 
solution in the solution supply tank for several hours before, a fatigue 
test, and this treatment was continued during the test. 
Purified argon was obtained by passing stock gas through drierite and then 
through furnace-heated copper turnings at about 450.degree. C. The 
purified argon was then supplied to the solution tank and the testing cell 
for solution deaeration and chamber flushing respectively. 
A (Princeton Applied Research), model 173 standard potentiostat was 
used to obtain precise potential measurement and control. It was also 
equipped with a model 175 Universal Programmer for carrying out the 
potential polarization (sweeping) test. 
The three-electrode method was employed for the tests to maintain accurate 
potential control. The working electrode was the specimen 47 itself, which 
was connected to the potentiostat (not shown) through the grip 49 and 
exited through outlet 41. The counter electrode 44 was a platinum gauze 
positioned near the bottom of the chamber. The reference electrode 53 was 
inserted through inlet 50 and consisted of a saturated calomel electrode 
coupled with a salt-bridge. 
The hysteresis loop of the deformation was monitored by a Tektronix model 
5223 digital oscilloscope. These loops were also recorded by an X-Y 
recorder for hardening and saturation analysis. The current transient wave 
form and the strain cycling wave form were recorded simultaneously by a 
Gould Brush 2400 two channel strip chart recorder. 
After polishing, the specimen 47 was cleaned in methanol, and dried. The 
surface of the specimen was then wrapped with PTFE tape at its ends, 
leaving a known area uncovered in the gauge length for the measurement of 
current density. 
After being mounted in the MTS machine, the specimen was cathodically 
treated at about -0.7 V (SCE) for 30 minutes to clean the exposed surface. 
During this period, purified argon was introduced into the chamber through 
the argon inlet 54 at the bottom of the chamber, to blow out the hydrogen 
bubbles from the specimen surface. The potential was then switched to that 
desired-for testing, and held steady for about 30 minutes to reach a 
stabilized current for beginning the test. 
The fatigue test was conducted under plastic strain amplitude control, with 
a sinusoidal wave form. The frequency of cycling was chosen to be about 
0.5 Hz. In most cases, the plastic strain amplitude was set at about 
0.001, which was determined according to the cyclic stress-strain curve 
(CSSC) obtained in air. In order to determine the contribution from 
elastic deformation to the transient current, some fatigue tests were 
performed at a total strain amplitude below the elastic limit. 
The electrochemical conditions of the tests were controlled at constant 
anodic potentials, which were chosen from the polarization curve obtained 
by a potential sweep test. The polarization curve of AISI 316L stainless 
steel, FIG. 5, consists of four distinct regions: cathodic, active, 
passive, and transpassive regions respectively. The active and passive 
regions proved to be the most interesting. In order to improve our 
understanding of electrochemical/ mechanical interactions, four potentials 
were chosen for performing corrosion fatigue tests, as indicated by the 
small triangles in FIG. 5. -0.25 V was chosen as the corrosion potential, 
-0.05 V as the active/passive transition potential, +0.3 V as the passive 
I potential and +0.6 V as the passive II potential, respectively. All of 
these potentials were measured at the saturated calomel electrode (SCE). 
One of the most important consequences of corrosion fatigue is that the 
corrosion rate of the material was shown to be greatly enhanced by the 
cyclic deformation. Accordingly, a variation in the corrosion current with 
applied strain was determined. Although the surface reaction will change 
with different applied potentials, some features of the current transient 
behavior, observed by analyzing the current data recorded at different 
potentials, were found to be common to the different potentials. 
During corrosion. fatigue tests, the transient current was observed to vary 
with mechanical cycling for all stainless steel polycrystalline specimens. 
Very small current levels were recorded. Such variations were observed at 
all four potentials shown in FIG. 5. A typical example of current behavior 
is shown in FIGS. 6a-6c. One polycrystalline specimen was cycled at a 
potential of -0.05 V (SCE) with a plastic strain amplitude of 
.epsilon..sub.p1 =1.times.10.sup.-3. The specimen was deformed in the 
tensile direction first. During the first cycle, the current showed a 
small transient (arrowed) towards the negative direction at the very start 
of deformation, but it then increased very quickly to a maximum value 
followed by a rapid reduction or decay. The frequency of the current 
cycling was found to be two times that of the mechanical cycling. 
As indicated by FIG. 6a, there are two current peaks in each fatigue cycle, 
which lie at positions a little earlier than the maximum values of the 
strains for both tension and compression. The capital letters "T" and "C" 
are used to identify the maximum values in the current versus time curves 
as a tension peak and a compression peak respectively. As the cycling 
proceeded, the height of the current peaks decreased gradually with the 
increased number of cycles. A relatively stable value of the current peak 
was reached after thousands of cycles. Apparently, these current peaks 
were produced by plastic deformation both in tension and compression. 
As shown in FIG. 6a, the tension peak was higher than the compression peak, 
especially for the beginning several cycles. This is believed to be caused 
by the well-known tension-compression asymmetry in stress, which favors 
plastic deformation in the tensile direction. 
Another important feature of the current transient curve is the different 
value of the minimum following each current peak shown in FIG. 6a. For 
convenience, the minimum after the tension peak is referred to as the 
T-valley, and the other minimum is referred to as the C-valley. It was 
found that the value of the T-valley was always higher than that of the 
C-valley, no matter which potentials were used, and no matter where in the 
number of cycles they were observed during the test. The difference 
between the values of the two valleys changed little during the test, 
except when the fatal cracks had developed and grew very quickly as shown 
in FIGS. 8b and 8c. 
The modification of the current transient curve by the two current valleys 
is indicative of the fatigue status of the specimen, since it has an 
almost direct correlation with the number of cycles to which the specimen 
has been subjected to. For example, in FIG. 6, the three columns of 
tracings record behavior at different numbers of cycles, the left column 
is at the start of cycling, and the 15th and 45th tensile strokes are 
labelled in the second and third columns to identify the cycle counts. 
Analysis was conducted with the current data obtained from a 
polycrystalline specimen, which was fatigued in the passive I region at 
about 1,000 cycles. In relating the current behavior to the components of 
the deformation, it is necessary to examine separately the effects of 
plastic strain and elastic strain. As shown in FIG. 7, the current 
transient can be divided into two components: current associated with 
plastic strain (CAPS) and current-associated with elastic strain (CAES). 
The CAPS has two peaks during one mechanical cycle, which correspond to 
the plastic deformation in tension and in compression respectively. The 
CAES has only one peak in each cycle which usually lies at the position 
between the maximum compression strain rate and the maximum compression 
strain. The values of these strains are plotted against time in FIG. 7. 
Also included in this figure is the current transient curve. 
From this curve, it can be seen that the major part of the current 
variation was caused by the plastic straining. At point A, which 
represents the yield point in the cycling, the plastic strain is maximum 
in compression. From point A to point B, the specimen undergoes plastic 
deformation from maximum compression to maximum tension. During this 
period, fresh metallic surface continuously emerged on the gauge section 
and, in association, the anodic current increased rapidly, i.e., the 
T-peak occurred in the current transient curve. From point B to point A', 
the plastic straining was temporarily halted, the specimen elastically 
relaxed from maximum tension to zero, and then underwent strain in 
compression. Correspondingly, the anodic current decayed rapidly to a 
minimum value (the T-valley in the current transient curve) From point A' 
to B', the specimen was plastically deformed in compression, and fresh 
surface again emerged as in the tensile period. Therefore, a C-peak showed 
up, followed by a C-valley when the cyclic process continued from point B' 
to A. The difference between the values of the T-valley and C-valley is 
due to the elastic stress or strain. The positions of the T-valley and 
C-valley lie at positions corresponding to the maximum strain rate on the 
elastic strain curve. 
It is possible in some structures where the electrochemical properties of 
the metal are of such a kind, or the fatigue damage so limited in area 
fraction, that the CAES could overwhelm the fatigue signature component of 
the measured current in a large background. In such,a case, a 
non-conductive coating could be applied to the specimen to suppress this 
background, and intrusion of the fatigue damage through the coating could 
be sensed. The same approach could be used to protect a corrosive 
sensitive specimen from potentially damaging effects of the fatigue 
sensing device. 
The following conclusions can be drawn here about the general features of 
the transient current. During cycling, the major part of the current 
variation comes from the plastic deformation, and is closely related to 
the exposure of the fresh metal surface to the corrosive solution. The 
reactivity of the newly exposed area with the solution is much higher than 
that of the pre-existing surface. The conclusion is that the differences 
in compositions and atomic arrangements between the newly formed area and 
the gross external surface of the specimen are responsible for the big 
difference in their reactivities. 
Another important feature of the current transient curve is that crack 
nucleation and growth can be monitored by close inspection of the shape of 
the current curve. An example of this shape effect is given in FIGS. 
8a-8c, in which a current-cycle recording is shown for polycrystalline 
specimen cycled in the passive I region. After 25,000 cycles in this 
specimen, cracks nucleated and began to grow. Accordingly, the current 
amplitude, j.sub.T or j.sub.C, began to increase (FIG. 8a). (Notice that 
the curve in FIG. 8a was recorded at a very low chart speed, therefore the 
current recording pen sweeps an area defined by the T-peak and C-valley.) 
At about 30,000 cycles, a fatal crack could be identified by the presence 
of a small "bump" located at the bottom of the C-valley (FIG. 8b). As the 
cycling proceeded, the bump gradually enlarged until final failure 
occurred (FIG. 8c). 
An explanation for this crack-related current transient is as follows. 
During the tensile period, from point A to B in FIG. 9, the specimen 
deforms elastically, and the crack opens by plastic flow at the tip thus 
exposing a fresh area to the solution, which in turn produces an instant 
increase in current, followed by a rapid current decay. As the specimen 
continues to be plastically deformed in tension, the T-peak develops in 
the current curve. The crack-opening peak and the T-peak are separated by 
a period of time. Therefore, they can be distinguished. As the specimen 
cycles into compression, the crack closes first, followed by plastic 
deformation. Since cracks tend to close as the specimen is unloaded from 
tension, one would expect a bump to precede the C-peak, like in tension. 
However, this behavior is not observed. Rather, from point C to D, a 
current transient is produced due to a change in surface status around the 
cracking area, such as film rupture. However, this crack-closure transient 
coincides with the C-peak. Therefore, no current variation is seen in the 
T-valley; and the height of the C-peak increases. The shape of the crack 
induced peak in the current is noticeable and can be much sharper than 
that of the straining current peaks under optimum conditions, showing that 
the crack-induced peak is an instantaneous reaction. Broadened 
distributions of current observed in the straining peaks, especially the 
T-peak, are associated with the sequential emergence of many slip steps. 
The above example demonstrating the correspondence between transient 
current and cyclic deformation in corrosion fatigue of an AISI 316L 
stainless steel leads to a number of conclusions. There is a strong 
interaction between the aqueous environment and the cyclic deformation, 
which can be perceived from transient current curves obtained during 
corrosion fatigue tests. The transient current can be divided into two 
components, the CAPS and the CAES respectively. The CAPS has two peaks in 
one mechanical cycle, corresponding to plastic deformation both in tension 
and in compression. The CAES varies periodically at a frequency the same 
as that of mechanical cycling. At a strain amplitude of 1.times.10.sup.-3, 
the CAPS causes the major part of the current transient, which is produced 
by metallic dissolution at slip steps where the atomic composition is 
quite different from that on the average surface. The decrease of the 
current peak during the softening stage is caused by a gradually reducing 
tendency in the selective dissolution of iron. The different values of the 
T-peak and the C-peak in the current curve are produced partly by the 
tension-compression asymmetry and partly by cyclic creep, which favors 
plastic deformation in the tensile direction. The beginning of corrosion 
fatigue failure by cracking can be monitored by close inspection of the 
transient current curve. Elastic strain or stress can cause current 
variation by changing the energetic status of the atoms on the specimen 
surface, which is important to corrosion fatigue at low strain amplitude. 
There is a possibility that a charging effect of the electric double layer 
has a noticeable contribution to the CAES. 
From the foregoing, it can be realized that this invention provides fatigue 
gauges and methods for determining the fatigue status of metallic elements 
at any point during their useful life. The electrochemical techniques used 
by this invention are highly sensitive and can even isolate the onset of 
fatigue cracking in specimens long before catastrophic failure. Although 
various embodiments have been illustrated, this was for the purpose of 
describing, and not limiting the invention. Various modifications, which 
will become apparent to one skilled in the art, are within the scope of 
this invention described in the attached claims.