WO2001031311A2

WO2001031311A2 - Methods and devices for evaluating fatigue damage

Info

Publication number: WO2001031311A2
Application number: PCT/US2000/029632
Authority: WO
Inventors: Laird Campbell; John J. Deluccia; Brian Tull; Yuan Feng Li
Original assignee: University Of Pennsylvania
Priority date: 1999-10-29
Filing date: 2000-10-27
Publication date: 2001-05-03
Also published as: AU3634201A; WO2001031311A3

Abstract

A method of determining the fatigue status of a metal-containing specimen (19), where the method utilized is benign to the fatigue properties of the tested specimen, in spite of the presence of an electrolyte (15), and the claimed method and the Electrochemical Fatigue Sensor (EFS) (10) device applying the claimed method can measure fatigue damage at different points during the service life of a metallic structure. The tested specimen is placed in an EFS cell containing an electrode (12) and an electrolyte, which does not cause any degradative effects to the tested specimen and is in contact with the tested specimen. The electrolyte is used with EFS device and method of the invention is capable of being incorporated in a gelling material. 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. By analyzing the elastic and plastic deformation transient current components the fatigue status and remaining fatigue life of a specimen are determined and/or estimated.

Description

METHODS AND DEVICES FOR EVALUA TING FA TIGUE DAMΛ GE

This application claims priority under 35 U.S.C. § 119(e) from a provisional

Application No. 60/162,334, filed on October 29, 1999 in the name of John J. DeLuccia,

Campbell Laird, Brian Tull and Yuan F. Li.

Field of the Invention

This invention relates to the general field of determining the degree to which

metallic materials suffer fatigue damage, and more particularly, to devices and 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 suffer a plastic component of deformation, in which the metal

undergoes deformation above its yield point, and a superimposed elastic component. The

degree to which the metal performs over the years that it is in service is largely affected by

the nature of these forces of strains, 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 alkalis 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.

Electrochemical Fatigue Sensors (EFSs) for measuring fatigue damage of a

metal structure, such as those described in U.S. Patent No. 5,419,201, the disclosure of

which is incoφorated herein by reference, may be used to measure fatigue damage at different points during the service life of a metallic structure. Unlike the other techniques

described above, an EFS may be used to detect fatigue before the onset of crack-like defects

without the need for previously affixing gauges. The EFS described in U.S. Patent No.

5,419,201 includes a module that contains an electrode and an electrolyte disposed in contact with a surface of the specimen or structure that is being examined.

The electrodes and an electrolyte used in electrochemical cells, however,

might degrade the fatigue properties of some metals. This is highly undesirable in many

practical applications, such as in determining the fatigue status and remaining fatigue life of

airplane components, turbine shafts and structural bridge components. It is important in these and other practical applications that the EFS allows for the non-destructive evaluation

of metals, that it responds to the types of damage which occur in commercial metals, and

that it be benign to the fatigue properties of the structure under evaluation, in spite of the

presence of an electrolyte.

Accordingly, it is important to provide a non-destructive method 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.

The three-electrode EFSs can be too expensive and too big to be

successfully utilized for testing outside the laboratory. Thus, there is a need for a less

expensive, more compact and rugged two-electrode device and method of measuring and

determining the fatigue life of a metal-containing specimen. Summary of the Invention

The main object of the present invention is to improve the aforementioned

EFS. Another object of this invention is to provide a benign method of

determining the fatigue status of the metallic specimen at just about any point during its

service life.

A further object of the present invention is to allow a variety of metals to be

tested with minimal corrosive effects. A still further object of this invention is to provide a benign determination of

the remaining fatigue life of various metal-containing structures, including aluminum

airplane components, turbine shafts and blades, and structural bridge components.

Still another object of this invention to provide a benign determination of the remaining fatigue life of various hardened commercial alloys, such as 7075 aluminum alloy, 4130 steel, and Ti-6A1-4V.

Another object of this invention is to provide a benign determination of a

fatigue status of other metal or alloy specimens.

A still further object of this invention is to provide a determination of the remaining fatigue life a metal or alloy specimen using a working electrode and a counter

electrode without the need for a reference electrode. Another object of this invention is to provide a determination of a fatigue

status of a metal or alloy specimen using a working electrode and a counter electrode

without the need for a reference electrode. With the two-electrodes structure, the counter

electrode is capable of providing passivity to the working electrode and eliminates the need for an electronic potentiostat.

Still another object of this invention is to benignly detect cracks or similar

flaws in new and existing structures that may be covered with conductive and non-

conductive coatings, paints, or oxide layers.

The foregoing and other features and advantages of the present invention

will become more apparent in light of the following detailed description of exemplary embodiments thereof, as illustrated in the accompanying drawings.

Brief Description of the Drawings

Fig. 1 is a schematic of a three-electrode fatigue gauge for measuring aluminum or steel potential against a saturated calomel electrode (SCE) reference according

to present invention;

Fig. 2 is a circuit diagram for a two-electrode electrochemical fatigue sensor

according to present invention;

Fig. 3 is a schematic for a two-electrode electrochemical fatigue sensor

according to present invention; Fig. 4a is a schematic for a three-electrode electrochemical fatigue sensor

according to present invention, which was used to test a round bar specimens of various

commercial metal alloys;

Fig. 4b is a schematic of a bar shaped fatigue specimen whose fatigue

status can be measured using a system built according to the present invention;

Figs. 5a and 5b show stress-life fatigue behavior of a hardened 4130 steel

specimens in the EFS electrolyte continually applied, as compared to behavior in air (indicated by dotted lines);

Fig. 5c shows stress-life fatigue behavior of a 7075 aluminum alloy

specimen in the EFS electrolyte continually applied, as compared to behavior in water and compared to behavior in air (scatter band indicated by dotted lines);

Fig. 6 shows EFS waveforms for A17075 aluminum alloy specimens cycled

at a stress of 200 MPa and a frequency of 2 Hz in a 0.3M H₃BO₃ + 0.075M Na₂B₄O₇ + 0.06M Na₂MoO₄ solution (pH = 8.4), with a fatigue life of 210,000 cycles;

Fig. 7a shows EFS waveforms for a 4130 steel specimens cycled at a stress

amplitude of 600 MPa at a frequency of 0.5 Hz in a 0.3M H₃BO₃ + 0.075M Na₂B₄O₇ +

0.06M Na₂MoO₄ solution (pH = 8.4), with fatigue life = 209,600 cycles;

Fig. 7b shows EFS waveforms for a 4130 steel specimen (S-50) cycled at a

stress amplitude of 600 MPa at a frequency of 0.5 Hz in a 0.3M H₃BO₃ + 0.075M Na₂B₄O₇ + 0.06M Na₂MoO₄ solution (pH = 8.4), with applied potential = 0.4 V (SCE),

with fatigue life = 209,600 cycles; Fig. 8a shows the EFS current components (peak current amplitudes)

associated with the elastic and plastic fatigue strain as a function of life, for a 4130 steel

specimen cycled at 600 MPa at a frequency of 0.5 Hz. in a 0.3M H₃BO₃ + 0.075M

Na₂B₄O + 0.06M Na₂MoO₄ solution (pH = 8.4), as a function of elapsed cycles;

Fig. 8b shows the EFS current components (peak current amplitudes) EFS

current components associated with the elastic and plastic fatigue strain as a function of life, for a 4130 steel specimen (S-50) cycled at a stress amplitude of 600 MPa at a

frequency of 0.5 Hz in a 0.3M H₃BO₃ + 0.075M Na₂B₄O₇ + 0.06M Na₂MoO₄ solution (pH

= 8.4), with applied potential = 0.4 V (SCE) and fatigue life = 209,600;

Figs. 9a, 9b and 9c show a replica representation of the fatal crack of an aluminum specimen at 78%, 87% and 94% of the fatigue life, respectively, after detection

of the formation of a crack by an EFS system built according to the invention;

Fig. 10 shows a crack detected by the sensor of an EFS system built

according to the invention, wherein the sensor is attached to the flat side of a 7075

aluminum alloy specimen having a blind hole on the other side, and wherein the specimen

is fatigued in electrolyte at a maximum stress of 150 Mpa, R=0, at a frequency of 1 Hz;

Fig. 1 1 shows a crack detected by the sensor of an EFS system built

according to the invention, wherein the sensor is attached to the flat side of a 4130 steel

specimen having a blind hole on the other side; Fig. 12 shows ionic bonds formed by platelets into so-called "House of

Cards" structure; and Fig. 13 illustrates the results of the anodic polarization of Aluminum Alloy

7075-T73 in a gelled steel electrolyte according to the invention at 5mV/sec scan rate.

Detailed Description of the Invention

The Electrochemical Fatigue Sensor (EFS) device and method, as generally described in U.S. Patent No. 5,419,201 of May 30, 1995, the disclosure of

which is incorporated by reference, involves a system that evaluates fatigue damage in

metal structural components through electrochemical current monitoring. To test the

device and method according to one embodiment of the invention, which is shown in Fig. 1, Al or steel specimens were set up in fatigue testing machines, and a laboratory

protocol, consisting of a three-electrode electrochemical cell and potentiostat, was used to obtain the required electrochemical current and characteristic EFS response. Heretofore,

EFS data was obtained through the use of a three-electrode laboratory protocol.

The three-electrode cell structure of the embodiment of the EFS device 10

shown in Fig. 1 involves an electrochemical circuit consisting of a working electrode 11, a counter electrode 12, and a reference electrode 13. The working electrode 11 in the electrolyte solution (or gelled electrolyte) 15 is in contact with the metal component 19

under test, and serves as the anode, while the counter electrode 12 serves as the cathode.

The full-cell potential at the surface of the working electrode is established with respect

to the reference electrode 13, and regulated through the use of the potentiostat 14. In this embodiment, the counter electrode is preferably a Platinum (Pt)

mesh, which is electrochemically inert and serves as a monitor of the electrochemical

current. The metal working electrode is coupled to the Pt counter electrode 12 through a

micro-ammeter 16 within the potentiostat 14, which may monitor and output the

electrochemical current signal to a signal processing and data collection equipment (not shown). When the structure undergoing evaluation is simultaneously stress-cycled, the

cell current fluctuates in some phase relationship with the stress, and with waveform

characteristics that yield information on the amplitude of the stress and the extent of the

associated fatigue damage. The current reflects changes in the strain localization and

responds actively to the presence of cracks, even when they are small. Thus, by capturing and feeding the electrochemical response of the EFS device to the signal processing and

data collection equipment, which may be done using a LabView software, or another

software capable of similar signal processing and data presentation, the fatigue status of

the tested sample may be visually presented, examined and analyzed.

The reference electrode 13 in the above-mentioned embodiment is the

Saturated Calomel Electrode (SCE), which serves to establish the full-cell electrochemical potential at the surface of the working electrode 11. The natural open

circuit potential between the SCE reference electrode and the working electrode is

insufficient to maintain the working electrode surface in a state of passivity, and thus the

terminals of the working electrode and SCE reference are connected to a potentiostat 14, which serves to regulate the operating full-cell potential in such a way that the metal surface is maintained in the passive state. Passivity, which is defined by the formation of

a thin protective oxide layer over the metal surface, is necessary for obtaining the

characteristic EFS response: As fatigue damage progresses, the passive oxide layer is breached, resulting in increasing electrochemical current activity.

While the three-electrode device, of the type shown in Fig.l, may be used

in the laboratory conditions, a successful commercial application of the EFS technology

to the field requires that the EFS device/system be simple, rugged, compact and cost- effective. Toward this end, the selection of an electrolyte is highly significant, i.e., it must be benign. This is particularly important in view of the objective to improve the

EFS's ability to perform durability assessments of aircraft and other practical structures

that require that the electrolyte used in the EFS to make these assessments must not

degrade durability. The benignity of the electrolyte is critically important since the aircraft alloys chosen for study (high strength aluminum, high strength steel, and

titanium), are susceptible to environmental cracking (stress corrosion cracking and

corrosion fatigue). The alloy designations that were tested by the devices operating in

accordance with the invention comprised 7075-T73511 aluminum alloy bar extrusions,

Rockwell "B" hardness 85.7 avg.; 4130 high strength low alloy steel, Rockwell "C" 36.9

avg.; and 6 A1-4V Titanium alloy, Rockwell "C" hardness 31.6 avg.

In order for an electrolyte to be successfully utilized in the EFS device to

test the fatigue life of alloys used on aircraft (and in other similar field applications) in accordance with the invention, it must fulfill the following conditions: • it must be sufficiently active to provide adequate current signals for

EFS measurements on the alloys that are being tested,

• it must be benign to the substrates of the aircraft and not cause any degradative effects, and

• it must be capable of being incorporated into a useful gel, in order to

avoid the inconvenience of working with liquid electrolyte on the

aircraft (or in other similar field applications). Since the EFS operates while electrochemically polarizing the alloys

within their passive regions (0.3 to 0.5 volts v. SCE electrode), the chosen electrolyte is

preferably mildly oxidizing (free of reducing anions) and within a pH region to preclude

corrosion. The types of corrosion that should be precluded consists of general corrosion,

pitting corrosion, meniscus (crevice) corrosion, potentiodynamic anodic polarization, and

effects on fatigue lives of the alloys specified. The following electrolyte meets all of the specified criteria:

0.3M H₃BO₃ + 0.075M Na₂B₄O₇. 10 H₂O + 0.06M Na₂MoO₄

This electrolyte is suitable for materials such as steel, aluminum, and titanium alloy. It

has a pH of 8.4 and can be successfully incorporated in an inorganic gel of Laponite

materials.

In accordance with another embodiment of the invention, which is shown

in Figs. 2 and 3, the three-electrode cell of Fig. 1 is replaced by a more compact and

rugged two-electrode system, which eliminates the use of expensive and bulky potentiostat and reference electrode. It should be noted that with development of a new

potentiostat, it is possible to electronically impose the required passive potential using

only a counter electrode and the working electrode. The counter electrode is preferably

made of platinum, which is known for its inertness and stability.

A simplified circuit and a schematic drawing of a two-electrode EFS

device according to the invention are shown in Figs 2 and 3, respectively. The EFS

device 20 is shown as incorporating the metal specimen under fatigue as the working

electrode 21 in an electrolyte solution 25 (which may also be a gelled electrolyte), a second driver electrode 22 (the charging/measuring electrode) and a simplified measuring

circuit with a micro-ammeter 23.

From the standpoint of the electrochemical cell design, the driver electrode

22 essentially functions as a counter electrode. However, unlike Pt counter electrode, the

driver electrode serves to maintain a half-cell potential at the surface of the metal, so that the metal surface is maintained in a state of passivity. Hence, the driver electrode

effectively replaces the potentiostat, the SCE reference electrode, the solution bridge, and

the Pt counter electrode. Use of a low-cost rugged driver electrode therefore represents a

major step forward in the adaptation of EFS technology to field applications.

Since the driver electrode 22 is capable of imposing its own natural potential on the metal anode over extended periods of time, it effectively functions as a

power electrode, replacing the potentiostat as a power source. The compound that is

preferably used as a driver electrode, fulfilling the requirements of being rugged and of being able to function over an extended period of time, is nickel/nickel oxide. It consists

of a nickel substrate with a layer of NiO deposited on the surface. The resulting electrode

is nonpolarizing, and therefore capable of imposing its own natural half-cell potential on

the surface of metal anodes, such as Al or steel, effectively replacing the potentiostat and reference electrode as means of imposing a required electrochemical potential at the

surface of the metal anodes.

Referring to Figs. 3, the schematic drawing shows a simplified, low cost,

rugged two-electrode system that incorporates a benign gelled electrolyte 25' enabling an

improved and commercially practical Electrochemical Fatigue Sensing (EFS) device. It

depicts a cell working on a flat surface, sealed by an "O" ring 26. It has been shown that

other than flat surfaces could be accommodated using a cell head that holds polymer foam capable of conforming to most industrial surfaces. The polymer foam and/or membrane

could incorporate the liquid or gelled electrolyte.

As described above, the compound that may be used as a driver electrode

is preferably nickel/nickel oxide (Ni/NiO). The following are some of its advantages:

• it maintains half cell potential within the passive region constant over a

long period of time;

• it is suitable for EFS applications (i.e. satisfies the passivity conditions for

the EFS and generates EFS signals (waveforms)) on steel and other high

strength materials; • it is operable in electrolyte in accordance with the invention (as further

discussed below), that is optimized for high strength materials;

• it remains stable and unpolarized for small galvanostatic currents - such as 10 microamps;

• the natural potential imposed by the Ni/NiO on steel in the electrolyte

(either in liquid or gelled form) in accordance with the invention, as

described below, does not degrade the fatigue properties of steel;

• natural potential imposed on steel in gelled electrolyte causes no weight loss or pitting.

EXAMPLE 1

The EFS devices/method of the invention were tested to determine the

ability of the device to read damage in the following hardened commercial alloys: 7075

aluminum alloy, 4130 steel and Ti-6A1-4V, which one might reasonably expect to be

most difficult to evaluate because damage tends to develop late in the fatigue life. In addition to testing the ability of the EFS device/system to detect damage

and small cracks in the tested samples, it was also tested whether the electrolytic medium

used in accordance with the invention does not degrade the fatigue properties of the

above-identified evaluated materials.

The following electrolyte composition was found to achieve the desired

criteria for aluminum, steel, and titanium alloy: 0.3 M H₃BO₃ + 0 075 M Na₂B₄O₇ + 0 06 MNa₂MoO₄

(pH = 8 4)

Varying amounts (± 10%) of each constituent could serve as benign electrolyte

compositions for most engmeeπng alloys. It is also understood that alternative electrolyte formulations may be devised to produce the appropπate EFS responses, without degrading the mechanical properties of the subject mateπal, as for example by using weak

acids that promote passivity and passivating anions other than molybdates, and which are

acceptable on environmental grounds For instance, tungstates and vanadates could be

used to accomplish the desired effect The chromates, however, might not be acceptable

on environmental grounds because of their carcinogenic qualities.

GEL TESTING

In order to be able to utilize the EFS device according to the current

mvention in the field applications, as for example in testing the fatigue status or

remaining fatigue life of aircraft parts, the electrolyte used with EFS device/system should be capable of being incorporated in a gelling mateπal such as Cab-o-sil (fumed silica) or Lapomte. Accordingly, the electrolyte was tested with gelling agent - Cab-o-sil

(fumed silica), where several composition vaπations and mixing procedures were

explored. The electrolyte gel was processed for 4 min., mixing at 500 rpm, with

composition being dependent on the electrolyte: Al. 8 5wt% Cab-o-sil, Steel, 9wt% Cab- The results of the tests conducted on the electrolyte gel showed that in a

sealed container, gel viscosity was generally stable for periods up to one month. The

formed gel dried in two hours in a laboratory. The pH of the electrolyte remained

constant, both before and after gellation, with conductivity and passivity behavior

accordingly maintained. The weight loss experiments conducted on a steel sample, which was exposed to the electrolyte gel for up to 11 days indicated that no pitting occurred in

the sample, and no weight loss in the sample.

LAPONITE TESTING

A synthetic hectorite mineral of mixed silicates provides thixotropic

rheology and suspending power that results in a gel when used at a level of approximately 0.5 to 2.0%. Having a clay type structure, LAPONITE will separate into many tiny

platelets, which under the correct conditions, will orient together with ionic bonds to give

a thixotropic structure. During the dispersion phase in deionized water, an electrical double layer forms around the platelets, resulting in repulsion between them and no structure buildup.

However, when an electrolyte (e.g., Na, Ca, Mg) is introduced from either the use of tap

water or ingredients in the formula, the double layer is reduced and attraction between the

platelets becomes possible. This results in ionic bonding, forming a "House of Cards" structure, as shown below in Fig. 12. In accordance with one goal of the invention, a successful LAPONITE gel was achieved with the electrolyte (pH = 8.4). Some of the advantages of using a LAPONITE gel are as follows:

• the gel is stable (indefinitely when sealed; and approximately one week

when open to air);

• the gel does not dry out;

• the gel maintains thixotropic behavior;

• the gel does not chemically change the electrolyte (i.e. it is inert); and

• the gel maintains clarity.

EFFECT OF GELLED ELECTROLYTE ON STEEL

During tests of the LAPONITE gel, the steel specimens were exposed to

the gelled electrolyte. The specimens experienced no weight loss in 10 days, and no

localized attack (pitting) was noted on the exposed specimens.

When a polarization cycle was imposed on the steel specimen using the

gelled electrolyte, a sufficiency stable passive region was noted with the absence of a

decreased pitting potential. This indicates that the gel formed with electrolyte, in accordance with the invention, does not cause pitting corrosion while being polarized.

The following figures indicate comparison of the average fatigue life of two steel samples that were tested in a gelled electrolyte in relation to an average life of a similar sample in

the air.

Average fatigue life in air for steel at 600 MPa, and 0.5 Hz — 100,000 cycles Fatigue life in a gelled electrolyte, polarized at 0.4 volts vs. sat. calomel at 600

MPa, and 0.5 Hz:

- sample 1 ~ 80,600 cycles - sample 2 — 120,200 cycles

Following two successful EFS runs on a steel sample in the gelled electrolyte, no discernible differences were found in the stress behavior or fatigue status of the exposed sample.

EFFECT OF GELLED ELECTROLYTE ON THE ALUMINUM ALLOY

One test that was conducted to test the gelled electrolyte for use in

accordance with the invention involved exposure of six aluminum coupons to the steel gel electrolyte for one week. The aluminum coupons used were made of a commercial

alloy that contains unwanted iron and manganese inclusions that serve as sites for pit

nucleation in any aqueous medium. The results revealed no significant weight loss in the

tested samples: 0.045 mm/yr (avg.). In addition, the microscopic examination revealed

only a few scattered pit nuclei on three of the coupons. An alternate exposure test was also conducted on the six aluminum

coupons. The coupons were totally immersed in the gel for four hours a day and then

removed and allowed to air dry for twenty hours. This cycle was repeated for five days,

and simulated a more severe environment because evaporation of water from the gel

concentrated the electrolyte. The results of the alternate immersion test indicated that coupons experienced no weight loss, no new pit nuclei formed, and existing pit nuclei did not grow.

The results of the anodic polarization of Aluminum Alloy 7075-T73 in a gelled steel electrolyte at 5mV/sec scan rate are shown in Fig. 13. A sufficiently passive

region (-.2v to + l .Ov) was noted, as well as the absence of a decreased pitting potential

(as shown). Additionally, the S-N plot, shown in Fig. 5c, indicated minimal effect of the

gelled electrolyte on the fatigue life of the aluminum sample, as compared to the fatigue

life of aluminum in the air (without presence of the gelled electrolyte).

To summarize, the results of various experiments that were conducted to

test the operation of EFS devices and the electrolyte in accordance with the invention have shown that an electrolyte for aluminum, steel and titanium has been successfully

formulated in the gelled state and that this electrolyte 1) maintains electrochemical

requirements for EFS measurements; and 2) remains stable for at least approximately one

week. This electrolyte may also serve for other metals, such as nickel-based alloys and

others because it 1) maintains electrochemical requirements for EFS measurements; 2) causes minimal corrosion on bare surfaces; 3) sustains successful EFS measurements.

EXAMPLE 2

The alloys chosen to study using the EFS device/system in accordance

with the invention are regarded as typical of those which would be used on conventional

jet aircraft and consist of the following: aluminum alloy 7075-T73511, with yield and ultimate strengths of 443 and 514 MPa, quenched and tempered 4130 steel with yield and

ultimate strengths of 1028 and 1092 MPa, and heat-treated Ti-6A1-4V in <χ/β form with yield and ultimate strengths of 892 and 958 MPa respectively.

As shown in Fig. 4b, ordinary round bar specimens with threaded grips, stress-free machined from stock of these metals, were cycled in electrohydraulic machines, usually under stress control and at low frequencies ( around 1 Hz ), using sinusoidal waveforms. Strain was measured using extensometers attached outside the gauge section, because it was desired to surround the specimens with a three-electrode EFS cell, which is shown in Fig. 4a.

For taking EFS measurements, the specimen was enveloped with a transparent cylindrical plastic cell, filled with electrolyte consisting of a buffered solution having pH adjusted to the passivity requirements of the metal, free of aggressive species, and equipped with a standard calomel reference electrode connected via a salt bridge. A diagram of a three-electrode EFS test cell used to test the specimen is shown in Figure 4a as having a fatigue specimen 49 in the electrolyte (not shown), operating as a working electrode, a counter electrode 42, connected to the platinum (Pt) mesh 42', and a reference electrode 43. Regular lock nuts were used with the threaded grips but these were covered with teflon caps and sealed. Instead of using a standard commercial potentiostat, which requires to electrically isolate the specimen from the test machine in order to obtain satisfactory readings of the very small transient cell currents, the redesigned potentiostatic system used m the test did not required such isolation Both the measurements of stress- strain response of the specimen and the electrochemical response were fed to a data acquisition system using LabView software, for data presentation and signal processing

RESULTS

Figure 5a shows the S/N behavior of hardened 4130 steel, which can be taken as typical of the three metals studied, for its response to the EFS environment while the specimens were undergoing interrogation by the EFS throughout life The dotted lines show the S/N behavior in laboratory air and a fatigue limit appears in the region of 550 MPa The plotted points show the fatigue lives of the EFS specimen The scatter in the lives of the EFS specimens fatigued at 600 MPa is entirely normal and, being grouped with the performance of specimens fatigued in air, which showed similar scatter, suggests no hint of an environmental effect Using the electrolyte descπbed above, the steel was tested by standard corrosion tests for both general corrosion and pitting behavior None was observed

Fig 5b shows S/N behavior of the same and other hardened 4130 steel specimens, that were also tested for their response to the EFS environment, while undergoing interrogation by the EFS throughout life The dotted lines show the S/N behavior in laboratory air, while the plotted points show the fatigue lives of the EFS specimen, as well as the lives of two specimens fatigued in tap water Similar studies of S/N behavior were conducted on A17075 specimens, the results of which are shown in Fig. 5c.

The tests indicated, within the constraints of engineering judgment, no significant adverse effect of the EFS environment on the fatigue behavior of the materials, provided that appropriate control is exerted over the chemistry of the electrolyte.

It is important to emphasize that, in the comparison of lives in air and EFS electrolyte, the EFS cell was operated throughout the lives of the specimens. In field applications, it is unlikely that the EFS would ever be used in continual monitoring. Periodic testing would be a more cost effective manner to employ the tool. In such circumstances, and assuming proper cleanup procedures are used after testing (for which well-established procedures are available), the EFS system in accordance with the invention offers no significant environmental threat to the fatigue life of the structure.

EFS WAVEFOM BEHAVIOR

The electrochemical current that flows through an EFS cell while the specimen is undergoing cycling consists of several parts: a DC component reflecting the general electrochemical reaction enforced by the applied voltage, and fluctuating components corresponding to the mechanical reaction of the specimen, including fatigue damage. One of these components reflects the elastic strain of the specimen and generally has the same frequency, although not necessarily the same phase, as the stress cycle. Another is of twice the frequency and is associated with cyclic plasticity. The doubling of the frequency is caused by the electrochemical reaction to fresh surfaces produced by the plasticity in both the tensile and compression reversals. Still higher harmonics can be associated with crack propagation.

The appearance of these components in the EFS signal depends on the stress level, the extent of the fatigue damage and the material. Typical EFS waveforms for aluminum 7075 alloy cycled at 200 MPa, at R = -1, and at a frequency of 2 Hz. are shown in Figure 6. Superimposed on the diagram is the stress cycle, which shows the relationship of the EFS response to the stress. The waveforms were selected from the period of life in which the crack was just beginning to propagate. As well known, cracks form late in the life of smooth specimens of this material. For most of the life, under conditions of passivity, the EFS signal is dominated by the doubled frequency associated with the cyclic plasticity. For much of this part of life the EFS peak associated with the compression reversal is dominant (note trace at 190,600 cycles). However, as the crack grows, the tensile peak gains in strength and becomes increasingly strong as the crack grows longer. The increase in the current associated with crack growth does not proceed smoothly, however. Note that, at 190,700 cycles, there is a jump in the peak current in tension, but at 191,200 cycles, it has fallen back again. We attribute this behavior to intermittent crack growth (or rather variable growth rate) when the crack is small, coupled with re-establishment of passivity in the fresh surfaces associated with cracking-induced plasticity. Crack growth does appear to continue as shown by the gradual increase of the tensile current at 192,600 cycles. Note that the current behavior during the compression reversals does not vary, in the cycle interval illustrated, while there is activity in the tensile reversal. Thus the EFS signal provides qualitative and quantitative information, having both scientific and practical content, about crack growth in the small crack regime of growth and well before the development of the long cracks that can be tolerated by aircraft structures.

The EFS behavior of 4130 steel differs from that of aluminum. As illustrated in Figure 7a and 7b , showing EFS waveform for 4130 steel cycled at a stress amplitude of 600 MPa at a frequency of 0.5 Hz, the current associated with elastic strain, in the fairly long life sampled, is dominant. A waveform rather similar to that of the stress cycle, but out of phase with respect to it, occurs for most of the life. When the fatal cracks form, a shoulder appears on the tensile side of the EFS peaks (cycle 150,100 and above) and this crack-associated peak increases, gradually but erratically, as the crack grows. Note that, like behavior in aluminum, the tensile current bursts and decays as the crack propagates and passivates again. While evidence of cyclic plasticity requires more advanced signal processing to illuminate it clearly, the crack damage produced can be seen in the EFS output. Fourier analysis of the EFS waveforms can reveal, through frequency association, the behavior of the various transient components of the EFS current, as a function of elapsed cycles. Figs. 8a and 8b show the EFS current components (peak values) associated with the elastic and plastic fatigue strain as a function of life, for a 4130 steel specimen cycled at 600 MPa at a frequency of 0.5 Hz. After a settling-in period at the onset of cycling, the AC components reach quiescent status that obtains as long as a crack does not initiate. Once crack(s) form, the currents show the instabilities visible in the waveforms selected for Figures 6 & 7. These instabilities, as noted above, are caused by frequent breakdowns of passivity by plasticity associated with crack propagation and the subsequent repeated reestablishment of the passivity. However, the AC transients are not the only parameters which provide information about fatigue damage. The phase shift between the elastic current component and the stress cycle is also illustrative of the damage. The phase relationship between these signals is quite stable (in the subject hard materials) before crack nucleation. After cracks form, and are subject to electrochemical instabilities, the phase shift provides the same kind of information as the transient currents.

One concern in the development of the EFS is whether or not signals will be accessible at the low cyclic stresses which are normally encountered in many engineering structures. Table 1 shows average values of the informative EFS signals obtained on 7075 aluminum alloy as a function of stress amplitude. EFS signals, which lie in the microamp region, are always low and become lower still as the stress amplitude

decreases. Nevertheless, as the table shows, they remain readable and distinct between

their components, and give adequate warning about crack formation, consistent with the behavior of the material, at the life percentages indicated.

TABLE 1

(STRESS EFFECT ON EFS MEASUREMENTS FOR Al 7075)

All current units = μA/cm"

When the EFS system built in accordance with the invention was tested with aluminum 7075 specimen, it detected a fatal crack of the specimen. Figs. 9a, 9b and

9c show the specimen at 78%, 87% and 94% of the fatigue life, respectively, after

detection of the formation of a crack by the EFS system. Fig. 10 shows a crack detected

by the sensor of an EFS system built according to the invention, wherein the sensor is

attached to the flat side of a 7075 aluminum alloy specimen having a blind hole on the other side, and where the specimen is fatigued in electrolyte at a maximum stress of 150 Mpa, R=0, at a frequency of 1 Hz. Similarly, Fig. 1 1 shows a crack detected by the sensor

of an EFS system built according to the invention, wherein the sensor is attached to the flat

side of a 4130 steel specimen having a blind hole on the other side.

In regard to the titanium samples, it should be noted that titanium is a

good passivator and EFS may easily be utilized in evaluation of its fatigue behavior, both

in the laboratory conditions and in the field applications.

To summarize, it has been determined and shown that EFS device/system

of the invention and the electrolyte can operate in accordance with the objective of the

invention in applying the device/method of the invention for non-destructive evaluation

of hardened metals, such as those described above and utilized in various commercial

structures.

Claims

We claim:

1 . A method of determining the fatigue status of a metal-containing specimen

that is benign to the fatigue properties of said specimen, comprising the steps of:

contacting said specimen with a cell containing an electrode and an electrolyte in contact

with said specimen;

applying a voltage between the specimen and said electrode; subjecting the specimen to a deformation cycle;

measuring a current passing through the electrolyte during said cycle;

analyzing elastic and plastic deformation transient current components to determine the

fatigue status of the specimen, said analyzing step comprising determining the

presence of a crack in the specimen by isolating a current spike in the current

curves; wherein said electrolyte does not degrade the fatigue properties of said specimen.

2. The method of Claim 1, wherein the step of measuring electrochemical current

passing through the electrolyte is accomplished using a reference electrode and a

potentiostat having a micro-ammeter.

3. The method of Claim 2, wherein said reference electrode is the Saturated

Calomel Electrode (SCE).

4. The method of Claim 1, wherein said electrode comprises a platinum (Pt) mesh

that is electrochemically inert.

5. The method of Claim 1, wherein the step of analyzing elastic and plastic

deformation transient current components is performed by a current signal processing

device and a data collection device, said devices operable to capture and process the

electrochemical response produced by said cell of said device, wherein the output from

said signal processing and data collection devices is indicative of the fatigue status of the metal-containing specimen.

6. The method of Claim 5, further comprising a step of determining the remaining fatigue life of a metallic specimen based on the fatigue status of said specimen.

7. The method of Claim 1, further comprising a step of determining whether said

specimen is in the rapid hardening, saturation, crack nucleation, or propagation stages of

fatigue deformation.

8. The method of Claim 7, wherein said step of determining whether said

fatigue is done by comparing the elastic and plastic deformation transient current

components of said specimen to the known elastic and plastic deformation transient current components of a measured current for a substantially similar metallic substance.

9. The method of Claim 1, wherein the monitoring of said current is performed

over a significant portion of the fatigue life of said specimen.

10. The method of Claim 1, wherein said specimen comprises 7075 aluminum

alloy.

11. The method of Claim 1, wherein said specimen comprises 4130 steel.

12. The method of Claim 1, wherein said specimen comprises Ti-6A1-4V.

13. The method of Claim 1, wherein said electrolyte comprises a liquid solution

of 0.3M H₃BO₃ + 0.075M Na₂B₄O₇ . 10 H₂O + 0.06M Na₂MoO₄, having pH value of about 8.4.

14. The method of Claim 1, wherein said electrolyte is a gelatinous substance

comprising 0.3M H₃BO₃ + 0.075M Na₂B₄O₇ + 0.06M Na₂MoO₄, having pH value of about 8.4.

15. The method of Claim 14, wherein said electrolyte is incoφorated in an

inorganic gel of Laponite materials.

16. An electrochemical fatigue sensor device for measuring and determining the fatigue status of a metal-containing specimen that is non-destructive to the fatigue properties of said specimen comprising:

a cell containing a working electrode and an electrolyte in contact with said specimen;

a counter electrode in said electrolyte;

a reference electrode regulated by a potentiostat for monitoring the electrochemical current of said device;

wherein said electrolyte does not degrade the fatigue properties of said specimen.

17. The device of Claim 16, wherein said working electrode serves as the anode

and said counter electrode serves as the cathode of said cell.

18. The device of Claim 16, wherein said reference electrode is the Saturated

Calomel Electrode (SCE) serving to establish a full-cell electrochemical potential at the surface of said working electrode.

19. The device of Claim 16, wherein said counter electrode comprises a platinum (Pt) mesh that is electrochemically inert.

20. The device of Claim 16, wherein said working electrode is coupled to said

counter electrode through the micro-ammeter.

21. The device of Claim 16, further comprising a current signal processing device

and a data collection device, said devices operable to capture and process the

said signal processing and data collection devices is indicative of the fatigue status of the

metal-containing specimen.

22. The device of Claim 21, wherein said current signal processing and data collection devices are further operable to determine the remaining fatigue life of a

metallic specimen based on the fatigue status of said specimen.

23. The device of Claim 16, wherein said specimen comprises 7075 aluminum

alloy.

24. The device of Claim 16, wherein said specimen comprises 4130 steel.

25. The device of Claim 16, wherein said specimen comprises Ti-6A1-4V.

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NY2 278219 04

26. The device of Claim 16, wherein said electrolyte comprises a liquid solution

27. The device of Claim 16, wherein said electrolyte is a gelatinous substance

comprising 0.3M H₃BO₃ + 0.075M Na₂B₄O₇ + 0.06M Na₂MoO₄, having pH value

of about 8.4.

28. The device of Claim 27, wherein said electrolyte is incoφorated in an

inorganic gel of Laponite materials.

29. An improved and simplified electrochemical fatigue sensor device for

measuring and determining the fatigue status of a metal-containing specimen that is non¬

destructive to the fatigue properties of said specimen comprising:

a cell containing an electrolyte in contact with said specimen; a driver electrode in said electrolyte; a measuring circuit with a micro-ammeter;

30. The device of Claim 29, wherein said specimen operates as a working

electrode and an anode of said cell and said driver electrode operates as a counter electrode and a cathode of said cell.

31. The device of Claim 30, wherein said driver electrode is operable to maintain a half-cell potential at the surface of the metal-containing specimen.

32. The device of Claim 29, wherein said driver electrode comprises nickel/nickel

oxide (Ni/NiO).

33. The device of Claim 29, further comprising a current signal processing device

and a data collection device, said devices operable to capture and process the

metal-containing specimen.

34. The device of Claim 33, wherein said current signal processing and data

collection devices are further operable to determine the remaining fatigue life of a

metallic specimen based on the fatigue status of said specimen.

35. The device of Claim 29, wherein said specimen comprises 7075 aluminum

alloy.

36. The device of Claim 29, wherein said specimen comprises 4130 steel.

37. The device of Claim 29, wherein said specimen comprises Ti-6A1-4V.

38. The device of Claim 29, wherein said electrolyte comprises a liquid solution

of 0.3M H₃BO₃ + 0.075M Na₂B₄O₇ . 10 H₂O + 0.06M Na₂MoO₄, having pH value

of about 8.4.

39. The device of Claim 29, wherein said electrolyte is a gelatinous substance

of about 8.4.

40. The device of Claim 39, wherein said electrolyte is incoφorated in an inorganic gel of Laponite materials.

41. A method of determination of a remaining fatigue life of a metal-containing

specimen that is benign to the fatigue properties of said specimen, comprising the steps

of: contacting said specimen with a cell having an electrode and an electrolyte in contact with

said specimen, said specimen and said electrode producing a galvanic reaction; subjecting the specimen to a deformation cycle;

measuring a galvanic current passing through said electrolyte during said cycle;

fatigue status of the specimen, said analyzing step comprising determining the

presence of a crack in the specimen by isolating a current spike in the current

42. The method of Claim 41, wherein said metal-containing specimen comprises

an anode and said electrode comprises a cathode of said galvanic reaction.

43. The method of Claim 41, wherein said electrode comprises nickel/nickel

oxide (Ni/NiO).

44. The method of Claim 41 , wherein the step of analyzing elastic and plastic

deformation transient current components is performed by a current signal processing device and a data collection device, said devices operable to capture and process the

metal-containing specimen.

45. The method of Claim 44, further comprising a step of determining the remaining fatigue life of a metallic specimen based on the fatigue status of said specimen.

46. The method of Claim 41, further comprising a step of determining whether

said specimen is in the rapid hardening, saturation, crack nucleation, or propagation

stages of fatigue deformation.

47. The method of Claim 46, wherein said step of determining whether said specimen is in the rapid hardening, saturation, crack nucleation, or propagation stages of

components of said specimen to the known elastic and plastic deformation transient

current components of a measured current for a substantially similar metallic substance.

48. The method of Claim 41, wherein the monitoring of said current is performed

over a significant portion of the fatigue life of said specimen.

49. The method of Claim 41, wherein said specimen comprises 7075 aluminum

alloy.

50. The method of Claim 41, wherein said specimen comprises 4130 steel.

51. The method of Claim 41, wherein said specimen comprises Ti-6A1-4V.

52. The method of Claim 41, wherein said electrolyte comprises a liquid solution

53. The method of Claim 41, wherein said electrolyte is a gelatinous substance comprising 0.3M H₃BO₃ + 0.075M Na₂B₄O₇ + 0.06M Na₂MoO₄, having pH value of about 8.4.

54. The method of Claim 53, wherein said electrolyte is incoφorated in an

inorganic gel of Laponite materials.