MX2008004270A - Electrochemical fatigue sensor system and methods - Google Patents

Abstract

A method and an electrochemical sensor device for determining a fatigue status of a growing crack in a substrate. The device includes an electrode formed from a stainless steel mesh and having a bottom surface that is coated with an adhesive layer and has a release paper attached to the adhesive layer. The adhesive layer is exposed through separation of the release paper from the adhesive layer. Contacting the adhesive layer to the substrate secures the device to the substrate and forms a cavity that contains an electrolyte and is partially bound by the substrate. The adhesive seals the bottom surface of the device to the substrate in order to prevent leakage of electrolyte from the cavity. When the substrate is subjected to cyclic loading, the fatigue status of the growing crack in the substrate is determined in accordance with a measured current between the reference electrode and the substrate.

Description

ELECTROCHEMICAL FATIGUE SENSOR SYSTEM AND METHODS TECHNICAL FIELD This invention relates generally to electrochemical fatigue sensing devices, systems and methods for using such devices.

BACKGROUND OF THE INVENTION With respect to Figure 1, a schematic diagram of an electrochemical fatigue sensing device (EFS) 10 is shown, according to the prior art. The EFS 10 device can be used to implement a non-destructive fatigue cracking inspection method to determine if the revised fatigue cracking is actively growing. For example, the EFS 10 device can be applied to a critical location of fatigue in a sample or laboratory structure to be inspected. The EFS device 10 consists of an electrolyte 12, sensor 14 and a potentiostat (not shown) to apply a constant polarizing voltage between the structure (substrate 16) and the sensor 14. The EFS 10 device operates according to the electrochemical principles. The structure is anodically polarized to create a protective and passive film on the surface to be tested. A polarizing voltage between the structure and the electrode produces a DC base current in the cell. If the structure to be challenged by means of the EFS suffers a cyclic tension, then the current flowing in the cell changes in a complex relation to the variation of the mechanical stress state. Therefore, an AC current is superimposed on the DC base current. Depending on the material of the structure and the loading conditions as well as the state of fatigue damage in the structure, the transient current of the cell provides information according to the state of the fatigue damage. The electrochemical conditions imposed during the interrogation of the EFS of a structure are designed to induce a passive stable oxide film on the surface of the material. During cyclic loading, the fatigue procedure causes microplasticity and localization of deformation in a very fine scale. The interaction of the cyclical slip and the passivity procedure cause temporary and repeated alterations of the passive films. These alterations, including dissolution and repassivation procedures, cause transient currents. Transient EFS currents are complex, and include cyclic changes in the electrical double layer at the metal interface and the EFS electrolyte, which generally has the same frequency as that of mechanical stress, but which has a complex phase relationship that depends of the specific metal interrogated. In addition, the alteration of the oxide films of the metal surface by the cyclic displacement causes an additional component of the transient current which has twice the frequency of the elastic current due to the effects of plasticity that occur during the tension parts and compression of the cycle. While fatigue damage develops with accumulated cycles and cracks are formed, cracks induce localized plasticity in different parts of the cycle due to fatigue from those in which a history of microplasticity occurs and in which cracks have not yet formed. Therefore, cracking induced plasticity includes higher harmonic components in the transient EFS current. The analysis and calibration of these current components allow the growth of fatigue cracking to be determined. Existing EFS devices, such as those shown in Figure 1, suffer from numerous disadvantages. For example, EFS devices are difficult to attach to a substrate and fill with electrolyte. Known EFS devices also suffer from low sensitivity and signal processing techniques to analyze the EFS signal that such devices generate also appear to be inadequate. This invention exhibits such deficiencies in the prior art.

BRIEF DESCRIPTION OF THE INVENTION This invention is directed to a method for determining a state of a growing cracking by fatigue in a substrate. An electrochemical sensor device is provided and includes an electrode formed from a stainless steel mesh. The electrochemical device has a lower surface that contacts the substrate. The bottom surface is coated with an adhesive layer and a release paper is bonded to the adhesive layer. The release paper is separated from the adhesive layer thereby exposing the adhesive layer. The electrochemical sensor device is secured to the substrate by contacting the adhesive layer with the substrate and thereby forming a cavity of the electrolyte partly bound by the substrate. The adhesive seals the bottom surface of the device to the substrate to prevent leakage of electrolyte from the cavity. The cavity is filled with the electrolyte. When the substrate is subject to the cyclic loading, the state of the fatigue growth cracking in the substrate is determined according to a current measured between the reference electrode and the substrate. According to another aspect, this invention is directed to an electrochemical sensor device for determining a state of a growing cracking by fatigue in a substrate. The system includes a reference electrode formed from a stainless steel mesh material that is substantially impermeable to an electrolyte. The reference electrode has a lower side which is oriented towards the substrate and an upper side which is oriented away from the substrate. At least one opening is provided in the mesh material, said at least one opening is sufficient in size to allow the electrolyte to flow through the reference electrode. A first electrolyte cavity is formed between the substrate and the lower side of the reference electrode. A second electrolyte cavity is formed between the upper side of the reference electrode and a cover of the device. An electrolyte inlet port is formed in a wall of the first electrolyte cavity. A purge outlet port is formed in a wall of the second electrolyte cavity. A sensor measures a current between the reference electrode and the substrate when the substrate is subject to cyclic loading. According to another aspect, this invention is directed to a method for determining a state of a growing cracking by fatigue in a location of fatigue in a substrate. A first electrochemical sensor device is provided which includes a first reference electrode. A second electrochemical sensor device including a second reference electrode is also provided. The first electrochemical sensor device is placed on the location of fatigue in the substrate, and a first current signal between the first reference electrode and the substrate is measured when the substrate is subject to cyclic loading. The second electrochemical sensor device is positioned at a location on the substrate where fatigue cracking is not likely, and a second current signal is measured between the second reference electrode and the substrate when the substrate is subject to cyclic loading. The state of fatigue growth cracking at the fatigue location is evaluated by comparing the information of the first and second current signals.

BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a schematic diagram of an electrochemical fatigue sensing device according to the prior art. Figure 2A is an isometric view of an EFS device in accordance with this invention. Figure 2B is a view of the parts of the EFS device shown in Figure 2A. Figure 2C is a top view of the EFS device shown in Figure 2B. Figure 3 is a diagram illustrating the early stages of the cracking initiation process. Figure 4 illustrates EFS FFT data for a growth crack of .254 millimeters in accordance with this invention. Figure 5 illustrates a comparison between EFS signals from a reference EFS device and an EFS device that monitors cracking according to the differential EFS techniques of this invention.

DETAILED DESCRIPTION OF THE INVENTION With respect to Figures 2A-2C, an EFS 100 device according to this invention is shown. The sensor includes reference electrode 110 which in one embodiment is a mesh made of 304 stainless steel. The sensor also includes sections 120, 130 which, in one embodiment, are each made of foam that has been coated in both sides with a pressure sensitive adhesive. In one embodiment, each of the sections 120, 130 has a surface area corresponding to a 2"x 2" square, or smaller. A release paper (not shown) is bonded to the adhesive on the bottom side 122 of the section 120. An electrolyte inlet port 150 is formed in a wall of the section 120. The electrolyte inlet port 150 is coupled to the filling tube 160 (for example, a plastic straw). A purge outlet port 170 is formed in a wall of the section 130. The purge outlet port 170 is coupled to the purge tube 180 (e.g., a plastic straw). The EFS 100 device also includes a transparent cover plate 190. The EFS 100 device is assembled by contacting the adhesive at the base of the surface 134 of the section 130 with the cover plate 190; which contacts the adhesive on the lower surface 132 of the section 130 with the upper surface 114 of the electrode 110; and contacting the adhesive with the upper surface 124 of the section 120 with the lower surface 112 of the electrode 110. Once assembled, the EFS 100 device is ready to be applied to a substrate to monitor a state of a cracking in growth by fatigue in the substrate. As mentioned above, the lower surface 122 of the section 120 is coated with an adhesive layer, and a libration paper is bonded to the adhesive layer. To apply the EFS 100 device to the substrate, the release paper is separated from the adhesive layer on the lower surface 122 of the section 120 whereby it exposes the adhesive layer on the lower surface 122 of the section 120. The EFS 100 device is secured to the substrate by contacting the adhesive layer with the substrate and thereby forming a lower cavity of the electrolyte attached at the bottom by the substrate, at the sides by the walls of section 120 and at the top by the electrode 110. The adhesive seals the bottom surface 122 of section 120 to the substrate to prevent leakage of electrolytes from the lower electrolyte cavity. The EFS 100 device also includes an upper electrolyte cavity joined at the bottom by the electrode 110, on the sides by the walls of the section 130 and at the top by the transparent cover 190. In one embodiment, the steel mesh The stainless steel used to form the electrode 110 is substantially impermeable to the electrolyte. At least one opening 116 (shown in Figure 2C) is provided in the mesh material, the opening 116 is sufficient in size to allow the electrolyte to flow through the reference electrode 110. After the EFS 100 device is fixed to the substrate as described above, the electrolyte is supplied (eg pumped) to the device by means of the filling tube 150. The electrolyte initially fills the lower cavity of the electrolyte. After the lower electrolyte cavity is filled, the electrolyte continues to be supplied by means of the filled tube 150 thereby causing the electrolyte to flow through the opening 116 from the lower cavity of the electrolyte into the upper electrolyte cavity. The procedure continues until the upper electrolyte cavity is also filled (for example, when the electrolyte begins to flow out of the purge tube 180). Once the filling process is completed, the tubes 160, 180 are perforated and both sides 112114 of the electrode 110 are covered with the electrolyte. During the filling procedure, the interior of the EFS 100 device can be monitored visually through the cover 190 to ensure that the device is filled with electrolyte and that there are no bubbles. In one embodiment, the electrolyte used to fill the EFS 100 device is: 1.2M H3BO3 + 0.3M Na2B4O7-10H2O + 0.24M Na2Mo04-2 H20 Those skilled in the art will understand that other electrolyte formulations may also be used. After the EFS 100 device is installed and filled, as described above, a potentiostat (not shown) is coupled to the reference electrode 110 and the substrate, to measure the current flow between the electrode 110 and the substrate. When the substrate is subject to cyclic loading, the state of fatigue growth cracking in the substrate can be terminated according to the current measured between the reference electrode 110 and the substrate (the EFS signal.) The EFS techniques of this The invention offers several advantages over other methods of non-destructive evaluation because it offers the potential to detect the growth of fatigue cracking and has the ability to detect very small cracks (.127 millimeters). Figure 3 illustrates the procedure of deviations that pile up to form intrusions and extrusions. Such intrusions and extrusions and the formation of cracking in early stages of cracking growth can be detected with the electrochemical fatigue sensors according to this invention. In the laboratory, it was found that when pure sinusoidal charge is used for fatigue samples, two dominant frequencies are found in the EFS signal. A fast Fourier transform (FFT) of the EFS data for a sample with a growth crack of .254 millimeters showed a frequency component 1 hz and 2 hz as shown in figure 4. The 1 hz component is due to the elastic deformations and the 2 hz component is due to localized plastic deformations. While the cracking increases and the beginning of growth of cracking increases, the magnitude of the second harmonic in 2hz increases. Under high load and before the local plasticity of fatigue cracking caused by the applied high load, similar secondary harmonics are produced. To differentiate between the plasticity caused by cracking and loading, a secondary reference sensor is used. The use of a primary sensor and a secondary sensor together is referred to as differential EFS. The differential EFS according to this invention uses two EFS 100 sensors, one as a reference sensor (R) and one as a crack measurement sensor (M) to determine a state of a fatigue growing crack in a fatigue location in a substrate. A first EFS device 100 (e.g. sensor device M) is placed over the fatigue location in the substrate, and a first current signal between the reference electrode in sensor device C and the substrate is measured when the substrate is subject to cyclical loading. A second EFS device 100 (for example the sensor device R) is placed at a location on the substrate where fatigue cracking is not likely and a second current signal between the reference electrode on reference sensor device and the substrate is measure when the substrate is subject to cyclic loading. The state of fatigue growth cracking at the fatigue location is evaluated by comparing the information of the first and second current signals. More specifically, using a signal that processes the signals that can be compared to determine if a cracking is present. By examining Figure 5, it can be seen that the measurement sensor M provides a magnitude larger than the reference signal indicating a cracking. Finally, those skilled in the art can observe that changes can be made in the embodiments described above without departing from the concept of the invention thereof. Therefore, it is understood that this invention is not limited to the particular embodiments described, but is intended to cover the modifications within the spirit and scope of this invention as defined in the appended claims.

Claims (3)

NOVELTY OF THE INVENTION CLAIMS

1. - A method for determining a state of a cracking growth by fatigue in a substrate comprising: a) providing an electrochemical sensor device that includes an electrode formed from a stainless steel mesh where the electrochemical device has a lower surface that is it comes in contact with the substrate, the bottom surface is coated with an adhesive layer, and a release paper is bonded to the adhesive layer; b) separating the release paper from the adhesive layer thereby exposing the adhesive layer; c) securing the electrochemical sensor device to the substrate by contacting the adhesive layer with the substrate and thereby forming an electrolyte cavity partly bound by the substrate where the adhesive seals the bottom surface of the substrate device to prevent the leakage of electrolytes from the cavity; d) filling the cavity with the electrolyte; and e) determining when the substrate is subject to the cyclic loading, the state of the cracking growth by fatigue in the substrate according to a current measured between the reference electrode and the substrate.

2. An electrochemical sensor device for determining a state of a cracking growth by fatigue in a substrate comprising: a) a reference electrode formed from a stainless steel mesh that is substantially impermeable to an electrode where the reference electrode has a lower side which is oriented to the substrate and an upper side which is oriented away from the substrate and where at least one opening is provided in the mesh material, said at least one opening being sufficient in size to allow the electrolyte to flow to through the reference electrode; b) a first electrolyte cavity formed between the substrate and the underside of the reference electrode; c) a second electrolyte cavity formed between the upper side of the reference electrode and a cover of the device; d) an electrolyte input port formed in a wall of the first electrolyte cavity; e) a purge outlet port formed in a wall of the second electrolyte cavity; and f) a sensor that measures a current between the reference electrode and the substrate when the substrate is subject to cyclic loading.

3. A method for determining a state of a growing cracking by fatigue in a fatigue location in a substrate comprising: a) providing a first electrochemical sensor device that includes a first reference electrode; b) providing a second electrochemical sensor device that includes a second reference electrode; c) placing the first electrochemical sensor device on the location of fatigue in the substrate and measuring a first current signal between the first reference electrode and the substrate when the substrate is subject to cyclic loading; d) placing the second electrochemical sensor device at a location on the substrate where fatigue cracking is not likely and measuring a second current signal between the second reference electrode and the substrate when the substrate is subject to cyclic loading and e) assess the state of fatigue growth cracking at the fatigue location when comparing the information from the first and second current signals.