Potential Drop Measurements Research Articles

A life prediction model for P110S steel in deep-well environments with H2S/CO2 coexistence based on multi-factor chemometric drive

A mechanistic-chemometrics model for life prediction of P110S steel in deep-well environments with H₂S/CO₂ coexistence was proposed. The model was developed by considering the interaction mechanism between uniform and pitting corrosion, then modified using a multi-factor chemometric drive incorporating temperature/pressure, flow velocity, stress. Finally, the pit-to-crack transition was modeled using finite element design and direct current potential drop measurements, completing the life prediction process. The model predicts a lifespan of 2.28–5.25 years at different well depths, and this result was validated with on-site data, indicating the model’s accuracy. The knowledge paradigm provided herein will assist in corrosion prediction.

Eddy current nondestructive evaluation of metallic plates electrical conductivity using artificial neural networks based inverse problem

The most used method and extensively studied in the literature for conductors’ characterization is Eddy Current Nondestructive Evaluation (ECNDE) due to its significant advantages, such as its ability to preserve the integrity of the structures or materials to be examined during manufacturing or a regular in-service nondestructive testing. Various approaches are developed for the Eddy Current measurement of the electrical conductivity. In the present work, the evaluation of the electrical conductivity is treated as an inverse problem. In pursuit of this aim, a combination is established between Eddy currents evaluation and artificial neural networks (ANN) to evaluate the electrical conductivity of homogeneous metallic plates from eddy-current probe impedance measurements. For this purpose, an experimental setup is developed, including a bobbin-coil probe, metallic plates (target), data acquisition and signal processing systems. Finally, experimental conductivity values of various metallic plates using ANN are compared with those obtained using four-point measurements of direct current potential drop (DCPD) made on the same plates and very good agreement is obtained.

Multiple potential drop measurement – A new method for crack detection and crack length measurement

The Direct Current Potential Drop Method (DCPDM) is widely used for crack detection and crack length measurement in fatigue experiments. Unfortunately, crack length measurements are only possible when a calibration for a specified crack initiation site and potential probe location is available. Investigations by Hartweg and Bär (2019) as well as Nahbein and Bär (2022) on notched round specimens equipped with three potential probes placed in a distance of 120° on the circumference have shown that the crack initiation site can be determined using a simple vector-based model. In this model the normal vector of a plane span by the three potentials is calculated in cylindrical coordinates. In the present work a more detailed investigation of crack initiation and propagation is undertaken to show the possibilities offered by this method. During fatigue tests with the simultaneous measurement of the three potential drops using amplifiers of the control electronics, overloads were introduced in fixed intervals to mark the crack front on the fracture surface. The measurement of both crack depth and fractured area allows a calibration of the potential drop measurement. The calibration was performed on the radius coordinate n of the normal vector using a function suggested by Tiedemann (2016). The multiple potential drop measurement enables an early detection of crack initiation as well as a location of the crack initiation site on the circumference of the specimen. Using the Tiedemann function fitted to the measured data, it is possible to determine the crack length independent of the crack initiation site on the circumference of the specimen.

Quantitative analysis of the structural health of railway turnouts using the acoustic emission technique

Rail defects such as fatigue cracks have been one of the leading root causes of a number of derailments in the past. Cracks that initiate and propagate below the surface are difficult to detect using traditional non-destructive testing (NDT) methods. Acoustic emission (AE) is a more effective method for detecting and monitoring crack growth in rails online. This study investigates the applicability of AE for quantifying damage propagation in austenitic cast manganese steel used in manufacturing railway turnouts. The relationship between AE and crack growth rate in austenitic cast manganese steel samples that were fatigue tested in a three-point bending configuration was investigated by evaluating the AE activity with respect to direct current potential drop (DCPD) measurements and scanning electron microscopy (SEM) fractographic analysis of the tested samples. From the results obtained, it was not possible to observe a clear relationship between AE activity and the actual crack growth rate. Based on the SEM fractographic analysis, this is likely due to the plasticity occurring at the tip of the fatigue crack in the tested samples. This is plausible since the cast manganese steel samples had been cut off from a plate that had not been previously work hardened. The effect of carbides present in the microstructure is an additional contributing factor. Further tests should be carried out on cast manganese steel samples that have been work hardened prior to fatigue testing.

Determination of Cracks using Multiple DC Potential Drop Measurements – Experimental Verification of an Advanced Model

The Direct Current Potential Drop Method (DCPDM) is a frequently used technique for crack detection and crack length measurement in fatigue experiments. Recent investigations have shown that with multiple potential drop measurements the location of crack initiation can be determined in single edge notch specimens (Wiehler and Bär, 2020), (Bär and Nahbein, 2022) and in round bars (Hartweg and Bär, 2019). In the present work a more detailed investigation of crack initiation and propagation is undertaken on round bars to advance the geometrical model by Hartweg and Bär (2019). Three potential probes were equipped on notched steel bars – at 0° (U1), at 120° (U2) and at 240° (U3). The three potentials were measured simultaneously during the fatigue tests using amplifiers of the control electronics. Moreover, the introduction of overloads was used to mark the crack front on the fracture surface in defined intervals to compare crack and size calculated from the potential drop measurements with the real crack size and geometry.In the case of an initiated single crack, the experimental set-up delivers different potentials depending on the distance of the probes to the crack. The geometrical model can be used not only for the determination of the crack angle like performed by Hartweg and Bär (2019) but also to calculate the crack size. On the measured data a function is fitted that allows the calculation of the crack length independent of the location of the crack initiation site. The experimental results have shown that it is possible to determine a crack with a size of less than 1 % of the specimen cross-sectional area.

Investigation of Crack Formation and Propagation in AA7475 using Multiple Potential Drop Measurement

The direct current potential drop method (DCPD) is a well-known method for crack detection and crack length measurement in fatigue experiments. Recent Investigations have shown that with multiple potential drop measurements the location of crack initiation can be determined in round bars (Hartweg and Bär, 2019) and single edge notched specimens (Wiehler and Bär, 2020).In this work a more detailed investigation of crack initiation and propagation is undertaken on single-edge notched specimens. The specimens were equipped with three potential probes – on the frontside (Ufront), on the backside (Uback) and on the narrow side (Unarrow) of the specimen. During the fatigue tests all three potentials were measured simultaneously using amplifiers of the control electronics. The crack front was marked on the fracture surface by introducing overloads in defined intervals to allow a direct comparison between the measured potential drop and the real crack location and crack front geometry.In a first step relative Potentials Pi (i= front, back and narrow) were calculated by normalizing the actual potential on the potential of the crack-free specimen. To localize the crack initiation site and to eliminate temperature effects quotients of the potentials were formed by dividing Pfront and Pback by Pnarrow, respectively. By comparing the runs of the potential quotients plotted over the cycle number, the time of crack initiation and the location of the crack initiation site can be determined. In addition, the run of the quotients gives information about the shape of the crack front of long fatigue cracks.

Measuring crack growth and related opening and closing stresses using continuous potential drop recording

To improve resolution of in-situ measurement of crack closure and opening in fatigue, potential drop (PD) measurement technique has been further developed to measure thousands of times through each load cycle with high precision. The results are interpreted with physical phenomena like strain, Poisson’s effect, piezo-resistivity, plasticity, and crack growth. Application of the technique to fatigue crack growth tests on aluminum 2024-T3 CCT specimens at different maximum stresses and stress ratios, demonstrate that indeed variations in PD can be associated to development of plasticity and crack opening and closure. Hence, the technique allows to measure timing and magnitude of crack opening and closure stresses in-situ in fatigue crack growth experiments.

Crack Detection and Crack Length Measurement with the DC Potential Drop Method–Possibilities, Challenges and New Developments

The direct current potential drop method (DCPDM) is widely used to determine the crack length in fatigue experiments. In practice, some special features of this method must be considered. Aside from general information on the experimental setup and calibration, some special features of the method, such as the influence of the ambient atmosphere and the application of the method to ferromagnetic materials, are presented and discussed. In addition, with the multiple potential drop measurement, a method is presented which improves the resolution of the DCPDM for detection of cracks and allows to determine crack initiation sites. The capabilities provided by this method are demonstrated on the basis of measurements undertaken on notched round bars and single edged notched specimens.

The use of potential drop measurements to predict the temperature distribution in a thin wire with current flowing through it

When a current is supplied to a thin wire having smaller heat capacity, the temperature of the wire easily increases due to the principle of Joule heating. The temperature distribution in the wire has constituted an important issue for thin wire application. This paper reports a method to predict the temperature distribution in a thin wire through which current is flowing. The potential drops at the surfaces of thin Cu wires with diameters of 25 µm and 100 µm were measured. For these measurements the points of contact were close together, enabling us to measure the temperature dependency of the electrical resistivity of the wire. On the other hand, potential drop measured between the points of contact much further apart provided the information on the temperature distribution in the wire. By assuming the symmetric and parabolic temperature distribution, the temperature distributions in the Cu wires of 25 µm and 100 µm thick were predicted using the potential drop measurements made with the points of contact much further apart. The temperature distributions predicted were in good agreement with those measured by infrared thermography. The validity of the proposed method was also verified by conducting a similar experiment on Fe wire having a diameter of 100 µm.

A Quasi-DC Potential Drop Measurement System for Material Testing

Potential drop (PD) measurements are well established for use in materials testing and are commonly used for crack growth and strain monitoring. Traditionally, the experimenter has a choice between employing direct current (dc) or alternating current (ac), both of which have strengths and limitations. The dc measurements are afflicted by competing spurious dc signals and, therefore, require large measurement currents (tens or hundreds of amperes) to improve the signal-to-noise ratio, which in turn leads to the significant resistive Joule heating. The ac measurements have superior noise performance due to the utilization of phase-sensitive detection and a lower spectral noise density but are subject to the skin effect and are, therefore, not well suited to high-accuracy scientific studies of ferromagnetic materials. In this paper, a quasi-dc monitoring system is presented which uses very low-frequency (0.3–30-Hz) current which combines the positive attributes of both dc and ac while mitigating the negatives. Bespoke equipment has been developed that is capable of low-noise measurements in the demanding quasi-dc regime. A creep crack growth test and fatigue test are used to compare noise performance and measurement power against alternative DCPD equipment. The combination of the quasi-dc methodology and the specially designed electronics yields exceptionally low-noise measurements using typically 100–400 mA; at 400 mA, the quasi-dc system achieves a 13-fold improvement in signal-to-noise ratio compared to a 25-A dc system. The reduction in measurement current from 25 A to 400 mA represents a ~3900-fold reduction in measurement power, effectively eliminating resistive heating and enabling much simpler experimental arrangements.

Crack Detection and Localization with Multiple Potential Drop Measurements

The direct current potential drop method (DCPD) is commonly used to measure the crack length in fatigue tests. Due to a propagating crack the cross section of the specimen is reduced leading to a densification of potential field lines, whereby the measured potential difference increases. First experiments on notched round bars have shown that the location of a crack can be determined with a multiple potential drop measurement (Hartweg and Bär, 2019).In this study single-edge notched specimens were equipped with three potential probes – on the frontside (Uf), on the backside (Ub) and on the narrow side (Un) of the specimen. During the fatigue test the three potentials were measured simultaneously using amplifiers of the control electronics. To enable a direct comparison between the potential drop measurement and the real crack geometry, the crack front was marked on the fracture surface by introducing overloads in defined intervals.The measured potential values were normalized on the starting potential and quotients of the potentials were formed (Pf/Pn and Pb/Pn). By plotting the potential quotients over the cycle number, the time as well as the location of the crack initiation site can be identified. In addition, it was possible to identify subsequent cracks and to determine the shape of the crack front of long fatigue cracks based on the measured potential data.

Corrosion behavior of Ag film lines in atmospheric conditions estimated by means of electrical resistance technique

Ag film lines have attracted considerable attention recently as a substitute for the next-generation of transparent conducting electrodes, and as such, ensuring the reliability of Ag film, i.e., their resistance to corrosion, is an important issue. Corrosion of these metallic film changes their electrical properties, leading to the malfunction of electrical devices. In this study, the authors examined the corrosion of 400 nm thick Ag film lines kept in atmospheric conditions for more than three years. The rate of corrosion over this period was determined by examining the changes in their electrical resistance, which was obtained from the potential drop measurements. It was confirmed that the rate of corrosion remained approximately constant for about one and a half years and then started to decreased monotonically with time. These results can be used to determine the reliability of Ag film lines.

Four-point transient potential drop measurements on metal plates

The transient potential drop method is based on measurement of the transient voltage on the surface of a conductor due to the injection of a pulsed current. In this paper, we consider an approximate analytical model for four-point transient potential drop measurements on plates that are thin compared to the probe separation. Experimental results show good agreement with the theory for measurements made on non-magnetic and ferromagnetic plates when an exponentially rising pulsed current is used as a source. To demonstrate possible applications of the theory we consider measurement of the conductivity and relative permeability of the materials as well as plate thickness.

Detection of an Electromotive Force during Resistance Spot Welding and Its Correlation with Nugget Growth

Abstract This article presents a report on an electromotive force that appears in the electromagnetically open circuit during resistance spot welding. We measured the potential drop between the Cu electrodes during the supply of an alternating current while keeping the area surrounding the electrical lines available for the potential drop measurement. We detected a unique electromotive force in the opposite direction to the back electromotive force induced by the change in the current. Here, we demonstrate that the change in the electromotive force is closely associated with nugget growth and the generation of spatter during welding. Based on the temperature measurement around the Cu electrode, we observed a temperature ripple during the welding process and consider this as the driving force behind the electromotive force.

Optimization of the DCPD technique for monitoring the crack propagation from notch root in localized plasticity

The Direct Current Potential Drop method is commonly used to monitor the crack growth during fatigue test when optical access to the crack is not possible. The reliability of this method depends on the calibration curve, which correlates the potential drop measurement with the crack length and shape. Using Finite Element Analysis and experimental approaches with heat tints, a procedure for the optimization of the calibration curve has been proposed. The influence of plastic deformation and of the presence of a secondary crack on the calibration curve has been studied. Finally, a test strategy, using two heat tints, has been developed in order to identify the first order parameters for the calculation of calibration curve. This strategy shows higher accuracy of crack length measurement and permit to balance experimental uncertainties out.

Influence of Process Parameters, Surface Topography and Corrosion Condition on the Fatigue Behavior of Steel/Aluminum Hybrid Joints Produced by Magnetic Pulse Welding

In this study, magnetic pulse welded steel/aluminum hybrid joints are investigated with the aim of optimizing the process parameters regarding the fatigue behavior. Changes in discharge current, acceleration distance, welding geometry as well as influences of surface topography and corrosion, are examined regarding fatigue life and damage mechanisms. Instrumented multiple amplitude tests combined with constant amplitude tests are carried out for assessing structure-property-relations in a resource-efficient manner. Stress-induced change in strain and alternating current potential drop measurement are well suited for reliable detection of damage initiation and estimation of the fatigue limit. Results reveal that the fatigue properties primarily depend on the imperfections of the weld seam, which are affected mostly by the discharge current and the surface topography. Corrosion shows to be a relevant factor since it decreases fatigue performance. Suitable process parameters are achieved when the fatigue strength of the weld seam lies above the weaker hybrid joint (aluminum). For S235JR and EN AW1050A-H14 (Al99.5) a suitable discharge current was found to be 349 kA at an acceleration distance of 1.5 mm.

Analysis of the crack location in notched steel bars with a multiple DC potential drop measurement

The direct current potential drop (DCPD) technique is a frequently used method to measure crack propagation in metallic materials. With a single potential probe attached to the specimen, the size of an initiated crack can be estimated. Therefore, this method is used in crack propagation experiments to determine the crack propagation rate. However, with a single potential probe it is difficult to detect small cracks and impossible to determine the location of the initiated crack.In the present study, three potential probes were attached to cyclic loaded notched steel bars. For the case of an initiated single crack, this experimental set-up delivers different potentials depending on the distance of the probe to the initiated crack. A geometric model to determine the position of the initiated crack from the increase of the individual probe signals is presented.The experimental verification of the model has shown that in case of a single crack, the position can be determined clearly, even in the early stages of crack propagation. Moreover, the sensitivity of the potential drop measurement for the detection of an initiated crack is enhanced by this method. In case of multiple crack initiation sites the scatter of the calculated angle can be used as a criterion for crack detection.

DCPD based detection of the transition from short to long crack propagation in fatigue experiments on the aluminum alloy 7475 T761

The cyclic lifetime of components can be divided into crack initiation, short crack growth until a through-thickness crack has built up, followed by long crack growth until final failure. For lifetime calculations of technical alloys based on fracture mechanics the calculation normally is limited to the propagation of long cracks. Unfortunately, long crack propagation covers only a minor part of the total lifetime. The propagation of short cracks extends over more than half of the service life and is therefore the focus of a lot of studies.For all calculations and researches an accurate crack length measurement is necessary, especially for short crack growth. With a high-resolution DC potential drop measurement, a crack can be detected very early, but it is not possible to display the real shape and location of the initial crack. Hence, this would be necessary for a full-featured detection of short crack propagation.Attaching multiple DC potential probes near the crack initiation site an analysis of the location and shape of the short cracks is possible. This paper shows the possibilities of short crack detection with two potential probes mounted near the notch on SEN specimens.

The influence of the dynamic magnetoelastic effect on potential drop measurements

Alternating Current Potential Drop (ACPD) measurements are routinely used for monitoring crack length in laboratory based fatigue tests, and so measurements will be taken on components which are exposed to cyclic dynamic stresses. It has been empirically observed that cyclic stresses cause a strong increase (above 10% is shown in this paper) in measured resistance that is both AC inspection frequency and loading frequency dependent. The excess resistance will result in erroneous measurements; this paper investigates the cause and provides recommendations to limit the influence. Applied stresses influence ACPD measurements through the magnetoelastic effect; elastic strain induces magnetization in ferromagnetic materials, which in turn influences the magnetic permeability and therefore skin depth. Further, it has recently been realised that cyclic magnetization results in a frequency dependent concentration of the magnetic flux at the surface of the component, and consequently a non-uniform spatial distribution of magnetic permeability. In this study it is found that the interaction between the non-uniform spatial distribution of both the current density and magnetic permeability results in significant non-linear modulation of the measurement signal. A combination of finite element simulations and experimental results are used to explore this phenomenon.

A Unified Potential Drop Calibration Function for Common Crack Growth Specimens

Calibration functions, used to determine crack extension from potential drop measurements, are not readily available for many common crack growth specimen types. This restricts testing to a limited number of specimen types, typically resulting in overly conservative material properties being used in residual life assessments. This paper presents a unified calibration function which can be applied to all common crack growth specimen types, mitigating this problem and avoiding the significant costs associated with the current conservative approach. Using finite element analysis, it has been demonstrated that Johnson’s calibration function can be applied to the seven most common crack growth specimen types: C(T), SEN(T), SEN(B), M(T), DEN(T), CS(T) and DC(T). A parametric study has been used to determine the optimum configuration of electrical current inputs and PD probes. Using the suggested configurations, the error in the measurement of crack extension is <6% for all specimen types, which is relatively small compared to other sources of error commonly associated with the potential drop technique.