Direct Current Potential Drop Method Research Articles

Calibrated the direct current potential drop method for fatigue crack propagation testing of nickel-based superalloy with film cooling hole

Turbine blades in aviation engines commonly feature a nickel-based superalloy structure integrated with film cooling holes to enhance inlet gas temperature. However, the presence of these film cooling holes often results in frequent occurrences of fracture failures nearby. The direct current potential drop (DCPD) method, renowned for its exceptional crack sensitivity, is frequently utilized for crack length monitoring. This study establishes a FEM model of the film cooling hole plate specimen to determine the optimal probe point location. Subsequently, a mapping relationship is derived to calibrate Johnson’s formula, accounting for the unequal crack lengths at the edges of film cooling holes. It is confirmed that the crack length measured by the DCPD method represents the cumulative crack lengths on both sides of the hole. Fatigue crack propagation experiments are then conducted, with crack length monitored using a microscope. The results affirm the successful application of the calibrated formula to the film cooling hole plate specimen, exhibiting an average error within 0.1 mm.

High-pressure corrosion fatigue experiments employing diverse methodologies for assessing rates of crack propagation

The new extreme faces strain gauges (XFS) technique, when compared to the direct current potential drop (DCPD) and back face strain gauge (BFS) methods, has shown superior accuracy and reliability in studying corrosion fatigue in high pressure. The XFS method serves a dual purpose by accurately measuring the crack length and acting as a mechanical load transducer. The results indicate that all three techniques can predict the final crack length effectively. However, BFS is unsuitable for high-pressure environments, and the current DCPD method can affect the crack propagation rate. The developed XFS methodology shows excellent performance in accurately evaluating corrosion fatigue studies.

A Study on the Propagation of Chloride-Induced Stress Corrosion Cracking for Austenitic Stainless Steel at Corrosion Environment

Austenitic stainless steels have been widely used in the petrochemical industry, nuclear power plants and the ship industry due to their high toughness and corrosion resistance. However, these materials are susceptible to environmentally-assisted degradation, such as chloride-induced stress corrosion cracking (CISCC). In this respect, this study investigated CISCC crack propagation rate for austenitic stainless steels, such as type 304L and type 316L, in the corrosion environment. Considering the environment of the spent nuclear fuel canister used in the dry storage system, the test chamber is always maintained at 3.5% NaCl and related humidity (RH) 95% at 60℃. For initiation of CISCC, a test procedure, which has nine pre-cracking procedures for a CT specimen, is suggested. In a test procedure, crack transitioning, which induces the environmental corrosion crack, is observed after mechanical precracking. To determine the CISCC crack length corresponding to each test step, the direct current potential drop (DCPD) method is measured in real-time. In addition, the CISCC crack observed after final fracture is measured using optical microscopy in order to improve the accuracy of the measured crack length. In the test results, CISCC crack resistance of type 316L is better than that of type 304L. Finally, the test results show that the CISCC crack propagation rate is 2.784E-11 m/s for type 304L and is 9.93E-11 m/s for type 316L.

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.

Microstructure-dependent corrosion fatigue crack growth behavior of Ti-6Al-4V alloy in simulated body fluid

The fatigue crack growth rate (FCGR) of Ti-6Al-4V alloys in simulated body fluid (SBF) was first determined by direct current potential drop (DCPD) method. The results show that the acceleration effect of SBF corrosion environment on FCGR of lamellar structure is far greater than that of equiaxed and bimodal structures, especially in Paris region, which is mainly due to the high corrosion sensitivity of the lamellar α/β phase boundary to SBF under fatigue loading.

A novel ultrasonic non-destructive testing methodology to monitor fatigue crack growth in compact tension specimens.

Detection and evaluation of fatigue cracks in structural materials are extremely important for life prediction analysis as part of the structural integrity procedure. In this article, we present a novel ultrasonic measurement methodology, based on the diffraction of elastic waves at the crack tips, to monitor the fatigue crack growth near the threshold regime in compact tension specimens at different load ratios. The diffraction phenomenon of ultrasonic waves from the crack tip is demonstrated using a finite element 2D wave propagation simulation. The applicability of this methodology has also been contrasted with that of the conventional direct current potential drop method. In addition, the crack morphology obtained in the ultrasonic c-scan imaging showed a variation in the crack propagation plane as a function of the cyclic loading parameters. The results suggest that this novel methodology is sensitive to fatigue cracks and can form the basis of in situ ultrasonic-based crack measurements in metallic and non-metallic materials.

Effect of temperature and medium environment on corrosion fatigue behavior of Inconel 625

AbstractInconel 625 is a nickel‐based alloy with exceptional creep and corrosion resistance properties. It has emerged as a candidate material for operational and structural use in critical and supercritical power generation plants. Supercritical power plants work at high temperatures, pressure and in an erosive environment that makes metallic materials more vulnerable to corrosion fatigue. This study aimed to evaluate the fatigue crack growth rate (FCGR) at elevated temperatures (550–650°C) in a steam and supercritical water environment. All the experiments were performed at a water oxygen level of 8000 ppb. A cyclic stress of 1 Hz frequency and sine waveform was applied for the test. A 0.6 stress ratio was applied with a maximum stress intensity factor of 32 MPa√m. A direct current potential drop method was applied to detect the crack growth rate (CGR). For the given parameters, fractographic analysis validated the role of corrosion‐fatigue interaction as the primary source of variation in the FCGR. Results revealed that the CGR was enhanced with a rise in temperature under all test environments. The influencing mechanism of temperature on CGR was also discussed.

Application on DCPD Real-Time Monitoring of Fatigue Crack Propagation

The formation, propagation, and detection of fatigue cracks are the key issues for the life extension of pressure pipelines and metal welded joints. Especially in the high-temperature and high-pressure water environment of nuclear power. Since the interference of measuring instruments and noise would affect the monitoring of potential signals that characterize crack growth. Combined with the relevant information and various methods of fatigue crack growth detection were analyzed, it can be seen that the Direct Current Potential Drop (DCPD) method has significant advantages in many crack detection methods. Firstly, the development history and current situation of fatigue crack research were analyzed. Secondly, its basic principles and key problems were summarized. Based on the existing crack detection methods, the real-time monitoring system was introduced, and the overall research status was evaluated and analyzed, which a reliable scientific basis for the numerical analysis of fatigue life and a reference were provided for the research on this field.

Effect of impact damage on electrical resistivity of C/C–SiC composites

Abstract The main objective of the present work was to study the low-velocity weight-drop impact damage behaviour of carbon – carbon silicon carbide (C/C– SiC) composites on electrical resistivity of the direct-current potential drop method. The results show that the electrical resistivity increases with increasing impact energy, which confirms that pre-microcracks of C/C– SiC composite grow very rapidly with a little amount of impact load and turn into a loss of conductivity of the material.

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.

Fracture mechanical investigation of preformed metal sheets using a novel CC-specimen

Joining is used in many areas of manufacturing where components are formed from individual parts or sheets into complex structures. A simple and common method applied in industry to join dissimilar and coated materials is clinching. This joining process does not require additional joining parts, which saves weight to reduce energy requirements and harmful emissions due to moving masses. In addition, the joining process is carried out without pre-punching and is therefore time-efficient. Damage caused by the joining process or by operational loading can lead to cracks near the joint, to crack propagation due to time-varying loading and subsequently to the complete failure of the component. To avoid these damage cases, knowledge about the fracture mechanical behavior of the clinched connection is required.Within the scope of this paper, a novel CC-specimen for fracture mechanical investigation of preformed metal sheets is developed and investigated. The new specimen geometry is taken directly from a clinched joint in order to identify a possible influence of the outer forming area of the joint on the fatigue crack growth rate. The clinched joint is placed at three different positions and is moved from the radius to the center of the C-shape at 3 mm intervals. In order to investigate the crack growth rate of this new specimen, it is necessary to numerically determine the geometry factor function for the stress intensity factor as well as the calibration curve for the crack length measurement with the direct current potential drop method beforehand. Finally, the results of the crack growth rate tests using the CC-specimen with different positions of the clinched joint are compared with the results of the base material.

Automated crack length measurement for mixed mode fatigue cracks using digital image correlation

It is well known that mode II leads to a kinking and mode III to a twisting of a fatigue crack. On the surfaces of an SEN specimen, an overlapping of these two crack loading modes occurs. Superimposed with mode I, this results in very complex fatigue crack paths and, in some cases, multiple crack branches. Thus, established methods for mode I crack growth measurement cannot be utilized. Therefore, the digital image correlation (DIC) technique is used to determine the crack length of mixed mode cracks on the surface of a specimen by means of the maximum principal strain. By Gehri et al. (2020) a concept is described to obtain the crack path by morphological thinning of the area on the surface, where the maximum principal strain exceeds a threshold value. In this paper, an extension of the provided MATLAB script is proposed that includes two criteria for measuring the crack length versus the number of load cycles (a-N-curve). For the first criterion, the crack opening is used to determine the current crack tip position and to obtain the crack length as a function of the number of load cycles. In the second criterion, a threshold value of the maximum principal strain is used for determining the number of cycles at the detected crack length. The new evaluation routine with both criteria is initially calibrated on mode I experiments using the direct current potential drop method. Moreover, mixed mode experiments under tension-torsion loading are conducted, to validate the method and compare the two proposed criteria. The experimental results in terms of the a-N-curves obtained with the marker load technique are in good agreement with the results using the presented method. Thus, the extended tool provides an evaluation routine to determine a-N-data for mixed mode fatigue crack growth tests independently from the crack orientation and possible branching.

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.

Qualitative Study On Cable Breakage of Nb3Sn CICC Based On Direct Current Potential Drop Method

Nb3Sn cable-in-conduit conductor (CICC) technology is widely applied to the large-scale superconducting magnets. Due to the multi-level composite structure, it is easy to cause partial damage or even strand breakage of the superconducting cable in the complex deformation mode. To evaluate the cable breakage of Nb3Sn CICC, direct current potential drop (DCPD) technique is proposed in this study. Serious of benchmark samples of cable with stage 3 were prepared and different breakage rates were designed in the cable samples. Potential difference (PD) obtained from the both side of the cable breakage was measured and analyzed. The experimental results indicated that direct current potential drop (DCPD) method is effective to qualitative measurement of cable breakage.

Crack Growth Signal Processing Approach Combining Wavelet Threshold Denoising and Variable Amplitude DCPD Technique

The direct current potential drop (DCPD) method is widely used in laboratory environments to monitor the crack initiation and propagation of specimens. In this study, an anti-interference signal processing approach, combining wavelet threshold denoising and a variable current amplitude DCPD signal synthesis technique, was proposed. Adaptive wavelet threshold denoising using Stein’s unbiased risk estimate was applied to the main potential drop signal and the reference potential signal under two different current amplitudes to reduce the interference caused by noise. Thereafter, noise-reduced signals were synthesized to eliminate the time-varying thermal electromotive force. The multiplicative interference signal was eliminated by normalizing the main potential drop signal and the reference potential drop signal. This signal processing approach was applied to the crack growth monitoring data of 316 L stainless steel compact tension specimens in a laboratory environment, and the signal processing results of static cracks and propagation cracks under different load conditions were analyzed. The results showed that the proposed approach can significantly improve the signal-to-noise ratio as well as the accuracy and resolution of the crack growth measurement.

Numerical calibration and experimental validation of the direct current potential drop (DCPD) method for fracture mechanics fatigue testing of single-edge-crack round bars

The Direct Current Potential Drop (DCPD) method has been investigated to estimate the crack size in axial fatigue tests of single-edge-crack round bars. The measurability, sensitivity, and reproducibility of the DCPD measurement have been evaluated by means of 3D electrical FE analyses for a range of geometrical configurations of current and potential probes. The accuracy of the numerical analyses has been checked against results of crack propagation experiments, where a compact clip-on gauge device seating both current and potential probes of the DCPD equipment has been adopted.

Understanding the High-Temperature Fatigue Crack Growth from Exceptional Nano-\u03b1 Phases and {10 bar{1} 2} Deformation Twins in Hot Deformed Titanium Alloy

In this study, the fatigue crack growth rate (FCGR) of Ti-6Al-4V alloy at 723 K was measured by direct current potential drop (DCPD) method, and exceptional nano-α phases and {10 bar{1} 2} deformation twins were newly found at the crack tip in Ti-6Al-4V alloy. The results showed that nano-α phases have Burgers orientation relationships (OR) (0001)α//(110)β, [ bar{2} 110]α//[ bar{1} 11]β with β phases. The terraced-structure interface consisted of (0 bar{1} 10)α//(1 bar{1} 2)β and (10 bar{1} 0)α//(1 bar{1} 0)β. Numerous dislocations accumulated in the β phase and became the diffused channels of O and V atoms. The α stabilizers (O element) diffused into the crystal lattice and β stabilizers (V element) spread out of the crystal lattice which accelerated the nano-α phases nucleation. <a> and <c + a> dislocations piled up at the primary α grain, interface and low angle grain boundaries (LAGBs), and dissociated into twinning dislocations to promote the twin nucleation. Dislocation transformation enabled nano-twins to grow through the primary α/β interface and strain transfer led that deformation twins nucleated in the adjacent primary α grains. With the effects of nano-α phases, LAGBs and twins, the resistance of crack propagation increased and the driving force decreased, and resulted in the low FCGR.Graphic

A customized electrical potential difference method for in situ monitoring of propagating cracks using a stochastic algorithm

In this paper, a modified direct current potential drop (DCPD) method for real-time measurement of the length, the inclination and the position of cracks is presented. Based on the proposed configuration, it is possible to process the data acquired by continuously measuring the change in the electrical resistance (potential drop) between specific points on the specimen in real time and correlate them with the propagation of the crack and thus identifying its crucial characteristics. Furthermore, many aspects that affect the electromagnetic field inside materials have been identified. In that way the influence of unwanted factors can be significantly reduced which has led to a better understanding of the relation between the implemented voltage values and the fracture itself. Therefore, conclusions are drawn about the structural integrity of any given specimen through a risk assessment after the crack characteristics have been calculated. In order to achieve this, a variety of techniques were implemented including the development of a stochastic algorithm along with a customized experimental layout so as to accomplish high accuracy for the prediction model as well as robustness towards other influencing parameters such as temperature and humidity.

Calibrating Johnson’s formula for applying DCPD method to an axial through-wall crack in a pipe

Direct current potential drop (DCPD) method is widely employed in the determination of crack length in conductive materials. It is based on the change of structure potential drop caused by the variation of the electrical resistance under a steady direct current. The accuracy of DCPD method depends on the calibration curve which describes the relationship between crack length and potential drop. Generally, the calibration curve for the crack in a plate sample is given by Johnson’s formula analytically. However, the traditional Johnson’s formula is only applicable to a center-cracked plate but not to a cracked pipe. In this paper, Johnson’s formula has to be calibrated to estimate the length of a through-the-thickness crack in a pipe. Considering the geometrical difference between a pipe sample and a plate sample, the calibrated Johnson’s formula is established based on the existence of parallel circuit in a pipe and the proposed unique reference sample. The calibration curve between axial through-wall crack length and potential drop is carried out by the calibrated Johnson’s formula. The calibrated formula has been verified for the axial through-wall crack in a pipe sample by numerical and experimental approaches. Compared with the traditional Johnson’s formula, the results illustrate that the present calibrated formula is successful to estimate the crack length in a pipe. This paper extends the applicability of the original formulation of Johnson’s formula to cracked pipes.

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.