Direct Current Potential Drop Technique Research Articles

Stress Corrosion Cracking of High-Strength Ni-Base Alloys in Pressurized Water Reactor Primary Water Containing KOH vs. LiOH

The stress corrosion cracking (SCC) behavior of high-strength Ni-base Alloys X-750 and 718 has been investigated to assess the effect of replacing LiOH with KOH as the pH moderator in pressurized water reactor (PWR) primary circuit coolant. The SCC initiation behavior of X-750 was evaluated by multitensile specimen constant load test in either LiOH- or KOH-containing PWR primary water. The SCC growth behavior of X-750 and Alloy 718 was evaluated on compact tension specimens with on-the-fly changes between LiOH- and KOH-containing water in three different PWR primary water chemistries. Direct current potential drop technique was used for in situ monitoring of crack initiation time and crack extension in SCC initiation and propagation tests, respectively. The results suggest no significant effect of KOH vs. LiOH on the SCC initiation or propagation of the tested materials.

Effect of deformation on crack extension measurement for compact tension specimen with the DCPD technique

Direct current potential drop (DCPD) technique is a crack growth monitoring technique widely used in the crack growth test of specimens in laboratory environment. Deformation of the specimen during test is one of the important factors that affect the accuracy and resolution. Because the effect of deformation on measurement is usually mixed with other factors, quantitative evaluation is difficult through experiments method only. In this work a novel quantitative estimation method combining experiment and finite element analysis has been proposed, and the effect of deformation on measurement for 316 L stainless steel compact tension specimen has been quantitatively evaluated. Through the tensile test and DCPD monitoring test of the non-cracked 316 L stainless steel plate-shaped specimen, the material mechanical property parameters and the change of potential drop caused by deformation were obtained. Then the slow tensile tests have been performed on compact tension specimens. Combined with the FE simulation of the experimental process, the effect of deformation on the potential drop under different crack length has been evaluated. The results show that the influence of deformation on the potential drop is a factor that must be considered for the crack growth monitoring experiment with high crack growth resolution. The proposed method can be used to quantitatively evaluate the influence of deformation, a factor that was usually ignored in the past, on the measurement of crack growth, which provides a method and basis for correction of crack measurement errors caused by deformation. In addition, the proposed method can be used to obtain a more accurate calibration function, which is of great significance to the improvement of the accuracy and resolution of crack growth monitoring.

Crack Initiation Detection in JAC780Y during Tensile Loading by Using Direct Current Potential Drop and Acoustic Emission Techniques

In this work, the non-destructive and fracture test techniques were studied for micro-crack detection in the advanced high strength steel (AHSS) notched sheets. Crack initiation of JAC780Y during tensile loading was investigated by Direct Current Potential Drop (DCPD) and Acoustic Emission (AE) techniques. The results confirm for the first time that both techniques can be induced to indicate micro-crack from fracture behavior of AHSS sheet in forming and AE technique can detect the crack initiation faster and more effective than DCPD.

Using Direct Current Potential Drop Technique to Estimate Fatigue Crack Growth Rates in Solid Bar Specimens under Environmental Assisted Fatigue in Simulated Pressurized Water Reactor Conditions

The direct current potential drop (DCPD) technique may be used in crack propagation tests to measure the crack growth rate (CGR). Potential probes attached to the specimen allow the variation of the crack length to be estimated. In this research, the DCPD technique using one single potential probe was applied to solid bar specimens (i.e., without any initial notch or crack) subjected to low-cycle fatigue testing in a simulated pressurized water reactor (PWR) environment. This particular analysis had two associated difficulties, the first one being the fact that crack initiation sites are not known beforehand, and the second one consisting in the experimental difficulties and conditioning factors associated with the simulation of the PWR environment. Nine solid bar specimens were tested to fatigue failure under different strain amplitudes and frequencies, while also measuring the corresponding DCPD signal during the fatigue process. It was observed that the initiation of multiple cracks was detected by the DCPD measurements. Moreover, as fatigue continued, one of the cracks became dominant and progressed to cause the specimen failure. The DCPD technique allowed the average CGR of the dominant crack to be estimated. Finally, the obtained average CGRs were validated by comparing them with average CGRs derived from striation spacing measurements, obtained from scanning electron microscopy (SEM) and from literature values gathered in the NUREG/CR-6909 document.

Damage evolution detection in a pipeline segment under bending by means of acoustic emission

A steel pipeline segment of 2.5 m length was subjected to quasi-static four-point bending load in three steps for studying the initial cracking and damage accumulation based on the Acoustic Emission (AE) technique and by the direct current potential drop (DCPD) technique. For the latter, a new post-test analysis method was established. AE is found more sensitive to crack initiation than DCPD. Formation of mesoscopic and macroscopic cracks as well as their closure and the resulting friction generate weighted peak frequencies below 400 kHz, whereas microscopic cracking produces AE with broad band spectra identifiable by weighted peak frequencies above 400 kHz. Critical states alike the maximum load level and the leak opening were accompanied by peak amplitudes above 85 dBAE. This rather fundamental study provides a data base for possibly developing advanced strategies of detection and alarm systems based on acoustic monitoring of pipelines, or at least, steel structures.

Determination of fatigue crack growth in the near-threshold regime using small-scale specimens

A complete description of the experimental procedure for characterizing the intrinsic fatigue crack propagation threshold (ΔKth,eff) as well as the fatigue crack growth rate (FCGR) in the near-threshold regime using small-scale specimens is presented. A comparative study is carried out on the high strength steel S960QL considering different single edge notch bend (SENB) specimen geometries. On one hand, the reference dimensions of 6 mm thickness (B) and 19 mm width (W) are analysed and referred to as conventional specimens. On the other hand, small-scale specimens with B = 3 mm and W = 4 and 6 mm are also considered. Several loading configurations (3-, 4- and 8-point bending) are used and a load ratio R = 0.8 is applied to avoid crack closure effects. The direct current potential drop (DCPD) technique is used to monitor the crack length. The reduced dimensions of the small-scale specimens imply the necessity of implementing a modified testing procedure compared to the recommendations of current standards for the generation of FCGR data. The results show a good agreement between tests conducted on different specimen sizes, which opens new perspectives in the use of small-scale specimens for characterizing the fatigue crack growth properties in metallic materials. Recommendations and limitations of the procedure are provided and discussed.

Influence of internal volumetric imperfections on the tearing resistance curve of welded Single Edge notched Tension (SENT) specimens

The acceptance of volumetric weld imperfections is subjected to workmanship guidelines, which are subjective quality criteria loosely related to fitness for purpose assessments. However, the relative impact of such imperfections may be accentuated in fracture toughness tests, given the relatively small dimensions of the test specimens. This study analyses the impact of weld volumetric imperfections on the outcome of fracture toughness tests. Single Edge Notched Tensile (SENT) specimens were machined from steel plates joined by robotic Metal Inert Gas (MIG) welding. The specimens contained different levels of volumetric imperfections which were characterized by high-resolution X-ray Computed Tomography (µCT) scans. Single specimen fracture toughness tests were conducted and crack growth was monitored by applying both the Unloading Compliance (UC) technique and the Direct Current Potential Drop (DCPD) technique. Tearing resistance curves (R-curves) were constructed based on UC and DCPD data. Results showed that specimens containing volumetric imperfections acceptable by workmanship guidelines may yield significantly inferior fracture initiation toughness values compared to imperfection-free specimens.

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.

Quasi-static and dynamic fracture toughness of a γ-TiAl alloy: Measurement techniques, fractography and interpretation

Intermetallic titanium aluminides (TiAl) offer an enormous potential for high temperature applications due to their high specific strength, creep and oxidation resistance. However, their use for structural components is limited by their low ductility and fracture toughness. During service, along with a high number of load cycles, the material may also be subjected to unexpected impact loads, which requires therefore knowledge about dynamic material parameters for a damage-tolerant component design. Here we present a feasibility study to determine the fracture toughness of a third generation TiAl-alloy over a wide range of loading rates. Quasi-static crack resistance curves (R-curves) were determined using the direct current potential drop (DCPD) technique. The dynamic experiments were performed with a drop tower with impact velocities ranging from 1 to 10 m/s on pre-cracked SENB specimens. To exclude the influence of inertial forces, the measurements were carried out with strain gauges applied in the elastic near-field of the crack tip. Despite a critical consideration of possible systematic errors of this measurement method, it was found that the dynamic fracture initiation toughness slightly increases up to loading rates of K̇I = 106 MPa√m/s. The fracture behaviour of the material and possible reasons for the toughness increase are discussed based on comprehensive fractographic investigations as well as crack path analyses within the microstructure.

Accelerated crack growth experiments of SS304H for dry storage canister in substitute ocean water – Effect of temperature

Spent nuclear fuels (SNFs) are expected to be stored in dry storage canisters (DSCs) for much longer time than earlier anticipated due to a lack of permanent deep geological repository. As a result, DSCs located in marine environment may undergo chloride induced stress corrosion cracking (SCC) during long-term storage of SNFs. We report here the effect of test temperature on accelerated crack growth experiments carried out on sensitized austenitic stainless steel SS304H in substitute ocean water using fracture mechanics approach. Wedge-opening loading specimens were selected for measurement of crack growth rates using direct-current potential drop technique. Average crack growth rates of 0.975 × 10−10 ± 9.528 × 10-12, 3.258 × 10−10 ± 9.551 × 10-11 and 1.580 × 10-9 ± 2.593 × 10−10 m/s were obtained for 22, 37 and 60 °C, respectively resulting in an activation energy of 60.9 kJ/mol corresponding to diffusion of hydrogen in steel. Intergranular crack propagation was noted along with formation of chromium-rich carbide precipitates at grain boundaries.

Numerical calibration of the direct current potential drop (DCPD) method in fracture mechanics fatigue tests

Fatigue strength and fracture mechanics concepts are linked together by the so-called cyclic R-curve, i.e. the crack size dependence of the fatigue crack propagation threshold. The cyclic R-curve requires to identify experimentally the extension of a propagating crack in the mechanically short crack regime. One of the most used techniques adopted in experimental tests is the direct current potential drop (DCPD) method, according to which the electrical resistance of the tested specimen increases due to crack propagation and the resulting electric potential change is used to derive the crack length thanks to proper calibration curves. In this work, the DCPD technique was applied in fracture mechanics fatigue tests of carbon steel bars weakened by a single-edge, semi-elliptical pre-crack. First, it was assumed that the semi-elliptical pre-crack propagates under fatigue axial loading in such a way to keep an iso-stress intensity factor (SIF) KI crack front. Accordingly, the corresponding crack pattern was derived by means of 3D structural FE analyses using the Peak Stress Method (PSM). Afterwards, the DCPD calibration curves were derived through 3D electrical FE analyses. The effects of the locations of the current and of the potential probes were investigated. Finally, a three-probe dual channel DCPD technique was applied to compensate any temperature variation of the tested specimen.

Quantitatively related acoustic emission signal with stress corrosion crack growth rate of sensitized 304 stainless steel in high-temperature water

Stress corrosion cracking (SCC) of sensitized 304 stainless steel in high-temperature water was in-situ monitored by coupling acoustic emission (AE) and direct current potential drop (DCPD) techniques. The AE signals were identified using novel recurrence quantification analysis (RQA) and k-mean cluster method. It was found that the RQA is feasible to identify the evolution of different AE waveforms generated by ligament tearing and plastic deformation of the crack tip. The AE cumulative hits rate was linear with the SCC crack growth rate, providing a possibility for applying the AE technique to quantitatively evaluate SCC in high-temperature water.

Analysis of crack geometry and location in notched bars by means of a three-probe potential drop technique

It is often necessary to define the crack initiation life of a fatigue tested component, generally at a given (short) crack length. The size of an initiated crack can be estimated by employing different experimental methods, one of which is the direct current potential drop (DCPD) technique. In the case of notched bars subjected to fatigue loadings, the crack configuration (i.e. circumferential or semi-elliptical) and location cannot be singled out by means of the potential drop method (PDM) operating with a single potential probe. In the present contribution, three potential probes are adopted to overcome this issue. The calibration curves reporting the three potential drops as a function of the crack size are derived by means of 3-dimensional electrical FE analyses. Two different crack configurations are analyzed: (i) circumferential and (ii) semi-elliptical surface cracks. The calibration curves have been validated by systematic comparison with experimental results, generated by fatigue testing of sharp as well as blunt notched specimens made of steel and a titanium alloy under pure axial loading. Finally, a procedure to assess the area, the configuration and the location of the initiated fatigue crack starting from the experimentally measured potential drops is discussed.

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.

Multi-Lead Direct Current Potential Drop Method for In Situ Health Monitoring of Ceramic Matrix Composites

The method of direct current potential drop (DCPD) can be utilized as an effective and convenient approach for in situ damage detection, and as a nondestructive evaluation technique. We present the results from use of a multiprobe DCPD technique for in situ damage detection in loading of a SiC/SiC composite. It is shown that in three different modes of loading (monotonic, fatigue, and cyclic load–unload), the sensing capabilities of DCPD technique compare well to the techniques of modal acoustic emission (AE) and digital image correlation (DIC). It was also found that DCPD technique provides a far earlier warning of failure under fatigue loading than the other two methods. In addition, we show that strategically placed multiple voltage leads on the specimen surface provide a promising way of qualitatively determining the crack initiation site. Therefore, the use of multiple lead DCPD method, together with other techniques, provides a viable option for sensing damage in ceramic matrix composites (CMCs) with complex geometries, and for applications at higher temperatures.

Notched Ti-6Al-4V titanium bars under multiaxial fatigue: Synthesis of crack initiation life based on the averaged strain energy density

The fatigue life of notched components subjected to multiaxial loading conditions may be influenced by extrinsic mechanisms operating during crack propagation phase, such as sliding contact, friction and meshing between crack surfaces. These extrinsic mechanisms may disrupt the applicability of local approaches, such as that based on the strain energy density (SED) averaged over a structural volume having size R0 and surrounding the crack initiation point. In the present contribution, the multiaxial fatigue behaviour of circumferentially notched specimens made of titanium grade 5 alloy, Ti-6Al-4V, has been analysed. Pure bending, pure torsion and in-phase as well as out-of-phase combined bending-torsion fatigue tests have been carried out on notched specimens characterized by two different root radii, namely 0.1 and 4 mm. Fatigue crack initiation and subsequent propagation have been monitored by adopting the direct current potential drop (DCPD) technique. In principle, crack initiation life has been defined when the initiated fatigue crack fractures the structural volume, which corresponds to a given potential drop increase calibrated by means of electrical finite element (FE) analyses. The structural volume size R0 has been determined by fatigue testing plain and sharp V-notched specimens under pure axial or pure torsion loadings. Finally, the averaged SED approach has been adopted to correlate the experimental fatigue results expressed in terms of crack initiation life, in order to reduce the effect of extrinsic mechanisms.

Calibration of the potential drop method by means of electric FE analyses and experimental validation for a range of crack shapes

AbstractIn experimental fatigue tests, it may be necessary to identify the onset of crack initiation, which is often defined at a given (short) crack length. Different experimental techniques are available to estimate the initiated crack size, one of which is the direct current potential drop technique. In this paper, the calibration curves reporting the potential drop change as a function of the crack depth have been derived by means of 3D electrical finite element analyses. Driven by previous experimental observations, two different crack shapes have been considered: (i) circumferential and (ii) semi‐elliptical surface cracks. Dealing with circumferential cracks, the effects of the three‐dimensional distribution of the electric current density and the temperature have been investigated. Concerning semi‐elliptical surface cracks, the effects of the crack aspect ratio and the potential probes locations have been analysed. Finally, the obtained calibration curves have been validated against experimental measurements, obtained by fatigue testing notched specimens under a selection of loading conditions.

Fatigue crack growth behavior of wrought γ-based TiAl alloy containing β-phase

The effect of microstructure containing β phase on the room temperature fatigue crack growth (FCG) behavior of a wrought TiAl alloy of Ti-43Al-4Nb-5V (at.%) was investigated. Three different microstructures which consist of α2/γ colonies grains where the grain boundaries are decorated with an enrichment of volume fraction VDP of β/γ duplex microstructure were designed. The effects of β/γ duplex on stress intensity threshold and Paris slope have been determined. Fatigue crack growth curve were measured using the direct current potential drop technique. Crack pathway and fracture surface were examined by scanning electron microscope combined with in-situ CCD camera with the aim to evaluate the fracture mechanisms. It was found then even with a high-volume fraction of β phase Vβ, it is possible to maintain the FCG properties as same as for fully lamellar microstructure. Thus, the presence of β phase could improve the stress intensity threshold and decrease the Paris slope comparing to its corresponding β-free duplex microstructure. However, with increasing volume fraction of β/γ duplex, the FCG properties decrease and if the volume fraction of γ grains become too high, the fatigue crack growth behavior is mainly driven by the γ phase.

Environmentally assisted crack growth rate of an austenitic steel TP347HFG in high‐temperature medium

The crack growth rate (CGR) tests of austenitic steel TP347HFG were conducted under <10‐8000 ppb dissolved oxygen (DO) in steam, the alternating environment of air and steam and supercritical water (SCW) at 600–650 °C. A constant stress intensity factor (K) method and direct current potential drop technique were used in the tests to evaluate its environmentally assisted cracking (EAC) properties as superheater and reheater boiler tubes in ultra‐supercritical power plants. CGR increases monotonically with temperature and DO content. CGR in SCW is greater than that in the high‐temperature steam which indicates that exposure pressure has an obvious effect on crack propagation. The varying CGR in different experimental environments show that exposure medium participates in the cracking process. Fracture surface and cross‐sectional surface of compact tension specimen were analyzed by scanning electron microscope. The mechanism of crack propagation in different exposure environment is further discussed.

Formability Prediction for Tube Hydroforming of Stainless Steel 304 Using Damage Mechanics Model

Tube hydroforming (THF) is an important manufacturing technology for producing tube components by means of fluid pressure. In comparison to other basic forming processes like deep drawing, forming steps can be reduced and more complex shape is allowed. In this work, it was aimed to establish the forming limit curve (FLC) of stainless steel tube grade 304 for the THF process by using finite element (FE) simulations coupled with the Gurson–Tvergaard–Needleman (GTN) damage model as failure criterion. The parameters of the GTN model were obtained by metallography analysis, tensile test, plane strain test of the examined steel in combination with the direct current potential drop (DCPD) and digital image correlation (DIC) techniques. These parameters were well verified by comparing the predicted FLC of steel sheet with the experimental FLC gathered from the Nakazima test. Then, the FLC of steel tube 304 was established by FE simulations coupled with the damage model of tube bulging tests. During the bulge tests, pressure and axial feed were properly controlled in order to generate the left-hand FLC, while pressure and external force needed to be simultaneously incorporated for the right-hand FLC. Finally, the FLC was applied to evaluate material formability in an industrial THF process of the steel tube.