Current Potential Drop Technique Research Articles

On the Application of Infrared Thermography and Potential Drop for the Accelerated Determination of an S-N Curve

The endurance approach has been used for fatigue design and assessment since many decades. In recent years, the use of infrared (IR) technology to assess fatigue damage has been gaining more interest. The infrared thermographic method reported in literature allows to define an entire S-N curve with a limited number of specimens. Inspired by that approach, a methodology based on the direct current potential drop (DCPD) technique is developed. It allows to determine an S-N curve in a quick manner and might potentially be used in application areas where IR is not feasible. The developed DCPD methodology and the established IR methodology have been applied to two high strength offshore steels. The so-obtained S-N curves show a very good correlation.

Creep strain measurements using an Alternating Current Potential Drop method: A Nondestructive approach

Online assessment of the state of creep damage of a component in service is very useful in ensuring safety of structures, estimation of remaining life for life extension programs. Thus, in-situ monitoring of the progress of creep damage in such components is very important and a challenging task. Alternating current potential drop (ACPD) technique along with Linear Variable Differential Transducers (LVDT’s) was employed to measure voltage drop during creep tests of Alloy 718 at 600-700°C and 600-750MPa. The ACPD voltage drop data were subsequently converted into creep strain using appropriate formula and compared with the creep strain measured using LVDT’s. It has been observed that the creep strains derived from the ACPD data match well with the creep strains measured using LVDT for all the test conditions. This study demonstrates that the ACPD technique has a potential to be employed as an effective non-destructive tool for in-situ monitoring of creep strain in components

Alternating Current Potential Drop Technique to Detect Pit-to-Crack Transition

ABSTRACTPitting and localized corrosion can occur under various conditions and accelerate the subsequent cracking and the failure of a component. Pit-to-crack transition is considered a critical transition process and has a significant impact on the total lifetime of a component. In this study, an alternating current potential drop (ACPD) system is built and applied the detection of the pit-to-crack transition. The results show that the ACPD system is capable and sensitive to crack initiation and that the sensitivity increased with increased frequency. Crack initiation sites from pits are characterized and discussed.

Determination of crack initiation and crack growth stress-life curves by fracture mechanics experiments and statistical analysis

It is well known that the fatigue lifetime of a notched component consists of the crack initiation lifetime (crack size equal to the characteristic microstructure size) and the crack growth lifetime. The crack growth lifetime is easily calculated if the initial crack size is known. A much more difficult task is to define a suitable initial crack size and to determine the initiation lifetime of such a crack.In the present contribution, a method is proposed to determine the crack initiation as well as the crack growth lifetime within a conventional fracture mechanics setup. To this purpose, single edge notched bending (SENB) specimens were manufactured with two different notch geometries. For measuring the crack length, the direct current potential drop (DCPD) technique was used. The DCPD results are not only used for crack growth assessment, but also for determining the point – or range, respectively – of crack initiation.By a statistical assessment of DCPD measurements at different load stress levels, it is possible to determine crack initiation and crack growth stress-life (S/N) curves, and in particular to assess the influence of the notch geometry on the crack initiation lifetime. The lower statistical tolerance limits of these curves may be used as criteria for mechanical design and determination of inspection intervals.

On-Line Monitoring of Environment-Assisted Cracking in Nuclear Piping Using Array Probe Direct Current Potential Drop

A direct current potential drop method utilizing array probes with measurement ends maintaining an equalized potential designated as equi-potential switching array probe direct current potential drop (ESAP-DCPD) technique has been developed earlier at Seoul National University. This paper validates ESAP-DCPD technique by showing consistency among experimental measurements, analytical solution and numerical predictions using finite element analysis (FEA) of electric field changes with crack growth in metals. In order to examine its viability as an on-line monitoring of environment assisted crack growth at the inner surface of piping welds, artificial inner surface cracks were introduced in a full-scale weldment mockup pipe and stainless steel metal mockup pipe. The weldment was joined by low alloy steel and stainless steel pipes. The pipes were monitored by using ESAP-DCPD in laboratory environments. Optimization of electrical wiring configuration has produced results with significantly reduced noise for adequately long period of time. Then optimized experimental results were compared with the FEA prediction results for the mockup to show a good agreement. Also a round-robin measurement has been made at three laboratories. It has been found that the developed ESAP-DCPD can detect circumferential cracks with a depth of 40 % of wall thickness in stainless steel with a good detectability for further growth behaviors. For axial cracks, however, the measurements showed poor detectability. Hence the developed ESAP-DCPD system can be used to monitor large circumferential cracks that existing non-destructive examination techniques often fail to detect until leakage takes place.

Creep Damage Evaluation of DS CM247 Nickel Base Superalloy Using Alternate Current Potential Drop Technique

The evolution of creep damage in a directionally solidified DS CM247 alloy during creep at 950 °C/250 MPa has been evaluated using alternate current potential drop (ACPD) technique in which a constant alternating current was passed through the specimen and corresponding potential drop was measured. Creep tests were interrupted at different strain levels in order to examine the microstructural degradation of the alloy during creep. The creep curve of the alloy exhibited limited primary and secondary creep regimes, but extended tertiary creep regime. Microstructural examination revealed the progressive directional coarsening of γ′ precipitates i.e., rafting and widening of γ matrix channel with accumulation of creep strain. The specimen exhibited a considerable amount of creep strain prior to fracture. Further, creep cavitation was observed only close to the fractured end of the specimens. Various factors responsible for the extended tertiary creep regime of the alloy are discussed. The potential of ACPD technique to be used as a NDE tool to monitor in service/online creep damage evaluation, has been established/demonstrated.

Novel Method for Sizing Metallic Bottom Crack Depth Using Multi-frequency Alternating Current Potential Drop Technique

Abstract Potential drop techniques are of two types: the direct current potential drop (DCPD) technique and alternating current potential drop (ACPD) technique, and both of them are used in nondestructive testing. ACPD, as a kind of valid method in sizing metal cracks, has been applied to evaluate metal structures. However, our review of most available approaches revealed that some improvements can be done in measuring depth of metal bottom crack by means of ACPD, such as accuracy and sensitivity of shallow crack. This paper studied a novel method which utilized the slope of voltage ratio-frequency curve to solve bottom crack depth by using a simple mathematic equation based on finite element analysis. It is found that voltage ratio varies linearly with frequency in the range of 5-15 Hz; this range is slightly higher than the equivalent frequency and lower than semi-permeable frequency. Simulation and experiment show that the novel method can measure the bottom crack depth accurately.

Single-specimen evaluation of tearing resistance in SENT testing

The evaluation of CTOD tearing resistance is required by several strain based defect assessment procedures. To that extent, the use of SENT testing is widely advised as the constraint matches that of pressurized pipes. This indicates the need for well validated testing methods. This paper discusses two single specimen techniques for the evaluation of tearing resistance using clamped SENT specimens: the unloading compliance (UC) and the direct current potential drop (DCPD) technique. From the results of tests carried out on a variety of both base metal and welded specimens, both methods show a comparable accuracy and scatter.

Comparison of girth weld tearing resistance obtained from Curved Wide Plate and Single Edge Notch Tensile testing

Curved Wide Plate and Single Edge Notch Tensile tests are commonly considered for girth weld defect assessment. It is however not clear which test yields the least conservative estimate of the tearing resistance. A combined numerical–experimental procedure has been established to measure the crack extension, based on the direct current potential drop technique. This procedure appears accurate with respect to testing standards. In addition, the tearing resistance of the SENT specimens appears noticeably lower compared to wide plate specimens. This implies that SENT testing introduces conservatism in a girth weld defect assessment.

Fracture toughness of irradiated Zr–2.5Nb pressure tube from Indian PHWR

Fracture toughness of irradiated Zr–2.5Nb alloy pressure tube, fabricated by the cold pilgering and stress relieving route, was evaluated using disk compact tension type specimens. These specimens were punched out from the irradiated pressure tube (S-07), which was in service for about 8 effective full power years of reactor operation in the Kakrapar Atomic Power Station-2 (KAPS-2). The tests were carried out remotely inside a lead shielded enclosure. Crack growth during the test was measured using the direct current potential drop technique. The irradiated pressure tube showed low fracture toughness at 25°C. The fracture toughness increased with increase in temperature up to 250°C but was practically unaffected with further increase in temperature up to 300°C. This paper discusses the fracture behavior of irradiated Indian pressure tube material and compares it with other data available.

Fatigue crack propagation behaviour of a nickel based superalloy at elevated temperatures

The fatigue crack growth behaviour of a nickel based superalloy at elevated temperatures 550, 650 and 710°C was studied. The direct current potential drop (DCPD) technique was used to measure the crack length during the test. It was found that the crack growth rates slightly increased with increasing temperatures, but the temperature effect on crack growth rates was less pronounced at higher ΔK ranges. At the same temperature the crack growth rates increased with increasing load ratios. The oxidation induced crack closure is considered to be a reason of the observed crack growth behaviour in this study.

Effect of temperature and loading mode on environmentally assisted crack growth of a forged 316L SS in oxygenated high-temperature water

The crack growth rates (CGRs) of a forged nuclear grade 316L SS piping material were measured at various temperatures and loading modes in oxygenated high-temperature water. A direct current potential drop technique was used to monitor the CGRs throughout the tests. The fracture surface of crack advance under stress corrosion conditions exhibited typical intergranular stress corrosion cracking characteristics. CGRs increased monotonically with increasing temperature. CGRs under trapezoidal waveform loading were moderately higher than those under constant K loading.

Fracture Toughness Evaluation of Zr-2.5Nb Pressure Tubes Manufactured Employing Double Forging for PHWR700

The need for improvement of the in-reactor performance of Zr-2.5 % Nb pressure tubes arose because of the higher than expected diametral creep rates and variability in axial elongation observed in some of the pressure tubes of 220 MWe Indian pressurized heavy water reactors (IPHWR220). A systematic study revealed that the pressure tubes used in various IPHWR220 had variations in microstructure, mechanical properties, texture, and chemical composition. The forthcoming PHWR700 is expected to operate at a higher temperature than that of IPHWR220 pressure tubes coupled with ∼3 % partial boiling toward the outlet end. The IPHWR220 pressure tubes were manufactured by NFC (Hyderabad, India) using the combination of extrusion-double pilgering with intermediate annealing and with 20 % final cold work. PHWR700 pressure tubes have been manufactured using a combination of forging-extrusion-single pilgering routes. The fracture behavior of cold-worked and stress-relieved Zr-2.5Nb pressure tube material manufactured employing a forging route for PHWR700 is described in this work as a function of hydrogen content (4 and 60 wppm), temperature (25°C–300°C), and sample location. Fracture toughness tests were carried out as per ASTM E1820-06 [ASTM E1820-06: Standard Test Method for Measurement of Fracture Toughness—Designation, Annual Book of ASTM Standards, ASTM International, West Conshohocken, PA] using curved compact tension specimens of 17 mm width. The crack growth was along the axial direction of the tube and was measured using the direct current potential drop (DCPD) technique. The fracture toughness of as-received samples show weak dependency on test temperature, whereas samples having 50 ppm hydrogen show typical S curve behavior and regain toughness above 150°C. The variation in fracture toughness across tube length was within scatter band.

Ductile damage measurement and necking analysis of metal sheets based on digital image correlation and direct current potential drop methods

In this study, a damage measurement procedure is proposed, which combines both digital image correlation and direct current potential drop techniques to evaluate quantitatively the ductile damage in metal sheets during uniaxial tensile loading. Digital image correlation and direct current potential drop techniques are applied to measure the full-field deformation and overall electrical resistance of the region of interest of the specimen during tensile loading. The basic principles, methodology and derivation process are presented in detail. A professional data processing system based on MATLAB is developed to characterize the deformation–resistance–damage relationship during uniaxial tensile loading. The ductile damage can be evaluated conveniently by the proposed approach, and the experimental results so obtained are consistent with those derived by using micro-hardness technique. The necking stage during uniaxial tension is discussed in detail, and it has been shown that the onsets of diffused necking and localized necking can be determined according to the relationship between the major principle true strain and loading steps. Further, the onset of the localized necking can be used to estimate accurately whether the material is going to rupture.

Crack Growth of a Polycrystalline Nickel Alloy under TMF Loading

Thermo-mechanical fatigue (TMF) is an important factor for consideration when designing aero engine components due to recent gas turbine development, thus understanding failure mechanisms through crack growth testing is imperative. In the current work, a TMF crack growth testing method has been developed utilising induction heating and direct current potential drop techniques for polycrystalline nickel-based superalloys, such as RR1000. Results have shown that in-phase (IP) testing produces accelerated crack growth rates compared with out-of-phase (OOP) due to increased temperature at peak stress and therefore increased time dependent crack growth. The ordering of the crack growth rates is supported by detailed fractographic analysis which shows intergranular crack growth in IP test specimens, and transgranular crack growth in 90° OOP and 180°OOP tests. Isothermal tests have also been carried out for comparison of crack growth rates at the point of peak stress in the TMF cycles.

Mutual Interaction between Fatigue Crack Initiation/Propagation and Microstructural Features in Cast Aluminum Alloys

The correlation between the microstructure, the mechanical properties and the fatigue life of the common aluminum cast alloy Al-7Si-0.3Mg (A356) was investigated. By variation the solution heat treatment temperatures and times the precipitation strengthening effect in the dendritic aluminum solid solution phase and the spheroidization of the eutectic silicon were modified. The results of fully reversed fatigues tests revealed an increase in the fatigue life of specimens that were heat treated at higher temperatures. This observation was supported by analyzing the fatigue crack propagation behavior using the direct current potential drop technique (DCPD). With (i) increasing heat treatment temperature, i.e., increasing dendritic α-Al strength and (ii) roundness of the eutectic silicon particles the resistance to technical fatigue crack initiation, expressed by the threshold value of the stress intensity range Kth, was shifted to higher values.

Fatigue crack growth from handling surface anomalies in a nickel based superalloy at high temperature

Aircraft engine manufacturers have to demonstrate that handling surface anomalies in sensitive areas of discs are not critical for in-service life of a component. Currently, the models used consider anomalies as long cracks propagating from the first cycle, which introduces a certain degree of conservatism when calculating the fatigue life of surface flaws. Preliminary studies have shown that the first stages of crack propagation from surface anomalies are responsible for the conservative results. Thus, the aim of the study is to characterize the crack propagation from typical surface anomalies and to establish a new crack growth model, which can account for the micro-propagation stage. To separate the effects of the geometry of the anomalies and the residual stress state after introduction of the surface flaws, two V-type anomalies are studied: scratches and dents. Different studies have shown that the residual stresses beneath the anomalies seem to control the fatigue life of samples exhibiting scratches and dents. In order to monitor the crack micro-propagation, a direct current potential drop technique, coupled with heat tints is used during fatigue tests at elevated temperature. Thermal treatments releasing the residual stresses are also used to decouple the effect of crack morphology and residual stresses.

Measurement of Full-field Ductile Damage based on Resistance Method

The measurement of damage evolution in metal materials during plastic deformation accurately and conveniently remains a challenging problem for current research. In this study, a convenient ductile damage measurement procedure is proposed. The damage value can be evaluated by continuous uniaxial tension with the digital image correlation and direct current potential drop techniques. The development of methodology and experimental setups is discussed in details. The ductile damage values obtained by the proposed approach are consistent with those obtained by the micro-hardness method. The full-field damage distribution can also be calculated besides the ductile damage evolution.

Fatigue crack growth from handling surface anomalies in a Nickel based superalloy at high temperature

Aircraft engine manufacturers have to demonstrate that handling surface anomalies in sensitive areas of discs are not critical for in-service life of a component. Currently, the models used consider anomalies as long cracks propagating from the first cycle, which introduces a certain degree of conservatism when calculating the fatigue life of surface flaws. Preliminary studies have shown that the first stages of crack propagation from surface anomalies are responsible for the conservative results. Thus, the aim of the study is to characterize the crack propagation from typical surface anomalies and to establish a new crack growth model, which can account for the micro-propagation stage. To separate the effects of the geometry of the anomalies and the residual stress state after introduction of the surface flaws, two V-type anomalies are studied: scratches and dents. Different studies have shown that the residual stresses beneath the anomalies seem to control the fatigue life of samples exhibiting scratches and dents. In order to monitor the crack micro-propagation, a direct current potential drop technique, coupled with heat tints is used during fatigue tests at elevated temperature. Thermal treatments releasing the residual stresses are also used to decouple the effect of crack morphology and residual stresses.

Comparison of J-parameters of cold worked and stress relieved Zr–2.5Nb pressure tube alloy determined using load normalization and direct current potential drop technique

The fracture toughness of Zr–2.5Nb pressure tube alloy used in Indian Pressurized Heavy Water Reactor was determined in the temperature range of 30–300°C using load normalization technique (LNT) and a comparison between the crack initiation and the crack propagation toughness parameters is presented with the values obtained using direct current potential drop (DCPD) technique. The dependence of error in J-parameters determined using LNT to that obtained using DCPD on temperature and final crack extensions was also investigated. Below 200°C, the error exhibited weak dependence on final crack extensions. Above 200°C the error in initiation toughness increased significantly while the error in propagation toughness was practically unaffected.