Discussion on Distributed Conductivity for Modeling Stress Corrosion Crack in Eddy Current Testing

A scheme to apply signals of pulsed eddy current testing (PECT) to reconstruct a deep stress corrosion crack (SCC) is proposed on the basis of a multi-layer and multi-frequency reconstruction strategy. First, a numerical method is introduced to extract conventional eddy current testing (ECT) signals of different frequencies from the PECT responses at different scanning points, which are necessary for multi-frequency ECT inversion. Second, the conventional fast forward solver for ECT signal simulation is upgraded to calculate the single-frequency pickup signal of a magnetic field by introducing a strategy that employs a tiny search coil. Using the multiple-frequency ECT signals and the upgraded fast signal simulator, we reconstructed the shape profiles and conductivity of an SCC at different depths layer-by-layer with a hybrid inversion scheme of the conjugate gradient and particle swarm optimisation. Several modelled SCCs of rectangular or stepwise shape in an SUS304 plate are reconstructed from simulated PECT signals with artificial noise. The reconstruction results show better precision in crack depth than the conventional ECT inversion method, which demonstrates the validity and efficiency of the proposed PECT inversion scheme.

Quantitative evaluation of crack profiles using Eddy Current Testing (ECT) signals often underestimates depth of a Stress Corrosion Crack (SCC) because of the crack complicated property, especially the conductivity property. In this paper, to improve reconstruction precision of crack depth, an SCC numerical model considering crack conductivity distribution is adopted. To solve the high-dimension optimization problem with multiple conductivity parameters, a hybrid inversion strategy combined Particle Swarm Optimization (PSO) algorithm with Powell direction acceleration method is proposed. Owing to overcoming local optimum and slow search speed in a high-dimension solution space of PSO algorithm, and solving the difficulty of initial value selection for Powell direction acceleration method, the hybrid inversion strategy can improve SCC reconstruction accuracy and numerical stability. Results from simulated and measured ECT signals of several artificial SCCs demonstrate that the proposed strategy is effective to improve reconstruction precision of crack depth.

The management of Stress Corrosion Cracking (SCC) represents one of the challenges for pipeline operating companies with pipelines potentially susceptible to SCC. In order to help better support the management of SCC, a graded performance specification for the high-resolution Electro-Magnetic Acoustical Transducer (EMAT) In-Line Inspection (ILI) technology is derived which provides higher levels of confidence for detection of crack-field anomalies with critical dimensions. This paper presents the process used to derive the graded performance values for the EMAT ILI technology with regard to SCC. The process covers the Probability of Detection (POD) and Probability of Identification (POI). A blind test was carried out to derive the graded performance specification. Test data set was compiled comprising EMAT data for several joints containing relevant anomalies and neighboring joints, some containing additional shallow SCC. These joints had been dug based on EMAT ILI data and all of the joints were evaluated with 360 degree Non-Destructive Examination (NDE) followed by destructive testing. For each target joint with relevant anomalies to be assessed, four additional joints were added in random order to generate a realistic density and distribution of anomalies. Furthermore, pipe joints with non-crack like anomalies, as well as pipe joints with mixed populations were included in the blind test data set to ensure a realistic feature population and to assess POI without side effects of a weighed feature population inside set of composed ILI data. The data set was then evaluated by multiple analysts and result from each analyst were evaluated and utilized to derive the POD and POI values for the graded specification. In addition, the full process of data analysis including team lead review was carried out for one of the analysts for comparison to the individual analyst results. Anomaly dimensions were compared against the true population to derive the POD and POI values. Furthermore, length and depth sizing performance was assessed.

The influence of minor alloying platinum group metal additions on stress corrosion cracking (SCC) of austenitic stainless steels has been investigated in simulated pressurised water reactor environments. Crack propagation studies, employing tensile specimens precracked in acidified potassium tetrathionate and subsequently loaded in proving rings revealed that 1 wt-% ruthenium additions improved the resistance of 304 stainless steels to SCC. Analytical transmission electron microscopy was used to characterise crack tips and surface oxides after high temperature SCC tests. Enrichment of ruthenium and molybdenum within the dual oxide surface layers was observed in the ruthenium modified stainless steel, which is likely to have occurred during potassium tetrathionate pre-exposure and promoted intergranular SCC mitigation. Palladium additions (1 wt-%) showed no beneficial effects to SCC, and this was associated with the formation of second phases PdMn particles upon sensitisation, which decreased the availability of Pd.

This study investigates the detection of stress corrosion cracking (SCC) using eddy current testing with bobbin and array probes. The conventional bobbin probe method has limitation in detecting some outer diameter (OD) initiated axial and circumferential cracks in SS304 tubes with a diameter of 25.4 mm and a thickness of 1.65 mm. In contrast, these cracks were detected by array probe technology at a higher probability of detection. This discrepancy led to a detailed examination of SCC detection in SS304 tubes by inducing Artificial SCC with varying depths.

Alloy 260 brass specimens under stress were exposed at room temperature to 15 N aqueous ammonia solution with 8 g/1 of cupric copper predissolved. This environment causes tarnishing of the brass surface and intergranular stress corrosion cracking. Scanning electron microscopy, energy dispersive X-ray analysis, and Auger electron spectroscopy were employed to study fractography, corrosion product composition and distribution within the stress corrosion crack, and fracture surface chemistry characteristic of stress corrosion cracking in this system. A thin oxidized film was detected by Auger spectroscopy at the leading edge of the propagating crack. With continued exposure to the corrosive environment, deposits form on the fracture surface, then coalesce to form a continuous tarnish film that is depleted of zinc. No bulk depletion of zinc was detected in the alloy at the stress corrosion crack leading edge. No evidence of noncrystallographic crack arrest marks was found on the intergranular fracture surface.

Stress corrosion cracking (SCC) continues to be a safety concern, mainly because it can remain undetected before a major pipeline failure occurs. SCC processes involve complex interactions between metallurgy, stress, external soil environment, and electrolyte chemistry beneath disbonded coatings. For these reasons, assessing SCC failure probability at any given location on a pipeline is difficult. In addition, the uncertainty in data makes the prediction of SCC challenging. The complex interactions that affect SCC failure probability can be modeled using Bayesian network models. The Bayesian network models link events by cause-consequence connections. The strengths of these connections are adjusted using expert knowledge, analytical models, and data from the field. An approach to predicting probability of High pH SCC failure using Bayesian networks was proposed in a previous publication.1 The previous paper discussed the effects of stress only. In this paper, the previously discussed model is extended to the evaluation of other factors that affect high pH SCC. The model can be used to assess the probability of failure due to SCC at different times for different pipeline segments.

A stress corrosion cracking (SCC) model has been adapted for performance prediction of high level radioactive-waste packages to be emplaced in the proposed Yucca Mountain repository. For waste packages of the proposed Yucca Mountain repository, the outer barrier material is the highly corrosion-resistant Alloy UNS-N06022 (Alloy 22), the environment is represented by aqueous brine films present on the surface of the waste package from dripping or deliquescence of soluble salts present in any surface deposits, and the tensile stress is principally from weld induced residual stress. SCC has historically been separated into ''initiation'' and ''propagation'' phases. Initiation of SCC will not occur on a smooth surface if the surface stress is below a threshold value defined as the threshold stress. Cracks can also initiate at and propagate from flaws (or defects) resulting from manufacturing processes (such as welding); or that develop from corrosion processes such as pitting or dissolution of inclusions. To account for crack propagation, the slip dissolution/film rupture (SDFR) model is adopted to provide mathematical formulae for prediction of the crack growth rate. Once the crack growth rate at an initiated SCC is determined, it can be used by the performance assessment to determine the time to through-wall penetration for the waste package. This paper presents the development of the SDFR crack growth rate model based on technical information in the literature as well as experimentally determined crack growth rates developed specifically for Alloy UNS-N06022 in environments relevant to high level radioactive-waste packages of the proposed Yucca Mountain radioactive-waste repository. In addition, a seismic damage related SCC crack opening area density model is briefly described.

Enhancements of eddy current testing techniques for quantitative nondestructive testing of key structural components of nuclear power plants

Stress corrosion cracking is a significant failure mechanism affecting carbon steels in seawater environments. This study aims to evaluate the effect of controlled plastic pre‐strain on the stress corrosion cracking behavior of ASTM A36 steel immersed in a 3.5 % sodium chloride solution. Four pre‐strain levels (2.5 %, 5 %, 7.5 %, and 10 %) were applied before testing using the slow strain rate testing method. Tensile tests were conducted to assess mechanical properties, and scanning electron microscopy was used to examine fracture surface morphology and crack propagation features. The experimental results revealed a progressive reduction in elongation from 20.0 % to 11.57 % and an increase in tensile strength from 418.02 MPa to 473.70 MPa with increasing pre‐strain, indicating strain hardening. stress corrosion cracking susceptibility, quantified by the ratio of area reduction in the corrosive environment to that in air, peaked at 12.5 for the 2.5 % pre‐strained specimen, while remaining constant at about 10 at higher pre‐strain levels. These findings highlight the complex influence of plastic deformation on stress corrosion cracking behavior and provide insights into crack initiation mechanisms in low‐carbon steels under simulated seawater exposure.

The effect of residual stress of multiple welding repairs in seamless API 5L X52 on the stress corrosion cracking (SCC) susceptibility was evaluated. Four repairs of the girth weld were evaluated through X-ray diffraction (XRD) on the internal face of the pipe to measure residual stresses in longitudinal and circumferential direction. Residual stresses in the circumferential and longitudinal direction reach values of about 77% and 58% of the UTS (460 MPa) respectively, approaching to the YS of the steel (360 MPa). It was observed that its magnitude increases as move away from weld center line. SCC susceptibility of X52 steel welded joints with the residual Original Research Article Contreras et al.; BJAST, 6(6): 621-634, 2015; Article no.BJAST.2015.117 622 stresses generated was evaluated through slow strain rate tests (SSRT) in a soil solution at low and high pH. Relation between mechanical properties and residual stresses on the SCC susceptibility was analyzed. SCC index obtained from the mechanical properties of SSRT indicate good SCC resistance of X52 steel exposed to soil solution at low and high pH. From these results, it is suggested that the region with high residual stresses before to generate cracks in the steel surface due to the combination of soil solution and the strain applied, should favor pitting formation and not cracking. Initiation of micro-cracks from these pits will depend on the geometrical form of the pit and the stresses state can be established at given pitting. It is suggested that initiation of micro-cracks from these pits may depend on the dissolution rate and morphology of the pitting and stresses on the bottom of the pitting. This favorable condition could appear increasing the soil exposition time of the steel and reducing the strain rate applied on the SSRT.

Eddy current testing is a nondestructive testing method, which is used to detect discontinuities and defects in conductive materials. Using this technique, two different types of artificial defects in a railhead were evaluated in order to analyze the relationship between different types of defects and eddy current signals, and to obtain data on the size of the rail surface defects and crack location. The actually used rail sample was also studied. Surface cracks and defects were clearly observed as amplitude and phase changes of detected signals. This study succeeds in quantitatively analyzing and discriminating the damage types.

To detect stress corrosion cracking (SCC) and evaluate its depth on the surface of Ni-base alloys, a microwave microscope was employed. An open-ended coaxial line sensor was used to increase the spatial resolution. The working frequency was 110 GHz and the standoff distance between the sensor and the sample was 60 μm. The shape of SCC was obtained by microwave imaging. By measuring the amplitude of reflection coefficient, detection of SCC was achieved. To evaluate SCC depth, the attenuation constant of microwave in the crack was decided by comparing the measured microwave signal with the depth of a reference SCC obtained by destructive testing. Finally, the distribution of the depth of another SCC was evaluated by the proposed equation with the obtained attenuation constant.

Aluminum 7075-T6 alloy has been one of the most stress corrosion cracking (SCC) susceptible material used as a structural material in aircraft. To control SCC in this material, a number of concepts were investigated. The chemical modifications of the processes at the crack tip and surfaces were most beneficial. A multifunctional chemical system, called DNBM, was developed. It inhibited SCC by modifying the crack-tip processes. Whereas, a versatile surface treatment, called CMT, was developed which not only inhibited SCC but also surface corrosion and pitting. The sustained load and double cantilever beam tests were used to study the SCC behavior.

Stress-corrosion cracking (SCC) has been found to be the cause of failure of 48-in. (1219.2-mm) and 30-in. (762-mm) diameter manganese bronze gate valve bodies, a 48-in.-diameter shaft cap, and 1½-in. (38-mm)-diameter nuts. All of these in-service failures occurred in New York City distribution shafts for Water Tunnels Nos. 1 and 2. The valves and shaft cap, which were made of Copper Development Association (CDA) Alloy C86500, failed in predominantly transgranular mode with some intergranular cracking. The nuts, made of CDA Alloy C86300, cracked in an intergranular mode only. With numerous manganese bronze castings presently in service in all 37 water distribution shafts in New York City, a stress-corrosion cracking test program has recently been initiated. The specimens used were of the modified compact wedge-loaded type made of CDA Alloy C86500. The purpose of the test was not only to find which distribution shafts contained an aggressive SCC environment, but to identify any corrosive agents in the shafts. All 37 shaft environments produced cracking in one or more of the specimens tested in 15 to 63 days. Most of the cracking occurred with the pH in the range from 7.5 to 9.5. The spillage of elemental mercury from flowmeters in the shafts plus the introduction of chemicals used for mercury cleanup accelerated the SCC investigation. Mn-bronze specimens were tested in jars containing ammonia, mercury, water, trisodium phosphate, and commercial mercury cleaning solutions. All chemical induced cracking in the specimens in a range from 1 to over 300 days, except mercury and trisodium phosphate. Scanning electron microscopy was used to characterize the crack pattern of several in-service failures and SCC test specimens. Energy-dispersive analysis was used to identify the corrosion products on the crack surfaces.