B. P. C. Rao

Detection of leakage magnetic flux from near-side and far-side defects in carbon steel plates using a giant magneto-resistive sensor

Giant magneto-resistive (GMR) sensors are attractive for magnetic flux leakage measurements, especially for the detection of shallow near-side cracks and deeply located defects. An optimized measurement system with magnetic yoke, GMR sensor and selective amplifier has been devised to detect the tangential component of leakage flux from various near-side notches and far-side notches (widths 0.5 mm and 1.0 mm, respectively) in 12 mm thick carbon steel plates. Far-side notches located at nearly 11 mm below the measurement surface have been detected with a good signal-to-noise ratio. The performance of the GMR sensor with lift off has also been studied for possible non-contact examination of hot surfaces and a lift off of 2 mm is expected to ensure the saturation-free detection of near-side as well as far-side notches.

An artificial neural network for eddy current testing of austenitic stainless steel welds

An artificial neural network is presented for on-line eddy current testing of austenitic stainless steel welds. Time-domain parameters that are functions of digitized in-phase and quadrature components of probe impedance are used as input to the neural network and the network output, in depth units, is evaluated and displayed continuously. The neural network is trained to recognize disturbing variables such as variations in weld microstructure, lift-off and edge-effect as well as notches of different depth. The neural network is able to detect and characterize longitudinal and transverse surface-breaking notches, despite the presence of disturbing variables.

Flexible GMR Sensor Array for Magnetic Flux Leakage Testing of Steel Track Ropes

This paper presents design and development of a flexible GMR sensor array for nondestructive detection of service-induced defects on the outer surface of 64 mm diameter steel track rope. The number of GMR elements and their locations within saddle-type magnetizing coils are optimized using a three dimensional finite element model. The performance of the sensor array has been evaluated by measuring the axial component of leakage flux from localized flaw (LF) and loss of metallic cross-sectional area (LMA) type defects introduced on the track rope. Studies reveal that the GMR sensor array can reliably detect both LF and LMA type defects in the track rope. The sensor array has a fast detection speed along the length of the track rope and does not require circumferential scanning. It is also possible to image defects using the array sensor for obtaining their spatial information.

Simulation of eddy current signals from multiple defects

The application of eddy current testing for structural integrity assessment of heat exchangers is a well established practice. Service induced multiple defects in heat exchanger tubes produce difficult-to-evaluate complicated impedance plane trajectories during periodic eddy current examinations. Finite element simulations carried out to analyse interactions in a two-defect system are discussed. Probable uncertainty in the interpretation of eddy current trajectories is highlighted. Systematic analysis of the results from these studies will simplify procedures for on-line evaluation of eddy current signals.

Non-destructive testing and evaluation for structural integrity

Reliable performance of a component or structure depends on pre-service quality of the component and in-service degradation of the component under operating conditions. The role of non-destructive evaluation (NDE) in ensuring pre-service quality and also monitoring in-service degradation to avoid premature failure of the components/structures is ever increasing. There are many NDE techniques based on various physical principles. The end objective of NDE is detection and characterisation of anomalies such as defects, stresses and microstructural degradations in materials. This is accomplished by establishing correlation between a nondestructively measured physical/derived parameter and quantitative information on defects/stresses/microstructures. The NDE information together with design parameters are taken into consideration for evaluation of integrity and life assessment of the components/structures. In this paper, a brief description of the physical concepts of NDE methods and the physical/derived parameters that are used for assessing defects, stresses and microstructures are given. A few case studies highlighting the importance of non-destructive testing and evaluation for structural integrity assessment are also discussed based on the investigations carried out at the authors’ laboratory. Emerging concepts like intelligent processing of materials, expert systems, neural networks, use of multisensors with fusion of data and exploitation of signal analysis and imaging approaches are also addressed in this paper.

Influence of crack length on crack depth measurement by an alternating current potential drop technique

An alternating current potential drop (ACPD) technique is used for sizing depth of surface cracks in metallic components. Crack depth estimations are prone to large deviations when ACPD measurements are made on very shallow and finite length cracks, especially in low conducting materials such as austenitic stainless steel (SS). Detailed studies have been carried out to investigate the influence of crack length and aspect ratio (length to depth) on depth estimation by performing measurements on electric discharge machined notches with the aspect ratio in the range of 1 to 40 in SS plates. In notches with finite length, an additional path for current to flow through the surface along the length is available causing the notch depths to be underestimated. The experimentally observed deviation in notch depth estimates is explained from a simple mathematical approach using the equivalent resistive circuit model based on the additional path available for the current to flow. A scheme is proposed to accurately measure the depth of cracks with finite lengths in SS components.

Three-Dimensional Finite Element Modeling of Remote Field Eddy Current Technique for Detection of Localized Defects

Three-dimensional finite element (FE) modeling of remote field eddy current (RFEC) technique has been performed to detect localized defects in ferromagnetic steam generator tubes. The excitation frequency, intercoil spacing, and the design of localized receiver coil have been optimized using this model. Amplitude and phase of RFEC signals of radial and axial receiver coils have been predicted for a through hole and two flat bottom holes (FBH). The detection sensitivity of the receiver coils has been compared, and the model has been validated for a few cases.

AN INTELLIGENT IMAGING SCHEME FOR AUTOMATED EDDY CURRENT TESTING

An intelligent imaging scheme involving artificial neural networks and image processing methods is presented for automated eddy current testing of austenitic stainless steel plates. A quick coarse-scan imaging is performed to identify all the defective regions, which are later subjected to detailed fine-scan imaging to produce accurate threedimensional pictures of the defects. Experimental studies confirm that the imaging scheme is insensitive to disturbing variables and is able to significantly reduce the imaging time as well as the data storage requirements. Details of the scheme and its applications are given.

Wavelet Transform–Based Denoising Method for Processing Eddy Current Signals

Wavelet transform (WT)–based denoising method is proposed for processing eddy current signals of thin-walled stainless steel fuel tubes with periodic wall thickness variations formed due to fluctuation in tube drawing process parameters. In this method, discrete wavelet transform with level-based threshold has been applied to selectively eliminate the noise due to periodic wall thickness variations towards meeting the quality assurance requirement of detection of holes larger than 0.3 mm diameter and linear defects deeper than 0.075 mm (20% wall thickness). The method has been applied to tubes having machined holes, longitudinal notches, and circumferential notches, and an overall improvement of 20 dB in signal-to-noise ratio has been observed. The method has been able to detect defects present anywhere in the wall thickness variation regions and also in tubes without any wall thickness variations.

Effect of Carbon Content on Eddy Current Response to Sensitization and Intergranular Corrosion In Simulated Heat-Affected Zone of Austenitic Stainless Steel

AISI Type 316 stainless steel (C = 0.055 wt. %) and type 316L stainless steel (C = 0.016 wt. %) were subjected to various heat treatments between 873 and 1073 K for various durations of time. The aged specimens were then subjected to ASTM A262 Practice E test in a boiling 16 % H2SO4 solution containing 100 g/l of CuSO4.6H2O for 24 h and followed by a bend test. Based on the appearance at the bend portion, the specimens were classified into four categories viz. unaffected, fissured, cracked and broken. Electrochemical potentiokinetic reactivation (EPR) technique was used to determine the degree of sensitization (DOS) for the various categories of specimens. Eddy current (EC) tests were carried out to determine the eddy current amplitudes for the various categories of specimens in both the as-aged and Strauss-test exposed condition. The results of the EPR tests and the EC measurements were then classified into these four categories of specimens, in the as-aged condition and Strauss-tested condition (prior to bending). The results indicated that it was possible to predict DOS in the as-aged conditions using EC technique, though the overall change in the EC amplitude was small. The EC response from the Strauss test-exposed specimens was much higher than that observed in the as-aged specimens. The results indicated that EC testing was sensitive enough to detect both sensitization and IGC. The differences in the response of the two sensitized steels to eddy currents indicated that their carbon content played a major role in altering the chemical composition and the geometrical features of the depleted regions.