The early detection of rolling-sliding contact fatigue cracks

Pearlite determination in plain carbon steel by eddy current method

Incomplete appreciation of eddy current limitations has contributed to both under-utilization and misapplication of the technique. Neither situation need arise if known limitations are recognized. Some, such as the skin-depth effect, are inherent to electromagnetic test methods and define the role of eddy-current inspection in the field of NDT. Others can be overcome with available technology, such as surface probes to find circumferential cracks in tubes, and magnetic saturation of ferromagnetic alloys to eliminate permeability effects. The variables responsible for limitations in eddy-current testing are discussed and alternative approaches, where they exist, are suggested. Limitations unique to testing ferromagnetic materials are highlighted. Areas with potential for further research and development are also identified.

Recognizing limitations in eddy-current testing

This paper is a contribution to the detection and characterisation of small cracks using Eddy Current Testing in the Non Destructive Evaluation framework. Small cracks are considered as incipient faults defined as gradual faults whose signature is weak and concealed by the noise. They are characterized by high signal to noise ratio and low fault to noise ratio. The detection and diagnosis of such faults is still an open challenge. For complex systems, model-based incipient fault detection and diagnosis (FDD) methods usually fail because of the inaccuracy of the model to describe all the phenomena and their interactions. Data-driven methods using statistical features are very promising as long as historical data are available. However in the case of incipient faults, there is not a significant variation of a single feature. The fault signature lies in the global variation of the signal properties. The proposed method relies on the Kullback-Leibler Divergence (KLD) as a nonparametric fault indicator. It measures the slight dissimilarities between the probability density functions of the current signal compared to the faultless or healthy one. Through experimental results, the KLD exhibits a higher sensitivity than the usual statistical features for the detection of small cracks (with dimensions in the order of 0.1 mm) realized in a nickel-based superalloy plate. Moreover, the detection is done with zero missed detection probability. Furthermore, the fault severity is assessed through the characteristics of the crack (surface, length, and depth). In the principal component analysis framework, the analysis of four statistical features (KLD, mean, variance, and maximum) dependency to the excitation frequency allows to discriminating among the cracks.

Statistical Approach for Nondestructive Incipient Crack Detection and Characterization Using Kullback-Leibler Divergence

Eddy current detection of changes in stainless steel after cold reduction

Eddy current manual, volume 2

Basic eddy current principles and standard inspection procedures relating to heat exchanger tubing are briefly summarized. Various revolutionary probes recently introduced by Chalk River Nuclear Laboratories of Atomic Energy of Canada Limited are described. Included are: probes for inspecting ferromagnetic tubing, probes for detecting circumferential cracks and probes for detecting in regions, such as the tubesheet expansion zone, where conventional probes are not effective.

Recent advances in probe design for eddy current testing of heat exchangers

Until recently, only impedance type probes were used for inservice eddy current inspection of heat exchanger tubing. The introduction of finned air conditioner tubes, ferromagnetic tubes, and a variety of service induced failures, has taxed the capability of eddy current testing. Availability of computer-aided multifrequency equipment has increased the inspectability of some of these components, but not all. This prompted R&D in probe design.

Many difficult or impossible inspection problems with impedance type probes can be overcome with transmit-receive probes containing separate excitation and receive coils. The excitation coil(s) induces eddy currents and the receive coil(s) detects the resultant magnetic field distortion from defects. This approach allows greater flexibility in probe design; size, shape, orientation and separation of transmit-receive coils are all parameters which can be optimized.

This paper describes various probes, and their performance, which were developed at the Chalk River Laboratories of Atomic Energy of Canada. Included are probes for detecting defects at tubesheet regions in heat exchanger tubes, defects at expansion-transition regions in finned tubes, and defects in ferromagnetic tubes. All of these probes can be used with conventional commercially available instruments.

Transmit-Receive Eddy Current Probes for Heat Exchanger Inspection

During eddy current inspection of zirconium alloy pressure tubes (111.0 mm outside diameter (OD) and 4 mm wall) installed in CANDU (CANada Deuterium Uranium) nuclear generating stations, with surface probes, signals indicating the possible presence of defects were observed. Further examination, in particular with ultrasonic techniques, revealed that no defects were discernible and that the signals were in fact due to narrow, axial bands of higher resistivity. Ultrasonic techniques used to separate resistivity bands from real defects are time-consuming, so a specialized eddy current probe capable of rapidly distinguishing between these signal sources was a desirable addition to the test equipment. This paper describes a successful probe design, how it works, and examples of signals obtained.

V.S. Cecco

Papers

Recognizing limitations in eddy-current testing

Eddy current manual, volume 2

Recent advances in probe design for eddy current testing of heat exchangers

Transmit-Receive Eddy Current Probes for Heat Exchanger Inspection

An eddy current probe for separating defects from resistivity variations in zirconium alloy tubes