Irradiation creep and growth of zirconium alloys: A critical review

The early detection of rolling-sliding contact fatigue cracks

Pearlite determination in plain carbon steel by eddy current method

A re-examination of the cottrell-stokes relation based on precision measurements of the activation volume

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

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

The pressure tubes of Canadian Deuterium Uranium (CANDU) reactors are monitored to evaluate their health. The main factors that affect their service life are changes in dimensions, shape: mechanical properties, and flaw tolerance. We have developed equipment and techniques to measure these properties and assess the health of the tubes. In this paper we describe the examination of the Zircaloy-2 pressure tubes of the Nuclear Power Demonstration (NPD) reactor. Their dimensional changes posed no problems. However, the combined effect of irradiation and the presence of radial hydrides caused the fracture toughness to decrease to such an extent that it would be difficult to assure leak-before-break in the future. Thus the reactor was closed down after 25 years of operation.

Evaluation of Zircaloy-2 Pressure Tubes from NPD

The recovery behavior of [001] Ni and Ni-Co single crystals was determined after prestraining in tension at room temperature to stress levels in stage II and stage III. Deformed specimens were subjected to various heat treatments, the associated changes in structure were monitored by compression testing. Isochronal annealing curves determined in this manner revealed two distinct regions associated with stage II and stage III deformation. Isothermal measurements revealed that region II recovery obeyed the Frank network growth kinetics with activation energies larger than that of self-diffusion. To be consistent with the dislocation climb model, the difference which increased with decreasing stacking fault energy should be an indirect measure of the minimum value for jog formation energy. On the other hand during region III recovery the activation energy was lower than that for self-diffusion although it also increased with decreasing stacking fault energy. The measured activation energies and the kinetics of the process are in keeping with a model of loop anneal by self-climb.

G. Van Drunen

Papers

Recognizing limitations in eddy-current testing

Evaluation of Zircaloy-2 Pressure Tubes from NPD

Recovery of ni and ni- co single crystals after varying amounts of cold work