Louis F. Coffin

The Application of Electric Current Computed Tomography to Defect Imaging in Metals

Electric Current Computed Tomography (ECCT) is a technique for producing images of the electrical resistivity profile within a body from measurements made on the body’s exterior. To make these measurements, an array of electrodes is attached to the surface of the body. Sets of current patterns are applied through these electrodes and the voltages needed to maintain these specified currents are measured and recorded. These applied currents and measured voltages are then used in a reconstruction algorithm to produce images that represent approximations to the electrical resistivity distribution in the interior of the body.

Instability effects in thermal fatigue

This paper considers shape changes which develop progressively in hourglass specimens subjected to fully reversed cyclic plastic strain applied in a variety of mechanical and thermal waveforms. While the strain at the minimum diameter is controlled, off-center locations subjected to cyclic plastic strain undergo ratcheting. Two major off-center shape changes have been observed: barreling and necking. The paper discusses the reasons for these changes.

Some Perspectives on Future Directions in Low Cycle Fatigue

This paper on low cycle fatigue (LCF) is divided into three parts. The first provides a brief historical perspective, the second deals with todays scene, and the third speculates on future LCF directions.

Localized Ductility Method for Evaluating Zircaloy-2 Cladding

The localized ductility method has been shown to be a useful laboratory method for evaluating the mechanical performance of fuel cladding under conditions that approximate that of pellet-clad interaction (PCI). Using the method on unirradiated annealed and cold-worked cladding, performance is determined in flowing gaseous iodine and cesium plus cadmium liquid metal. In the case of iodine, increasing temperature and decreasing strain rate enhance the embrittlement, as does a texture in which the basal pole is in the direction of the applied stress. Further, microcracking is not found to occur below a finite plastic strain. The behavior is consistent with a film rupture mechanism for chemical-assisted cracking. For cesium plus cadmium, cracking is found in Zircaloy-2 above a threshold level of stress as indicated by the performance of constant crosshead and predefected constant load tests.

Role of damage tolerance and fatigue crack growth in the power generation industry

The problem of intergranular stress-corrosion cracking (IGSCC) in boiling water reactor (BWR) piping is discussed and the body of work undertaken in the authors laboratory to solve that problem is described. Particular attention is given to the development of electrical potential crack monitoring techniques and their application to surface crack growth, particularly under conditions approaching those found in service. The important role of water chemistry and its control is described in this context. The concept and description of sensors to monitor in situ the degree of damage containment from intergranular stress-corrosion cracking is then described, with reference to use in piping components and other types of monitoring. Finally, a concept for the life management of structures is described where damage processes are identified and monitored in situ using appropriate sensors to measure the damage rate continuously. In conjunction with damage monitoring, an applicable life prediction model is used to forecast the future damage state and to assist in the decision process for appropriate required actions to optimize plant operation.