Abstract
Potential drop (PD) measurements are well established for use in materials testing and are commonly used for crack growth and strain monitoring. Traditionally, the experimenter has a choice between employing direct current (dc) or alternating current (ac), both of which have strengths and limitations. The dc measurements are afflicted by competing spurious dc signals and, therefore, require large measurement currents (tens or hundreds of amperes) to improve the signal-to-noise ratio, which in turn leads to the significant resistive Joule heating. The ac measurements have superior noise performance due to the utilization of phase-sensitive detection and a lower spectral noise density but are subject to the skin effect and are, therefore, not well suited to high-accuracy scientific studies of ferromagnetic materials. In this paper, a quasi-dc monitoring system is presented which uses very low-frequency (0.3–30-Hz) current which combines the positive attributes of both dc and ac while mitigating the negatives. Bespoke equipment has been developed that is capable of low-noise measurements in the demanding quasi-dc regime. A creep crack growth test and fatigue test are used to compare noise performance and measurement power against alternative DCPD equipment. The combination of the quasi-dc methodology and the specially designed electronics yields exceptionally low-noise measurements using typically 100–400 mA; at 400 mA, the quasi-dc system achieves a 13-fold improvement in signal-to-noise ratio compared to a 25-A dc system. The reduction in measurement current from 25 A to 400 mA represents a ~3900-fold reduction in measurement power, effectively eliminating resistive heating and enabling much simpler experimental arrangements.
Highlights
O VER the last decades, potential drop (PD) methods have become well-established in a number of nondestructive evaluation (NDE) industry applications [1], [2] and ubiquitous in laboratory-based material testing [3]–[9]
The choice depends on the skin effect, which is not present in dc measurements but it is utilized in ac measurements, as will be explained in the remainder of this section
An exactly equivalent component and measurement arrangement as the creep crack growth test was utilized for a room temperature fatigue test
Summary
Abstract— Potential drop (PD) measurements are well established for use in materials testing and are commonly used for crack growth and strain monitoring. The dc measurements are afflicted by competing spurious dc signals and, require large measurement currents (tens or hundreds of amperes) to improve the signalto-noise ratio, which in turn leads to the significant resistive Joule heating. A creep crack growth test and fatigue test are used to compare noise performance and measurement power against alternative DCPD equipment. The combination of the quasi-dc methodology and the specially designed electronics yields exceptionally low-noise measurements using typically 100–400 mA; at 400 mA, the quasi-dc system achieves a 13-fold improvement in signal-to-noise ratio compared to a 25-A dc system. The reduction in measurement current from 25 A to 400 mA represents a ∼3900-fold reduction in measurement power, effectively eliminating resistive heating and enabling much simpler experimental arrangements
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