Selecting Suitable Probes Distances for Sizing Deep Surface Cracks Using the DCPD Technique

Abstract

In this study, the way to enhance the sensitivity of evaluating deep surface cracks by DCPD technique using four probes is considered. The potential drops across two-dimensional cracks having different depths are analyzed by the three-dimensional finite-element method. The effect of the distance between current input and output probes and the distance between measuring probes on the change in potential drops are analyzed for a wide range of crack depths. By extending the distance between current input and output probes, the change in potential drop with the change in the depth of deeper crack becomes large. But the voltage of potential drop becomes small to measure. Finally, the way to select the appropriate distances between the probes for the measuring sensor is shown from the viewpoints of sensitivity and the required current.

Measurement of Potential Drop Distribution by Scanning the Closely Coupled Probes Sensor for Sensitive NDE of Shallow Surface Cracks

“…The value of 2a evaluated by the present technique (14.0 mm) and that measured were judged to be in good In the present study, to realize the quantitative NDE of shallow cracks, the sensor used has narrow spacing between the probes, for example, the current probes and the potential drop measuring probes distances were 6 mm and 3 mm, respectively. Wider space of the four-point probes achieves deeper penetration depth of current, 16) and in contrast, the micron-scale four-point probes, which have been recently fabricated, 17,18) focus the current on the surface. The present NDE technique based on the potential drop profiles measured by the four-point probes is applicable for NDE of wide range of cracks, micron-scale to several millimeter, provided that the suitable distances of four-point probes are selected.…”

Potential difference methods for measuring crack growth: A review

“…If this point is taken to its conclusion, calibration becomes a function only dependent on specimen geometry and the probe arrangement, meaning that geometrically similar test-pieces share identical calibrations. There are several methods to determine a reference potential value; measuring across a specified crack length such as the initial notch [6,40,42,55,117,140,153,155,170], measuring on the initially un-cracked test-piece [117,119,164], measuring on a separate reference specimen placed in the same condition as the test-piece [11,45,127,134] and measuring in a region remote from the crack so that the current field is independent of the crack and remains homogenous (as mentioned for the dual pair potential probes) [3,48,51,76,77,81,82,102,183,184]. Normalisations using a singular reference value allows the elimination of the effects of inherent material properties but excludes sustaining errors due to the variations of temperature and current during testing [151].…”