Shridhar Nath

Optimization of Self-Nulling Eddy Current Probe for the Detection of Shallow Fatigue Cracks

The Self-Nulling Eddy Current Probe has been the focus of much research during the past several years [1–7]. Developed under NASA’s Airframe Structural Integrity Program, past research has focused on applying the Self-Nulling Probe technology to the inspection of damage to thin aluminum airframe skins. As a result of this work prototype fatigue crack detectors, single and multi-layer thickness gauges, and a system for the detection of cracks under installed fasteners have been developed[l–2,5–7]. The probe has also been successful at detecting surface flaws in thick bulk materials, for which a commercial instrument has been produced and marketed by Kramer Branson, Inc.-This paper will explore the fatigue crack detection mechanism of the Self-Nulling Probe for shallow flaws in thick materials as compared to that of through cracks in thin skins. The resulting change in the performance of the Self-Nulling Probe will then be detailed, and proposed modifications to optimize the performance of the probe for the detection of shallow fatigue cracks enumerated.

Characteristics of Frequency Domain Spectrum of Self-Nulling Eddy Current Probe Output

Since its introduction several years ago, the self-nulling eddy current probe [1–3] technology has been one of the focal points of the aging aircraft related R&D effort. Numerous application areas have broadened the scope of the probe which has also helped in better understanding the underlying principle. As the technology matures, however, deeper understanding on the various details related to the self nulling effect is needed to overcome difficulties associated with the current field tests and expand its application areas. A particular problem to be addressed is in differentiating the effect of small, shallow surface cracks from that of probe wobble during automated data acquisition operation.