A New Instrument for the Detection of Fatigue Cracks under Airframe Rivets

Detection of Sublayer Fatigue Cracks under Airframe Rivets

“…The Rotating Probe System then measures the magnetic field associated with the perturbed current flow while filtering off deviations due to probe misalignment, rivet tilt or misfit, and lift-off. A more detailed description of the operation of the system can be found elsewhere [2,4]. o 0 j R;", Hood The experimental system designed to detect fatigue cracks buried under rivet heads has been developed into a field ready prototype system.…”

“…The filter is based upon knowledge of the probe/flaw interaction, which can be used to determine the spatial frequency at which the flaw will occur. As shown in [4], an estimate of the spatial flaw frequency is given by, (2) where fr is the rotation frequency, R rev is the radius of probe revolution, and P fp is the induced eddy current distribution on the sample surface, or probe footprint. Voltage variations caused by the sources mentioned above will typically occur at the probe rotation frequency (except for lift-off which causes a DC shift) while, for 21tR rev > P fp ' the spatial flaw frequency will be greater than the rotation frequency.…”

“…The Rotating Self-Nulling Probe System developed as a part of the National Aeronautics and Space Administration's Airframe Structural Integrity Program has been shown in the past to provide a quick and reliable method to detect fatigue cracks hidden under airframe rivets [1][2][3]. The previous research has concentrated on detecting cracks in the top layer of an airframe lap-splice joint, obtaining a 90% probability of detection for 0.032" long top layer fatigue cracks buried under airframe rivets [2,3].…”

“…The previous research has concentrated on detecting cracks in the top layer of an airframe lap-splice joint, obtaining a 90% probability of detection for 0.032" long top layer fatigue cracks buried under airframe rivets [2,3]. Although this was a major accomplishment, the question of detectability of fatigue cracks in the lower layers of the joint has been left open.…”

Development and Testing of Prototype Giant Magnetoresistive (GMR) Rotating Probe System

“…Previous research in this area at NASA LaRC has resulted in the development of the Self-Nulling Probe Based Rotating Probe System, which has been shown to have excellent fatigue crack detection capabilities for near surface damage [1][2][3], and has been commercialized by Foerster Instruments Inc [4]. Continued development activities at NASA have focused on improving the deep flaw detection capabilities of the system.…”