Applications of Photoinductive Imaging

Photoinductive Imaging for Bolt Hole Corner Crack Inspection

“…The resulting temperature fluctuation induces a highly localized change in the conductivity and permeability of the metal, which in turn induces a change in the impedance of the eddy current probe. It can be shown from Auld's reciprocity relation [7,8] that this impedance change is (2) where V is the entire volume of the metal, E and H are the electric and magnetic field intensities in the metal when laser beam is on, £' and "ii' are the electric and magnetic field intensities in the metal when laser beam is off, (0 is the angular frequency of eddy currents, LlT is the ac temperature change in the metal, and (J and 11 are the conductivity and permeability of the metal. be replaced by the unperturbed fields with good accuracy.…”

“…[1,2] It provides an imaging technique with microscopic resolution using eddy current sensors. We have applied this technique to characterize comer cracks at the edge of a bolt hole.…”

“…Thermal waves produce a localized modulation of electrical conductivity in the specimen, which can be detected through its effect on the impedance of a nearby eddy current coil. Moulder, and others [2][3][4] showed that this new technique can also be used to calibrate and characterize eddy current probes. This probe characterization method offers a means to determine the electric field intensity of eddy current probes, a quantity that is directly related to their performance for flaw detection and characterization.…”

High Resolution Photoinductive Imaging

“…Because the principles and equipment of photoinductive imaging have been covered in detail in prior volumes of this series [1, 2,3], the emphasis here will be on the images that can be produced, with only a brief introduction to the principles. Gas cell detection, which is used for comparison with the photoinductive images, has also been covered in detail elsewhere [4].…”

“…For the gas cell scans, the cell, with its hearing aid microphone and window to admit the laser radiation [4] is sealed to the sample surface with a water soluble glue. For the photoinductive scans, the small printed circuit of the differential coils [2] is mounted under the microscope objective.…”