Surface Cracks

Rivets or holes show as smooth circular forms.  In the MOI image shown here, there are two rivets with no cracks and two rivets with cracks.  Cracks can be seen around the complete circumference of the rivet site in one pass of the imager over the aircraft.  Surface images are viewed using higher frequencies (50 - 100 kHz).  To penetrate more deeply into the metal, the inspector must lower the frequency level on the power unit.  Images are viewed in the PVS headset by the inspector, or on an optional outside TV monitor (NTSC or PAL compatible) that connects to the control unit.

Subsurface Corrosion

This is an MOI 308/3 image of corrosion on the aircraft bellyskin.  Frequency has been set to 5 kHz.  Corrosion images are irregular in shape and show as dark shadows between rivets or around rivet sites.  In this sample, there is a chem-milled step which shows as a dark line shape to the right of the corroded area.  Since the MOI is an area inspection device instead of a point by point device (such as an eddy current probe) the MOI can rapidly locate corroded areas.

Subsurface Cracks

This is third layer crack image that is 0.084" from the surface.  The frequency has been set to 5 kHz resulting in a more diffuse image.  Since surface cracks will also be visible, it is important that the inspector perform a high frequency scan first and mark surface defects.  When the frequency is lowered, new crack images are subsurface defects.