Compton Backscatter Tomography
for imaging aircraft skins
X-ray backscatter, particularly Compton scattering, bounces off the object rather than passing through it. Systems using Compton backscatter imaging have been successful in imaging the insides of objects where access is restricted to one side. The apparatus shown was optimized to allow the micrometer-accuracy measurement of aircraft skin layers. Corrosion is very common in aircraft. The FAA allows planes to fly so long as at least 90% of the thickness of each of the skin layers remain. This apparatus and technique was devised by Larry Lawson working at Northwestern University as part of the FAA's Aging Aircraft Program.
Compton Backscatter Tomography is a continuing topic of research at the national level. Current interest is directed toward imaging of multiply scattered photons since these have been useful in identifying landmines.
Using the first generation prototype to measure corrosion metal loss on a Boeing 737 at Sandia National Laboratory
See SAMPLE SCANS
Imaging head under construction shown with graduate student, Nohyu Kim. Kim used Lawson's Compton Backscatter Tomography instruments to analyze multilayer impact fractures of organic composites.
Aircraft skins are often assembled by rivetting overlapping pieces together to make a "lap splice"(right),somewhat like on a lapstrake boat. A backscatter density profile through a lap splice (left) shows the thickness of the layers and the presence of a low density faying strip, between the two layers of metal, used to seal the joint. The dashed line is an optimized noise-free fit to the density profile.
Strips of metal called "tear strips" may be attached to the inside of a lap splice. The density profile (left) shows, from left to right: 1)upper skin, 2)faying strip, 3)lower skin, 4) bonding material, 5)tear strip and 6) possibly a layer of metallic paint. The dashed line indicates a noise-free fit. The three-layer appearance of the lower skin may indicate that it is clad or simply be an artifact resulting from noise. The physical appearance of the lap splice, with the profiled area shown as if cut away, is at the right.
This x-ray study (left) is that of a layer of skin rivetted to a rib or stringer made of hat channel as shown at the right. In this case the profiled material was actually cut out with a core drill and examined in the scanning electron microscope (SEM) to verify dimensional accuracy and compare layers. The SEM picture is shown under the scan at the left.