The Use of Neutron Tomographic Techniques for the Detection of Corrosion Damage in Aircraft Structures

The presence of hydrogen in metals and alloys may result in significant changes in their mechanical properties. Such hydrogen may be introduced as the result of environmental factors connected with the use of such materials. Thus, the detection and quantitative characterization of hydrogen in metals is a significant problem in non-destructive testing. No presently available nondestructive method of imaging can reliably detect hydrogen or other low atomic number atoms in thick metal components. Neutron imaging has high sensitivity to hydrogen because of the large interaction probability (cross section) of neutrons with the hydrogen nucleus, Radiography with neutrons exploits this large cross section to image hydrogen containing corrosion products and moisture in metals but is limited due to the effective averaging of the hydrogen density over the entire particle path length; thus internal details may be hidden. Alternatively, tomographic imaging using radiographs (projections) from multiple angles allows reconstruction of the object’s internal properties. Quantitative determination of total interaction probability can be made over the resolution element of the tomogram and related directly to the spatial distribution of hydrogen. We have recently developed (Lanza, et. al., 1991), a neutron tomographic imaging system specifically for the detection of hydrogen and have applied it (McFarland, et. al., 1995) to studies of the embrittlement of turbine blades by hydrogen. Further, we have used an accelerator based neutron source to demonstrate in preliminary studies the usefulness of this technique in detection of corrosion in aircraft structures. (Shi, 1995) These results demonstrate a significant role for neutron tomography in the detection of hydrogen in critical aircraft components but also point toward applications in other areas where hydrogen embrittlement may be significant such as in pipe walls or nuclear fuel rod cladding. (Barritt, 1994).