Section 3.1.3.2. Corrosion Specimen Selection and Design
In the case of a crack
detection assessment, representative cracks can be grown quite successfully
in the laboratory. Since methods of
corrosion growth are not well established, most notably for hidden corrosion,
at present it is necessary to include real aircraft pieces with real corrosion
in the specimen sets to be used in NDI
capability demonstrations. Finding
specimens with appropriate levels of corrosion is not a trivial
problem. Potential specimens can be
obtained from obsolete aircraft and from depots. While such specimens may contain real corrosion, they are not
necessarily representative for a particular application. Further, a “good” NDI system for detecting
hidden corrosion would be needed to select the specimens with varying degrees
of corrosion damage. On the other hand,
this situation does not eliminate the need for engineered and manufactured
specimens. These specimens provide a
level of control not available with the aircraft specimens. The type, location, and size of the defect
(as measured by the chosen metric) can be controlled. The particulars of the engineered specimens must be determined
from the specific metric chosen and the
application. For thickness loss between
layers, engineered specimens might
include machined out areas of various depths and lateral dimensions. Experiment
objectives also impact specimen designs. For example, a spatial resolution test would require a specially
designed and manufactured specimen.