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.