The work currently being done on the laser shearography project consists of baselining the instrument for use in detecting debonds in the spray-on foam insulation (SOFI) on the external tank. The baselining is being conducted on test panels with programmed debonds and voids. The test panels consist of a 24-inch-square aluminum substrate covered with a primer. The panels are then sprayed with SOFI to an average thickness of approximately 3 inches and are then planed off to a uniform thickness of 1.5 inches. There are currently four completed test panels with programmed debonds and voids. The debonds and voids range in size and geometry in order to maximize the test information. The excitation mode will primarily be vacuum, as preliminary results indicate this is the most efficient method. While holding the foam thickness constant, each test panel will be run incrementally through a range of pressures (0 to 25 inches of water) to determine if the defects present on the panel can be detected. The foam thickness of each panel will then be reduced by 0.250 inch, and the process will be repeated until the foam thickness of each panel is reduced to 0.5 inch. The data will then be assembled into a three-dimensional "detection matrix," with one dimension being pressure differential, one dimension being defect size, and the third dimension being foam thickness. The elements of this matrix will indicate whether or not a defect was detected and, perhaps on a qualitative sliding scale, the level of visibility of the defect. With this detection matrix, an operator should be able to determine what size defect should be visible (if present) at a given foam thickness and pressure. This should provide a first step to standardizing the instrument for use on foam applications. Other data will also be taken. This data includes the shear vector, location, extent, aspect ratio, and orientation. This data, combined with that used to construct the detection matrix, should provide a database sufficient to perform the statistical analyses necessary to produce a meaningful probability-of-detection curve. Together, the detection matrix and probability-of-detection curve will allow the user to state within a certain confidence level that the area under observation is free of defects.
A vacuum chamber of sufficient size to fit a 24-inch-square test panel was constructed from 3/4-inch Plexiglas. The vacuum chamber is fitted with a vacuum gage so the pressure differential can be monitored during testing. Testing has begun on the four test panels, and the detection matrix is being constructed. Debonds on the order of half the foam thickness in diameter were observed with pressure differentials as small as 2 inches of water (0.07 pound per square inch). Also, while the vacuum chamber was operational, other areas of laser shearography application were investigated. Test panels consisting of 1-inch-thick ablative material and K5NA were inspected, and debonds were successfully detected in both these materials at pressure differentials similar to those previously discussed. These preliminary tests indicate there are numerous other areas of application for laser shearography at KSC. Future plans for this work include qualifying the instrument for use on flight hardware, inspecting flight hardware, defining new areas of application, image processing to yield more quantitative information, and perhaps building specialized laser shearography heads to facilitate use of laser shearography technology in areas of difficult access.
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