Electronic shearography for detecting disbonds in lattice/skin structures

Aluminum structures with an integral lattice (rib system) and skin currently are used for a variety of large space structures (Titan and Delta series) due to their structural efficiency. Various components of space structures will be manufactured out of graphite/epoxy composites in the future. Interstages of rockets, decks of small satellites (MightySat program), payload shrouds, solar cell substrates, rocket motor casings, etc., are being fabricated at the Air Force Phillips Laboratory. Tests have shown that the principal mode of initial failure in these structures involves separation between the skin and the ribs, during fabrication or loading. The redundancy of the rib system in these structures leads to alternate load paths that make it difficult to detect such defects. The fabrication, integration, and testing often are carried out in different parts of the country. It is therefore important to have a method of detecting and possibly quantifying the integrity of the rib/skin interface. The objective of this paper is to demonstrate that electronic shearography (ES) can be used to detect disbonds in rib/skin "Isogrid" structures. This laser-based interferometry technique provides fringe patterns that represent full-field displacement gradients. The ruggedness and portability of the system make it a prime candidate for in-service inspection of large structures. The observed fringe patterns change dramatically for disbonded ribs that form a basis of rapidly detecting disbonds over a large area. The expected fringe patterns can be quantified and compared with results from finite-element (FEM) analyses of the structure. A commercial FEM code was used with orthotropic material properties that are representative of the composites used.