NDE of damage in thick stitched composites using laser-based ultrasound with optical fibers for remote generation and detection

As design engineers implement new composite systems such as thick, load bea ring, composite structures, they must have certifiable confidence in the st ructure's durability and worthiness. This confidence builds from understand ing the structural response and failure characteristics of simple component s loaded in testing machines to tests on full scale sections. Nondestructiv e evaluation is an important element that can provide quantitative informat ion on the damage initiation, propagation, and final failure modes for the composite structural components. Although ultrasound is generally accepted as a test method, the use of conventional ultrasound for in situ monitoring of damage during tests of large structures is not practical. The use of la sers to generate and defect ultrasound extends the application of ultrasoun d to in situ sensing of damage in a deformed structure remotely and without contact. The goal of this research is to utilize this technology to monito r damage progression during testing. This paper describes and demonstrates the application of laser based ultras ound to quantify damage in thick stitched composite structural elements. Th is method involves using a Q-switched laser to generate a rapid, local ther mal strain on the surface of the structure, causing the generation of ultra sonic waves into the material. A second laser used with a Fabry-Perot inter ferometer detects the surface deflections. The use of fiber optics provides safety for eyes and a convenient method of delivering the laser over long distances to the specimens. The composite material comprises several stacks of graphite assembled by stitching through the laminate thickness, then in filtrated with a resin and cured. The specimens used for these nondestructi ve evaluation studies had either impact damage or skin/stiffener interlamin ar failure. Although little or no surface damage was visible, internal dama ge was detected by laser based ultrasound.