Nondestructive Strain Measurement System Used to Determine Surface Strain on Fibers

摘要

Small-diameter structural fibers are being considered as reinforcements for high temperature ceramic matrix composite materials, and thus they require characterization. At the NASA Lewis Research Center, a nondestructive optical technique was used to determine surface strain on a structural fiber, in real time, as it was pulled in a tensile test machine. With this technique, interference or speckle patterns from the laser illuminated fiber test specimen are recorded. As the fiber is pulled, its speckle pattern shifts in proportion to the strain, translation, and rotation components of the sample deformation. Shifting speckle patterns are detected in real time by two linear charge-coupled discharge (CCD) camera arrays, and the images are processed by a hardware correlator. Surface strain is selectively detected on fibers with diameters on the order of 100 mm and can be resolved to 19 microstrain. This system was designed to be robust and compact and generally does not require surface preparation of the structural fibers. For strain detection, two laser beams are positioned incident on the structural fiber being tested, as shown in the photograph, where the test specimen is mounted in a tensile test machine via two coupons. As the fiber is pulled, the speckle pattern produced from each laser beam is detected by one of two CCD arrays located inside the tube on the right side of the photograph.