Nondestructive Testing of Thin Composite Structures for Subsurface Defects Detection Using Dynamic Laser Speckles

ABSTRACT A novel nondestructive testing method for subsurface defects detection in thin composite structures using dynamic laser speckles is proposed. In this method, a laminated composite panel containing a subsurface defect is excited by a frequency scanned ultrasonic (US) wave and is illuminated by an expanded laser beam. If one of resonant frequencies of the defect coincides with the US frequency, a local area (a region of interest or ROI) of the panel optically rough surface, placed directly above the defect, begins to vibrate, and the sequences of difference speckle patterns containing the spatial response from the defect are recorded. The formation of this response is caused by both decorrelation and speckle blurring within the local speckle pattern generated by the vibrating ROI at its opposite tilts. The accumulation of difference speckle patterns increases the intensity of the spatial response. This method differs from similar ones in that defects are detected using dynamic speckle patterns of a composite rough surface, illuminated by a single expanded laser beam. The verification of the proposed method was performed using a hybrid optical-digital experimental breadboard to test composite panels containing artificial subsurface defects, as well as a real defect.