Development of an optical fiber-guided robotic laser ultrasonic system for aeronautical composite structure testing

This paper proposes a novel all optical fiber-guided robotic laser ultrasonic system for nondestructive testing of aeronautical composite structures. A 1064 nm Nd:YAG pulse laser coupled with an optical fiber is used to generate ultrasonic signals, and a laser interferometer based on two-wave mixing in photorefractive polymers is used to measure the signals. A precise six-axis articulated robot is used as the scanning mechanism for laser ultrasonic imaging of defects in composite structures. A composite specimen with simulated internal delamination and another one with impact damage are prepared for experiments. The broadband ultrasonic signals generated by the pulse laser are measured by the laser interferometer, the signal to noise ratio is improved by a preamplifier and the narrowband signals in specific frequency ranges are extracted by a filter. The reflection and attenuation of laser ultrasonic signals induced by structure defects are monitored based on the pulse echo method. Typical C-scan imaging of the composite specimens with preset defects are realized using the fiber-guided laser ultrasonic system, and the shape, size and location of the defects are imaged clearly. The results proved that the proposed optical fiber-guided robotic laser ultrasonic system is effective for the nondestructive testing of composite structures.