Research On Nondestructive Testing Of Materials Based On Low Optical Coherence

Cracks are widely found in the metal parts of aircraft,ships,high pressure vessels and other equipment,and the microcosmic shape of the crack occupies most of the entire life cycle,so the detection of micro-cracks is of great significance.Based on the principle of low coherence,a set of high precision metal crack detection system is designed,which has the characteristics of high measurement accuracy,fast imaging speed,non-contact and no damage.The system is divided into the scanning part of the metal crack surface,the receiving conversion part of the crack detection information and software to noise demodulation part.This article on the system design is as follows:(1)Due to the micro-arc scan sampling of the scanning galvanometer,the obtained demodulation signal itself has a certain error,which not only affects the detection accuracy of the system but also submerges the real damage signal when the crack damage is small and greatly reduces the system performance.In this paper,the signal acquisition method of arc compensation is proposed,and the error introduced by galvanometer scanning is optimized to only a few microns in size,which has been successfully improved by an order of magnitude.(2)The microscopic cracks on the metal surface are extremely small,so the detection system needs to have very high detection sensitivity.In this paper,we use the interference signal phase extraction method to optimize the design of demodulation,and use peak demodulation method for optical path length gain,not only improve the measurement accuracy of the system,but also improve the system's measurement stability;(3)Because of the roughness of the metal surface,the recognition of crack detection will be affected.In this paper,the lens parameters are selected by the software simulation of the optical lens and the threshold parameters are introduced into the image reconstruction algorithm,and successful realization of the three-dimensional reconstruction of different types of metal micro-cracks imaging.Through the above-mentioned improved design,the fatigue crack was detected by using the frequency-domain signal analysis method,and the 3D reconstruction image of the micro-crack was successfully obtained.The resulting reconstructed crack size is better than 20 ?m and the accuracy of vertical crack detection is up to 5 ?m.