Fatigue damage detection and growth monitoring for composite structure using coda wave interferometry

This paper presents the development of a fatigue damage detection and growth monitoring technique based on coda wave interferometry (CWI) for composite structures. The time domain distortion (TD) component of coda waves, which is typically ignored as noise in the literature, is used and experimentally validated. First, a widely used signal processing method for CWI called the stretching method is applied, and TD component is extracted in the time domain. Next, threshold values are statistically estimated using noise levels of the signal and compared with the amplitude of TD component for detecting early-stage fatigue damage. A damage index (DI) is then obtained from TD component and used to monitor fatigue damage growth. The experimental results of the damage detection and growth monitoring are validated through visual inspection during the fatigue test. Finally, the developed technique's performance is evaluated by comparing it to conventional ultrasonic and CWI-based composite fatigue damage assessment methods. The uniqueness of this study lies in (1) detection and growth monitoring of the early-stage composite fatigue damage, (2) extraction and use of TD component of CWI and investigation of the correlation with the damage existence and growth, and (3) experimental validation and performance examination by comparing with the conventional ultrasonic and CWI-based composite fatigue damage assessment methods.