Physics induced terahertz time-of-flight compensation method for accurate thickness measurement and full-field imaging of damage in composite

Accurate and efficient damage thickness estimation and imaging are critical for assessing the structural integrity of composite in service. Terahertz (THz) technique, as an advanced nondestructive testing (NDT) method, has emerged considerable potentials in the quantitative characterization of composite materials. The primary challenge in damage thickness measurement lies in precisely extracting the time-of-flight (TOF) of damage from the measured THz signal. However, existing TOF extraction methods are susceptible to the complex interferences in THz signal such as dispersion, overlaps, multiple reflection, and noise, and the performance is limited by the expert experience and prior knowledge of damage characteristics. In this work, to address these limitations, a novel physics induced THz TOF compensation method is proposed for the accurate thickness measurement and full-field imaging of damage in composite. The method leverages the propagation time difference of THz wave through the damaged and non-damaged regions, and thereby mitigating the impact of the human interventions and THz signal interferences. Initially, the improved physical propagation model of THz wave in composite is established to describe the effects lift-off distance and surface evenness of sample on THz testing performance. Subsequently, the THz TOF compensation method is developed to accurately estimate the damage thickness based on THz propagation characteristic of THz wave. In addition, a high-resolution full-field damage imaging approach is proposed, utilizing the estimated damage thickness to visualize the damage distribution across the composite structure. Overall, the proposed method provides a novel insight for the accurate damage thickness measurement under various complex scenarios in THz NDT, offering significant advancements in the automation and online monitoring of composite materials.