DAMAGE DETECTION IN COMPOSITE STRUCTURES VIA SCANNING LASER DOPPLER VIBROMETRY USING LINEAR AND NONLINEAR ULTRASONIC WAVE FEATURES

This study presents the modeling and experimental investigation of damage detection and evaluation methods based on the linear and nonlinear features of ultrasonic guided waves while they interact with possible structural damage sites. Such wave damage interaction features are visualized via the full field imaging capability of SLDV. First, numerical modeling is conducted to develop an in-depth understanding of the mechanism behind the wave damage interactions. A coupled-filed transient dynamic finite element model is constructed with a simulated delamination area. The modeling of Contact Acoustic Nonlinearity (CAN) is realized by defining the contact surfaces at the delamination area. The linear ultrasonic features such as the trapped modes as well as the nonlinear features such as the mixed frequency response are illustrated using the numerical simulation. Based on the numerical study, experimental investigations are further conducted. Experiments are performed to explore linear ultrasonic technique for damage quantification. For the linear case, the detection utilizes a short tone burst in both spatial and temporal domain, generated by a Piezoelectric Wafer Active Sensor (PWAS); the trapped wave energy and a directional vector field technique are adopted to visualize an impact damage in a carbon fiber composite plate. The nonlinear detection methodology combines a continuous resonant low frequency harmonic pumping wave with a high frequency tone burst probing wave, simultaneously generated by two PWAS transducers on both sides of the specimen to take advantage of the mixed nonlinear interactions between the vibro-acoustic waves and the structural damage. This study shows that both linear and nonlinear ultrasonic techniques possess great application potential for the damage detection and quantification in composite structures. The paper finishes with summary, concluding remarks, and suggestions for future work.