Damage Detection in Composite Structures via Scanning Laser Doppler Vibrometry Using Linear and Nonlinear Ultrasonic Wave Features

Composite materials are taking an increasingly significant role in high-performance engineering structures such as aircrafts, ships, and automobiles. Strong and lightweight, composite structures also exhibit a superb fatigue behavior. However, the failure mechanism of composite materials is rather complex, giving rise to the difficulty of their damage progress and residual life prediction. Thus, the development of damage detection and quantification methodologies for composites is of vital importance. Ultrasonic guided waves have been investigated as a class of powerful tool for Nondestructive Evaluation (NDE) and Structural Health Monitoring (SHM). They can propagate long distances covering a large inspection area without much energy loss. Scanning Laser Doppler Vibrometry has been widely adopted for wave field measurement and imaging, due to its capability of conducting dense sensing data collection.

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. Two sets of experiments are performed to explore linear and nonlinear ultrasonic technique for damage quantification, respectively. The first experiment 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 second experiment combines a continuous low frequency harmonic pumping wave with a high frequency tone burst probing wave, simultaneously generated by two PWAS transducers on the 2 sides of the specimen to take advantage of the mixed nonlinear interaction 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.