Implementation of nonlinear acoustic techniques for crack detection in a slender beam specimen

Abstract

Techniques that analyze nonlinear transformations of high frequency vibration signals, such as harmonic distortions and frequency modulations, termed nonlinear acoustic techniques (NAT), offer unique advantages in detecting and characterizing structural damage. Linear techniques are limited in their ability to detect small incipient damage and false indications caused by environmental variability and structural features of comparable size to damage. Defects with contact surfaces, such as cracks and delaminations, lead to strong nonlinear behavior in the form of nonlinear frequency interactions. The advantage of NAT over traditional linear techniques in detecting incipient small-scale nonlinear damage is demonstrated by initiating and identifying a fatigue crack in notched beam specimens. Impact-modulation (IM) is utilized to identify frequency modulation caused by the initiation of fatigue cracks. Piezo-stack actuators and modal impact hammers are used to generate structural excitations measured using high frequency accelerometers. Practical implementation issues of NAT are discussed, such as characterizing the inherent nonlinearities of electronics, actuators and sensors for reliable defect characterization.