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.
Fatigue tests on a stabilizer bar link of an automotive suspension system are used to initiate a crack and
grow the crack size. During these tests, slow sine sweeps are used to extract narrowband restoring forces across the stabilizer bar link. The restoring forces are shown to characterize the nonlinear changes in component internal forces due to crack growth. Broadband frequency response domain techniques are used to analyze the durability response data. Nonlinear frequency domain models of the dynamic transmissibility across the cracked region are shown to change as a function of crack growth. Higher order spectra are used to show the increase in nonlinear coupling of response frequency components with the appearance and growth of the crack. It is shown that crack growth can be detected and characterized by the changes in nonlinear indicators.
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