Nonlinear Acoustic Measurements for NDE Applications: Waves Versus Vibrations

Majority of acoustic instruments widely used in industry and technology for non-destructive evaluation (NDE) make use of a linear elastic response of materials. The nonlinear approach to ultrasonic NDE is concerned with nonlinear material response, which is inherently related to the frequency changes of the input signal, and is a new technology for monitoring of deterioration in material properties and diagnostics of damage. The application field now includes both the nonlinear wave and nonlinear vibration modes. The former is based on the assumption and is applicable to the case studies of the distributed material nonlinearity. It profits from accumulation of the wave nonlinear response along the propagation distance and relies on the higher harmonic signals. A strong nonlinear response of non-bonded interfaces in planar defects introduces the nonlinearity localized in the defect area where the vibration nonlinearity steps up. The concept of local defect resonance (LDR) combined with its nonlinearity identifies a nonlinear inclusion as a nonlinear oscillator and brings about different dynamic and frequency scenarios in vibration nonlinear phenomena. The LDR-induced trapping of the nonlinearity generates a defect-selective nonlinearity and conditions for efficient and even noncontact nonlinear diagnostic imaging of damage.