Fatigue crack inspection and characterisation using non-collinear shear wave mixing

In recent years, nonlinear ultrasonic techniques have been developed rapidly for defect detection and characterization at its early development stage. In this paper, non-collinear shear wave mixing is used for fatigue crack detection and characterization, exploring how orientation and size may be determined. This is achieved through measuring the amplitude of the generated longitudinal wave from a crack of interest. This amplitude is a function of the interaction angle and frequency ratio of two incident shear waves and the resulting amplitude parameter space is termed the nonlinear fingerprint of the crack. Numerical analysis of the nonlinear interaction between two incident shear waves and cracks was performed using two-dimensional finite element models to explore a broader parameter space than possible experimentally. It is shown that the interaction angle which leads to the maximum generated longitudinal wave amplitude is related to the orientation angle of the crack. To confirm these conclusions the model was validated using experimental measurements of vertical fatigue cracks grown using 3-point bending tests from initiation notches. The polarity flipping method was used to improve signal to noise ratio in such measurements. It is shown that there is a good agreement between the experimentally measured fingerprints and those simulated using finite element methods. Finally, an approximate method for sizing fatigue cracks was introduced that uses multiple non-collinear measurements along the crack length to determine its extent. As expected, the measured sizes from the proposed method indicate greater crack lengths than seen with linear ultrasonic phased array images due to the closed fatigue cracks being undetectable with linear arrays.