Precise localization of closed cracks: A nonlinear ultrasonic total focusing imaging technique integrated with phase coherence

Closed cracks in in-service structural materials generate extremely weak ultrasonic reflection/scattering signals, making them difficult to detect effectively using conventional linear ultrasonic testing methods. This paper develops a nonlinear ultrasonic total focusing method integrated with phase coherence to achieve high-sensitivity detection and precise localization of closed cracks. The approach enhances incident wave energy through parallel excitation and extracts high signal-to-noise ratio (SNR) nonlinear harmonic signals using pulse inversion. On this basis, a total focusing algorithm based on nonlinear responses is constructed, and phase coherence weighting is introduced to significantly suppress noise and clutter, thereby improving the imaging SNR and defect localization accuracy. To validate the effectiveness of the method, systematic experimental studies were conducted on aluminum specimens containing fatigue-induced closed cracks (including two cases of single and double cracks). The results show that the proposed method can clearly image closed crack structures and accurately locate their propagation tips. The study also reveals that closure stress affects the intensity of the nonlinear response of closed cracks, and the ultrasonic energy at the crack is a key factor determining the localization accuracy of crack tips. By integrating nonlinear ultrasound with phase-coherent total focusing imaging, this method effectively achieves high-sensitivity, visual detection and quantitative evaluation of small closed cracks inside materials.