Nonlinear Ultrasonic Techniques in Non-Destructive Testing: A Systematic Review

Non-destructive testing (NDT) has undergone significant advancements over recent decades, with ultrasonic techniques playing a central role in the evaluation and monitoring of material integrity. Conventional linear ultrasonic methods are widely used due to their robustness and simplicity; however, their sensitivity to early-stage damage, such as microcracking, remains limited. In this context, nonlinear ultrasonic (NLU) techniques have emerged as a promising alternative, offering enhanced sensitivity to subtle microstructural changes in tested materials. Despite their potential, NLU methods are still relatively recent compared to traditional ultrasonic approaches, and challenges related to theoretical understanding, signal interpretation, and result standardization continue to limit their widespread application in engineering practice. To address this gap, this study presents a systematic review of NLU techniques, focusing on their development, main applications, and advances in NDT. The results identify the most common applications of NLU strategies and highlight the predominant approaches for signal analysis, including Higher Harmonic Generation and Nonlinear Wave Modulation. In addition, the review provides a comprehensive overview of the most adopted signal types, excitation frequency ranges, emerging trends in signal processing, numerical modeling, and nonlinear ultrasonic imaging. Overall, this study emphasizes the growing relevance of NLU within the evolution of NDT techniques and highlights its potential as a powerful tool for early damage detection, while also identifying key challenges that must be addressed for its broader implementation.