Research on non-collinear mixing test method of concrete nonlinear ultrasound

Considering that the nonlinear ultrasonic mixing method is seldom used in concrete detection, for the feasibility of the non-collinear mixing method detecting concrete, this study conducts ultrasonic nonlinear research on sum frequency longitudinal wave scattered by two intersecting ultrasonic transverse waves. Based on non-collinear mixing theory, the paper analyses the angle between two incident fundamental frequency waves and the relationship between incident angle and frequency ratio to form non-collinear mixing testing system. Testing on concrete beams, the relationship between the nonlinear coefficient and the respective frequency and amplitude of the incident fundamental waves have been explored. The results show that mixing wave appeared only when the frequency ratio of two fundamental waves was in the range of 0.54–1.87. The nonlinear coefficient gradually decreases with the increase of frequency and amplitude of the fundamental waves. The amplitudes of the fundamental and the mixing wave increase with the rising of the frequency of the fundamental wave. Compared with the test results of the higher harmonic ratios method, the nonlinear coefficient of non-collinear mixing method is more sensitive to the change of fundamental frequency and amplitude, which verified the feasibility of non-collinear mixing method in detecting concrete damage.