Concrete Microcracks Detection under Compressive Load Based on Nonlinear Ultrasonics Modulation with Broadband Excitation

The initiation and propagation of microcrack damage in concrete under compressive load have a great influence on structures that require high durability. This paper theoretically analyzed the nonlinear acoustic modulation mechanism in concrete at the initial stage of compressive cracking and explored the relationship between microcrack density and nonlinear response variation. Nonlinear behavior and microcrack development of concrete were simulated. The opening/closing state of microcracks under different cracking states and the propagation characteristics of nonlinear acoustic modulated signals were investigated. It was found that the change in microcrack density measured by the DI (damage index) of the nonlinear acoustic field technology was much more significant than that measured by DIC (digital image correlation). The experimental results showed the good feasibility of microcrack detection with width of 13–80 μm. Moreover, the experiment found that when the peak value of DI is between 0.02 and 0.2, microcracks occur in concrete.