Adapting 1-D phononic crystals to contact nonlinear ultrasonics for enhancing damage detection in concrete

We demonstrate the enhancement of contact nonlinear ultrasonic testing by implementing the 1-D phononic crystal layers. Nonlinear ultrasonic testing (NLUT) can detect sub-wavelength defects in concrete using the second harmonic generation method (SHG). SHG method is based on the generation of the second harmonic signal in solids when the first harmonic wave interacts with defects. Due to the inherent heterogeneous characteristic of concrete, the typical ultrasonic frequency for NLUT in concrete is 50 - 100 kHz. Frequency higher than 100 kHz may introduce high nonlinearity due to the multiple scattering in concrete, which would mask defect-related nonlinearity to be detected by NLUT. A 1-D phononic crystal is placed between the sensor and concrete surface to enhance the signal quality to block the signal beyond 100 kHz exhibited from instrumentation and couplant. The 1-D phononic crystal structure consists of a periodical arrangement of composite layers that can tune the band gap 100 kHz-200 kHz. The binary composite system configuration using the mixture of steel and aluminum is numerically studied. The periodic array exhibiting a band gap near 100kHz-200kHz is selected for experimental validation. The sensitivity of NLUT is improved by detecting the sub-wavelength inclusion in concrete. The results demonstrated that the nonlinearity due to defects becomes more apparent by removing the instrumentation-induced nonlinearity in concrete.