The integration of superlattices and immersion nonlinear ultrasonics to enhance damage detection threshold

摘要

We demonstrate the enhancement of immersion nonlinear ultrasonic testing (NLUT) by exploiting superlattices (SLs). NLUT can detect sub-wavelength micro-structural changes in solids by measuring the fundamental and second harmonic frequencies. The amplitude of second harmonic frequency increases with the presence of defects or other heterogeneities. The immersion NLUT is beneficial as water provides a consistent coupling condition; however, water generates high non-linearity that can mask the weak non-linearity originated from the micro-structural features in solids. In this research, SLs are proposed to remove the non-linearity arisen from water and experimental instruments. The SLs made of a periodic arrangement of composite layers can provide a band gap to restrict the propagation of a specific range of frequencies between transmitter and receiver. The periodic arrangement of solid-fluid layers is numerically designed and experimentally adapted to the immersion NLUT. Our results imply that the periodic array of 100 μm thick glass and 100 μm thick water layers provides a band gap that blocks 4.5 MHz (the second harmonic frequency), while this periodic structure passes 2.25 MHz (the first harmonic frequency). The improvement in the sensitivity of the NLUT is demonstrated through detecting the micro-structural changes associated with plastic deformation in aluminum 1100 specimens. It is revealed that the proposed methodology enhances the damage detection sensitivity of immersion NLUT by an order of magnitude as compared to the current practice.