Simultaneous measurement of layer thicknesses in thin layered materials using the phase of ultrasonic reflection coefficient spectrum

Abstract

The wide application of layered materials proposes more requests for their mechanical performance. The measurement of layer thicknesses is an important part of performance evaluation for the layered materials. Ultrasonic testing is an effective method for thickness measurement. However, in thin layered materials, the layer thicknesses are immeasurable due to the overlap of ultrasonic echoes from interlayer interfaces. This work presented an ultrasonic method for simultaneous measurement of layer thicknesses in thin layered materials using the phase of ultrasonic reflection coefficient spectrum (PRCS) at normal incidence, and the other method using the magnitude of ultrasonic reflection coefficient spectrum (MRCS) was also investigated for comparison. The inversion algorithm based on the particle swarm optimization (PSO) was used to avoid convergence to local minima. Both the sensitivities to the thickness parameters and effects of frequency bandwidth of the transducer were studied for PRCS and MRCS. The results indicate that PRCS is sensitive to the layer thicknesses and larger frequency bandwidth is favorable for convergence zones to be produced. Compared with MRCS, the narrower convergence zones can be obtained using PRCS. The experimental measurement results of the thin Al-Ti bi-layered material confirm that the measurement of layer thicknesses using PRCS is more accurate than MRCS. The relative errors of the Al layer and the Ti layer between the PSO estimated values and the real values are −3.77% and +1.91% using PRCS, respectively.