Characterization of bonding quality of composites with the nonlinear modulation method

Abstract

The nonlinear ultrasonic nondestructive testing technique is emerging as a valuable tool for material characterization and diagnostics. The most widely used nonlinear method is based on measurements of harmonic distortion of the probing ultrasonic wave. Another approach, developed by the authors, utilizes effect of modulation of ultrasound by low-frequency vibration. This method is more sensitive to various material and structural flaws, such as cracks, debonding, delamination, etc. It is also much easier to implement in practice. The present work focuses on characterization of bonding quality of composite plates used in the aerospace industry. The test was performed on 14 samples with various (known) degrees of bonding. The ultrasonic frequencies in the range of 50 kHz to 1.5 MHz were examined to determine the most sensitive frequencies. The modulating vibrations were in much lower frequency range up to 1 kHz. It was found that the average (across the frequency range) normalized modulation level is proportional to the bonding quality and can be used for its quantitative assessment.