NDE for Compression Loaded Composite Structures Using a Nonlinear Ultrasound Method

Abstract

The work looked at determining a nonlinear ultrasound acoustic moment method for structures that are under high load and thus damage detection is more difficult. Previous work using acoustic moments have focused mainly on the health of adhesive joints and bonds. This work focuses on developing a methodology that uses both the linear acoustic moment and nonlinear acoustic moments of a given signal to help determine the damage present in a compression loaded specimen. A rig was used that allowed the adjustment of the pressure between two flat composite plates, while the contact between the two plates was used to simulate a composite joint with a kissing bond defect. Two piezoelectric sensors were used to send and receive the input and output signals. The acoustic moment was based on the integral of the Power Spectral Density (PSD) function and tests were conducted in three steps: (i) initially the 1st, 2nd and 3rd harmonics of the output PSD function were located, (ii) then they were isolated by filtering out other noise and responses thus leaving the peaks of the respective harmonics, and (iii) finally the respective harmonics were evaluated using integration, giving the respective acoustic moments. The results showed that nonlinear acoustic moments (2nd and 3rd harmonic responses) responded more adversely to increases in load than that of the linear acoustic moment (1st harmonic). This was clearly seen as the rate of decrease in the above described compression loaded system for the nonlinear acoustic moments as load increased, was larger than that of the linear acoustic moment. The work provides a solid platform for the proposed acoustic moment method suggested, and shows that the relationship between the linear response and the conversion of energy to generate the nonlinear responses can provide useful information about damage in compression loaded systems.