Modal acoustic emission analysis of mode-I and mode-II fracture of adhesively-bonded joints.

Acoustic emission (AE) testing has previously been demonstrated to be well suited to detecting failure in adhesively-bonded joints. In this work, the relationship between the fracture-mode of adhesively-bonded specimens and the acoustic wave-modes excited by their failure is investigated. AE instrumented Double-Cantilever-Beam (Mode-I fracture) and Lap-Shear (Mode-II fracture) tests are conducted on similar adhesively-bonded aluminium specimens. Linear source-location is used to identify the source-to-sensor propagation distance of each recorded hit, theoretical dispersion curves are used to identify regions of the signal corresponding to the symmetric and asymmetric wave modes, and peak wavelet-transform coefficients for the wave-modes are compared between the two fracturemodes. It is demonstrated that while both fracture-modes generate AE dominated by the asymmetric mode, the symmetric mode is generally much more significant during Mode-II fracture than Mode-I. While significant scatter and overlap in results prevents the ratio of peak-wavelet transform coefficients from being a robust single classifier for differentiation between fracture-modes in most cases, other modal analysis methods, or integration of this parameter into multi-parameter methods in future work may result in more reliable differentiation. Understanding of the wave-modes excited by the different fracture-modes also has implications for source-location, as identification of the correct modes is critical for selection of suitable wave velocities.