Validating sideband peak count-index (SPC-I) technique as a hybrid linear/nonlinear ultrasonic technique through...

The sideband peak count-index (SPC-I) technique has been widely used to assess material nonlinearity. However, its ability to capture true nonlinear responses of materials and structures has not convinced some researchers yet. This is because SPC-I variations can be generated by both linear scattering effects and nonlinear responses. Since changes in linear scattering modes can also influence SPC-I values, it is sometimes difficult to determine whether the observed variations stem from material nonlinearity or purely from linear scatterings. This work investigates whether SPC-I is sensitive to the variation in material nonlinearity. It also gives clear guidelines on how the upper and lower bounds of the moving threshold line should be set for capturing the linear and nonlinear variations separately, or together as a hybrid technique. Numerical simulations are conducted using an isotropic nonlinear elastic material (a hyperelastic material) modeled as a "Murnaghan material" characterized by second- and third-order elastic constants (SOECs and TOECs). Linear elastic (classic theory of linear elasticity) and nonlinear elastic plates are excited at different amplitudes for comparison and the amplification factor (AF) for these excitations is defined as the ratio of the input ultrasonic excitation to its baseline value (the first input excitation value). Numerical results demonstrate that for different AF values, SPC-I computed from normalized spectral plots remains constant for linear materials but varies for nonlinear materials. This trend is also observed for nonlinear elastoplastic materials in another sets of numerical modeling. Experimental measurements on aluminum and carbon fiber composite plates confirm the findings of the numerical modeling based observations that SPC-I can capture thenonlinear responses, reinforcing its significance in material characterization and damage detection. Finally, this capability of SPC-I is used for solving a real-world structural health monitoring (SHM) problem - monitoring bolt-loosening - where SPC-I responses and resonance frequency shifts are shown to follow similar trends. Both techniques are capable of tracking evolving contact conditions from bolt loosening while the SPC-I technique is easier to implement.