A critical note on the sideband peak count-index technique: Failure for nonlinear damage characterization of impacted CFRP plates

It is widely accepted, that nonlinear elastodynamic methods are superior to linear methods in detecting early stages of material deterioration. A number of recently developed methods based on counting spectral peaks are reported to be simple, robust but nevertheless very sensitive to nonlinearities and thus appropriate to indicate early damage. We applied systematically one of the methods, the sideband peak count index (SPC-I), to a series of increasingly damaged carbon fiber reinforced plastic (CFRP) plates. Our data leads to other conclusions. The SPC-I values are influenced by (usually undocumented) variations in the index calculation procedure, which is not acceptable for a robust method. Moreover, the behavior of the index when the ultrasound amplitude is varied excludes material nonlinearity as a direct and significant contributor to the index value. To clarify the apparent contradiction of our results with the previously published statements, it is recommended that (a) our data is re-evaluated by independent researchers and (b) the experiments already published are repeated or (if sufficient data is available) re-evaluated as well. • The sideband peak count-index technique (SPC-I) is systematically studied for carbon fiber reinforced plastic (CFRP) plates with varying degrees of impact damage. • CFRP is a typical fiber reinforced plastic (FRP) studied previously by SPC-I. • Although there are variations in the index, there is no systematic change with increasing damage, as would be expected for a damage index. • Instead, slight changes in the evaluation procedure have a substantial effect on the index and its variation across the various severely damaged plates. • Our data also exclude a substantial influence of nonlinearity to the index. • Our results disagree with conclusions drawn in many previous publications about SPC-I and related methods.