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

TL;DR

This paper critically evaluates the Sideband Peak Count Index (SPC-I) as a nonlinear elastodynamic damage indicator for CFRP plates. By systematically varying damage, excitation amplitude, and measurement/evaluation parameters, the authors show that SPC-I lacks a robust, monotonic relationship with damage and is highly sensitive to procedural choices, challenging claims of its nonlinearity-based diagnostic value. They find no convincing evidence of frequency mixing or reliable reciprocity-based signatures, and argue that many reported SPC-I outcomes may arise from setup or analysis details rather than material nonlinearity. The study emphasizes the need for independent replication, standardized protocols, and data transparency to validate or refute SPC-I’s applicability across materials and applications.

Abstract

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 are reported to be particularly 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 different 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 contradicts 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 are re-evaluated by independent researchers and (b) the experiments already published are repeated or (if sufficient data is availThe paper has been updated for submission to NDT&E.able) also re-evaluated.

Figures (11)

• Figure 1: Ultrasonic C-scans of the plates. The plates are impacted with the energy given in the images. The full scan size is 160 x 110 mm².
• Figure 2: Positioning of the piezoelectric discs (white) on the plates. The annotation Ch 1, Ch 2 etc. refers to the channels of the MAS electronics connected to the corresponding discs. By switching the channels Ch 1 $\Leftrightarrow$ Ch 4 and Ch 2 $\Leftrightarrow$ Ch 3 the ultrasound propagation direction in the plate could be changed keeping the same electronic channels for both the excitation and the measurement. This will suppress the influence of the electronics when changing the propagation direction.
• Figure 3: RC1 signal shape of the excitation signal calculated according \ref{['eq:2']}(left) and its spectrum (right).
• Figure 4: Photo of the experimental arrangement. On the left: the MAS electronics, in the middle: a 20 dB preamplifier, on the right: the plate supported on soft foams for the measurements. The photo shows an early measurement, where only two piezo discs where glued on the plates. Later all plates were completed with five piezodiscs as shown in Fig.\ref{['fig_piezo_positions']}.
• Figure 5: First 150 µs of the time signal (left) and normalized spectrum (right) for one of the measurements of Section 3 (plate 10J, actor 2, receiver 3, amplitude 20 %, amplification 22 dB, total recorded signal length 804 µs). The grid lines in the spectrum at $f_{min} = 10$ kHz and $f_{max} = 700$ kHz indicate the start and the end frequency for the peak detection.