Suppression of Dry-Coupled Rubber Layer Interference in Ultrasonic Thickness Measurement: A Comparative Study of Empirical Mode Decomposition Variants

In dry-coupled ultrasonic thickness measurement, thick rubber layers introduce high-amplitude parasitic echoes that obscure defect signals and degrade thickness accuracy. Existing methods struggle to resolve overlap-ping echoes under variable coupling conditions and non-stationary noise. This study proposes a novel dual-criterion framework integrating energy contribution and statistical impulsivity metrics to isolate specimen re-flections from coupling-layer interference. By decomposing A-scan signals into Intrinsic Mode Functions (IMFs), the framework employs energy contribution thresholds (>85%) and kurtosis indices (>3) to autonomously select IMFs containing valid specimen echoes. Hybrid time-frequency thresholding further suppresses interference through amplitude filtering and spectral focusing. Experimental results demonstrate the framework’s robustness, achieving 92.3% thickness accuracy for 5 mm steel specimens with 5 mm rubber coupling, outperforming conventional methods by up to 18.7%. The dual-criterion approach reduces operator dependency by 37% and maintains ΔT < 0.03 mm under surface roughness up to 6.3 μm, offering a practical solution for industrial nondestructive testing with thick dry-coupled interfaces.