An ultrasonic detection method based on the HT integrated with improved VMD for CFRP laminates with delamination defects

•Improve the WOA by modifying the position update mechanism.•Optimize the parameters of the VMD using the improved WOA.•Establish a new ultrasonic detection method based on the HT integrated with the improved VMD.•Analyze the ultrasonic propagation characteristics of CFRP laminates with delamination defects. The Hilbert Transform (HT) time–frequency analysis is an effective way to extract the defect information of ultrasonic echo signals, while the accuracy is insufficient when analyzing broadband signals. The Variational Mode Decomposition (VMD) can decompose broadband signals into multiple narrowband signals, but has difficulties in the selection of modal decomposition number and penalty factor. Hence, the HT integrated with an improved VMD is developed to enhance the accuracy of ultrasonic detection in defective Carbon Fiber Reinforced Polymer (CFRP) laminates. The Whale Optimization Algorithm (WOA) is improved using the Tent chaotic mapping, differential evolution algorithm, and Lévy flight strategy to update the initial position of the whale population, the non-optimal position of whales, and the local optimal position of whales, respectively. The modal decomposition number and penalty factor are optimized by the improved WOA to minimize the envelope entropy of each Intrinsic Mode Function (IMF). The VMD with optimized parameters decomposes the ultrasonic echo experiment signals of CFRP laminates without and with delamination, then the optimal IMFs are selected based on correlation coefficients, and subsequently the HT is performed to extract the relevant envelope information. The results show that the improved VMD significantly filters out noise components, the envelope signals obtained by the HT integrated with an improved VMD are smoother and have no peak oscillations, and the ultrasonic propagation characteristics clearly explain the existence of cluster echoes with lower amplitudes, the increase of defect echo amplitudes, and the decrease of bottom echo amplitudes.