Zhiyang Deng

The complex laminate architecture and electrical anisotropy of carbon fiber-reinforced polymer (CFRP) composites make defect detection in the filament-wound overwrap of hydrogen storage cylinders highly challenging. This study presents a CFRP defect detection approach based on alternating current field measurement. First, a theoretical model was established to analyze the electromagnetic propagation characteristics of the carbon fiber winding layers. Following this, the finite element method was employed to simulate the eddy current (EC) distribution and to clarify the effect of electrical anisotropy on signal characteristics. Second, an enhanced probe capable of effectively guiding EC flow along the thickness direction was designed and optimized. By varying the defect depth, characteristic signals were extracted, and a quantitative relationship between signal parameters and defect depth was established. Finally, an experimental system for carbon fiber composites was constructed, and validation experiments were carried out. The results demonstrate that the proposed method can effectively detect surface and buried cracks in 4 mm-thick multidirectional CFRP laminates, achieving a depth resolution of 0.5 mm. Least-squares regression confirmed a strong linear correlation between signal features and defect depth. The method enables detection and quantitative evaluation of crack defects in filament-wound carbon fiber layers, providing a theoretical basis for the safe operation of carbon fiber-reinforced pressure equipment such as hydrogen storage cylinders.

In pipeline systems such as oil transportation, defects on both the internal and external surfaces of thick-walled pipes can lead to distinct negative effects. It is crucial to detect and differentiate between these two types of defects simultaneously. Traditional methods for differentiation are generally only applicable to thin-walled ferromagnetic components and are not effective for thick-walled components. This study proposes a new method based on the slope ratio of stepped-lift-off signals to distinguish between internal and external defects. The method utilizes the principle of permeability perturbation and differentiates internal and external defects by comparing the attenuation rates of the secondary magnetic fields. Initially, a finite element simulation model is employed to explore the influence of probe parameters under direct current magnetization conditions on permeability perturbation signals, as well as how the signals vary with different lift-off values. This allows for the development of a threshold selection method for distinguishing internal and external defects. Subsequently, a stepped-lift-off multi-coil probe was fabricated and tested in experiments designed to detect internal and external defects in plates of varying thicknesses and dimensions. Experimental results demonstrate that by appropriately setting the slope ratio threshold, the method can effectively identify internal and external defects on steel plates of varying thicknesses and sizes. Additionally, the influence of coil size on the method’s precision is further investigated, offering valuable insights for enhancing the overall performance of the method and guiding future research.

Electromagnetic acoustic transducer (EMAT) typically employ rigid permanent magnets (PMs) or flexible magnetizing coils (FMCs) energized by high pulse currents to provide a bias magnetic field. However, the PMs results in a bulkier transducer, whereas the FMCs requires accurate timing control of the magnetizing current. Based on traditional static bias magnetostrictive shear horizontal (SH) wave theory, this paper designs a novel dual alternating-current (AC) magnetostrictive SH wave transducer and investigates the design parameters influencing its operational efficiency. The working mechanism of the dual AC magnetostrictive SH wave transducer is derived, followed by the development of a finite element model for the SH wave transducer under AC magnetization. The effects of parameters such as magnetizing current amplitude, frequency, excitation current amplitude, and wire length on the excitation wave amplitude are examined. It is observed that with increasing magnetization frequency, the wave amplitude first increases rapidly, then slightly decreases, and finally stabilizes, approaching the direct current (DC) magnetization state. An experimental system is subsequently built, and the experimental results regarding the effect of magnetization frequency on the excitation wave amplitude are found to be in good agreement with the simulation results, validating the feasibility of the dual AC magnetostrictive method for exciting SH waves. Further defect detection experiments are conducted on steel plates with hole and crack defects of varying thickness. The results show that when the magnetization frequency and current are set to 10 kHz and 30 A, respectively, only the proposed transducer can effectively detect a 3 mm hole defect. For a 20 mm × 1 mm × 2 mm crack defect, the signal-to-noise ratio (SNR) of the new transducer increases by more than three times. When the plate thickness is increased to 3 mm, the SNR for both transducers decreases, but the SNR of the new transducer increases by more than 2.5 times compared to the permanent magnetized transducer.

Magnetic permeability perturbation testing (MPPT) based on DC magnetization is effective for deep-buried defect detection. However, ferromagnetic materials with large thicknesses require larger magnetization devices, which hinders the lightweight design of in-service inspection equipment. This paper proposes a magnetic permeability perturbation testing based on pulse magnetization (PMPPT) for buried defects. Under the influence of pulse magnetization, information about the internal defects is mainly transferred to the surface layer of the material through the magnetic permeability perturbation (MPP), and then detected by sensors. The magnetic permeability perturbation within the material under pulsed magnetization is analyzed, and the feasibility of this method is verified by simulations and experiments. Experimental results demonstrate effective detection of defects located at a depth of 1 mm on the backside of a specimen with a thickness of 12 mm. Furthermore, the effects of pulse magnetization parameters, including frequency, duty cycle, and amplitude, on the detected signals are investigated. The results indicate that increasing the pulse current amplitude enhances the detection signal, while a lower pulse square wave frequency is more effective for thicker samples. By adjusting the amplitude and duty cycle of the pulse current, the detection signal can be improved compared to DC magnetization. PMPPT can contribute to reducing the weight of in-service inspection equipment and holds significant potential for engineering applications.