Consistent detection method for internal cracks in uneven wall thickness regions of thick-walled steel pipes based on impedance gradient analysis

Crack detection in uneven wall thickness regions remains challenging. This paper proposes a consistent detection method based on impedance gradient analysis. The approach employs a transformer detection model to analyse the relationship between internal magnetic permeability and detection coil impedance. Simulation studies reveal that impedance detection signals exhibit distinct waveform characteristics in thinned and thickened regions. Further analysis of the impedance signal variation rate relative to the probe displacement defines an impedance gradient ratio P. Results show that P<0 in thinned regions while P>0 in thickened regions, enabling accurate identification of wall thickness conditions. Peak-valley value versus depth fitting curve intercept compensation is then applied to defect signals under different degrees of wall thickness unevenness, achieving consistent crack detection in both thinned and thickened regions. Experimental results demonstrate that this method achieves consistent detection of 0.5 mm internal cracks in both thinned and thickened regions of 10-mm-thick walls with 5%–20% thickness variations, effectively eliminating thickness unevenness interference.