Research On Quantitative Detection Of Hidden Cracks Based On Pulsed Alternating Current Field Measurement Technology

According to the detection requirements of corrosion fatigue crack under the protecting coating and internal defect of multi-structure,a pulsed alternating current field measurement(PACFM)method and system are present based on ACFM technology to achieve classification and quantification of hidden crack using low frequency pulse excitation technique.By theoretical analysis,numerical modeling and experiments,the transient response signal of hidden crack is analyzed and a new feature is extracted based on principal component anslysis.Classification algorithm and quantification algorithm is proposed based on the new feature.The PACFM test is set up,and the classification and quantification experiments of surface cracks and hidden cracks are carried out.The main works are summarized as follows:(1)Theoretical and simulation models of PACFMAccording to the theory of classical electromagnetism,the theoretical model of PACFM is present based on ACFM using low frequency pulse excitation technique.The finite element method(FEM)is employed to analyze the longitudinal distribution of electromagnetic field and the propagation of transient field.The distorted electromagnetic field around the hidden crack is mainly studied.(2)Classification and quantification analysisBased on the simulation model of PACFM,the transient response signals of surface cracks and hidden cracks are picked up and analyzed in time domain,frequency domain and time-frequency domain.A new feature is extracted by the feature analysis and extraction technology.The classification algorithm and quantification algorithm of hidden depth are defined based on the new feature.(3)Development of PACFM system and hidden crack detection experimentsThe pulse excitation generator is designed and the signal conditioning circuit is optimum designed.The software based on the classification algorithm and quantification algorithm is developed.A complete system integrated with probe,signal conditioning module and software is set up and the crack test experiments are carried out.The results show that the PACFM system can detect the hidden crack of aluminium with the hidden depth of 5mm,and it can classify surface crack and hidden crack correctly as well as quantify the hidden depth.