Research On Evolution Of Piezomagnetic Field Of Deep Sea Pipeline Steel Under Cyclic Tensile Load

Deep sea pipelines always suffer from sophisticated loading conditions and they are inevitably affected by cyclic loadings during the long-term services,which easily induces fatigue failure.Magnetic detection method based on piezomagnetic effect can characterize the stress state and damage situation of ferromagnetic components by piezomagnetic signals,and make early diagnosis of defects formation and damages occurrence in ferromagnetic components.This is a promising non-destructive testing technology.Based on the cyclic tension tests of deep sea pipeline steels,the evolution of piezomagnetic field of deep sea pipeline steels has been quantitatively studied in this paper.Combining with the explanation of the relevant experimental phenomena from the micro-mechanism level,this paper also put forward a stress evaluation method based on the magneto-mechanical hysteresis curves,which provided theoretical support for the quantitative application of piezomagnetic effect in the field of ocean engineering detection.Firstly,the equal-amplitude cyclic tension tests were carried out and the effect of stress loading history on the piezomagnetic field was studied.Results showed that the small stress history had limited influence on the piezomagnetic field under cyclic loading,while the large stress history would assimilate the magneto-mechanical hysteresis curves of the subsequent cyclic loading process with a smaller stress amplitude.Secondly,variable-amplitude cyclic tension tests were carried out to investigate the influence of measuring distance of piezomagnetic field on the evolution of piezomagnetic field in deep sea pipeline steels.It was found that in a certain range,the enlargement of measuring distance would increase the value of piezomagnetic field and decrease the amplitude of piezomagnetic field.This phenomenon could be explained by the equivalent method of finite-length electrified solenoid.Finally,the corresponding relationship between stress and piezomagnetic field in the cyclic loading process with variable amplitudes after the pre-loading with large stress amplitude was studied.A new stress evaluation method based on piezomagnetic effect was established with utilization of the quantitative relationship between the slope and the second derivative of the magneto-mechanical hysteresis curve and cyclic stress.The predicted results were in good agreement with the experimental results,which proved the accuracy and validity of this stress evaluation method.