A novel sensor to measure the biased pulse magnetic response in steel stay cable for the detection of surface and internal flaws

•Biased pulse magnetic response in steel stay cable are measured for flaws assessment.•The novel sensor possess the characters of flexibility and low power consumption.•Both the surface notches and internal broken wire flaws in the cable is detectable. A new type of sensor is designed to measure the biased pulse magnetic response in a large-diameter steel stay cable for the assessment of both surface and internal flaws. The sensor deploys two parallel-connected flexible flat coils (FFC) fed with a biased pulse current as the electromagnet for cable magnetization. A tunnel magneto-resistive (TMR) device and two series-connected sensing coils are used to measure the surface magnetic flux leakage (MFL) and the main-flux variation in the defective cable, respectively. A comparative study between the weak and near-saturated magnetization states of a defective cable is performed by finite element simulation tools to investigate the flaw detection ability of both the surface MFL and main-flux measurement methods With the optimized implementation plan and installation locations of the magnetic sensing elements, a prototype of the sensor and a biased pulse current supply is developed for proof-of-concept experiments. The MFL induced by a surface wire notch with the minimal size of 2mm in width and 1mm in depth can be detected by the TMR device with a lift-off distance of 8mm to the notch. To achieve the quantitative evaluation of multiple internal broken wires (MIBW), main-flux measurements are applied to reveal the effect of the MIBW flaws on the estimated magnetic induction intensity of the cable. The approximate linear dependency of the magnetic induction intensity on the loss rate of the cross-section area can be concluded at the rising and falling edges of the pulsed current. The maximum magnetic induction intensity may act as the feature parameter to characterize the extent of the MIBW flaws with the high accuracy. Compared with the traditional yoke magnetizing sensor, the novel sensor utilizing the FFCs improves the detection flexibility and decreases the weight. The application of the biased pulse current not only reduces the power consumption for coil heating during long-term inspection but also allows the sensor to detect both the surface and internal flaws in the cables.