# A Method for Detecting Surface Defects in Railhead by Magnetic Flux Leakage
## 1 Introduction  
In recent years, the development of rail transport is rapid. Heavier duty, higher density, and faster running are current development trend. It proposes higher requirements for the safety and reliability of the rail. A rail in commission is inevitable to encounter rolling, wear, and shock of train wheelsets and harsh environment, so that it may be damaged [1]. A small defect in the surface of the railhead is a common early defect [2].  
Some non-destructive testing techniques such as ultrasonic, eddy current, and MFL, are used in detection of rail defects at practical inspection vehicle speeds [3]. Ultrasonic detection uses a water wheel probe, which is in rolling contact with the rail, and is used on flaw detection trains [4]. Ultrasonic detection technology has reflected clutter on the near-surface, so it can only detect defects inside a rail, making the surface or near-surface of a rail becomes a blind spot for detection, which poses a safety hazard. In addition, there are significant requirements for the flatness, cleanliness, and geometry of the rail surface during the inspection process.  
The eddy current detection method can detect the surface defects in the railhead and achieve rapid detection, and without the need to add any exchange agent, it can achieve routine detection in the harsh detection environment. However, the eddy current detection is limited by the skin effect. Usually, it can only detect surface opening defects, but cannot detect buried defects near the surface of a rail. Variable eddy current detection signals are also susceptible to interference from the railway environment, which imposes more significant requirements on signal processing circuits and algorithms [5].  
MFL is an electromagnetic testing technology developed from magnetic particle testing technology. MFL can detect the surface damage of ferromagnetic materials such as steel pipe [6], and has the advantages of high sensitivity, high speed, low requirement for surface cleanliness, low cost, and simple operation. MFL has been successfully applied to the detection of detection of detects in rails [7]. However, some shortcomings in the MFL detection also exist, such as the poor detection effect of the internal defects of the steel rail, and the detection results are easily affected by the magnetic field excitation and lift-off.  
When MFL technology is used to detect the surface defects, the detection system is often subject to various interferences, so the signal contains a lot of noise [8]. The vertical distance between the magnetic sensor and the workpiece is called lift-off, and the leakage magnetic field varies from lift-off. When the probe travels on the surface of a workpiece, due to vibration and other factors, the lift-off may change, resulting in the signal change of the sensor and interfere the detection. Lift-off interference is affected by amplitude and frequency of vibration, workpiece surface state and other factors, and its spectrum often overlaps with the defects signal. In addition to lift-off interference, detection speed, and random factors also bring interference [9].