*2.2. Experimental System*

The MMM detection system, shown in Figure 2, consists of three parts. The metal magnetic memory detector—EMS-2003, shown in Figure 2b, is used to store *H*p(*y*) signal. The three-dimensional electrically controlled displacement system, shown in Figure 2c, is used to move the two-channel pen sensor probe with a constant scanning speed and lift-off. The displacement system is made of aluminum alloy, so the influence of material magnetism on *H*p(*y*) signal can be ignored. The two-channel pen sensor probe, shown in Figure 2d, is used to collect *H*p(*y*) signal. For matching the sampling rate of EMS-2003 and the displacement system, the scanning speed of the sensor probe is optimized, and 40 mm/s is optimal. Before collecting *H*p(*y*) signal, the samples are firstly placed horizontally on the platform of the displacement system, and then the *H*p(*y*) signal is collected along detection lines from initial position to ultimate position, as shown in Figure 1. The static tensile test is done by using a CMT2502 testing machine, shown in Figure 2a, the dynamic load error of which is ±1.0%, meeting the requirements of the experiment.

**Figure 2.** Metal magnetic memory detection system for stress evaluation. (**a**) CMT2502 testing machine. (**b**) EMS-2003. (**c**) Three-dimensional electrically controlled displacement system. (**d**) Two-channel pen sensor probe.

Lots of experiments pointed out that lift-off of the sensor probe is an important factor for the amplitude of the *H*p(*y*) signal, so the best optimal lift-off is discussed in this study. For the experiment, the sample with the same width and different depths of crack was prepared by using the same technology, which was employed in Figure 1. Increasing the lift-off gradually, the *H*p(*y*) signals were collected and shown in Figure 3. It can be seen that when lift-off is lower than 5.0 mm, the mutation of *H*p(*y*) signal, at the location of the groove, is clear, and the amplitude of the *H*p(*y*) signal is high. As lift-off increased, that mutation becomes smaller and smaller, and the amplitude of the *H*p(*y*) signal

also becomes lower. After lift-off reaches 15.0 mm, and as lift-off increases further, that mutation disappears and that amplitude becomes lower. With comprehensive consideration, 1.0 mm is seen as the best optimal lift-off of sensor probe in this study.

**Figure 3.** *H*p(*y*) signal corresponding to different lift-offs of sensor probe.
