Abstract: Metal magnetic memory technology is a new nondestructive testing method. To explore a new diagnostic method based on magnetic gradient testing, a series of tensile tests on low carbon steel bars are carried out in geomagnetic field environment. A self-made special detector is used to measure the magnetic gradient above the material surface. The results show that the relation between the magnetic gradient and the stress varies with different measurement methods. The magnetic gradient has no specific relation to tensile stress if the sample is placed on the tensile testing machine and measured. If the sample is taken off from the machine and measured, the magnetic gradient shows a linear relation with the maximum stress that the sample was subjected to recently. It offers a new method for nondestructive measurement of stress by measuring the magnetic gradient on the ferromagnetic component surface. In addition, magnetic memory signal will gradually decrease after the sample is taken off and laid aside, which shows that the detection of stress concentration by using metal magnetic memory technique is time dependent.

Key words: Magnetic gradient, Magnetic memory testing, Neck contraction, Stress concentration