Process of magnetized ferromagnetic materials due to the irreversible movement of magnetic domain barkhausen effect and magnetostrictive phenomenon, the formation of magnetic acoustic emission (MAE). Ono et al. Magnetic acoustic emission behavior of ferromagnetic materials are studied, the preliminary theory of magnetic acoustic emission model is established, laid the groundwork for the further research of magnetic acoustic emission. Due to the sensitivity of the magnetic acoustic emission stress, thus opens up a magnetic acoustic emission technology application prospect in terms of stress measurement.
Nondestructive testing residual stress in the most widely with X shooting method, due to its penetration depth is too shallow (only 10 orders of magnitude), only a very thin surface residual stress, and the more expensive equipment, as a result, we determined by MAE 16 mn steel welded plate surface residual stress distribution, the basic consistent results and blind hole method. 
1 experimental conditions
1.1 experiment device
Using acoustic emission instrument is shenyang electronic production by SF - 02 type a, tensile test system in domestic WJ - 10 universal material testing machine. Magnetizing apparatus using homemade u-shaped coil, silicon steel sheet core between them, the magnetization voltage relation with intensity of magnetic field hospital for:
Unit, the unit of magnetization is KA. M
1.2 the experimental process
Will first 50 hz alternating current through the transformer to the magnetizing apparatus coil, coil produces a court's magnetic field, make the magnetization of the sample, to produce magnetic acoustic emission signal, the signal by the resonance frequency of 120 kh PZJ - 5 type piezoelectric crystal appraise is converted to electrical signals, then the gain of 40 db preamplifier amplification junior rusheng launch device, through the filter, andadministrative (ratio adjustable range 1 ~ 59 db), signal processing, after losing to the x-y recorder function (also can be directly after andadministrative output to tape recorder, through signal processing, to print out the results of the analysis), with computer data processing, to determine crack location, area analysis to eliminate noise.
1.3 test materials
16 Mn steel well is divided into: 0.17% C and 0.27% Si, Mn, 1.36% S 0.028%, 0.016% P, the rest of Fe.
Material organization for ferrite, pearlite and annealed MnS inclusions. Tensile samples at the beginning of the thin neck in the middle of the plate sample, size is 232 mmx30mmx4. 5 mm, middle thin neck to 16 mm wide. 

2 the result of the experiment and analysis
2.1 the basic rule of magnetic acoustic emission
When tensile sample, the movement of magnetic domain wall is hampered by the stress and weak, as shown in figure 1, 16 mn steel under rolling condition, the greater the stress c, the lower the amplitude of magnetic acoustic emission. Figure 1, the EN for energy count rate, namely reflect MAE; For displacement in the experiment, using the ringing count rate and stress, the relationship between study of MAE in 16 mn steel fine structure (figure 2), found that there are laws, when H = 7 ka. When ringing count rate of RN and strain curve in a hour in a maximum, and then with the up and down; When H = 3.5 KA, curve in RN linear decline with increase.
In the actual process of stretching, should consider stress within a certain range can also stimulate the acoustic emission, when stress is small, the intensity of magnetic acoustic emission value rise with the increase of stress; Magnetic acoustic emission intensity reaches a certain value, the value decreased with increasing stress.
According to theoretical analysis, we propose a MAE amplitude formula, type of stress; H. As the external magnetic field; Item 5 for the magnetic acoustic emission effects caused by stress excitation; Is stress make domain wall movement or magnetization vector rotates, the volume caused by inelastic strain. With (1) can explain the phenomenon appears in figure 2 

A. when the magnetic field strength is small, no matter what stress values, always have, namely the item 5 contribution is very small, when the magnetic field strength is larger, if stress is lesser, have sparked by the stress at this time of magnetic acoustic emission effect is significant; Only when the stress exceeds a certain value, have MAE elastic stress wave belong to continuous wave function when number, since the power spectrum density analysis, can reflect the change of the stress wave energy, the power spectral density (PSD) is defined as, time frequency of f of MAE signal amplitude square, the average of the square of the average filter output values.
From 16 mn steel stretching from the power line (see figure 3) can also see, early stretch, the G score also appeared a peak, and then decreased, the 1 and figure 2 MAE curve has a good correspondence. 

2.2 stress calibration
Good relationship before measuring stress, must advance calibration curve (see figure 4), the calibration method is: under the constant magnetic field, the selection and test sample structure of the state of the same material as the calibration samples, measured under different stress state is known of the count rate EN magnetic acoustic emission energy, to make the calibration curve of the material.
In actual measurement, it must be to the same selection and calibration of measuring conditions, such as magnetization selected threshold value V voltage, instruments, and gain value, etc.
And then measuring the energy of the unknown sample on some point count rate EN, contrast calibration curve, the residual stress value can look up to that point, also can find the mathematical expression by EN - 0, type, EN. = 400 x (0.38) when the count rate of energy; For (1) the contribution of the sum of the first four; Equivalent to (1) the type of item 5. Due to the EN and (3) type 0 is not a simple linear relationship, iterative method can be used to calculate the value of the (3) and (1) form, which proved the accuracy of (1) type. 

2.3 weldments residual stress test
Weldments is 296 mmx280mm16mn steel, weld the central, magnetization direction parallel to the weld, position of measurement point is magnetizing apparatus, near the weld. Agent point location in the position of 40 mm from the center of magnetizer, and with the direction of magnetization, respectively, and the orientation of the magnetization voltage, measure each point count rate of energy value of acoustic emission, application of calibration curve or formula (3) can be calculated value and the measured value and the stress of the blind hole method to measure the corresponding points listed in table 1. 

Unit for the determination of magnetization curve at the same time can be set stress value plus or minus, we use spectrum analysis, in accordance with the magnetic acoustic emission waves change with field case, determine the main frequency of the stress is negative, if when the plus a magnetic field changes, the magnetic acoustic emission wave basically remain unchanged, the main frequency can be concluded that the sample is in tension stress state; If the main frequency oscillation type change, it is pressure.
Three precision analysis
A. because magnetic field lines closed trend, make the magnetization is limited to local, when smaller or magnetic field coil is more concentrated, might be more accurately measured value. We have to, within the effective area of the magnetic field on the general specimen measurement can reach the level of comparative accuracy.
B. In this paper, the measured value or half quantitative, accuracy is not high, by the blind hole method, the error is less than 20%, with the blind hole method measuring accuracy also has a lot to do.
C. measurement method is simple, no damage to work; Calibration method needs further discussion, the compressive and tensile stresses and acoustic emission intensity calibration curve should be separate. Applied mathematics formula to calculate the stress measurement automation and is worth extending.