CN108362768B - Stress non-contact weak magnetic detection method - Google Patents

Stress non-contact weak magnetic detection method Download PDF

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CN108362768B
CN108362768B CN201810170269.2A CN201810170269A CN108362768B CN 108362768 B CN108362768 B CN 108362768B CN 201810170269 A CN201810170269 A CN 201810170269A CN 108362768 B CN108362768 B CN 108362768B
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CN108362768A (en
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刘斌
马泽宇
王缔
何璐瑶
张贺
任建
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Shenyang University of Technology
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    • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
    • G01N27/00—Investigating or analysing materials by the use of electric, electrochemical, or magnetic means
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Abstract

A stress non-contact weak magnetic detection method belongs to the technical field of non-contact detection of a ferromagnetic metal member stress concentration area, and particularly relates to a stress non-contact weak magnetic detection method. The invention provides a stress non-contact weak magnetic detection method. The invention comprises the following steps: step 1): establishing a magneto-mechanical coupling model; step 2): establishing a J-A stress magnetization model; step 3): establishing a magnetic signal space scattered field model; also included are validation experiments.

Description

Stress non-contact weak magnetic detection method
Technical Field
The invention belongs to the technical field of non-contact detection of a stress concentration area of a ferromagnetic metal member, and particularly relates to a stress non-contact weak magnetic detection method.
Background
In the working process of a large ferromagnetic metal component in a complex environment, fatigue damage with different degrees can be generated, the oil and gas pipeline causes stress damage due to the construction, corrosion, external force action and other reasons in the operation process, and the defects of deformation, corrosion, cracks and the like are generated, so that the pipeline condition is gradually worsened, thereby burying important potential safety hazards and threatening the national property and the life safety of people.
The metal magnetic memory detection method is a practical and available detection method for the stress deformation state of a structure as a novel nondestructive detection method in the 21 st century, and can accurately detect and early warn stress damage and corrosion of a pipeline. So far, the metal magnetic memory detection technology has been developed to the present that non-contact detection can be carried out, and many international experts have devoted themselves to research on the generation mechanism of the magnetic memory signal and have achieved certain results, but the research on the quantitative calculation lack system for carrying out non-contact detection on the stress concentration degree through the strength of the magnetic memory signal limits the application of the magnetic memory method in the non-contact detection engineering technical field in the weak magnetic environment.
Disclosure of Invention
The invention aims at the problems and provides a stress non-contact weak magnetic detection method.
In order to achieve the purpose, the invention adopts the following technical scheme, and the invention comprises the following steps:
step 1): establishing a magnetomechanical coupling model
When no external force acts, the magnetic induction intensity of the ferromagnetic material is B1，
μ0μ1＝B1/H (1)
In the formula, mu0Is a vacuum permeability, mu1The magnetic material has relative magnetic conductivity under no stress, and the external magnetic field intensity is H; when external force acts, the magnetic induction intensity of the ferromagnetic material is B2The energy increment of the ferromagnet under the action of the external force is as follows:
Figure BDA0001585612980000021
wherein, W1、W2The energy of the ferromagnetic material before and after being subjected to external force is respectively; when the ferromagnetic body is acted by external force, the crystal is correspondingly phase-changed, and the external stress action increases the stress energy in the ferromagnetic crystal to generate a magnetoelastic effect; when the ferromagnetic material only has spontaneous magnetization deformation, the anisotropy of the generalized magnetocrystalline of the ferromagnetic body is as follows:
FK＝Fk(αi)+Fela(Aik0)+Fms(αiAik0)
Figure BDA0001585612980000022
wherein alpha is1、α2、α3Is the spin vector direction cosine; a. theik0Is a spontaneous deformation tensor; k0、K1、K2Is the magnetocrystalline anisotropy constant; fk(αi) Ideal magnetocrystalline anisotropy without considering deformation; fela(Aik0) Is pure elastic energy; fms(αiAik0) Is magnetic elastic energy;
when the ferromagnet is acted by external force, the crystal is correspondingly deformed, and at the moment, the crystal not only has magnetoelastic energy caused by spontaneous deformation, but also has magnetic stress energy caused by deformation generated by the external force;
the total deformation tensor of the crystal is expressed as:
Figure BDA0001585612980000023
Figure BDA0001585612980000024
is the deformation tensor under the stress;
the total free energy is:
F(Aik)＝FK(αiAik)-∑Aikσik
＝Fk(αi)+Fela(Aik)+Fms(αiAik)-∑Aikσik (5)
expressed in equilibrium as:
Figure BDA0001585612980000025
Figure BDA0001585612980000031
various coefficients of magnetic crystals in different crystal axis directions; c2、C3Modulus of elasticity along the crystal axis; sigmaik＝σγiγkAnd gamma is the direction complementary rotation of the external stress intensity of the external stress.
Wherein λ is100、λ111Magnetic field of easy and hard axes respectivelyCoefficient of expansion; from the equations (3) and (6), the stress anisotropy performance expression is:
Figure BDA0001585612980000032
in isotropic magnetostrictive materials: lambda [ alpha ]100＝λ111When the magnetization direction is the same as the stress, theta is 0, and the amount of change of the magnetoelastic energy of the ferromagnetic body under the action of the external force is as follows:
Figure BDA0001585612980000033
and sigma is stress, and lambda is the magnetostriction coefficient of the material.
The law of conservation of energy is known as follows: the variation of magnetization energy per unit volume | Δ W | is equal to the variation of magnetic stress energy caused by external stress | Δ W |1I.e. that
ΔW＝-ΔW1 (9)
Figure BDA0001585612980000034
Magnetostriction coefficient of ferromagnetic material
Figure BDA0001585612980000035
Substituting formula (10) to obtain:
Figure BDA0001585612980000036
in the formula BmSaturation induction, λ, for ferromagnetic materialsmFor a saturated magnetostriction coefficient, the relative permeability of the ferromagnetic material under stress solved by equation (11) is:
Figure BDA0001585612980000037
in the formula BmSaturation induction, λ, for ferromagnetic materialsmIs a saturated magnetostriction coefficient, mu0Is a vacuum permeability, mu1The initial permeability of the material, and σ is the applied stress.
Step 2): establishing J-A stress magnetization model
A magnetic field is applied to the ferromagnetic object, and the object is magnetized and shows certain magnetism;
the following relationship exists between the magnetization M and the applied magnetic field H:
M＝χH (13)
wherein χ is the magnetic susceptibility of the material;
under the combined action of an external magnetic field H and an external stress sigma, the total energy of the system is A, the magnetization effect of the external stress is equivalent to the effect of an external magnetic field and is expressed by a magnetostriction coefficient lambda, then,
Figure BDA0001585612980000041
wherein, alpha is a coupling parameter constant; coefficient of magnetostriction
Figure BDA0001585612980000042
Is taken as lambda ═ gamma1M2+γ2M4；
The total energy of the system is differentiated to the magnetization intensity to obtain a corresponding equivalent magnetic field, and H isσAn equivalent magnetic field for the stress action, then
Figure BDA0001585612980000043
The magnetization consists of reversible and irreversible magnetizations, i.e.:
M＝Mirr+Mrev (16)
wherein M isirrIs irreversible magnetization, MrevFor reversible magnetization, the two sides of the above formula are simultaneously derived from the stress energy to obtain:
Figure BDA0001585612980000044
wherein M isan＝Ms[coth(He/a-a/He)]For no hysteresis magnetization, He＝H+αM+3σdλ/2μ0And dM simplifying the derivative relation of the magnetization to the stress energy to obtain a relation model of the magnetization and the stress:
Figure BDA0001585612980000051
eliminating the irreversible magnetization to obtain a relation model of the magnetization and the stress
Figure BDA0001585612980000052
M is the magnetization, ManFor hysteresis-free magnetization, σ is the stress, c is the reversible coefficient, and ε is a constant related to the stress properties of the material.
Obtaining the magnetic susceptibility of the material of the stress action region by the formulas (19) and (13);
step 3): establishing a magnetic signal space scattered field model
It can be known from ferromagnetism that when the magnetic medium is not saturated, the cumulative surface density of magnetic charges on the end face of the magnetization region is:
Qm＝χmμ0SH
ρ＝Qm/S＝χmμ0H (20)
wherein: qmThe magnetic charge excited at the end face at the initial moment of magnetization, in Wb/m2；
H-magnetization field strength, unit A/m;
χmthe magnetic susceptibility of the material, being a dimensionless pure number, is a function of H;
μ0vacuum magnetic permeability, equal to 4 π × 10-7H/m；
S-end surface area, unit m2；
ρ——Magnetic charge density at the initial instant of magnetization, unit Wb/m2；
Cracks are initiated and developed in a strip-shaped area in the test piece, the damage is uniform, and the magnetization characteristic of the fatigue damage concentrated area is equivalent to the magnetic charge uniformly distributed on the end face of the area; establishing a three-dimensional magnetic dipole model by taking the surface center of the ferromagnetic material stress concentration region as an origin;
the magnetization direction is parallel to the X axis, and only the magnetic charge effect on the side wall parallel to the YOZ plane is considered; magnetic charge areal density of ρxDefining the coordinate of a three-dimensional field point as P (x, y, z) and the coordinate of a magnetic charge point as (x)m，ym，zm) (ii) a The magnetic charge surface infinitesimal dymdzmThe scattered magnetic field strength generated at the point in space P is:
Figure BDA0001585612980000061
let the wall position be xmAnd 0 to-dy and-dz to dz, and performing binary integration on the magnetic field to obtain three coordinate axis direction components of a local magnetic field H', wherein the three coordinate axis direction components are respectively as follows:
Figure BDA0001585612980000062
Figure BDA0001585612980000063
Figure BDA0001585612980000064
solving equations (22) - (24) yields:
Figure BDA0001585612980000065
Figure BDA0001585612980000066
Figure BDA0001585612980000067
the local magnetic field H 'is a synthetic magnetic field under the combined action of the positive magnetic charge band and the negative magnetic charge band, so that H' is:
Figure BDA0001585612980000068
Figure BDA0001585612980000071
in the formula, ex,ey,ezUnit vectors in three directions of X, Y, Z coordinate axes;
from the formula (28), the intensity distribution and the transfer characteristic of the distorted magnetic field at different spatial positions are calculated by modeling and vector superposition of the magnetic charges in the finite long stress concentration region in the orthogonal magnetization field.
As a preferred solution, the invention further comprises a step of calculating the spatial distribution portion of the scattered field, comprising the steps of:
1) mathematical model calculation
Setting the magnetic permeability mu of the steel plate, the width 2b (2Dx), the depth H (Dy) and the length (2Dz) of the rectangular stress concentration area, the external excitation magnetic field along the X-axis direction and the strength H0Calculating the path along the X axis;
and calculating the magnetic signal component transfer characteristics of the stress region or the defect part of the ferromagnetic material in the applied orthogonal magnetization field through a two-dimensional magnetic dipole model.
2) Finite element model solution
Establishing a magnetic memory effect finite element model in a three-dimensional magnetic equilibrium field:
establishing a three-dimensional Cartesian rectangular coordinate system by taking the center of the surface of the steel plate as an origin; a stress concentration area is arranged on the surface of the middle part of the model, and the depth direction is along the-Y axis;
distributing the magnetic vectors of the external magnetic field along the X direction, adding a vertical boundary condition to the three-dimensional air field model, calculating a boundary freedom constant according to the required external magnetic field intensity and the space size, and setting constraint to be applied to two side surfaces parallel to YOZ to obtain a balanced magnetic field H0;
looking up a magnetic field simulation calculation result near a stress concentration area, and locally amplifying a magnetic field intensity cloud chart of an XOY section;
taking a distance from the left side to the right side of the stress concentration area as a center respectively to serve as a calculation path, taking a distance from the surface of the steel plate as a change range of the lift-off value, mapping and calculating the magnetic field intensity, and analyzing the spatial distribution condition of magnetic signals;
as another preferred scheme, the method takes different lift-off values along the surface of the test piece, and respectively calculates the tangential component Hx and the normal component Hy of the magnetic signal;
checking whether the tangential component of the magnetic signal at the stress area center (namely, z is 0) of the orthogonal magnetization component has a maximum value, and detecting whether a signal curve is symmetrical about a y axis along an X axis path; checking whether the amplitude is reduced and changed along with the increase of the lift-off value at a certain distance; checking whether the propagation distance is increased and the attenuation amplitude is reduced or not, and checking whether the stress concentration area is difficult to identify or not as the propagation distance is increased;
checking whether normal components of the magnetic field distributed along the X axis pass through zero points or not, whether positive and negative wave peaks appear or not and whether the curve is symmetrical about an original point or not at the center of the stress area;
checking whether the amplitude is reduced or not and the variation value is reduced or not along with the increase of the lift-off value; the propagation distance is increased, whether the peak-to-peak value change amplitude is reduced or not and whether the stress area magnetic signal curve tends to be smooth or not are judged; calculating the magnetic signal component transfer characteristics of the stress region or the defect part of the ferromagnetic material in the applied orthogonal magnetization field; checking whether the H-X curves of the horizontal and normal components are in clear monotone decreasing change along with the increase of the lift-off value;
if both are true, then there is a stress concentration condition.
As another preferred aspect, the stress concentration region is considered difficult to identify when the amplitude of the region signal is less than 5% of the amplitude of the material surface signal as the propagation distance increases.
As another preferred scheme, the invention checks whether the amplitude is reduced and changes value as the lift-off value is increased from 1mm to 6 mm.
As another preferred scheme, the invention checks the magnetic field simulation calculation result near the stress concentration region, locally amplifies a magnetic field intensity cloud chart of the XOY cross section, checks whether the stress concentration region is magnetized, whether an obvious leakage magnetic field exists on the surface of the steel plate and in the air, whether the magnetic field is attenuated in the surface propagation process of the steel plate, and whether a magnetic signal obtains the maximum absolute value in the stress concentration region; the cloud chart shows whether the sign of the normal component on the surface of the steel plate changes or not and whether the normal component has a maximum negative value and a maximum positive value or not; whether it is affected by the magnetization direction, the tangential component being greater than the normal component; if both are true, then there is a stress concentration condition.
As another preferred scheme, the method takes a distance from the stress concentration region to the left and right as a calculation path, takes a distance from the surface of the steel plate as a change range of the lift-off value, maps and calculates the magnetic field intensity, and analyzes the spatial distribution condition of the magnetic signal;
checking the calculation result of the tangential component Hx of the magnetic memory signal to determine whether a peak appears in the stress concentration region; at a certain distance from the surface, checking the maximum value of the magnetic signal Hx, increasing the lift-off value to a certain distance, and checking the amplitude of a tangential signal curve; checking the attenuation percentage of each certain distance of the signal amplitude by taking the signal amplitude as a standard when the certain distance of the surface is taken as a lift-off value; the tangential signal of the magnetic field in the stress concentration area is transmitted in the air, and with the increase of the transmission distance, whether the amplitude is reduced or not, whether the curve is flat or not and whether the signal is reduced fastest or not on the surface of the material are judged;
checking the calculation result of the normal component Hy of the magnetic memory signal, whether the magnetic signal in the stress concentration area crosses the zero point or not, and whether a positive peak and a negative peak exist or not; increasing the lift-off value, and checking the change value of the magnetic signal normal component Hy peak value in the stress concentration area; checking the descending percentage of each fixed distance of the curve peak value by taking the initial peak value as a standard; the normal component of the magnetic memory signal in the ferromagnetic material stress concentration region is transmitted in the air, whether the curve degree is gradually reduced or not and whether the signal attenuation speed is reduced or not along with the increase of the distance or not are judged;
checking whether the normal component peak value of the curve corresponding to a certain distance on the surface is smaller than the tangential component peak value, the propagation distance is the same, and whether the tangential component amplitude of the stress concentration area is attenuated more greatly;
whether the amplitude of a magnetic signal generated by the stress concentration region is reduced or not in the non-ferromagnetic medium, whether the curve fluctuation amplitude is reduced or not, whether the signal characteristic of the stress concentration region is weakened or not, whether the attenuation on the near surface of the material is faster or not and whether the stress concentration region can be obviously identified from a signal propagation curve chart or not;
if both are true, then there is a stress concentration condition.
As another preferred aspect, the stress concentration zone of the present invention is considered to be clearly identifiable from the signal propagation plot when the signal amplitude of the stress concentration zone is greater than 5% of the material surface.
As another preferred scheme, at the position of 10mm distance from the surface, the maximum value of the magnetic signal Hx is checked, the lift-off value is increased to 22mm, and the tangential signal curve amplitude is checked; checking the attenuation percentage of each 2mm of the signal amplitude by taking the signal amplitude as a standard when the lift-off value is 10 mm;
the lift-off value is increased from 10mm to 22mm, the change value of the magnetic signal normal component Hy peak value in the stress concentration area is checked, and the percentage reduction of the curve peak value every 2mm is checked by taking the lift-off value of 10mm as a standard;
it is checked whether the 10mm corresponding curve normal component peak is smaller than the tangential component peak.
As another preferable scheme, the magnetic permeability of the steel plate is 500H/m, the width 2b (2Dx) of the rectangular stress concentration area is 2mm, the depth H (Dy) is 2mm, the length (2Dz) is 16mm, the external excitation magnetic field is along the X-axis direction, and the strength H0 is 40Am-1Calculating the path along the X axis to be-20 mm; different lift values are taken along the surface of the test piece, and y is changed from 1mm to 6 mm.
As another preferred scheme, the steel plate is made of X80 type steel, has the magnetic permeability of 300H/m and the size of 400mm multiplied by 50mm multiplied by 18mm, and a three-dimensional Cartesian rectangular coordinate system is established by taking the surface center of the steel plate as an origin; the middle surface of the model is provided with a stress concentration area with the size of 16mm 2mm, and the depth direction is along the-Y axis.
As another preferable scheme, the magnetic field intensity of the equalizing magnetic field is 40Am-1。
In another preferred embodiment, the left and right of the stress concentration area are respectively taken as 50mm as a calculation path, the distance between the calculation path and the surface of the steel plate is 10 mm-20 mm as a change range (interval is 2mm), the total 7 paths (7 dotted lines at the upper part of the graph 4) are mapped and calculated on the magnetic field intensity, and the spatial distribution condition of the magnetic signals is analyzed.
As another preferred scheme, the invention also includes the verification experiment, this experiment uses X80 type plate steel as the experimental test piece, make a crackle in advance on the surface of middle part of the steel, the length direction of the crackle is perpendicular to length direction of the test piece, when the test piece receives the external force to act on, there is serious stress concentration in the tip of the crackle, place the test piece along the direction of geomagnetic field;
a detection probe of the metal magnetic memory detection device sweeps through a crack tip stress concentration area along the length direction of the test piece, and a tangential component and a normal component of a magnetic field are detected; changing the thickness of a cushion layer (namely the distance between a probe and the surface of a test piece), sequentially scanning the variable magnetic field of the stress concentration area, and performing repeatability verification;
looking up the scanning results of the magnetic field intensity tangential component Hx and the normal component Hy in the stress concentration area of the X80 steel plate;
for the tangential component Hx scanning result, checking whether the magnetic signal curve has a maximum value in the stress concentration section; when the surface of the steel plate has no lift-off, observing the maximum value of the tangential component, increasing the lift-off value to a certain distance, and observing the decrease value of the maximum value of the tangential component;
for the tangential component Hy scanning result, observing whether the positive and negative changes of the magnetic signal curve in the stress concentration section occur or not and whether positive and negative wave peaks appear or not; when the surface of the steel plate is not lifted away, the normal component curve wave peak value is observed; when apart from steel sheet surface certain distance in the air, inspect the decline value of crest, inspect whether the curve tends to the gentleness, whether still have the crest trough, whether the trend of change is the same with the simulation result.
As another preferred embodiment, the size of the X80 type plate-shaped steel material is 450mm by 50mm by 18.6 mm.
As another preferred scheme, a crack of 20mm multiplied by 0.5m multiplied by 1mm is made on the surface of the middle part of the steel part in advance.
As another preferred scheme, TSC-2M-8 type metal magnetic memory detection equipment is adopted by the metal magnetic memory detection equipment.
Secondly, the lift-off value is increased to 30mm, and the tangential component maximum value drop value is observed;
the value of the peak of the wave is reduced when the distance between the steel plate and the air is 30 mm.
In addition, the scanning range of the tangential component Hx and the normal component Hy is as follows: the thickness of the steel plate is-50 mm in the X-axis direction (namely 50mm is taken along the left and right directions of the X direction by taking the center of the stress concentration area as an original point), and the thickness of the steel plate is 0-30 mm in the Y-axis direction (namely the distance range of 30mm above the surface of the steel plate), so that obvious and complete signal change can be obtained.
The invention has the beneficial effects.
The invention establishes a scattered field mathematical model through a magneto mechanical coupling model and a J-A stress magnetization model, and provides a stress non-contact weak magnetic detection quantitative algorithm for the early stage of the crack damage of the ferromagnetic material in an orthogonal field; and the stress concentration degree detection of the crack initiation area is realized.
Drawings
The invention is further described with reference to the following figures and detailed description. The scope of the invention is not limited to the following expressions.
FIG. 1 is a schematic diagram of a three-dimensional surface magnetic charge model according to the present invention.
FIG. 2 is a diagram of the transfer characteristics of the tangential component of the mathematical dimensional model of the present invention.
FIG. 3 is a diagram of normal component transfer characteristics of a mathematical dimensional model of the present invention.
Fig. 4 is a schematic view of a stress concentrating ferromagnetic material according to the present invention.
FIG. 5 is a diagram of the uniform applied cross-magnetization field of the present invention.
FIG. 6 is a cloud view of the distribution of the magnetization in the tangential space of the present invention.
FIG. 7 is a cloud view of the normal spatial magnetization distribution of the present invention.
FIG. 8 is a diagram of the magnetic signal tangential component distribution for the stress concentration zone of the present invention.
FIG. 9 is a diagram of the magnetic signal tangential component distribution for the stress concentration zone of the present invention.
FIG. 10 is an enlarged view of the X80 test specimen and the crack portion.
FIG. 11 is a diagram of the memory signal tangential component in the air layer of the present invention.
FIG. 12 is a diagram of normal components of magnetic memory signals in an air layer according to the present invention.
Detailed Description
The invention comprises the following steps:
step 1): establishing a magnetomechanical coupling model
When no external force acts, the magnetic induction intensity of the ferromagnetic material is B1，
μ0μ1＝B1/H (1)
In the formula, mu0Is a vacuum permeability, mu1The magnetic material has relative magnetic conductivity under no stress, and the external magnetic field intensity is H; when external force acts, the magnetic induction intensity of the ferromagnetic material is B2The energy increment of the ferromagnet under the action of the external force is as follows:
Figure BDA0001585612980000121
wherein, W1、W2Respectively the energy of the ferromagnetic material before and after being subjected to the external force.
When the ferromagnetic body is acted by external force, the crystal is correspondingly phase-changed, and the external stress action increases the stress energy in the ferromagnetic crystal to generate a magnetoelastic effect; when the ferromagnetic material only has spontaneous magnetization deformation, the anisotropy of the generalized magnetocrystalline of the ferromagnetic body is as follows:
FK＝Fk(αi)+Fela(Aik0)+Fms(αiAik0)
Figure BDA0001585612980000131
wherein alpha is1、α2、α3Is the spin vector direction cosine; a. theik0Is a spontaneous deformation tensor; k0、K1、K2Is the magnetocrystalline anisotropy constant; fk(αi) Ideal magnetocrystalline anisotropy without considering deformation; fela(Aik0) Is pure elastic energy; fms(αiAik0) Is magnetic elastic energy;
when the ferromagnet is acted by external force, the crystal is correspondingly deformed, and at the moment, the crystal not only has magnetoelastic energy caused by spontaneous deformation, but also has magnetic stress energy caused by deformation generated by the external force;
the total deformation tensor of the crystal is expressed as:
Figure BDA0001585612980000132
Figure BDA0001585612980000133
is the deformation tensor under the stress;
the total free energy is:
F(Aik)＝FK(αiAik)-∑Aikσik
＝Fk(αi)+Fela(Aik)+Fms(αiAik)-∑Aikσik (5)
expressed in equilibrium as:
Figure BDA0001585612980000134
Figure BDA0001585612980000135
various coefficients of magnetic crystals in different crystal axis directions; c2、C3Modulus of elasticity along the crystal axis; sigmaik＝σγiγkAnd gamma is the direction complementary rotation of the external stress intensity of the external stress.
Wherein λ is100、λ111Magnetism having easy and hard axes respectivelyCoefficient of magnetostriction; from the equations (3) and (6), the stress anisotropy performance expression is:
Figure BDA0001585612980000141
in isotropic magnetostrictive materials: lambda [ alpha ]100＝λ111When the magnetization direction is the same as the stress, theta is 0, and the amount of change of the magnetoelastic energy of the ferromagnetic body under the action of the external force is as follows:
Figure BDA0001585612980000142
and sigma is stress, and lambda is the magnetostriction coefficient of the material.
The law of conservation of energy is known as follows: the variation of magnetization energy per unit volume | Δ W | is equal to the variation of magnetic stress energy caused by external stress | Δ W |1I.e. that
ΔW＝-ΔW1 (9)
Figure BDA0001585612980000143
Magnetostriction coefficient of ferromagnetic material
Figure BDA0001585612980000144
Substituting formula (10) to obtain:
Figure BDA0001585612980000145
in the formula BmSaturation induction, λ, for ferromagnetic materialsmFor a saturated magnetostriction coefficient, the relative permeability of the ferromagnetic material under stress solved by equation (11) is:
Figure BDA0001585612980000146
in the formula BmSaturation induction, λ, for ferromagnetic materialsmIs a saturated magnetostriction coefficient, mu0Is a vacuum permeability, mu1The initial permeability of the material, and σ is the applied stress.
Step 2): establishing J-A stress magnetization model
A magnetic field is applied to the ferromagnetic object, and the object is magnetized and shows certain magnetism;
the following relationship exists between the magnetization M and the applied magnetic field H:
M＝χH (13)
wherein χ is the magnetic susceptibility of the material;
under the combined action of an external magnetic field H and an external stress sigma, the total energy of the system is A, the magnetization effect of the external stress is equivalent to the effect of an external magnetic field and is expressed by a magnetostriction coefficient lambda, then,
Figure BDA0001585612980000151
wherein, alpha is a coupling parameter constant; coefficient of magnetostriction
Figure BDA0001585612980000152
Is taken as lambda ═ gamma1M2+γ2M4；
The total energy of the system is differentiated to the magnetization intensity to obtain a corresponding equivalent magnetic field, and H isσAn equivalent magnetic field for the stress action, then
Figure BDA0001585612980000153
The magnetization consists of reversible and irreversible magnetizations, i.e.:
M＝Mirr+Mrev (16)
wherein M isirrIs irreversible magnetization, MrevFor reversible magnetization, the two sides of the above formula are simultaneously derived from the stress energy to obtain:
Figure BDA0001585612980000154
wherein M isan＝Ms[coth(He/a-a/He)]For no hysteresis magnetization, He＝H+αM+3σdλ/2μ0And dM simplifying the derivative relation of the magnetization to the stress energy to obtain a relation model of the magnetization and the stress:
Figure BDA0001585612980000155
eliminating the irreversible magnetization to obtain a relation model of the magnetization and the stress
Figure BDA0001585612980000156
Obtaining the magnetic susceptibility of the material of the stress action region by the formulas (19) and (13);
step 3): establishing a magnetic signal space scattered field model
It can be known from ferromagnetism that when the magnetic medium is not saturated, the cumulative surface density of magnetic charges on the end face of the magnetization region is:
Qm＝χmμ0SH
ρ＝Qm/S＝χmμ0H (20)
wherein: qmThe magnetic charge excited at the end face at the initial moment of magnetization, in Wb/m2；
H-magnetization field strength, unit A/m;
χmthe magnetic susceptibility of the material, being a dimensionless pure number, is a function of H;
μ0vacuum magnetic permeability, equal to 4 π × 10-7H/m；
S-end surface area, unit m2；
Rho-magnetic charge density at the initial instant of magnetization, unit Wb/m2；
Cracks are initiated and developed in a strip-shaped area in the test piece, the damage is uniform, and the magnetization characteristic of the fatigue damage concentrated area is equivalent to the magnetic charge uniformly distributed on the end face of the area; establishing a three-dimensional magnetic dipole model by taking the surface center of the ferromagnetic material stress concentration region as an origin, as shown in figure 1;
the magnetization direction is parallel to the X axis, and only the magnetic charge effect on the side wall parallel to the YOZ plane is considered; magnetic charge areal density of ρxDefining the coordinate of a three-dimensional field point as P (x, y, z) and the coordinate of a magnetic charge point as (x)m，ym，zm) (ii) a The magnetic charge surface infinitesimal dymdzmThe scattered magnetic field strength generated at the point in space P is:
Figure BDA0001585612980000161
let the wall position be xmAnd 0 to-dy and-dz to dz, and performing binary integration on the magnetic field to obtain three coordinate axis direction components of a local magnetic field H', wherein the three coordinate axis direction components are respectively as follows:
Figure BDA0001585612980000162
Figure BDA0001585612980000171
Figure BDA0001585612980000172
solving equations (22) - (24) yields:
Figure BDA0001585612980000173
Figure BDA0001585612980000174
Figure BDA0001585612980000175
the local magnetic field H 'is a synthetic magnetic field under the combined action of the positive magnetic charge band and the negative magnetic charge band, so that H' is:
Figure BDA0001585612980000176
in the formula, ex,ey,ezUnit vectors in three directions of X, Y, Z coordinate axes;
the magnetic charge modeling and vector superposition of a finite long stress concentration region in the orthogonal magnetization field are used for calculating the intensity distribution and the transfer characteristic of the distorted magnetic field at different spatial positions;
and (3) establishing a function formula of the stress intensity, the detection lift-off value and the magnetic signal intensity of a crack initiation stress concentration region in the orthogonal magnetization field by combining the steps 2) and 3).
The invention also includes a portion for calculating the spatial distribution of the scattered field, comprising the steps of:
1) mathematical model calculation
The magnetic signal in the stress concentration area has the same characteristics as the magnetic signal near the rectangular defect area, and the magnetic signal has the same characteristics with the longer defect (D)z>5Dx) The three-dimensional magnetic field is infinitely close to the action field of the two-dimensional magnetic charge model, and the two-dimensional magnetic charge model is used for calculating and explaining the magnetic signal propagation rule of the three-dimensional finite long stress concentration region.
Setting the magnetic permeability mu of the steel plate, the width 2b (2Dx), the depth H (Dy) and the length (2Dz) of the rectangular stress concentration area, the external excitation magnetic field along the X-axis direction and the strength H0Calculating the path along the X axis; taking different lift-off values along the surface of the test piece, and respectively calculating a tangential component Hx and a normal component Hy of the magnetic signal; as shown in fig. 2;
as can be seen from fig. 2, the tangential component of the magnetic signal at the center of the stress region of the orthogonal magnetization component (i.e., z ═ 0) has a maximum value, and the detection signal curve along the X-axis path is symmetrical about the y-axis; the amplitude is increased from 111Am along with the increase of the lift-off value from 1mm to 6mm-1Reduced to 8Am-1(ii) a The propagation distance is increased, the attenuation amplitude is reduced along with the propagation distance, and the signals in the stress area are not obvious;
at the center of the stress region, the normal component of the magnetic field distributed along the X axis crosses zero, and positive and negative peaks appear, and the curve is symmetrical about the origin, as shown in FIG. 3; the amplitude increases from 103Am with increasing lift-off value-1Reduced to 8Am-1(ii) a The propagation distance is increased, the change amplitude of the peak value is reduced, and the magnetic signal curve of the stress area tends to be smooth;
and calculating the magnetic signal component transfer characteristics of the stress region or the defect part of the ferromagnetic material in the applied orthogonal magnetization field through a two-dimensional magnetic dipole model. The H-X curves of the horizontal component and the normal component are in clear monotone decreasing change along with the increase of the lift-off value;
2) finite element model solution
The method establishes a magnetic memory effect finite element model in a three-dimensional magnetic equilibrium field, and is used for analyzing the propagation characteristics of magnetic memory signals;
establishing a three-dimensional Cartesian rectangular coordinate system by taking the center of the surface of the steel plate as an origin; a stress concentration area is arranged on the surface of the middle part of the model, and the depth direction is along the-Y axis;
the magnetic vectors of the external magnetic field are distributed along the X direction, a vertical boundary condition is added to the three-dimensional air field model, a boundary degree of freedom constant is calculated according to the required external magnetic field intensity and the space size, constraint is arranged and applied to two side surfaces parallel to YOZ, and a balanced magnetic field H0 is obtained as shown in FIG. 5;
in order to clearly view the magnetic field simulation calculation result near the stress concentration area, a magnetic field intensity cloud chart of an XOY section is enlarged locally, as shown in FIG. 6; the stress concentration area is magnetized, an obvious leakage magnetic field exists on the surface of the steel plate and in the air, the magnetic field is attenuated in the transmission process of the surface of the steel plate, and the magnetic signal obtains the maximum absolute value in the stress concentration area; FIG. 7 is a cloud chart showing the sign change of the normal component at the surface of the steel plate, having a maximum negative value and a maximum positive value; influenced by the magnetization direction, the tangential component is greater than the normal component;
taking a distance from the left side to the right side of the stress concentration area as a center respectively to serve as a calculation path, taking a distance from the surface of the steel plate as a change range of the lift-off value, mapping and calculating the magnetic field intensity, and analyzing the spatial distribution condition of magnetic signals;
magnetic markerThe calculation result of the tangential component Hx of the memory signal is shown in FIG. 8, and a peak appears in a stress concentration region; the maximum value of the magnetic signal Hx is 113.96Am at the surface 10mm distance-1The lift-off value is increased to 22mm, and the amplitudes of the tangential signal curves are 69.39, 29.28, 18.55, 11.13, 3.67 and-1.77 Am respectively-1(ii) a The signal amplitude is attenuated by 39.1%, 35.2%, 9.4%, 6.5% and 4.8% every 2mm by taking the signal amplitude at the lift-off value of 10mm as a standard; the tangential signal of the magnetic field in the stress concentration area is transmitted in the air, the amplitude is reduced along with the increase of the transmission distance, the curve is flat and straight, and the signal is reduced fastest on the surface of the material;
the calculation result of the normal component Hy of the magnetic memory signal is shown in FIG. 9, and the zero crossing point of the magnetic signal in the stress concentration region has a positive peak and a negative peak; the lift-off value is increased from 10mm to 22mm, and the peak values of the normal components Hy of the magnetic signals in the stress concentration areas are 41.8, 28.52, 23.13, 22.36, 19.65, 16.93 and 12.42Am respectively-1Taking an initial peak value (lifted off 10mm) as a standard, the peak value of the curve is decreased by 31.7%, 12.9%, 1.85%, 6.5% and 3% in sequence every 2 mm; the normal component of the magnetic memory signal in the ferromagnetic material stress concentration region is transmitted in the air, the curve degree is gradually reduced, and the signal attenuation speed is reduced along with the increase of the distance;
the normal component peak value of the curve corresponding to 10mm is smaller than the tangential component peak value, the propagation distance is the same, and the tangential component amplitude attenuation of the stress concentration area is larger;
the magnetic signal that stress concentration district produced propagates amplitude in non-ferromagnetic medium and reduces, and curve fluctuation amplitude descends, and the signal characteristic in stress concentration district weakens, and near the surface decay is very fast at the material, and stress concentration district still can discern.
The magnetic permeability of the steel plate is 500H/m, the width 2b (2Dx) of the rectangular stress concentration area is 2mm, the depth H (Dy) is 2mm, the length (2Dz) is 16mm, the strength H0 is 40Am along the X-axis direction of an external excitation magnetic field-1Calculating the path along the X axis to be-20 mm; different lift values are taken along the surface of the test piece, and y is changed from 1mm to 6 mm.
The steel plate is made of X80 section steel, has magnetic conductivity of 300H/m and size of 400mm multiplied by 50mm multiplied by 18mm, and a three-dimensional Cartesian rectangular coordinate system is established by taking the surface center of the steel plate as an origin; the middle surface of the model is provided with a stress concentration area with the size of 16mm 2mm, and the depth direction is along the-Y axis.
The magnetic field intensity of the equalizing magnetic field is 40Am-1。
And taking 50mm from the left and right of the stress concentration area as a center respectively to serve as calculation paths, taking 10 mm-20 mm from the surface of the steel plate as a change range (interval of 2mm) of a lift-off value, mapping and calculating the magnetic field intensity, and analyzing the spatial distribution condition of magnetic signals.
The invention also includes a verification experiment, which adopts X80 plate steel as an experiment test piece, and a crack is pre-made on the middle surface of the steel piece, as shown in figure 10. The length direction of the crack is vertical to the length direction of the test piece, when the test piece is acted by external force, the tip of the crack has serious stress concentration, and the test piece is placed along the direction of the geomagnetic field;
a detection probe of the metal magnetic memory detection device sweeps through a crack tip stress concentration area along the length direction of the test piece, and a tangential component and a normal component of a magnetic field are detected; changing the thickness of a cushion layer (namely the distance between a probe and the surface of a test piece), sequentially scanning the variable magnetic field of the stress concentration area, and performing repeatability verification;
and (5) scanning results of the tangential component Hx and the normal component Hy of the magnetic field strength in the stress concentration area of the X80 steel plate are checked.
As shown in fig. 11, the magnetic signal curve exhibits a maximum in the stress concentration section. When the surface of the steel plate is not lifted off, the maximum value of the tangential component is 26.38Am-1The lift-off value is increased to 30mm, and the tangential component is reduced to-4.12 Am-1。
The normal component Hy of the magnetic field strength in the stress concentration region is shown in fig. 12, and the sign of the magnetic signal curve in the stress concentration region changes, so that positive and negative peaks appear. When the surface of the steel plate is not lifted off, the wave crest of the normal component curve is 9.5Am-1When the air lift-off is 30mm, the wave crest is reduced to-2.62 Am-1And the curve tends to be flat, but the wave crest and the wave trough still exist, the variation trend is the same as the simulation result, and the correctness is verified.
The simulation (which can adopt ANSYS simulation software) unit type selects a magnetic vector solid 96, three cubes with the sizes of 16mm 2mm, 400mm 50mm 18mm and 2000 x 2000 are respectively built by taking an original point as a center, Boolean operation enables the three cubes to be overlapped, three material attributes of MURX500, MGXX 10000, MURX300 and MURX1 are respectively built corresponding to the three cubes in material model, corresponding distribution is carried out in mesh, constraint 0 is applied to the right side of the third cube after grid division, constraint 160000 is applied to the left side, and a checking result is solved.
Under certain stress concentration degree, namely, the magnetic permeability of the stress concentration area is certain, the magnetic signal intensity and the lift-off value have a one-to-one corresponding relation, the magnetic permeability of the stress intensity and the magnetic permeability of the load area have a corresponding relation, and the damage condition of the stress concentration area before crack initiation is obtained by combining the detection signal intensity and the lift-off value. Because the material is subjected to the magnetostrictive effect caused by an external load, a stress concentration area is generated, the magnetic conductivity and the magnetization intensity of the material are changed, the stress, the magnetic conductivity and the magnetization intensity have a one-to-one correspondence relation according to a magnetic coupling model, and then the corresponding magnetic field intensity can be calculated by a magnetic dipole model formula, namely, the stress and the magnetic signal correspond to each other one by one, so that the stress concentration degree of the material can be calculated according to the magnetic field intensity, when the stress is greater than the yield limit, the material is subjected to plastic deformation, cracks are further initiated, and potential safety hazards are generated.
The X80 type plate steel size is 450mm 50mm 18.6 mm.
A crack of 20mm multiplied by 0.5m multiplied by 1mm is made on the surface of the middle part of the steel part in advance.
The metal magnetic memory detection equipment adopts TSC-2M-8 type metal magnetic memory detection equipment.
It should be understood that the detailed description of the present invention is only for illustrating the present invention and is not limited by the technical solutions described in the embodiments of the present invention, and those skilled in the art should understand that the present invention can be modified or substituted equally to achieve the same technical effects; as long as the use requirements are met, the method is within the protection scope of the invention.

Claims (3)

1. A stress non-contact weak magnetic detection method is characterized by comprising the following steps:
step 1): establishing a magnetomechanical coupling model:
when no external force acts, the magnetic induction intensity of the ferromagnetic material is B1，
μ0μ1＝B1/H (1)
In the formula, mu0Is a vacuum permeability, mu1The magnetic material has relative magnetic conductivity under no stress, and the external magnetic field intensity is H; when external force acts, the magnetic induction intensity of the ferromagnetic material is B2The energy increment of the ferromagnet under the action of the external force is as follows:
Figure FDA0003137840740000011
wherein, W1、W2The energy of the ferromagnetic material before and after being subjected to external force is respectively; when the ferromagnetic body is acted by external force, the crystal is correspondingly phase-changed, and the external stress action increases the stress energy in the ferromagnetic crystal to generate a magnetoelastic effect; when the ferromagnetic material only has spontaneous magnetization deformation, the anisotropy of the generalized magnetocrystalline of the ferromagnetic body is as follows:
FK＝Fk(αi)+Fela(Aik0)+Fms(αiAik0)
Figure FDA0003137840740000012
wherein alpha is1、α2、α3Is the spin vector direction cosine; a. theik0Is a spontaneous deformation tensor; k0、K1、K2Is the magnetocrystalline anisotropy constant; fk(αi) Ideal magnetocrystalline anisotropy without considering deformation; fela(Aik0) Is pure elastic energy; fms(αiAik0) Is magnetic elastic energy;
when the ferromagnet is acted by external force, the crystal is correspondingly deformed, and at the moment, the crystal not only has magnetoelastic energy caused by spontaneous deformation, but also has magnetic stress energy caused by deformation generated by the external force;
the total deformation tensor of the crystal is expressed as:
Figure FDA0003137840740000013
Figure FDA0003137840740000014
is the deformation tensor under the stress;
the total free energy is:
F(Aik)＝FK(αiAik)-∑Aikσik
＝Fk(αi)+Fela(Aik)+Fms(αiAik)-∑Aikσik (5)
expressed in equilibrium as:
Figure FDA0003137840740000021
Figure FDA0003137840740000022
various coefficients of magnetic crystals in different crystal axis directions; c2、C3Modulus of elasticity along the crystal axis; sigmaik＝σγiγkGamma is the direction of the external stress intensity;
wherein λ is100、λ111Magnetostriction coefficients of an easy magnetization axis and a hard magnetization axis respectively; from the equations (3) and (6), the stress anisotropy performance expression is:
Figure FDA0003137840740000023
in isotropic magnetostrictive materials: lambda [ alpha ]100＝λ111λ when direction of magnetizationWhen the stress is in the same direction, theta is 0, and the amount of change of the magnetoelastic energy of the ferromagnetic body under the action of the external force is as follows:
Figure FDA0003137840740000024
sigma is external stress, and lambda is the magnetostriction coefficient of the material;
the law of conservation of energy is known as follows: the variation of magnetization energy per unit volume | Δ W | is equal to the variation of magnetic stress energy caused by external stress | Δ W |1I, i.e.:
ΔW＝-ΔW1 (9)
Figure FDA0003137840740000025
magnetostriction coefficient of ferromagnetic material
Figure FDA0003137840740000026
Substituting formula (10) to obtain:
Figure FDA0003137840740000031
in the formula BmSaturation induction, λ, for ferromagnetic materialsmFor a saturated magnetostriction coefficient, the relative permeability of the ferromagnetic material under stress solved by equation (11) is:
Figure FDA0003137840740000032
in the formula BmSaturation induction, λ, for ferromagnetic materialsmIs a saturated magnetostriction coefficient, mu0Is a vacuum permeability, mu1The initial magnetic conductivity of the material is shown, and sigma is an external stress;
step 2): J-A stress magnetization model:
a magnetic field is applied to the ferromagnetic object, and the object is magnetized and shows certain magnetism;
the following relationship exists between the magnetization M and the applied magnetic field H:
M＝χH (13)
wherein χ is the magnetic susceptibility of the material;
under the combined action of an external magnetic field H and an external stress sigma, if the total energy of the system is A, the magnetization effect of the external stress can be equivalent to the effect of an external magnetic field and can be expressed by a magnetostriction coefficient lambda, then,
Figure FDA0003137840740000033
wherein, alpha is a coupling parameter constant; coefficient of magnetostriction
Figure FDA0003137840740000034
Is taken as lambda ═ gamma1M2+γ2M4；
The total energy of the system is differentiated according to the magnetization intensity, so that a corresponding equivalent magnetic field can be obtained, and H isσAn equivalent magnetic field for the stress action, then
Figure FDA0003137840740000035
The magnetization consists of reversible and irreversible magnetizations, i.e.:
M＝Mirr+Mrev (16)
wherein M isirrIs irreversible magnetization, MrevFor reversible magnetization, the two sides of the above formula are simultaneously derived from the stress energy to obtain:
Figure FDA0003137840740000041
wherein M isan＝Ms[coth(He/a-a/He)]Is magnetization without hysteresisDegree He＝H+αM+3σdλ/2μ0And dM simplifying the derivative relation of the magnetization to the stress energy to obtain a relation model of the magnetization and the stress:
Figure FDA0003137840740000042
eliminating the irreversible magnetization to obtain a relation model of the magnetization and the stress
Figure FDA0003137840740000043
M is the magnetization, ManThe magnetization intensity is hysteresis-free, sigma is external stress, c is a reversible coefficient, and epsilon is a constant related to the stress property of the material;
obtaining the magnetic susceptibility of the material of the stress action region by the formulas (19) and (13);
step 3): establishing a magnetic signal space scattering field model:
it can be known from ferromagnetism that when the magnetic medium is not saturated, the cumulative surface density of magnetic charges on the end face of the magnetization region is:
Qm＝χmμ0SH
ρ＝Qm/S＝χmμ0H (20)
wherein: qmThe magnetic charge excited at the end face at the initial moment of magnetization, in Wb/m2；
H-magnetization field strength, unit A/m;
χmthe magnetic susceptibility of the material, being a dimensionless pure number, is a function of H;
μ0vacuum magnetic permeability, equal to 4 π × 10-7H/m；
S-end surface area, unit m2；
Rho-magnetic charge density at the initial instant of magnetization, unit Wb/m2；
Cracks are initiated and developed in a strip-shaped area in the test piece, the damage is uniform, and the magnetization characteristic of the fatigue damage concentrated area is equivalent to the magnetic charge uniformly distributed on the end face of the area; establishing a three-dimensional magnetic dipole model by taking the surface center of the ferromagnetic material stress concentration region as an origin;
since the magnetization direction is parallel to the X-axis, only the effect of the magnetic charges on the side walls parallel to the YOZ plane is considered; magnetic charge areal density of ρxDefining the coordinate of a three-dimensional field point as P (x, y, z) and the coordinate of a magnetic charge point as (x)m，ym，zm) (ii) a The magnetic charge surface infinitesimal dymdzmThe scattered magnetic field strength generated at the point in space P is:
Figure FDA0003137840740000051
let the wall position be xmAnd 0 to-dy and-dz to dz, and performing binary integration on the magnetic field to obtain three coordinate axis direction components of a local magnetic field H', wherein the three coordinate axis direction components are respectively as follows:
Figure FDA0003137840740000052
Figure FDA0003137840740000053
Figure FDA0003137840740000054
solving equations (22) - (24) yields:
Figure FDA0003137840740000055
Figure FDA0003137840740000056
Figure FDA0003137840740000057
Figure FDA0003137840740000061
the local magnetic field H 'is a synthetic magnetic field under the combined action of the positive magnetic charge band and the negative magnetic charge band, so that H' is:
Figure FDA0003137840740000062
in the formula, ex,ey,ezUnit vectors in three directions of X, Y, Z coordinate axes;
from the formula (28), the intensity distribution and the transfer characteristic of the distorted magnetic field at different spatial positions are calculated by modeling and vector superposition of the magnetic charges of the finite long stress concentration region in the orthogonal magnetization field;
the method also comprises a verification experiment, wherein an X80 plate-shaped steel material is adopted as an experiment test piece, a crack is pre-manufactured on the middle surface of the steel piece, the length direction of the crack is vertical to the length direction of the test piece, when the test piece is acted by an external force, the tip of the crack has serious stress concentration, and the test piece is placed along the direction of the geomagnetic field;
a detection probe of the metal magnetic memory detection device sweeps through a crack tip stress concentration area along the length direction of the test piece, and a tangential component and a normal component of a magnetic field are detected; changing the thickness of a cushion layer, namely the distance between a probe and the surface of a test piece, sequentially scanning the variable magnetic field of the stress concentration area, and verifying the repeatability;
looking up the scanning results of the magnetic field intensity tangential component Hx and the normal component Hy in the stress concentration area of the X80 steel plate;
for the tangential component Hx scanning result, checking whether the magnetic signal curve has a maximum value in the stress concentration section; when the surface of the steel plate has no lift-off, observing the maximum value of the tangential component, increasing the lift-off value to a certain distance, and observing the decrease value of the maximum value of the tangential component;
for the tangential component Hy scanning result, observing whether the positive and negative changes of the magnetic signal curve in the stress concentration section occur or not and whether positive and negative wave peaks appear or not; when the surface of the steel plate is not lifted away, the normal component curve wave peak value is observed; when apart from steel sheet surface certain distance in the air, inspect the decline value of crest, inspect whether the curve tends to the gentleness, whether still have the crest trough, whether the trend of change is the same with the simulation result.
2. The method of claim 1, wherein the dimension of the X80 steel plate is 450mm by 50mm by 18.6 mm.
3. The method for detecting the non-contact weak magnetism of the stress according to claim 1, characterized in that a crack of 20mm x 0.5m x 1mm is made on the surface of the middle part of the steel part in advance.
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