Fundamental principle of magnetic memory testing

1.Metal magnetic memory method is a nondestructive method,its fundamental principle is record and analyze leakage magnetic distribution condition in components and device stress concentration area,magnetic leakage field reflects magnetization intensity has  inreversible change toward the work load on the principle stress direction,and accessories and welding joint in its manufacturing and cooling in earth magnetic field, its metal organization and manufacturing technology heredity。Metal magnetic memory method in testing ,using natural magnetization intensity,and components and device metal practical transform and metal organization change,express results in the form of magnetic memory

2.Magnetic memory effect

The main source of Mechanical parts and metal components have damage ,is microcosmic and macroscopic mechanical stress,in stress concentration area,development of corrosion 、tired、 and creep process is the most furious。Mechanical stress and ferromagnetic material and magnetization phenomenon and residual magentism condition have direct connection，under the condition of magnetic effect,magnetic permeability decreased at flaw position,workpiece surface leakage magnetic field increased，the property of ferromagnetic material is called magnetic mechanical effect。Magnetic mechanical effect makes ferromagnetic metal workpiece surface magnetic field enhanced ,this enhanced magnetic field memorizes flaw of parts or stress concentration position,this is “ magnetic memory” effect.

3.Testing principle

As we all know,ferromagnetic component processing cooling ,hardening process ,the places of cooling hardening is more furious can form change,at the section that Hp=0 ,will produce fast speed approach,and cause slight crack--,form latter components damage origination point or stress concentration line,when stress concentration line is  prependicular to external load acting force,trigger component breakage must happened in stress concentration line，if stress concentration line axis distributes or stress concentration  is low,the position of change and component  breakage is always misalign 。Nevertheless,along with load acting force is increasing,and will happen stress concentration line shifts to another position。Therefore instantly disclose on-service metal components stress concentration line  is very important。

Engineering parts produce slight crack due to lack of tired.creep deformation  will lead to exist stress concentration line at flaw position，researches show that::ferromagnetic metal part exists magnetic mechanical effect,its surface distribution of magnetic field has certain relationship with stress load,so it can by magnetic field distribution condition of testing parts surface indirectly diagnose on part flaw or stress concentration position。

Ferromagnetic part flaw or stress concentration area magnetic field tangential component Hp (X) has max value,normal component Hp(y) changes symbol and has zero value，like graph 1,in practical application ,we complete test on parts by testing normal component

Metal component  stress concentration area is source of generating and developing flaw。Metal component is processed by metal material,the generation of its internal stress concentration area is depended on its manufacture technology，in process ,metal material is always being fusion 、Forging、 heat treatment and other processing technology,when metal material much more than 768℃,its remained magnetism is disappeared,but follows,metal material is gradually cooling in earth magnetic field.under magnetic mechanical effect,at the same time of producing crystallogrpphic,generating magnetic organization, cooling process generates metal component is microstructure,including particle shape and size 、particle homogeneity that forming metal,have or not impurity or flaw,all will be expressed by the form of magnetic memory。 Forming “inheritance”property of this parts

Under condition of existing earth magnetic field，flaw and impurity in metal component will appear magnetic domain fixed point and have leakage magnetic field。In flaw concentrated area and internal stress concentrated area,the magnetic permeability of metal is the lowest,and generating max leakage magnetic field on the surface。In stress concentrated  area，the magnetic field tangential component Hp(x) has max value,but normal component Hp(y) changes symbol and have zero value。Degree of Stress concentration can according to Hp(x)change to judge，this value is corresponding to  component surface domain 。The direction of Closure magnetic domain is along with axis that hard to magnetize,so Hp(x)value express anisotropy energy that decided by organization with inhomogeneity。

Metal part stress concentration line is not only closely to “inheritance ”property of part, and but also have relationship with on-service load and direction and size of acting force。Experiment proof,component suffered from external force in process of operation,due to external load acting force has different direction and position，strengthen and weaken stress concentration line of component, if stress concentration line of component is mutually perpendicular with load acting force，then can decrease stress concentration line intensity，when intensity decreases to a certain limitation the component will be damaged

Theory and experiment are show that damage of metal component and its inborn “inheritance” properties is related to acquired on-service working load ，in the process of generating and developing flaw,stress concentration is source,is early stage e­xpression of component damage。To eliminate device accident at sprout state，it is necessary to constant test stress concentration line of detecting parts。  IDEAP0201  intelligent metal magnetic memory diagnostic apparatus is a new type of nondestructive testing system that based on above principlesOn-service component of device,due to relation of structure inheritance  and load ,magnetic memory is expressed by accumulation。In service,the size and direction of components load acting force will cause metal magnetization changed on magnitude and direction。 Magnetic memory diagnostic techniques is according to this principle to be developed。Experiment proof that under geomagnetic function ,flaw of on-service ferromagnetic workpiece and slag inclusion,will produce domain unification phenomenon,and appear leakage magnetic field on it。At flaw position or internal stress relative concentrated place,metal magnetic permeability is min,its magnetic field tangential component has max value，but normal component changes symbol，has zero value。Scanning test to workpiece surface leakage magnetic field normal component,then make sure stress concentration area，consequently indirectly judge possibility of the ferromagnetic workpiece exist flaw。 To more directly illuminate components failure testing ability between magnetic memory diagnostic technique and traditional NDT testing technique,use graph 1 to express。Graph 2 shows developing process of metal microstructure discontinuity change,its horizontal coordinates shows time of workpiece usage,vertical coordinates shows along with continuation of time  the degree of flaw is increasing。According to MMM method testing principle ,generally said,if normal testing method can detect more than or equal to T0 flaw,but MMM not only detect more than or equal to To flaw,but also find flaw that smaller that To.so MMM method is a nondestructive testing technique that realizing metal failure early stage diagnostic。

Advantages

1. No requirement on preparation to detected objects (clean surface)

2.Do not need to do manual magnetization,because it utilize natural magnetization intensity generated by workpiece production and using process

3.Metal magnetic memory not only test device that is running ,but also test device that is repairing 

4.Metal magnetic memory method --only method to make sure stress concentration area at 1mm precision

5.Prevent from damaged source before generating loss

Main application

Mechanical manufacturer test metal and welding joint quality

All industrial sector manufacturing,repairing and tube 、container,any structure and  workpiece

Jack-up and rotating machinery

Research metal mechanical properties in laboratory