Fundamental difference of the metal magnetic memory method from other known magnetic non-destructive testing methods. Totals and prospects of the method development
Dr., Professor A.A. Dubov
In 2003 the "Control. Diagnostics" journal published an article highlighting the fundamental features of the metal magnetic memory (MMM) method . In this article, the author of the method answered the typical questions that arose and were discussed at SRI IN MSPU "Spektr" Scientific-Technical Council chaired by RSNTTD President V.V. Klyuev (July, 2003). Meeting of the Scientific-Technical Council was specially devoted to the MMM method.
Due to the fact that in circles of "magnetologists" involved in investigation and development of various magnetic non-destructive testing methods (NDT) the questions still arise related to characteristics and physical bases of the MMM method, there is a long-felt need to consider this topic once again in a popular form in the author's interpretation.
In particular, some most common questions on the MMM method are highlighted in E.S. Gorkunov's article published in the "Defectoscopy" journal . Some comments on the contents of this article should be noted.
The abstract of this article states that "equipment stress-strain state (SSS) assessment based on the metal magnetic memory (MMM) method of without taking into account the conditions of residual magnetization (RM) state formation will have low reliability". Further the article lists various RM formation conditions and types, among which the most stable thermoremanent magnetization is listed, the formation of which occurs in products in the course of their various productions during the metal cooling below the Curie point. Exactly thermoremanent magnetization that reflects structural and process history of products is mainly used in the MMM method during the express quality sorting of engineering products. During operation the products thermoremanent magnetization exposed to workloads is redistributed and provides information on local stress concentration zones (SCZs) - sources of damages development.
Further the article considers the mechanisms of products RM variation under the conditions of application of external permanent and variable magnetic fields, which are not directly related to the MMM method technology (see Figures 1-7 provided in the article ).
The mechanism of RM (metal magnetic memory) formation is discussed in detail in the book on the physical bases of the MMM method . Judging by the contents of article , the author, although formally, makes a reference to the above mentioned book, but it is obvious that he did not study it properly.
In the article section "RM resistance to the effect of elastic various-type strains" the author considers the mechanisms of ferromagnets RM variation in conditions of elastic strain from the position of formed ideas of magnetoelastic effect (m.e.e.). In conditions of actual practice products work not only in the area of elastic strain (this area is limited to the proportional limit, which is approximately equal to 0.3-0.5 of the yield strength for carbon steels). The article does not consider at all magnetoplastics and RM formation in conditions of shear strain (the main type of strain in products under the action of workloads). Vector distribution of RM and magnetic stray fields in ferromagnetic products volume, i.e. the formation mechanism of the internal magnetic field, recorded on the inspected product's surface, due to macro characteristics of its stress-strain state is not considered either.
In the same section the author of article  confirms the possibility of "remembering" the residual magnetization of maximum stresses acting directly on products. However, in this case, as the author states, it is necessary to know the RM distribution in products before and after stress application. This is exactly the way the experts, who use the MMM method in practice, act. Indeed, very often, when the MMM method is applied in practice, the RM distribution in products or equipment components in the initial state before the load application is not known. In these cases the MMM method uses the prerequisite that all the structural elements in the initial state have approximately the same magnetization, the same shape (standard size), the same material. Based on this prerequisite, after the workload effect magnetic anomalies in stress concentration zones (SCZs) occur in products and, thus, by measuring the distribution of RM and magnetic stray fields in SCZs and outside these zones it becomes possible to assess the products' and structural elements' state. In addition, it was experimentally found that after load application to a product in the very first cycle the initial RM changes irreversibly (“RM history” practically disappears) and in subsequent cycles of the same load the new values of RM and SMSF, respectively, remain unchanged. In the course of external load application and removal a magnetomechanical hysteresis loop, stably corresponding to the load value, occurs in the product SCZs. A situation occurs when the structure itself and its elements after the workload removal display their weak points due to the “magnetic memory of the metal”. Currently, Energodiagnostika Co. Ltd. (Moscow) developed about 60 various guideline documents and techniques that are used in practice to assess the actual stress-strain state of equipment and structures. The author of article  and other experts are recommended to study the experience of the MMM method practical application described in paper  and at the web site http://www.energodiagnostika.ru.
Figures 10-14 in E.S. Gorkunov's article  consider dependencies of RM variation during the effect on ferromagnetic specimens of different steel grades of tensile, compressive and cyclic alternating loads obtained in laboratory conditions. At that artificial magnetization of specimens is used and inhomogeneity of the metal structure is not taken into account. Therefore such study results are not suitable for use in the MMM method, which uses the residual magnetization formed under the effect of applied loads in a weak magnetic field of the earth (or workshop).
The book on the physical bases of the MMM method  provides a magnetomechanical diagram developed for the first time (there are no analogues in the theory of magnetism) and reflecting in energy terms formation of RM (or residual induction) in ferromagnets when exposed to stresses of different levels and signs and various-intensity external magnetic fields. The process of magnetization formation in ferromagnets should be considered exactly in energy terms, i.e. taking into account the weight energy contribution of mechanical stresses and the external magnetic field.
In Сonclusion the authors of the article  expresses doubt that in conditions when RM variation in a ferromagnet is simultaneously influenced by several factors (stress, temperature, external field intensity, structure, corrosion, etc.), it is impossible to unambiguously assess the stress-strain state of products using the MMM method parameters. Many experts – "magnetologists" express this doubt. To answer this question, the author of the MMM method had to carry out much experimental work and theoretical studies, as a result of which the following was established.
If we consider the effect of basic factors He, T, σ on magnetization variation ΔМ of a specifically shaped ferromagnet and weight on the initial state M0, we obtain the functional ratio in the following form:
ΔМ(Не, Т, σ) = (1 + kНе) × (1 + kT) × (1 + kσ) × М0 =
= (1 + kНе + kT + kσ + kHe × kT + kHe × kσ + kT × kσ + kHe × kT × kσ) × М0 , (1)
where ki – are non-linear functions reflecting the effect of each of the specified factors on variation of ΔМ and mutual influence of these factors on each other; Не – external magnetic field; Т – temperature; σ – stress.
And if we follow the path of the traditional approach in the study of these non-linear functions effect on variation of a ferromagnet magnetization, these studies will take a long time and a large amount of experimental work. Actually, many experts, who study the processes of ferromagnets magnetization, follow exactly this path.
Based on analysis results of a large amount of experimental work carried out during the MMM method development, an energy approach to studying the effect of the factors specified in ratio (1) on variation of the ferromagnet magnetization ΔМ was proposed.
This energy approach is based on the idea that each ferromagnet of a certain shape and material has a specific energy consumption characterizing its limiting resistance to external stresses and failure. In experimental studies it was found that the limiting state, occurring during the ferromagnet failure, has the same energy characteristics, i.e. the ferromagnet's failure energy and, accordingly, the maximum variation of its magnetization ΔМ based on ratio (1), irrespective of different combinations of factors and physical effects that brought it to this state, is the same and is an energy constant. At that the time to reach the limiting state even for the same-type products, depending on various combinations of factors, may be different. A more detailed description of the energy approach in the MMM method is described in the article .
The effect of demagnetization factor and various random disturbances on RM distribution in products that occur in practice (impacts, work hardening, surface roughness, the effect of external magnetic fields) and the methodology of offsetting from them are discussed in the Training Handbook .
Another most common question that we hear in the circle of "magnetologists" and other experts in the field of NDT is: "What for and why was MMM named so?" The inspection instruments in the MMM method use mainly flux-gate sensors; therefore experts often suggest to call the MMM method simply the flux-gate NDT method. In this connection it should be stated that in the author's opinion it is principally incorrect to name any NDT method by the type of the sensor used. If this logic is followed, then, for example, the acoustic emission method and various vibration diagnostics methods that use piezoelectric transducers should be also called "piezoelectric conversion" methods. NDT methods should be named and classified by the type of physical fields and effects used in any of the methods.
Currently, in accordance with GOST 18353 "Non-destructive testing. Classification of types and methods", one can count more than 100 different NDT methods with titles, in which sensor types and used physical effects are chaotically reflected and interlaced without their clear classification on the physical basis. Classifying of the known NDT methods according to the type of physical fields, the following types can be obtained:
At the same time, the well-known and commonly used methods like optical, radio wave, X-ray, acoustic, holographic, capillary, electrical resistance methods, strain gauge, as well methods of moire, nets, photoelasticity and others have not disappeared; they occupied their places within these five types. Article  considers in more detail the offers on the new system of NDT methods classification.
Now let us consider in a popular form some provisions relating to the MMM method name and its physical bases.
The concept of the "magnetic memory of metal" was first introduced by the author in 1994, and it had not been used in the technical literature before that time. The following terms and concepts were known: "magnetic memory of the earth" in the archaeological studies; "magnetic memory" in sound recording; “shape memory effect” caused by structural-phase transformations oriented by internal stresses in metal products.
Based on the established relation of dislocation processes to the physics of magnetic phenomena in metals of products, the concept of the "magnetic memory of metal" was introduced and a new method of diagnostics was developed. The MMM method uniqueness is that it is based on the use of the self-magnetic stray field (SMSF). SMSF occurs due to formation of domain boundaries at high-density dislocation clusters (dislocation walls) under the effect of workloads. It is impossible to obtain the source of information like self-magnetic field in working structures under any conditions with artificial magnetization. Such information forms and can be obtained only in a small external field like the magnetic field of the earth in loaded structures, when the strain energy by order exceeds the energy of the external magnetic field. SMSF, formed under the effect of workloads, is simultaneously a measure of metal coercivity. Practical works show that the MMM method can be applied both during the equipment operation, and after ерy workloads relief during maintenance. Due to the "magnetic dislocation hysteresis", the magnetic texture formed under the effect of workloads after their relief seems to get "frozen". Thus, a unique opportunity is provided to assess the equipment’s actual stress-strain state and to detect at an early stage areas of maximum metal damaging zone by reading this information using specialized instruments.
The physical bases of SMSF occurrence are fundamentally different compared to magnetic leakage fields (MLF) occurring on product defects during their artificial magnetization used in well-known magnetic NDT methods. SMSF occurs in local zones (from 0.1 to tens of microns) on the surface and deep layers of the products metal. Study of SMSF and physical bases of its occurrence before the "birth" of the MMM method (the 90s of the last century) was not carried out and such problem was not set! SMSF occurrence is conditioned by interaction of force fields with electromagnetic fields of microparticles consistently constituting the atom, the primitive crystalline lattice, then its unit cell, the very lattice, the domain, and finally, a group of domains, provided that the crystalline lattice is imperfect. The mechanism of ferromagnetic products self-magnetization and SMSF occurrence in them, taking into account the quantum field energy that causes hysteresis, is described in detail in paper .
SMSF also occur on new engineering products immediately after their fabrication. It is known that at ferromagnet heating above the Curie point (for example, for iron ТC ⩰ 770°С) and its subsequent cooling even in the weak external magnetic field of the earth it acquires the magnetization level that can be achieved at normal temperature only in a high-intensity magnetic field. As a rule, exactly under such conditions the "natural" (thermoremanent) magnetization is formed during fabrication of engineering products. The mechanism of the product's actual magnetic texture occurrence (melting, forging, heat treatment, welding) takes place directly after crystallization during cooling below the Curie point. And the actual products cooling process is, as a rule, non-uniform. The external metal layers cool faster than the internal ones. Thermal stresses occur across the product volume that cause shear strain of the crystalline lattice and form the appropriate magnetic texture. n areas with the highest concentration of crystalline lattice defects (for example, dislocation clusters) and structure inhomogeneities the domain boundaries (DB) pinning nodes are formed with product surface outcropping along the glide pads, as a rule, in the form of the normal SMSF component sign alternation lines (Н=0 lines). Industrial studies found that formed in this way thermoremanent magnetization reflects structural and process history of the product, and the lines Н=0 correspond to residual stress concentration lines. A number of techniques for engineering products quality control using the MMM method were developed.
Based on analysis of the results of experimental studies and practical diagnostics, a conclusion was made on the presence of at least three types of physical effects underlying the MMM method. One of the effects is magnetoelastic, and it seems to be thoroughly studied. The second effect – is magnetoplastics characterizing the process of magnetic fields interaction with dislocations and their clusters . The third effect – is the phenomenon of SMSF occurrence of in conditions of simultaneous effect of external loads and weak geomagnetic field on a ferromagnet.
If the mechanisms of these effects manifestation are considered in experimental studies in relation to the criteria of modern fracture mechanics , it becomes obvious that the practical use of the MMM method represents a new direction in the technical diagnostics. The MMM method combines the potentials of non-destructive testing, fracture mechanics and materials science. Equipment and structures inspection provides another unique opportunity of appearance of strain waves caused by the effect of workloads, structural inhomogeneity and the geometrical dimensions of the inspection object [8, 9]. Thus, to the author's opinion, the magnetic memory of metal - is a new physical phenomenon that provides the possibility to reveal rheological properties of the products material: force-torque stresses, response to the effect of external forces and relaxation, geometric displacements during strain processes. The metal magnetic memory effect, as a new physical phenomenon, requires further study.
The role of the magnetic field of the earth in the MMM method should be specially noted. Geomagnetic field, which is present everywhere, is a prerequisite for formation of the magnetomechanical information used in the equipment diagnostics. Due to the strained crystalline lattice magnetic moments precession in the magnetic field of the earth, the ferromagnetic product’s actual SSS displays at the macro level.
The magnetomechanical diagram, showing the results of force and magnetic fields energy interaction during magnetization of ferromagnetic products, was developed in paper . The boundaries of force and magnetic fields predominant influence are shown. At the intensity of the external field H ≤ 1200 A/m the graphs of the force and the magnetic components sensitivity variation coincide regardless of the stress-strain state of products. And at much lower intensity of the external field, for example, at the value of the external magnetic field of the earth (approx. 50 A/m) the magnetization process id determined by the force field, and the internal magnetic field direction in a ferromagnet traces the direction of the developing glide in the entire range of the force effect, right up to failure. Papers [3, 7] present the ideas of the formation of magnetomechanical domains in terms of energy interaction of force and magnetic fields.
Based on the above, it is proposed to single out the MMM method among all magnetic NDT methods as a separate type of inspection similar to the AE method, which was singled out among the acoustic NDT methods.
In conclusion let us note some totals of the MMM method development and implementation in Russia and other countries. As of January 2020, besides Russia, the MMM method and the appropriate inspection instruments were implemented in 44 countries of the world. Energodiagnostika Co. Ltd. Certification Body trained in the MMM method:
- more than 2600 experts in Russia;
- more than 500 experts in China;
- 85 experts in Poland;
- more than 230 experts in other countries.
The program of experts training and certification in the MM method is agreed with the Federal Service of Environmental, Technological and Nuclear Supervision (Rostekhnadzor). This Program and the Training Handbook were reviewed by experts from different countries within the framework of the International Institute of Welding (IIW) Commission "Quality Control of Welding" and adopted as an IIW document No.V-1347-06.
Based on the international standards on the MMM method (ISO 24497-1:2007(Е), 24497-2:2007(Е), 24497-3:2007(Е)), national standards in Russia, Bulgaria, Canada, China, Iran, Italy, Korea, Mongolia, Poland and Ukraine were put into effect.
There are more than 60 effective guidance documents and industry standards in Russia provided methodical guidelines on application of the MMM method in the equipment diagnostics.
In recent years, the non-contact magnetometric diagnostics (NCMD) of oil and gas pipelines, water pipelines, and heating network lines buried under the soil layer is increasingly implemented in Russia. This direction developed by a number of enterprises becomes the main method in the inspection of hard-to-access pipeline sections. NCMD is based on diagnostic parameters developed and used in the MMM method.
The MMM method development, which represents a new direction in technical diagnostics, is a broad front. Simultaneously with the methodology development, instruments and scanning devices are upgraded, the experts training process is improved, and experimental work in the laboratory and industrial conditions is conducted.
In some countries, where the MMM method is most broadly implemented (Russia, China, Poland, Czech Republic, Hungary, Germany), the scientific-research and educational institutes carry out active studied of structural and mechanical properties of the specimens metal using the physical method of MMM. The MMM method provides unique opportunities in experimental studies of physical-mechanical and strength properties of the metal both in the laboratory and industrial conditions.
1. A.A. Dubov. Fundamental distinctive features of the metal magnetic memory method and inspection instruments compared to the known magnetic non-destructive testing methods // Control. Diagnostics, 2003, No.12. pp. 27-29.
2. E.S. Gorkunov. Different states of residual magnetization and their resistance to external effects. To the issue of the "magnetic memory of metal" // Defektoskopia, 2014, No.11. pp. 3-21.
3. V.T. Vlasov, A.A. Dubov. Physical bases of the metal magnetic memory method. Moscow: ZAO "Tisso", 2004, 424 p.
4. A.A. Dubov, Al.A. Dubov, S.M. Kolokolnikov. Metal magnetic memory method and inspection instruments. Training Handbook – 5th Edition. Moscow: Publishing House "Spektr", 2012. 395 p.
5. A.A. Dubov. Energodiagnostics – a physical basis of the metal magnetic memory method // Territory of NDT, 2014, No.2. pp. 46-49.
6. A.A. Dubov, V.T. Vlasov. On classification of methods // NDT World, 2007, No.8. pp. 63-64.
7. V.T. Vlasov, A.A. Dubov. Physical theory of the "strain-failure" process. Part I. Physical criteria of metal's limiting states. Moscow: Publishing House "Spektr", 2013. 488 p.
8. A.A. Dubov, Al.A. Dubov. The effect of the pipes and vessels strain waves occurrence in experimental studies using the metal magnetic memory method // Territory of NDT, 2016, No.1. pp. 55-60.
9. M.I. Kozhinov. Laws of self-magnetic stray field anomalies formation in conditions of thin-walled cylindrical tanks stability loss // Materials strain and failure, 2015, No.5. pp. 43-47.