The Metal Magnetic Memory Method -
a New Trend in Engineering
Traditional methods and means of diagnostics (ultrasonic inspection, magnetic
particle inspection, X-ray) are oriented to detecting the already developed
defects and by their designation can not prevent sudden fatigue damages of
equipment, which are the main reasons of failures and sources of maintenance
It is known that stress concentration (SC) zones, in which corrosion, fatigue
and creep processes develop most intensively, are the main sources of damages
occurrence in operating structures. Consequently, detecting SC zones is one of
the most important tasks of equipment and structures diagnostics.
Variations of metal properties (corrosion, fatigue, creep) in SC zones are
the processes preceding operating damaging. Metal magnetization, reflecting the
actual stress-strained state of pipelines, equipment and structures, changes
At present a principally new method of equipment and structures diagnostics
based on using of metal magnetic memory has been developed and implemented
successfully in practice. The MMM method unites the potential opportunities of
non-destructive testing (NDT) and fracture mechanics due to which it has a
number of significant advantages over other methods at inspection of industrial
Basic practical advantages of the new diagnostics method as compared to the
known magnetic and other traditional methods of non-destructive testing (NDT)
application of the method does not require special magnetizing devices as
the phenomenon of equipment and structures units magnetization in the process
of their operation is used;
locations of stress concentration due to working loads, which are unknown
beforehand, are determined in the course of their inspection;
metal dressing or any other preparation of the test surface is not
small-sized instruments, self-contained power supply, recording systems and
a memory unit up to 32 Mb are used to perform inspection by the proposed
special scanning devices allow testing of pipelines, vessels and equipment
in the express-control mode at a speed of 100 m/hr and more.
The MMM method is the most suitable practical NDT method at assessment of
actual stress-strained state. Therefore application of the new diagnostics
method is the most effective for equipment units life assessment.
The suggested diagnostic method, based on application of metal magnetic
memory, allows performing an integral evaluation of a unit state considering the
metal quality, actual operating conditions and its structural features.
The main task of the MMM method is detecting on the test object of
the most dangerous sections and units characterized by SC zones. Then, using,
for example, UT in SC zones, the presence of a specific defect is detected.
Based on calibration strength calculations of the most stressed units, detected
by the MMM method, the equipment actual life assessment is carried
Besides that, MMM and the appropriate inspection devices allow:
carrying out early diagnostics of fatigue damages and predicting equipment
documenting inspection results and making the equipment state data
performing express grading of new and old parts by their susceptibility to
detecting the future crack location and propagation direction on the test
object with accuracy up to 1 mm as well as recording of the already formed
inspecting in some cases of pipelines and vessels without insulation
What is principally new in the suggested inspection method?
From the analysis of known magnetic methods the following obligatory
conditions of their application are arise. Firstly, the magnetizing devices
should be necessarily used, and secondly, the known magnetic methods can be
applied effectively only on condition that locations of stress concentration and
defects in the control object are known in advance. Besides, the known magnetic
inspection methods require, as a rule, metal dressing and other preparatory
operations. It is obvious that application of conventional magnetic inspection
methods in extended structures and on equipment at such conditions is
practically impossible. For example, the task of specially magnetizing the tube
system, whose length on a modern power boiler approaches 500 km, is unreal. It
is impossible to know in advance stress concentration zones (the main sources of
damages development) on each boiler tube due to different process, design and
operating factors influencing their formation.
It is known at the same time that most of metal structures and equipment of
ferromagnetic materials is susceptible to "self-magnetization" in the magnetic
field of the Earth under influence of working loads.
The figure shows the scheme of magneto-elastic effect action (ΔBr - residual induction's change; Δσ - cyclic
load's change; Нe - external
magnetic field). If a cyclic load σ acts in some area of a structure and an
external field is present (for example, the magnetic field of the Earth), the
residual induction and residual magnetization growth occurs in this area.
Fig.1. The scheme of magneto-elastic effect
The phenomenon of equipment and structures "self-magnetization" is fought
against everywhere (shipbuilding, power engineering, ball bearing and other
industries). Upon studying this magnetization phenomenon on the example of
boiler tubes, it was suggested for the first time to use it for the purposes of
engineering diagnostics. At equipment and structures "self-magnetization"
various magnetostriction effects appear. However, the new inspection method uses
an aftereffect (in all varieties of magnetostriction effects), which becomes
apparent as the metal magnetic memory to actual strains and structural changes
in equipment metal. The more detailed information on the principal differences
of the MMM method from other known magnetic NDT methods can be found in a A.
A. Dubov "Principal Features of the Metal Magnetic Memory Method and Inspection
Tools as Compared to Known Magnetic NDT methods".
Metal magnetic memory is an aftereffect, which becomes apparent in
the form of residual magnetization of products and welded joints metal formed in
the course of their fabrication and cooling in a weak magnetic field or in the
form of irreversible changing of products magnetization in stress concentration
and damaging zones due to working loads.
Note: A weak magnetic field is a geomagnetic field and other
low-intensity external fields. The more clear boundary between weak and strong
magnetic fields is considered in the book "The Physical Bases of the Metal
Magnetic Memory Method" by Vlasov V.T., Dubov A.A. M.: ZAO "TISSO", 2004.
The metal magnetic memory method is a non-destructive testing method
based on registration and analysis of self-magnetic leakage fields (SMLF)
distribution on products surface for determination of stress concentration
zones, defects, metal and welded joints structure inhomogeneity.
The self-magnetic leakage field of a product is a magnetic leakage
field occurring on the product surface in the zones of dislocations stable
slipbands under the influence of operational or residual stresses or in zones of
maximum inhomogeneity of metal structure in new products.
For individual items, products and welded joints MMM is based on
registration of own leakage magnetic fields occurring in residual stress
concentration zones after their fabrication and cooling in the magnetic field of
the Earth. During fabrication of any ferromagnetic products (fusion, forging,
heat and mechanical treatment) the mechanism of real magnetic texture formation
takes place simultaneously with solidification at cooling, as a rule, in the
magnetic field of the Earth. In areas of the maximum lattice defects
concentration (for example, dislocation clusters) and structural inhomogeneities
domain boundaries occur with exit to the product surface in the form of SMLF
normal component sign alternation lines. These lines correspond to the part
section with the maximum magnetic resistance and characterize the maximum metal
structure inhomogeneity zone and, accordingly, the internal stresses maximum
concentration zone (SCZs).
Currently more than 40 guidance documents and inspection techniques has been
developed and practically applied in power engineering, chemical, petrochemical,
oil- and gas-refining, oil, gas and other Russian industries. The complex of
researches for theoretical substantiation of the method is conducted jointly
with a number of Russian institutes. The quantitative and qualitative criteria
allowing performing early diagnostics of equipment fatigue damages and life
assessment using the MMM method are developed.
During the period from1990 to 2008 the experts of Energodiagnostika Co. Ltd
carried out industrial researches with a state assessment of more than 310 steam
and hot-water boilers, more than 220 steam and gas turbines, more than 200
vessels and apparatuses, more than 500 km of various process purpose pipelines;
the quality inspection of machine-building products at more than 50 plants and
companies both in Russia and other countries is carried out; experimental
control of a rails and wheel sets on railway transport enterprises, bridge
structures, hoisting mechanisms and other technical objects is performed.
Based on the results of 2008, diagnostic companies and organizations, which
bought instruments and passed training at "Energodiagnostika Co. Ltd"
Certification Center, applied the MMM method at equipment diagnostics at more
than 1000 enterprises of Russia. Besides Russia, the method was implemented at a
number of enterprises of 25 countries: Argentina, Angola, Australia,
Bulgaria, Byelorussia, Canada, China, Finland, Germany, India, Iraq, Iran,
Israel, Kazakhstan, Latvia, Lithuania, Macedonia, Moldova, Mongolia, Montenegro,
Poland, Serbia, South Korea, Ukraine, USA.
The following Russian standards were prepared and put into effect:
52005-2003.Non-destructive testing. Metal magnetic memory method. General
52081-2003.Non-destructive testing. Metal magnetic memory method. Terms
52330-2005.Non-destructive testing. Stressed-strained state test of
industrial objects and transport. General requirements.
ST RWS 004-03.Non-destructive testing.
Welded joints of equipment and constructions. Method of metal magnetic
During the period from 1994 till 2008 42 IIW documents with positive
resolutions on the metal magnetic memory method were issued.
The International Standard ISO 24497-1:2007(E), 24497-2:2007(E), 24497-3:2007(E) on the metal magnetic memory method is approved in 2007 as a result of positive
voting among 18 IIW member countries and more than 10 ISO Committee
Significant experience of industrial and laboratory investigations,
availability of techniques, guidance documents, scientific and technical reports
allowed developing the normative-technical documentation (NTD) on certification
of the metal magnetic memory method, inspection devices and personnel. Besides
the techniques and GD, the normative-technical documentation includes: the
requirements to technical knowledge of the experts studying the MMM method; the
program of Level I, II and III experts training (approved by the State
Engineering Supervision (Rostechnadzor) of Russia); passports and technical
specifications to inspection instruments; operating manuals, techniques for
inspection instruments calibration and testing; the user's manual to the
software for computer processing of results; training handbook.
1. Dubov A.A., Dubov Al.A., Kolokolnikov S.M. Method of
metal magnetic memory and inspection instruments. Training handbook. Moscow: ZAO
"TISSO", 2008, 365p.
2. Vlasov V.T., Dubov A.A. Physical theory of the
"strain-failure" process. Part I. Physical criteria of metal's limiting states.
Moscow: ZAO "TISSO", 2007, 517p.
3. Vlasov V.T., Dubov A.A. Physical bases of the metal
magnetic memory method. Moscow: ZAO "TISSO", 2004, 389p.
4. Dubov A.A. I.C. 2029263. Patent of Russia and the C.I.S.
countries. Method for residual stresses determination in products made of
ferromagnetic materials. List of Inventions, No.5, 1995.
5. Proceedings of the First, the Second, the Third and the
Fourth International Scientific-Technical Conferences "Equipment and structures
diagnostics using the metal magnetic memory". Papers and summary to papers.
Moscow: Energodiagnostika Co. Ltd, 1999, 2001, 2003, 2007.
6. Dubov A.A. Diagnostics of boiler tubes using the metal
magnetic memory. Moscow: Energoatomizdat, 1995.
7. Dubov A.A. Diagnostics of turbine equipment using the
metal magnetic memory. Moscow: Energodiagnostika Co. Ltd, 1999.
8. Dubov A.A. Diagnostics of pipelines, equipment and
structures using the metal magnetic memory. Collection of papers and reports.
Moscow: Energodiagnostika Co. Ltd, 2001.
9. Dubov A.A. Investigation of metal properties using the
magnetic memory method // Physical metallurgy and thermal treatment of metals,
10. Dubov A.A. Express method of welding stresses inspection
// Welding fabrication, No.11, 1996.
11. Dubov A.A. Diagnostics of rails fatigue damaging using
the metal magnetic memory // In the world of NDT, No.5, 1999.
12. Goritzky V.M., Dubov A.A., Demin E.A. Investigation of
steel samples structural damaging using the metal magnetic memory method //
Testing. Diagnostics, No.7, 2000.
13. Dubov A.A. The problems of the ageing equipment life
assessment // Labour safety in industry, No.12, 2002, pp.30-38.
14. Dubov A.A. The method of metal limiting state
determining and equipment life assessment by magnetic diagnostic parameters //
Testing. Diagnostics, No.5, 2003.