Method of Metal Magnetic Memory
Method of Metal Magnetic Memory is method of a nondestructive inspection based on registration and analysis of distribution of self-magnetic leakage fields (SMLF) arising on products and equipment in stress concentration zones (SCZ) and metal defects. At that, SMLF reflect irreversible change of a magnetization in a direction of maximal stresses action from working loads. Also SMLF reflect structural and technological heredity of products and welded joints after their manufacturing and cooling in a weak magnetic field (as a rule, magnetic field of the Earth). A natural magnetization and aftereffect, which appears as magnetic memory of metal to actual strains and structural changes in products and equipment metal, are used in the MMM method.
Physical fundamentals of MMM method:
- Magneto-elastic and magneto-mechanical effects.
- Effect of formation of domains and domain boundaries on dislocations clusters in stress concentration zones (magnetoplasticity).
- Effect of magnetic field leakage by structural and mechanical heterogeneities at metal natural magnetization.
Magnetic parameters used at inspection:
- Normal and/or tangential component of proper magnetic leakage fields Нр;
- Magnetic field gradient on length (dНр/dx) or on base between measurement channels.
What is inspected in metal and welded joints by MMM method?
- Stressed-strained state (SSS), structure heterogeneity and progressing defects.
What is determined by MMM method?
- Stress concentration zones – basic sources of damages development.
- Micro and macro-defects on the metal surface and in deep layers of metal.
Advantages of MMM method:
- Any special preparation (metal dressing and the like) of the object to be examined is not required.
- Special magnetization is not required because the natural magnetization formed at manufacturing and working of products is used.
- MMM method can be used both at operation of the object to be examined and at its repair.
- MMM method is unique NDT method to detect stress concentration zones and defects on the surface and in the deep layers of metal within 1mm in express-inspection mode.
- At the inspection by MMM method the small-size instruments having self-contained supply, recording device, microprocessor and up to 32MB memory unit are used.
- MMM method allows to carry out 100% quality control of engineering products and their sorting in mass line production.
- Increasing of inspection efficiency and reliability at combination of the MMM method with conventional NDT methods.
Differences from conventional NDT methods (ultrasonic inspection, magnetic particle inspection, X-ray, etc.):
- MMM is method of early diagnostics of fatigue damages and pre-defect state of metal.
- MMM is unique method answering to the question: "In which location to perform metal sampling" at the equipment lifetime assessment.
- MMM is second passive method after acoustic emission (AE), at which the information on structures radiation is used. Thus MMM, besides early defect detection additionally gives the information on actual stressed-strained state and allows revealing the reason of developing damaging.
- At the inspection by MMM method the parameters of proper magnetic leakage fields due to dislocations accumulation in stress concentration zones are measured unlike other known magnetic methods, at which scatterings of the artificially created magnetic fields on defects are measured.
Area of MMM method application:
- Quality control of metal and welded joints of products at engineering industry plants.
- Pipelines, vessels, equipment, any constructions and products (from ferromagnetic and paramagnetic austenitic material) in all industries at manufacturing, repair and maintenance.
- Hoisting devices and running mechanisms.
- Research of metal mechanical properties in laboratory conditions.