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Method for determining the stress-strain state of metal (Coercive method)


Stress-strain state (SSS) of steel constructions is one of the determining factors in assessing the reliability of lifting facilities in operation.

With the introduction of production of welded steel structures need to assess SSS becoming more relevant at all stages of the product life cycle, as rates of aging equipment far ahead of the pace of technological upgrading.

Over the past twenty years due to physical aging over 85% of crane fleet went beyond normal period of operation, which provides industrial safety threat. Moreover, after the collapse of the USSR and the effects of financial and economic crises instead of bankrupt enterprises, new liberated areas, but with a different output profile, the owners of which have become major consumers in the "secondary market" cheap crane technology is unknown how many years were in operation.

As one of the ways to solve this problem on the basis of years of research practice has spread diagnosis of steel structures lifting facilities for the examination of industrial safety on the basis of measurements of the magnetic properties of the metal - the coercive force (Hc).

Theoretical bases of evaluation of the stress-strain state of bearing metal crane

1. The relationship of magnetic and mechanical properties of the structural condition of steels and alloys

In assessing the stress-strain state of steel structures magnetic diagnostics techniques (non-destructive testing) are based on the relationship of magnetic, electromagnetic and mechanical properties of steels and alloys, which are determined by the structural state, chemical and phase composition of the substance.

Thus, the non-destructive magnetic control method is used in the presence of stable pair or multiple probabilistic relationships between the controlled quality metal and magnetic characteristics of steel.

Correlation between the magnetic properties and mechanical properties - quality indicators defined on the basis of the information set for each grade of steel or grade groups, differing mainly carbon and "magnetically hard" or "soft magnetic" alloying elements.

2. Physical basis of magnetic testing.

Non-destructive magnetic testing of mechanical properties of ferromagnetic materials based on the correlation between magnetic and physical-mechanical properties when they both depend on the same factors: chemical composition, mode of heat treatment, plastic deformation, micro-and macroscopic stresses, dislocations and other.

Micro-and macro-defects structures accumulate in the metal during cyclic loading in tension, compression, bending or twisting, like collect and store information that is uniquely associated with the maximum values ​​operating loads, causing the metal structure steel structural parts serves as a kind of memory sensor peak force. A number of magnetic parameters, uniquely associated with the number of violations of the metal structure, thus, is a kind of display mode power design.

As the main controlled magnetic parameter was chosen value of the coercive force Hc, as it is uniquely related to the residual plastic deformation under static and cyclic loading of metal structures during operation.

All the projected lifetime of lifting structures (LS) can be divided into stages according to the degree of accumulation of residual strains and micro-elements in the most loaded metal structures. Local standards divide lifting structures on three modes of operation:

  1. "safe" operation, when the metal works in the elastic region of loading diagrams and maximum voltage does not exceed the physical limit of elasticity of steel.
  2. "controlled" operation, where parts of the metal work in the elastic-plastic loading, and the maximum residual stresses reach a physical limit strength of steel.
    When operating in a "controlled" mode required:
    • organization of permanent visual control over the elements of metal structures caught in the area of elastic-plastic deformation;
    • during the regular complete technical examination (CTE) to conduct magnetic (coercimetric) control latching measurement results of the National Assembly in the passport of the LS, which will monitor the dynamics of the fatigue stress-strain state of the metal.
  3. "critical" mode of operation, where parts of the of the LS work in the elastic-plastic and plastic regions of loading diagrams and maximum stresses exceed the yield strength of steel, resulting in fatigue cracks appear.
    When operating in "critical" mode requires:
    • make strengthening elements, since several times increases the risk of technical failure;
    • avoid overloading the PC;
    • limit the term of the next survey to 1 year.

Since 2012 carrying out magnetic coercimetric control introduced by local standards as a mandatory milestone at carrying out expert examination of load-bearing elements of metal structures gantry cranes, that are out of operating age.