Microplastic Limit of Steels as a Means of Fatigue Limit Determination

The experimental determination of fatigue limit in case of steels represents an expensive and time consuming process not speaking about the necessity of a sufficient volume of representative material necessary for the specimens manufacturing, manufacturing of numerous test specimens, the need of an expensive (electrohydraulic) test equipment etc. Microplastic strains occurring at stresses below the yield point play a significant role at cyclic loading of metals within high cycle fatigue region. The microplastic strains cannot be measured by common methods due to the discontinuity of their distribution and a varying character of their level in the metal structure, but however, owing to the magnetoelastic effect the threshold stress at which dislocation pile-up stresses begin to obstruct the magnetic domains in rotation to the direction of the tensile stress can be determined by means of changes in magnetic permeability during tensile loading. In the paper, the changes of the magnetic permeability were measured indirectly – by means of the changes of the electrical impedance (a.c. resistance). The measurement was performed on the normalized low-carbon structural steel CSN 41 1375. The microplastic limit was determined by evaluation the ΔR-σ records. A comparison with the fatigue limit in a reversed fatigue loading showed that the MPL was below the fatigue limit. The microplastic limit can thus be considered to separate non- damaging and damaging cyclic stresses. The paper presents the results of the abovementioned characteristics what may shorten and make cheeper the determination of the fatigue limit and non-damaging cyclic stresses of structural materials.