Application of the Lm-Hardness Method to Assess the Current Material State of Structural Elements.

The paper presents the results of experimental studies of the possibility to assess the damage process of steels of different classes by using the LM-hardness method under cyclic loading at different cycle characteristics (stress amplitude, mean stress, loading frequency). Fatigue damage of metal under cyclic loading has been assessed by the method outlined in the PNAE standard. It is shown that the discrepancy between the recommended tabulated values and experimentally obtained values of mechanical characteristics of the material has a significant influence on the character of the calculated fatigue curve of the material, which increases with the number of cycles. Analysis of the dependence of stress ratio on the number of operation cycles shows that reduction of stress ratio decreases the durability of the studied material due to increase in the range of stresses in the cycle. It is obtained that increase of constant strain amplitude in the cycle relative to the established level by 10–15% corresponds to initial section of plastic deformation durability loss under cyclic loading. The homogeneity parameter determined in this case will characterize the limiting state of the material, which precedes its failure. Based on the analysis of experimental results, it is found that under conditions of realized loading, the damage kinetics curves for all investigated materials has a linear character. In different materials, the linear sections have different slopes. It can be assumed that the indicated slope of this curve characterizes the conditions of equilibrium between the processes of strain-hardening and softening of metal, achieved at the stage of steady creep. The presented results give grounds for using the homogeneity coefficient according to the LM-hardness method to assess the material condition of structural elements during the current monitoring of their condition. [ABSTRACT FROM AUTHOR]