SYMBIOSIS OF SURFACE HARDNESS OF STRUCTURE AND MATERIAL FATIGUE – PROJECTION OF REMAINING LIFETIME

The existence of plastic deformations on the surface part causes an increase in the hardness on the surface. The level of hardness is increased on the part of the structure that is subjected to higher loads, i.e. the existence of plastic deformations (cracks, fissures, fractures). Measuring hardness in this case is a diagnostic technique that can prevent the appearance of cracks on the structure. Hardness measurement can be used to determine the optimal service life of a steel structure in dynamically complex structures. Hardness measurement can also be linked to material fatigue – the longer the service life of a structure, with cyclic loads over time, the greater the plasticity of the material. The hardness automatically increases due to the increased number of cycles. The more the plasticity of the material increases, the greater the hardness, i.e. the greater the likelihood of cracks appearing in the structure. The upper rotary platform of the SRs2000+VR bucket wheel excavator was used for hardness testing. The platform has the shape of a cylinder with reinforcements and its function is to transfer all loads, both static and dynamic, from the upper rotary structure of the excavator to the lower structure of the excavator. The operational safety and service life (especially from a risk perspective) depend on the integrity of the structure of this part of the excavator structure. Under static and dynamic loading, the hardness value changes. All measured values ​​are greater than the nominal value, indicating an increase in the hardness or ductility of the structural steel (to a greater or lesser extent). The measured values ​​range from 6.3 HRc to 16.3 HRc. This hardness level corresponds to a tensile strength of 585 MPa to 705 MPa. Compared to the nominal, initial hardness for steel St52-3 of 575 MPa, large deviations are observed. The increased level of measured hardness indicates an increased level of plasticity of the parts of the turntable structure, which can also be characterized as material fatigue. Differences in hardness provide information about the degree of fatigue of the structure, i.e. for identifying parts of the structure where there is an increased level of probability of expected failures.