For critical structures in the oil and gas refining, petrochemical and chemical industries, it is necessary to determine the value of mechanical characteristics. In the present paper we consider issues related to the determination of the mechanical characteristics of structural materials using compact samples. Basically, current values of the mechanical properties of materials are determined by measuring hardness using existing empirical dependences that combine hardness values with yield limit and tensile strength values. But such dependencies are not universal and require experimental verification. The need for compact samples application occurs during determination of the reasons of equipment failure, when the size of a fragment of broken down equipment does not allow producing standard samples for traditional testing performance. The application of empirical dependencies for determination of hardness mechanical characteristics is complicated by the fact that it is necessary to take into account the degree of the material degradation. This, in its turn, requires additional studies. The key issue in determining the accuracy of measurements on compact samples is the role of the surface in the formation of fracture focal points. The experiments were carried out on thin steel samples and the mechanical characteristics were determined on Instron 8801 dynamometer unit. It was shown that in case of increase of thickness of the samples that underwent tension testing, the fracture work varies according to the power law.
It is showed by experiments that the use of compact specimens for determination of fracture work of steel that undertook cyclic deformations enables to identify metal with a number of cumulative fatigue damages. The yield limit determined in the first series of experiments was used for cycling parameters determination: 0.9 from the yield limit was set for damage accumulation in the elastoplastic stress range. At the same time an average cycle load was 8.9 kN, and an amplitude was 8.6 kN. It is demonstrated that specific fracture work increases upon the increase of compact specimens’ thickness. The tendency of specific fracture work decrease is observed upon fatigue damage accumulation. Thin specimens are particularly sensitive to damage accumulation. The results of the experiments indirectly support the hypothesis of fractional-dimensional surface layer existence.
It is necessary to evaluate possible changes of mechanical characteristics while determining reasons of equipment destruction as a result of implementation of emergency situation. Limited amount of the material collected at the local fracture point requires the use of compact testing specimens. Practical implementation of the method requires establishment of dependence setting correlation between characteristics which were determined under the standard method and the method that uses compacts specimens. In the present research as exemplified by the steel 20 for recalculation of values of yield limit determined by different methods.
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