Cyclic impact of external parameters on the structure, such as the daily change of temperature, change of operating parameters (pressure, temperature, volumes of loaded raw material) may lead to the brittle fracture of the equipment material with fast propagation of the main crack. In view of this, there is a need to use the nondestructive test methods which allow determining the critical level of damage accumulation. The article considers the pattern of change in properties as a result of low-cyclic loading by studying the parameters of acoustic signal. Changes in acoustic emission (AE) parameters were measured in the process of uniaxial tension of specimens with different degrees of accumulated damage. Upon processing of these AE parameters, stages of damage accumulation in cyclic fatigue were determined. Thus, is was determined that acoustic-emission diagnostics is sensitive to the said changes and may be used for assessment of the damage degree of 09G2S steel specimens.
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.
Nowadays, a change of material properties is evaluated on the basis of the results of "traditional" methods (for example, tensile testing and impact bending test). Unfortunately, "traditional" mechanical testing requires quite large samples, that leads to a damage of studied objects integrity. That is why testing with the application of compact samples, which allows converting obtained results into regular parameters, such as tensile properties and fracture resistance, was developed. Due to the novelty of this method, influence of different factors on testing results is understudied. Here arises the aim to study the influence of the level of accumulated damage on testing results. Regular changes of small punch testing tensile diagrams, depending on the number of worked out cycles, are under study in the present paper. Based on the study results, algorithm for determination of mechanical properties of steel 20 with different levels of cumulative fatigue damage with the application of compact samples was developed.
The equipment of oil refineries and other hazardous production facilities operate under high pressures and temperatures. Such operation conditions require continuous control and equipment remaining operation life period assessment. The existing methods of diagnostics are based on probabilistic remaining life assessment and use data regarding wall thickness variation during the operation process. The present article presents the method of accumulated damage assessment and its approximation to the limiting state, based on electromagnetic processes studying by means of eddy current control method. The main purpose of studies was determination of optimal value of input signal frequency, which could the most informative for determination of regularity of electric signal parameters change depending on the level of accumulated damages. Steel grade 09Г2С samples were used as the subject of studies. The samples were exposed to static tension under constant rate and during the process of samples deformation we measured the value of electric signal under three frequencies: 100 Hz, 10 kHz, and 1 MHz Based on the obtained results we prepared output signal voltage-relative elongation dependencies, which showed that accumulation of plastic deformations in metal leads to reduction of signal amplitude. Particularly interesting was dependence under 1 MHz frequency, under which electromagnetic processes occur in subsurface and surface layers. This dependence was of some regular nature, which was described by means of the sinusoidal function. Graph of the obtained function qualitatively describes the experimental dependence. On the basis of obtained results we can make a conclusion that optimal input signal frequency is within megahertz range, under which difference between the sinusoidal function graph and the empirical curve is minimum.
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