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Analiza zmian histerezy cyklicznej w badaniach trwałości niskocyklowej stali.

Kłysz S.

Zagadnienia Eksploatacji Maszyn

2002

Vol. 37, nr 2

79-93

Poddano analizie wyniki badań wytrzymałościowych i trwałości niskocyklowej próbek ze stali 18G2A oraz St3SY, opisując je równaniem Mansona-Coffina. Badania wykonano na próbkach klepsydrowych przy sterowaniu odkształceniem Delta epsilon=const. Oceniono zmiany pól powierzchni pętli histerezy uzyskanych w tych badaniach i przedstawiono algorytm numeryczny do obliczania pól powierzchni tych pętli. Analizowano zależności zmiany pól powierzchni pętli histerezy w procesie niskocyklowego zmęczenia, istotne z punktu widzenia kryteriów energetycznych trwałości.

Low cycle fatigue (LCF) data are very important for estimation of structural components durability both in design or operating time stages and in service life extension analysis. The hysteresis loops recorded during cyclic loading contain some information about the fatigue fracture of tested material. Their shape and characteristic dimensions during low cycle fatigue testing depend on the type of material and loading conditions. The loop width at stress level equal zero corresponds to plastic strain range determining life in low cycle fatigue - according to Manson-Coffin equation. Changes of material properties under cyclic load conditions (also under overloads) are among the topics of many conferences and papers and are necessary for fatigue analysis, especially damage cumulation and durability analysis. In this paper some results of strength tests and low cycle fatigue tests of two type of constructional steel in transverse strain Delta epsilon=const conditions are presented. A calculation algorithm for analysis of hysteresis areas, one based on the recorded LCF tests data has been shown. Some trends of changes of the hysteresis-loop areas during the LCF tests have been evaluated. Tensile tests were conducted on 6,4x12,5 mm plate specimens and fatigue tests carried on the 6 mm diameter hour-glass specimens. The hydraulic MTS Systems, type 810.23 was used in this investigation. The assumed stress ratio R was -1, frequency was 1 Hz. Longitudinal strains were measured with the 632.02F-30 extensometer and transverse strains with the 632.31F-24 one. To compare two investigated steels, a statistical analysis of difference significance by means of the T-Student tests was conducted using the tensile test results. The low cycle fatigue test results for the range of transverse strain from 0.167% to 1% have been shown for two types of steels. The Manson-Coffin curve was plotted for this data. The change of stress amplitude vs. number of cycles relationships and some specific quality of the LCF tests were presented. On the grounds of these results a cyclic stress-strain curve was estimated. For both tested steels the changes of sum of hysteresis-loop areas vs. number of cycles during the LCF tests and some effects of material softening phenomena were presented. The relations between fatigue life and size of hysteresis-loop areas or sum of hysteresis loop areas were estimated. The presented results are useful to analyse the energy criteria of fracture.