Papers by Author: Jan Kohout

Abstract: The paper presents the results of research focused on assessment of the influence of loading cycle asymmetry on fatigue limit values. For tests two heats of unalloyed nodular cast iron were used. Test bars made of keel blocks were heat treated in salt bathes (austenitization at 900 °C during 1 hour, isothermal transformation at 380 and 400 °C) and loaded at symmetrical, repeating and pulsating loading cycles at room temperature. Evaluation of fatigue properties was based on the determination of S-N curves in high-cycle region including the fatigue limit assessment for 107 cycles to fracture. Fatigue and static tests were completed by metallographic and quantitative phase analysis. Most important result obtained from the presented study is that the dependence of stress amplitude on mean stress cannot be approached by the linear relation but by general power law with exponent lower than 1 (i.e. the Haigh diagram has convex shape).

Abstract: The structure of austempered ductile iron (ADI) matrix and consequently its mechanical properties are influenced by the heat treatment conditions, above all by the temperature and dwell length of isothermal transformation. The paper is focused on deeper understanding the interrelation between matrix mixture composition and static mechanical properties of ADI in dependence on the isothermal transformation dwell. Practical aim of the paper is to find the optimal isothermal transformation dwell range for ADI isothermally transformed at the temperature of 380 °C with emphasis on the level of static mechanical properties in tension. Microstructure and mechanical properties changes that proceed during isothermal transformation are observed and evaluated for the transformation dwells of 2, 5, 10, 25, 60, 120, 270, and 540 minutes.

Abstract: Most families of S-N curves determined at various temperatures present certain general regularities on whose basis the Basquin equation describing finite-life S-N curves can be generalized for various temperatures. This equation can be represented by straight lines with common slope if log-log fit for stress vs. temperature dependence is used. Deviations from these straight lines (anomalies) are evidence that additional degradation mechanisms are effective besides fatigue, whose temperature dependences differ from the mentioned temperature dependence of fatigue strength. In high-temperature region it is most often cyclic creep, in low-temperature region athermal processes of plastic deformation can play significant role in fatigue failure.

Abstract: Two ADI heats transformed at temperatures of 400 and 380 °C during temporal range from 2 minutes to 9 hours were studied in details, with emphasis on structure composition and mechanical properties (tensile and fatigue properties were determined). In the case of the shortest dwells the level of mechanical properties is influenced by martensite, which occurs in the structure as a result of subsequent cooling. UTS and yield stress increase slightly with the dwell of isothermal transformation while the values of elongation to fracture as well as of fatigue limit are very closely dependent on the amount of the retained austenite in the microstructure.

Abstract: The steady-state creep rate increases with temperature according to the Arrhenius equation and its increase with applied stress is mostly described by the power law. Combining both these laws, equation ) exp( RT Qa n − µ s e& is obtained, where apparent activation energy a Q and stress sensitivity parameter n are considered to be constants. But most measurements show some dependence of activation energy on applied stress and of stress sensitivity parameter on temperature. An equation respecting these facts is derived in the paper and verified using some published results of creep tests. Based on this derived equation, the dependence of yield stress on temperature and strain rate and an equation describing the relaxation curves are deduced.