Roman Petráš

Basic Mechanisms Leading to Fatigue Failure of Structural Materials

Abstract General features of the damage evolution in cyclic loading of structural materials are summarized. The attention is paid to the comparison of the damage mechanisms in materials that are used for service both at room and at elevated temperatures, namely austenitic stainless steel Sanicro 25. Principal mechanisms leading to fatigue fracture at room and at elevated temperature are documented. While cyclic slip localization is a decisive process in the initiation of fatigue cracks at room temperature, the localized oxidation plays an important role in isothermal high temperature cyclic loading. Specific mechanisms of the early fatigue damage in thermomechanical cyclic loading are studied. The in-phase thermomechanical loading leads to the intergranular crack initiation due to preferred grain boundary oxidation and intergranular crack growth. The out-of-phase thermomechanical loading results in oxide cracking and localized oxidation of the metal in the area of cracked oxides and to transgranular crack growth. The damage mechanisms can explain differences in fatigue life under various loading and temperature conditions.

Hysteresis Loop Analysis in Cyclically Strained Materials

The generalized statistical theory of the hysteresis loop is adopted to describe the stress-strain relations, preferably in cyclic straining. The effective stress and the distribution of the internal critical stresses in cyclic straining are evaluated in two materials cycled at room and at elevated temperatures using the analysis of the hysteresis loop shape. The evolution of the shape of the probability density function of the internal critical stresses yields deeper insight into the mechanisms of cyclic plastic straining. It indicates the important role of cyclic plastic strain localization in room temperature fatigue softening. The approximation of the probability density function by Weibull distribution leads to the assessment of the effective and internal stresses and allows the simulation of the relations between the stress and strain in case of different cyclic histories.

Crack Initiation in Austenitic Stainless Steel Sanicro 25 Subjected to Thermomechanical Fatigue

Thermomechanical fatigue experiments were performed with austenitic stainless Sanicro 25 steel. Several amplitudes of mechanical strain in a wide temperature interval (250-700 °C) were applied to the specimens. Mechanical response was recorded and fatigue lives were obtained. Scanning electron microscopy combined with FIB technique was used to study the mechanism of crack initiation in in-phase and in out-of-phase thermomechanical cycling. Different mechanisms of the crack initiation were found in these two types of loading. During in-phase loading fatigue cracks start in grain boundaries by cracking of the oxide. Cracks grew preferentially along grain boundaries which resulted in rapid crack initiation and low fatigue life. In out-of-phase loading multiple cracks perpendicular to the stress axis developed only after sufficiently thick oxide layer was formed and cracked in low temperature loading half-cycle. The cracks in oxide allowed localized repeated oxidation and finally also cracking. The cracks grow transgranularly and result in longer fatigue life.

Cyclic plastic response of nickel-based superalloy at room and at elevated temperatures*

Abstract Nickel-based cast IN 738LC superalloy has been cycled at increasing strain amplitudes at room temperature and at 800 °C. Hysteresis loops were analyzed using general statistical theory of the hysteresis loop. Dislocation structures of specimens cycled at these two temperatures were studied. They revealed localization of the cyclic plastic strain in the thin bands which are rich in dislocations. The analysis of the loop shapes yields effective stresses of the matrix and of the precipitates and the probability density function of the critical internal stresses at both temperatures. It allows to find the sources of the high cyclic stress.