Papers by Keyword: Cyclic Deformation Behavior

Abstract: The cyclic deformation behavior of the austenite and ferrite phase in an austenite-ferrite duplex stainless steel was studied by using the load-controlled cyclic nanoindentation approach. The results showed that the maximum penetration depth onto both austenite and ferrite phases increased continuously while the rate decreased gradually and finally reached to a constant during the repeated indentation. Both transient state and quasi-steady state were observed for the penetration depth per cycle on both of the austenite and ferrite phases with the increased cycles. By contrast, both the penetration depth and rate per cycle into the austenite phase were larger than those into the ferrite phase. This was ascribed to the stress-induced densification in the austenite and ferrite phases and strain-induced transformed martensite in the austenite phase.

Abstract: High frequency fatigue tests were carried out with a 20 kHz ultrasonic testing facility to investigate the cyclic deformation behavior of Ti6Al4V in the Very High Cycle Fatigue (VHCF) regime in detail. The S,N_f -curve at the stress ratio R = -1 shows a significant decrease of the stress amplitude and a change from surface to subsurface failures in the VHCF regime for more than 10⁷ cycles. Microscopic investigations of the distribution of the α-and β-phase of Ti6Al4V indicate that inhomogeneities in the phase distribution are reasons for the internal crack initiation. Scanning electron microscopy as well as light microscopy were used to investigate the internal crack initiation phenomenon in the VHCF-regime. Beside the primary fatigue crack additional defects like micro cracks and crack clusters were observed in the fatigued specimens. SEM-investigations of specimens which were loaded up to 10¹⁰ cycles without failure show irreversible microstructural changes inside the specimens. Two step tests were performed to evaluate the influence of internal fatigue induced defects observed in specimens which did not fail within 10¹⁰ cycles.

Abstract: Stress-controlled load increase and constant amplitude tests have been carried out in a temperature range of -60°C < T < 20°C at the aluminium alloy AlMg4.5Mn (EN AW-5083). Therefore a recently developed climate chamber which operates with liquid nitrogen was mounted on a servo-hydraulic fatigue testing machine to realize the required low temperatures. Beside conventional mechanical hysteresis measurements, electrical resistance and temperature measurements are used to characterize the fatigue behavior. Furthermore, with these methods, the endurance limit was successfully estimated in a load increase test. Woehler curves were determined with constant amplitude tests at different temperatures. The conventionally determined endurance limit corresponds with the value from the load increase test.

Abstract: Mechanical stress-strain hysteresis, temperature and electrical resistance measurements were performed for the microstructure-related characterization of the fatigue behavior and for the fatigue life calculation of metals. The proceeding fatigue damage was evaluated using the change of the load-free electrical resistance, which is strongly influenced by the defect density of the individual material state. A new test procedure was applied for the fatigue assessment under random loading on the basis of cyclic deformation curves, similar to single step loading. A physically based fatigue life calculation “PHYBAL” was developed, which requires only three fatigue tests for the rapid and nevertheless precise calculation of S-N (Woehler) and fatigue life curves.

Abstract: In the present paper, the relation between fatigue crack growth threshold and material’s cyclic saturation behavior is investigated. The dislocation-free zone (DFZ) model is used to determine the dislocation distribution ahead of the crack tip. A cohesive zone model is developed to determine the stress field of the DFZ under cyclic loading. The effect of cyclic loading makes the plastic zone hardening (or softening), which raises the stress level in DFZ, and may lead to fracture. It is found that the near threshold characteristics are mostly determined by the cyclic deformation behavior of the material, and might be theoretically determinable from the standard cyclic loading test.