Andrei Grigorescu

Simulation of the Interaction of Plastic Deformation in Shear Bands with Deformation-Induced Martensitic Phase Transformation in the VHCF Regime

The experimental observation of the microstructural deformation behavior of a metastable austenitic stainless steel tested at the real VHCF limit indicates that plastic deformation is localized and accumulated in shear bands and martensite formation occurs at grain boundaries and intersecting shear bands. Based on these observations a microstructure-sensitive model is proposed that accounts for the accumulation of plastic deformation in shear bands (allowing irreversible plastic sliding deformation) and considers nucleation and growth of deformation-induced martensite at intersecting shear bands. The model is numerically solved using the two-dimensional (2-D) boundary element method. By using this method, real simulated 2-D microstructures can be reproduced and the microstructural deformation behavior can be investigated within the microstructural morphology. Results show that simulation of shear band evolution is in good agreement with experimental observations and that prediction of sites of deformation-induced martensite formation is possible in many cases. The analysis of simulated shear stresses in most critical slip systems under the influence of plastic deformation due to microstructural changes contributes to a better understanding of the interaction of plastic deformation in shear bands with deformation-induced martensitic phase transformation in the VHCF regime.

Effect of Geometry and Distribution of Inclusions on the VHCF Properties of a Metastable Austenitic Stainless Steel

The effect of inclusions on the VHCF properties of a metastable austenitic stainless steel in undeformed and predeformed condition was studied. The material contains an inhomogeneous distribution of elongated oxide inclusions. TEM investigations of foils extracted by means of FIB technique show that the stress concentration at the inclusions is compensated by plastic deformation in the austenite phase preventing internal crack initiation in the VHCF regime for the non-predeformed, i.e., almost martensite-free condition. The effect of the spatial distribution and geometry of the inclusions on the VHCF strength was systematically investigated for the predeformed condition. Samples were monotonically predeformed at -80°C resulting in a martensite content of about 60% and then fatigued in high frequency testing machines. Since mechanical components are in practice subjected to complex cyclic loading situations, samples were tested both parallel and transversal to the rolling direction, in order to cover a broad field of applications. The higher notch sensitivity of the martensite phase leads to internal crack initiation from inclusions supported by the formation of a fine granular area (FGA). The change in testing direction perpendicular to the rolling direction reduces the number of cycles to failure due to the increased stress intensity factor at inclusions which leads to internal crack initiation without the formation of a fine granular area. These findings are discussed on the basis of a detailed microstructural characterization of the material focusing on the effect of martensite content, the inclusion morphology with respect to the rolling direction and the load axis applied

Numerical investigation of the influence of shear band localization on the resonant behavior in the VHCF regime

The monitored resonant behavior of fatigue specimens of metastable austenitic stainless steel (AISI304) is correlated with its damage accumulation in the very high cycle fatigue (VHCF) regime. The resonant behavior is studied experimentally and shows a distinct transient characteristic. Microscopic examinations indicate that during VHCF a localized plastic deformation in shear bands arises on the specimen surface. Hence, this work focuses on the effect of damage accumulation in shear bands on the resonant behavior of AISI304 in the VHCF regime. A microstructural simulation model is proposed that takes into account specific mechanisms in shear bands proven by experimental results. The simulation model is solved numerically using the two-dimensional boundary element method and the resonant behavior is characterized by evaluating the force-displacement hysteresis loop. Simulation of shear bands agrees well with microscopic examinations and plastic deformation in shear bands influences the transient characteristic of the resonant behavior.

Development of a Probabilistic Model for the Prediction of Fatigue Life in the Very High Cycle Fatigue (VHCF) Range Based on Inclusion Population

The aim of the present work is to develop a statistical approach for the correlation between the quality of metallic materials with respect to the size and arrangement of inclusions and fatigue life in the VHCF regime by using the example of an austenitic stainless steel AISI 304. For this purpose, the size and location of about 60000 inclusions on cross sections of AISI 304 sheet in both longitudinal and transversal directions were measured and subsequently modeled using conventional statistical functions. In this way a statistical model of inclusion population in AISI 304 was created. The model forms a database for the subsequent statistical prediction of inclusion distribution in fatigue specimens and the corresponding fatigue lives. By applying the extreme value theory the biggest measured inclusions were used in order to predict the maximum inclusion size in the highest stressed volume of fatigue specimens and the results were compared with the failure-relevant inclusions. The location of the crack initiating inclusions was defined based on the modeled inclusion population and the stress distribution in the fatigue specimen, using the probabilistic Monte Carlo framework. Reasonable agreement was obtained between modeling and experimental results.