Modeling of Multi-Axial Fatigue Damage Under Non-Proportional Variable-Amplitude Loading Conditions

This dissertation presents a new path-dependent non-proportional multi-axial fatigue model which is formulated in an incremental form of moment of load path (MLP) on σ-√β τ plane, ε-√(β^ε ) γ plane or K plane (i.e K_I-√(β^K ) K_III plane), depending upon if fatigue evaluation is performed in terms of stress-life, strain-life or crack growth life for a given application of interest. The total fatigue damage of an arbitrary non-proportional load path is partitioned into two parts: one is due to proportional part (defined with respect to a reference path) of the load path and the other is due to non-proportional part. The proportional part can be related to an effective stress range, e.g., the distance in σ-√β τ plane. The non-proportional part can be shown to be related to an integral form of strain energy densities contributed by normal and shear deformation with each being weighted by a path-dependent function. Furthermore, a dimensionless representation of non-proportional damage, termed as load path non-proportionality factor, is used for construction of a MLP based equivalent stress or strain or stress intensity factor range. A material sensitivity parameter to load-path non-proportionality is also considered in the proposed model for modeling different materials and their sensitivities to non-proportional loading. A general procedure for determining material sensitivity to load-path non-proportionality is also proposed and demonstrated. The MLP based fatigue damage model is implemented as an integral part of a recently developed “path-dependent maximum range” or PDMR cycle counting procedure for performing both cycle counting for arbitrary non-proportional multiaxial loading histories and computing MLP based fatigue damage as each half cycle is being counted. With this new capability, comprehensive validations of the newly developed model are performed by analyzing multi-axial fatigue test data of various forms (from simple to rather complex load paths) of non-proportional loading in four major areas: (a) structural steels; (b) different series of aluminum alloys; (c) various types of welded structural components; and (d) mixed mode fatigue crack growth in carbon steel and stainless steel. In addition to achieving a good correlation of test data in all four areas above, it is also found that structural steels are more sensitive to non-proportional loading than aluminum alloys. Different grades of aluminum alloys exhibit different levels of sensitivity to non-proportional loading, showing an approximately linear relationship between material ductility and material sensitivity to load-path non-proportionality. MLP based fatigue damage model is further extended for applications in structural life evaluation of welded structures by formulating an equivalent structural stress parameter that takes into consideration of plate thickness effects, bending ratio effects, and residual stress effects. The effectiveness of the equivalent stress parameter in correlating different components from various sources is proven by its ability to collapse all test data into a much narrower band than those with existing methods given in international codes and standards (e.g., IIW and Eurocode 3). Finally, non-proportional mixed mode fatigue crack growth problem is examined by extending the MLP model to stress intensity plane or K plane. A two-parameter mixed-mode crack growth model is then proposed to take into account of both load-path non-proportionality induced damage and mean stress effects computed in K plane. Analysis of available existing crack growth data subjected to non-proportional loading shows that the proposed model provides the most effective data correlation than any other available models.PHDNaval Architecture & Marine EngineeringUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttps://deepblue.lib.umich.edu/bitstream/2027.42/136940/1/meijifa_1.pd

Effects of Crystalline Anisotropy and Indenter Size on Nanoindentation by Multiscale Simulation

Nanoindentation processes in single crystal Ag thin film under different crystallographic orientations and various indenter widths are simulated by the quasicontinuum method. The nanoindentation deformation processes under influences of crystalline anisotropy and indenter size are investigated about hardness, load distribution, critical load for first dislocation emission and strain energy under the indenter. The simulation results are compared with previous experimental results and Rice-Thomson (R-T) dislocation model solution. It is shown that entirely different dislocation activities are presented under the effect of crystalline anisotropy during nanoindentation. The sharp load drops in the load–displacement curves are caused by the different dislocation activities. Both crystalline anisotropy and indenter size are found to have distinct effect on hardness, contact stress distribution, critical load for first dislocation emission and strain energy under the indenter. The above quantities are decreased at the indenter into Ag thin film along the crystal orientation with more favorable slip directions that easy trigger slip systems; whereas those will increase at the indenter into Ag thin film along the crystal orientation with less or without favorable slip directions that hard trigger slip systems. The results are shown to be in good agreement with experimental results and R-T dislocation model solution

Review of Power Sharing Control Strategies for Islanding Operation of AC Microgrids

Microgrid is a new concept for future energy distribution system that enables renewable energy integration. It generally consists of multiple distributed generators (DGs) that are usually interfaced to the grid through power inverters. For the islanding operation of AC microgrids, two important tasks are to share the load demand among multiple parallel connected inverters proportionately and maintain the voltage and frequency stabilities. This paper reviews and categorizes various approaches of power sharing control principles. Simultaneously, the control schemes are graphically illustrated. Moreover, various control approaches are compared in terms of their respective advantages and disadvantages. Finally, the paper presents the future trends