On the fatigue strength calculation of the welded shell structures from high-strength steels under low-cycle loading. Part 1: Estimation at the initial stage of fatigue failure

The normative methods for calculating the fatigue strength of welded joints are of limited use for low-cycle loads, as they do not take into account the possible variation in the asymmetry of the operating stress cycle, differences in the expected level of residual stresses, and the possible variety of joint geometry. Estimation procedures have been developed for shell structures made of high-strength steels subjected to external and internal pressure. They were based on experimental data on the resistance to fatigue fracture, physical modeling of individual stages of fatigue damage, and generalization of the results of numerical studies of the FEM of the stress-strain state.

Numerical Analysis of Dynamic Characteristics of Thermowell Based on Two-Way Thermo-Fluid-Solid Coupling

Because thermowells are prone to fatigue damage in petroleum cracking gas pipelines, in this paper, the LES method is used to simulate the flow around thermowells through two-way thermal-fluid-solid coupling, the internal causes of thermowell damage are explored, and measures for improving the thermowell safety are proposed. According to this study, when high-speed, high-temperature gas passes the thermocouple bushing, the main factors affecting the structural safety of the thermocouple bushing are the alternating stress caused by the vortex falling off, the thermal stress cycle due to the temperature gradient, and the pressure gradient impacted by the gas. Furthermore, this paper proposes improving the thermowell safety by installing the interference devices and optimizing the installation angle. The improvement measures were studied by conducting a two-way thermal fluid-structure coupling simulation. The results of this study show that after installing the interference device and optimizing the installation angle the displacement deformation of the thermowell and the equivalent stress is reduced by 57.2% and 72.1%, respectively, which indicates the safety improvement of the thermowell structure and the effectiveness of the method. The research contents of this paper can provide guidance for the installation and use of thermocouple bushing.

Stress-cycle driven evolution of faulting in a plate boundary transition zone

<p>Upper plate faults along the Cascadia subduction margin of North America go through a 3 stage evolution over millions of years as a consequence of the migrating Mendocino Triple Junction (MTJ). Initially, NE-directed cyclic shortening produced by the Cascadia subduction earthquake cycle drives reverse dip-slip motion on trench-parallel faults. As the triple junction moves north, NNW-shortening associated with the Mendocino Crustal Conveyor (MCC, Furlong & Govers, 1999) is superimposed on the cyclic subduction-earthquake-cycle regional stress field. As the triple junction migrates further north, and these faults transfer from the subduction to transform plate boundary, they become part of the San Andreas system and are loaded by right-lateral shear. In this work we investigate how the faulting behavior in northern California evolves through time from first being driven by cyclic subduction zone stresses (superimposed on a NNW-oriented shortening field) to eventually forming the primary structures within a dominantly strike-slip stress regime.</p><p>We decompose the observed horizontal GPS velocity field in southern Cascadia to determine a subduction coupling component and a NNW-directed displacement component to separate the subduction cycle effects from other tectonic effects on the behavior of upper plate faulting and its evolution through time. Since the MCC processes acts over millions of years, we assume that the effects associated with the NNW-directed signal can be represented by a constant stress field over subduction earthquake cycle timescales. Early in the subduction earthquake cycle, the principal stresses north of the MTJ are oriented in this NNW-SSE direction and rotate clockwise as the subduction component increases. This stress cycle then resets following each large megathrust event. Coulomb stress analyses indicate that the cyclic nature of the regional stress field, changes the likelihood of faulting and slip behavior on faults in southern Cascadia over time intervals of 100s of years. Trench-parallel faults are most likely to exhibit right-lateral or oblique motion early in the seismic cycle, however by ~100-200 years following a megathrust event, they are more likely to exhibit reverse dip-slip motion as the stress effects from the subduction component increase.</p><p>Though the NNW-oriented displacement field is assumed to be temporally constant over subduction earthquake cycle timescales, the spatial extent of this deformation field constrains strain localization within the upper plate. For example, a steep decrease in GPS velocities from SW to NE in southernmost Cascadia indicates right-lateral strain is accumulating adjacent to the relatively rigid Klamath Mountain Province. This region of localized right-lateral shear coincides with the location of the development of several regional-scale right-lateral strike slip faults. We hypothesize these faults, formed within the subduction regime, evolve to become regional-scale 'San Andreas-type' plate boundary faults. Understanding the implications of such time- and space-variable stress regimes provides insight into interpreting geologic estimates of the slip history of faults along the Cascadia and northern San Andreas margins of North America, and also a framework for understanding how a new plate boundary develops following a major change in plate interaction.</p>

Thermal Stress Cycle Simulation in Laser Cladding Process of Ni-Based Coating on H13 Steel

In order to improve the work efficiency and save resources in the process of laser cladding on the H13 steel surface, based on COMSOL, by combining computer simulation and experiment, a plane continuous heat source model was used to simulate and analyze the temperature and stress field. The optimal power and scanning speed were obtained. It is found in the simulation process that the thermal sampling points stress increases with the increase of laser power and scanning speed. Because of the existence of solid–liquid phase variation in the laser cladding process, there are two peaks in the maximum thermal stress cycle curve of the sample points located in the molten pool, and the starting and ending time of each sample point’s peak value is basically the same. When the sample point is outside the molten pool, because the metal at the corresponding location is not melted, so there is no obvious peak value in the thermal stress cycle curve. With the increase of cladding layer depth corresponding to each sample point, the variation range of the two alternating thermal stress peaks increases first and then decreases, while the duration increases. According to the peak value of alternating thermal stress at the sampling point, the molten pool depth can be predicted. The residual stress analysis of the cladding layer is carried out according to the analysis results of temperature field and stress field. Through the actual cladding experiment, it is found that the depth of molten pool in the simulation results is basically consistent with the experimental results. All simulation results are verified through actual cladding experiments.

Integrated Numerical-Experimental Assessment of the Effect of the AZ31B Anisotropic Behaviour in Extended-Surface Treatments by Laser Shock Processing

In recent years, an increasing interest in designing magnesium biomedical implants has been presented due to its biocompatibility, and great effort has been employed in characterizing it experimentally. However, its complex anisotropic behaviour, which is observed in rolled alloys, leads to a lack of reliable numerical simulation results concerning residual stress predictions. In this paper, a new model is proposed to focus on anisotropic material hardening behaviour in Mg base (in particular AZ31B as a representative alloy) materials, in which the particular stress cycle involved in Laser Shock Processing (LSP) treatments is considered. Numerical predictions in high extended coverage areas obtained by means of the implemented model are presented, showing that the realistic material’s complex anisotropic behaviour can be appropriately computed and—much more importantly—it shows a particular non-conventional behaviour regarding extended areas processing strategies.

Microstress cycle and contact fatigue of spiral bevel gears by rolling-sliding of asperity contact

The rolling contact fatigue (RCF) model is commonly used to predict the contact fatigue life when the sliding is insignificant in contact surfaces. However, many studies reveal that the sliding, compared to the rolling state, can lead to a considerable reduction of the fatigue life and an excessive increase of the pitting area, which result from the microscopic stress cycle growth caused by the sliding of the asperity contact. This suggests that fatigue life in the rolling-sliding condition can be overestimated based only on the RCF model. The rubbing surfaces of spiral bevel gears are subject to typical rolling-sliding motion. This paper aims to study the mechanism of the micro stress cycle along the meshing path and provide a reasonable method for predicting the fatigue life in spiral bevel gears. The microscopic stress cycle equation is derived with the consideration of gear meshing parameters. The combination of the RCF model and asperity stress cycle is developed to calculate the fatigue life in spiral bevel gears. We find that the contact fatigue life decreases significantly compared with that obtained from the RCF model. There is strong evidence that the microscopic stress cycle is remarkably increased by the rolling-sliding motion of the asperity contact, which is consistent with the experimental data in previous literature. In addition, the fatigue life under different assembling misalignments are investigated and the results demonstrate the important role of misalignments on fatigue life.