Institute Of Mechanics,Chinese Academy of Sciences
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An energy-based virtual element method framework for polycrystalline plasticity
No full textDeformation in an aggregate of single crystalline grains, in conventional crystal plasticity finite element methods, is modeled through the collective behavior of individual ones each consisting of multiple elements. Such a procedure often entails tedious preparedness such as grain-level discretization and post-processing to obtain meaningful collective behavior. Virtual Element Method (VEM) which is designed to take care of a multifaceted three-dimensional body as one element, offers a promising alternative for crystal plasticity modeling. In this paper, we implement a phenomenological, rate-dependent crystal plasticity model within the framework of the energy-based VEM. By integrating nested iterative algorithms with automatic differentiation techniques, the numerical procedure efficiently simulates both single-crystal and polycrystalline solids under various loading conditions. With three numerical examples, we demonstrate the ability of VEM based crystal plasticity modeling to predict stress-strain responses, texture evolutions, and deformation mechanisms in face-centered cubic crystals. This numerical procedure shows a promising method of modeling polycrystalline solids composed of a vast amount of grains, and paves a new route of two-scale modeling which bridges microstructures with macroscopic mechanical behavior of solids
Defects-Induced Fatigue Failure Behavior and Life Prediction of Laser Powder Bed Fusion GH4169 Superalloy in High and Very-High Cycle Regimes
No full textLaser powder bed fusion (LPBF) is a leading method for fabricating high-strength materials such as GH4169 superalloy. However, their deformation behavior must be thoroughly analyzed before they can be used in critical engineering applications. The fatigue behavior of LPBF GH4169, particularly in the very-high cycle regime, is not yet fully understood. Therefore, uniaxial fatigue tests are conducted to investigate high and very-high cycle fracture analysis and associated failure behaviors in the solution-aged condition under two stress ratios at room temperature. Subsequently, scanning electron microscopy, electron backscattered diffraction, and transmission electron microscopy are employed to observe the dislocation structures and fracture mechanism. Fracture surfaces reveal that cracks nucleate through a single site, typically on the surface or within the interior, driven by manufacturing or crystallographic defects. Interior failure is evident at lower stress amplitudes, with two distinct crack nucleation modes identified. Dislocation accumulation and interactions with delta phases are revealed. Furthermore, the coupling reinforcement of strengthening phases enhances the fatigue performance. Moreover, the threshold values as well as the transition sizes from small to long cracks increase as failures shift from surface to interior. Finally, a fatigue life prediction model is developed, demonstrating a strong correlation to the experimental results
Heterogeneous microstructures by combining laser additive manufacturing with auxiliary ultrasound field for strength-ductility betterment
No full textHeterogenization is broadly employed to metallic alloys to improve their strength and ductility. In the current work, taking advantage of layer-wise melting by laser additive manufacturing (LAM) and grain refinement by auxiliary ultrasound field (USF), we combine LAM with USF to realize heterogeneous structures in 316 L stainless steel (SS). The as-built heterogeneous 316 L SS exhibits hierarchical microstructures spanning multiple length scales, ranging from millimeter-sized grains and micrometer-scale intragranular cellular domains down to nanoparticles of Fe-Cr sigma-phase and L1(2) precipitates heterogeneously dispersed within the cellular matrix. This unique spatial distribution of microstructures with a broad size-span and chemical fluctuations give rise to an ultra-high tensile strength in heterogeneous 316 L SS with a ultimate strength up to 1 GPa and an elongation of similar to 12.5 %. We further reveal that the synergy effect from hierarchical twins and metastable cells on dislocation gliding gives rise to the superior strength-ductility performance. The demonstration of using LAM with auxiliary USF to achieve controllable hierarchical heterogeneous microstructure paves a practical way for microstructure design and manufacturing
Dynamic evolution of oil droplet population in non-Newtonian swirling flow field
No full textPolymer flooding is widely applied in offshore oil recovery, but the resulting non-Newtonian, high-viscosity mixtures pose challenges for rapid oil-water separation in compact vane-type swirl separators. To investigate the dynamic evolution of oil droplet clusters in such complex flows and improve separation efficiency, this study developed a coupled simulation model integrating the Eulerian multiphase model, power-law rheological model, population balance model, and Reynolds stress model turbulence model. The model captures the breakup and coalescence behavior of oil droplets in non-Newtonian swirling flow fields under varying polymer viscosity coefficient, rheological index, inlet liquid velocity, and inlet oil-phase concentration. The results show that increasing the polymer viscosity coefficient leads to larger average droplet diameters (D32, D90, D5) across the pipe section, but a gradual decrease in centerline D32. A lower rheological index reduces average D32 and D5, but increases centerline D32 and slightly increases average D90. As inlet liquid velocity rises, average D32 increases, while centerline D32 and D90 first rise then fall, peaking at 1.2 m/s; D5 decreases steadily. Higher inlet oil-phase concentration enhances collision frequency, resulting in an overall increase in droplet sizes. These findings elucidate the interaction between rheology and flow conditions in governing droplet cluster dynamics within swirling flows, thereby enabling the directional optimization of swirl-vane separators for non-Newtonian fluids
Experimental characterization of cyclic stress softening for isotropic soft magnetorheological elastomer
No full textSoft magnetorheological elastomer (s-MRE) is fabricated by dispersing magnetic particles within the elastomer matrix. Its mechanical properties dramatically change under the applied magnetic field, exhibiting a typical magnetorheological (MR) effect. This property provides s-MRE with the potential for application in vibration control. During vibration control application scenarios, s-MRE-based vibration control devices are often subjected to asymmetric cyclic loads, for instance, the structure's self-weight and dynamic loads (e.g., wind or traffic loads), which result in a decay of the stress response with cyclic loading (mainly short cycle loading), referred to as the cyclic stress softening. If the influence of cyclic stress softening is ignored, the evaluation of s-MRE-based vibration control devices may exhibit unwarranted optimism and lack sufficient reliability, which may cause safety hazards. However, a literature review reveals limited investigation on the cyclic stress softening of s-MRE. To address this research gap and enhance the performance evaluation of s-MRE-based devices, this work experimentally characterizes the influence of mechanical loads and magnetic fields on the cyclic stress softening of isotropic s-MRE. The test results indicate that peak strain plays a dominant role in the occurrence of cyclic stress softening and an increased strain rate further amplifies this effect. Furthermore, the presence of a magnetic field positively influences the mechanical behaviour of isotropic s-MRE and concurrently intensifies the cyclic stress softening effect. This research enhances the understanding of the cyclic stress softening in s-MRE, thereby benefiting the long-term advancement of s-MRE-based development and design
Damage and microstructure evolution for rolling contact fatigue of a CSS-42L gear steel under mixed lubrication
No full textIn this study, a plasto-elastohydrodynamic lubrication model is employed to analyze the contact stress distribution in rolling contact fatigue (RCF) of a CSS-42L steel, which takes into account the effects of surface roughness and plastic deformation. Numerical simulation reveals that the maximum von Mises stress occurs at subsurface and it is not significantly affected by the presence of lubricating oil due to the relatively low surface roughness of the sample. The experiments are conducted under a mixed lubrication state, i.e., the hydrodynamic lubricant film and rough surface asperity contact coexists. Numerical analysis further indicates that the hydrodynamic lubricant film dominates the contact zone. It is supported by experimental observations that subsurface spalling pits are the main cause for the RCF failure in CSS-42L steel. In contrast, the oxidative wear damage from solid-solid asperity contact primarily leads to micro-pitting on the sample surface. Microstructure characterization indicates that the high-density carbide effectively impedes the RCF crack growth by influencing the path of crack propagation. The refinement of microstructure is also observed during the RCF, which is attributed to the cyclic plastic deformation caused by repetitive high contact stress
Failure mechanism of the inner ring raceway in the backing bearing for multi-roll rolling mills
No full textBacking bearings are crucial components of multi-roll rolling mills, and their failure mechanisms are diverse due to the complex service loads. The failure mechanism of the inner ring raceway in the backing bearing was investigated. The wear surface of the inner ring raceway presented typical abrasive wear morphology. The central wear region experiences more severe wear, with larger wear pits, while the wear in the edge regions is lighter, and the wear pits are smaller. The inclusion type and size analysis indicated that the size and distribution of inclusions meet the standard requirements. The gradient hardness distribution of the raceway surface can enhance the fatigue resistance of the inner ring. It has been confirmed that the damage to the inner ring is caused by abrasive wear rather than rolling contact fatigue. The failure mechanism of the inner raceway was proposed. First, the impact process causes hard contact between the rollers and the raceway surface, which results in the generation of abrasive particles. Then, three-body wear between the rollers and the raceway occurred after the abrasive particles enter the lubricating oil. Due to the higher stress in the center and lower stress on the edge on the inner raceway, continuous abrasive wear has led to the formation of a central wear region and edge regions
Failure mechanism of the inner ring raceway in the backing bearing for multi-roll rolling mills
No full textBacking bearings are crucial components of multi-roll rolling mills, and their failure mechanisms are diverse due to the complex service loads. The failure mechanism of the inner ring raceway in the backing bearing was investigated. The wear surface of the inner ring raceway presented typical abrasive wear morphology. The central wear region experiences more severe wear, with larger wear pits, while the wear in the edge regions is lighter, and the wear pits are smaller. The inclusion type and size analysis indicated that the size and distribution of inclusions meet the standard requirements. The gradient hardness distribution of the raceway surface can enhance the fatigue resistance of the inner ring. It has been confirmed that the damage to the inner ring is caused by abrasive wear rather than rolling contact fatigue. The failure mechanism of the inner raceway was proposed. First, the impact process causes hard contact between the rollers and the raceway surface, which results in the generation of abrasive particles. Then, three-body wear between the rollers and the raceway occurred after the abrasive particles enter the lubricating oil. Due to the higher stress in the center and lower stress on the edge on the inner raceway, continuous abrasive wear has led to the formation of a central wear region and edge regions
Deterioration mechanism of adhesion properties of FRP-soil interface induced by moisture
No full textThe deterioration of fiber-reinforced polymer (FRP)-soil interfacial adhesion due to water intrusion has been a core issue in geotechnical engineering, but its microscopic mechanism remains unclear. In this study, molecular dynamics (MD) simulation method is employed to reveal the microscopic deterioration mechanism of water on adhesion properties of epoxy-quartz (i.e., FRP-soil subsystem) interface, the structural and dynamic characteristics of interlayer water film. The steered molecular dynamics pulling simulation and the modified Bell's model are used to evaluate the adhesion energy of epoxy-quartz interface in dry and wet cases. The simulation results show that (1) the interfacial water film weakens adhesion strength of epoxy-quartz interface, playing a dual role in "interface isolation" and "lubrication", aggravating the interfacial debonding. (2) The work of adhesion, maximum pulling force, potential of mean force, and adhesion energy of dry system are significantly higher than those of wet system. (3) The interlayer water film has a distinct layered structure: bound, free, and sparse water layers, which have different angle orientations and density distributions. (4) The diffusion coefficient increases with the rising thickness of free water layer, which may trigger a capillary-seepage effect and aggravate interface deterioration. This study provides atomic-scale insights into moisture-induced FRP-soil interface failure mechanism
Machine learning revealed force-stress-fatigue damage correlation of high-speed train bogies
No full textClarifying the correlation of multi-level mechanical parameters of structures in complex dynamic systems is a prerequisite for determining the accruing fatigue damage. In this paper, we adopt the independent component analysis algorithm in unsupervised learning and tap the latent correlation between measured forces and stresses of high-speed train bogies. It is revealed that there exists a strong correlation between the vertical force and the stress at the junction of the transverse beam and the side frame, a site prone to fatigue. Stresses reconstructed by strongly correlated independent components account for more than 70% of the fatigue damage, which in turn supports the finding that the vertical forces are the main contribution to the fatigue damage at the junction of the transverse beam and the side frame. This strong correlation between vertical forces and stresses effectively reduce the error in fatigue damage prediction and provide insights into fatigue life enhancement of critical structures of dynamic systems beyond high-speed trains