A research on the mechanism and model of surface micro-damage in grinding hardening

Grinding hardening can obtain hardening effect besides fine machining effect on the surface of workpiece. A certain degree of surface micro-damage will be formed during grinding hardening process which mainly contains micro-crack, melting coating, decarburization, and surface scratch. In order to study the micro-damage of grinding hardening, the paper carries on a grinding hardening experiment and the characteristics of micro-damage are observed. The generation mechanism of micro-crack is revealed. It is formed by the accumulation of dislocation pile-up in the condition of grinding hardening process. When the grinding depth reaches a certain value, the surface micro-crack will be produced obviously and become more serious with the increasing of grinding depth. Other micro-damage containing surface melting coating, decarburization, and surface scratch are also studied and their relevance with micro-crack is revealed. Based on the grinding hardening theory and pile-up dislocation theory, a model for micro-crack of grinding hardening is established. The calculating result of the model accords with the experimental result in general.

Correlation between Microstructural Change and Irradiation Hardening Behavior of He-Irradiated V–Cr–Ti Alloys with Low Ti Addition

V–4Cr–xTi (x = 0 to 4) alloys were used to investigate the additional effect of Cr, Ti and interstitial impurities on the microstructural evolution in He-irradiated V–Cr–Ti alloys to minimize radioactivity after fusion neutron irradiation. Transmission electron microscopy and atom probe tomography were carried out to the He-irradiated specimens at 500 °C with 0.5 dpa at peak damage. A flash electro-polishing method for the FIB-extracted specimen was established for the ion-irradiated vanadium alloys. The microstructural evolution of the irradiation-induced titanium-oxycarbonitride, Ti(CON) precipitates was observed and was influenced by the effect of Ti addition on the Ti(CON) precipitation. Apparent Ti(CON) precipitates formed in V-4Cr-xTi with 2% addition of Ti. In the V-4Cr-1Ti alloy, a high density Ti enriched cluster was formed. The origin of the irradiation hardening increase resulted from the size distribution of Ti(CON) precipitation from the dispersed barrier-hardening theory.

MODELING OF THE COOL CREEPIN THE FRAMEWORK OF THE HARDENING THEORIES ON THE STEPWISE LOADING

In the framework of the hardening hypothesis, the creep behavior of a titanium alloy at room temperature is modeled at stepwise stress change both increasing and decreasing load. Methods for the identification of material constants and functions included in constitutive equations of the hardening theories are considered in detail. The prospects for the computational-experimental method proposed by Rybakina for determination of material constants in some versions of the hardening theory are noted. The results of the creep modeling of titanium alloy confirm the existing theoretical possibilities and limitations of the hardening hypothesis fully.

A State-Dependent Constitutive Model for Gas Hydrate-Bearing Sediments Considering Cementing Effect

This paper presents a state-dependent constitutive model for gas hydrate-bearing sediments (GHBS), considering the cementing effect for simulating the stress–strain behavior of GHBS. In this work, to consider the influence of hydrate on matrix samples in theory, some representative GHBS laboratory tests were analyzed, and it was found that GHBS has obvious state-related characteristics. At the same time, it was found that GHBS has high bonding strength. In order to describe these characteristics of GHBS, the cementation strength related to hydrate saturation is introduced in the framework of a sand state correlation model. In addition, in order to accurately reflect the influence of cementation on the hardening law of GHBS, the degradation rate of cementation strength is introduced, and the mixed hardening theory is adopted to establish the constitutive model. The model presented in this paper reproduces the experimental results of Masui et al. and Miyazaki et al., and the prediction performance of the model is satisfactory, which proves the rationality of this work.

Modeling of Yield Strength in Retrogression of RRA Treatment of Spray Formed Al-Zn-Mg-Cu Alloy

7075 alloy is the most typical Al-Zn-Mg-Cu alloy and widely used in industry. In the present study the regularities of the change for the size and volume fraction of the precipitates in the retrogression process and the effects of aging time on strengthening of spray formed 7075 alloy were investigated based on the thermodynamics, aging kinetics and hardening theory. The results show that there was a relationship between the parameters of retrogression treatment and yield strength of the alloy. A unified model is presented to establish the quantitative relations between the retrogression process and the yield strengths of spray formed 7075 alloy from the perspective of combining micro and macro.

Load Controlled Cyclic Loading of Transversely Isotropic Cylindrical Vessels Based on the Anisotropic Kinematic Hardening Models

This study evaluates the plastic responses of thick cylinders made of transversely isotropic materials under mechanical cyclic loads, using the kinematic hardening theory of plasticity. The Hill yield criterion is adapted to the kinematic hardening theory of plasticity. The constitutive equations of plastic strains are obtained using the adapted yield criterion. The flow rule based on the kinematic hardening theory of plasticity associated with the Hill yield criterion is represented to evaluate the cyclic behavior of transversely isotropic cylindrical vessels. A numerical method is proposed to calculate the stresses and plastic strains in this structure due to the cycling of pressure at its inside surface. The numerical solution is validated simplifying the results with those of isotropic materials. Using the proposed method, the effect of anisotropy on ratcheting and shakedown response of the vessel is evaluated. It has been shown that the ratcheting or shakedown response of the vessel and the rate of ratcheting are highly affected by the anisotropy ratio. The numerical results of this paper show that the yield strength ratio, which is affected by initial work hardening of the metal, may control the ratcheting behavior of the cylindrical vessels.

A CDM Model and its FE Implementation for 2D C/SiC Composite under Tension and Shear Loads

2D C/SiC ceramic matrix composite (CMC) displays significant damage characteristic coupled with inelastic strain under tension and shear loads, which should be considered in the constitutive model. In this study, a continuum damage mechanics (CDM) model was proposed for this material, in which the process degradation of the material property was described by introducing a set of scalar damage variables, and the damage-coupling effect was also considered. Meanwhile, isotropic hardening theory was applied to form the evolution rule of inelastic strains. The model was then implemented into the UMAT in ABAQUS software and validated by comparison between the simulation and experiment results.