Xianghong Xu

Ductile to brittle transition in dynamic fracture of brittle bulk metallic glass

We report an unusual transition from a locally ductile to a pure brittle fracture in the dynamic fracture of brittle Mg65Cu20Gd10 bulk metallic glass. The fractographic evolution from a dimple structure to a periodic corrugation pattern and then to the mirror zone along the crack propagation direction during the dynamic fracture process is discussed within the framework of the meniscus instability of the fracture process zone. This work might provide an important clue in understanding of the energy dissipation mechanism for dynamic crack propagation in brittle glassy materials

Temperature dependent dynamics transition of intermittent plastic flow in a metallic glass. II. Dynamics analysis

By reducing the testing temperatures down to the temperature well below the glassy transition temperature, the serrated flow behaviour during plastic deformation of a Zr-based metallic glass was experimentally investigated and the results were presented in Part I of the present paper. It shows that the yield strength, the plastic deformation ability, the density of shear bands of the metallic glass increase with decreasing temperature. In order to understand the mechanisms for the changes of the mechanical behaviour at low temperatures, in Part II of this study, the stress-time sequence in the plastic strain regime is characterized by a comprehensive dynamical and statistical analysis. The stress-time sequence is found to exhibit a chaotic state at high temperatures (>203 K), whereas a self-organized critical state is obtained at low temperatures (<= 203 K) due to the freezing effect. The reasons for the transition between these two distinct spatio-temporal dynamical states are elucidated by investigating the effect of temperature on the deformation units (shear transformation zones) and the elastic interactions between neighbouring shear bands. The results demonstrate that the low temperatures results in an enhancement of the interactions between the elastic strain fields initiated by neighbouring shear bands, which is primarily responsible for the enhanced plasticity of the metallic glass and a dynamics transition.

Dimension limit for thermal shock failure

We analytically present the characteristic dimensional limit below which the thermal shock failure of ceramics never occurs. This limit, together with the critical temperature difference, separates the state space of the ceramics under thermal shock into two parts – the cracked and the uncracked. Based on the water-quench tests of ceramics, we experimentally proved that when the states of ceramics are in the uncracked region, the ceramics do not produce any cracks during thermal shock. The results provide a guide to prevent thermal shock failure in ceramic.

Experimental Evidence of Critical Sensitivity in Catastrophe

The paper presents an experimental study on critical sensitivity in rocks. Critical sensitivity means that the response of a system to external controlling variable may become significantly sensitive as the system approaches its catastrophic rupture point. It is found that the sensitivities measured by responses on three scales (sample scale, locally macroscopic scales and mesoscopic scale) display increase prior to catastrophic transition point. These experimental results do support the concept that critical sensitivity might be a common precursory feature of catastrophe. Furthermore, our previous theoretical model is extended to explore the fluctuations in critical sensitivity in the rock tests.

Effect of temperature-dependent surface heat transfer coefficient on the maximum surface stress in ceramics during quenching

Abstract We study the difference in the maximum stress on a cylinder surface σmax using the measured surface heat transfer coefficient hm instead of its average value ha during quenching. In the quenching temperatures of 200, 300, 400, 500, 600 and 800°C, the maximum surface stress σmmax calculated by hm is always smaller than σamax calculated by ha, except in the case of 800°C; while the time to reach σmax calculated by hm (fmmax) is always earlier than that by ha (famax). It is inconsistent with the traditional view that σmax increases with increasing Biot number and the time to reach σmax decreases with increasing Biot number. Other temperature-dependent properties also have a small effect on the trend of their mutual ratios with quenching temperatures. Such a difference between the two maximum surface stresses is caused by the dramatic variation of hm with temperature, which needs to be considered in engineering analysis.

Effect of grain size on thermal shock property of alumina ceramic

Ceramic has a great broad application in high-temperature environment due to its favorable mechanical, antioxidant and corrosion resistance properties. However, it tends to exhibit severe crack or fail under thermal shock resulting from its inherent brittleness. Microstructure property is a vital factor and plays a critical role in influencing thermal shock property of ceramic. The present study experimentally tested and characterized thermal-shock crack and residual strength of ceramic under different quench temperature, while two kinds of alumina ceramics with different grain size are employed. A two-dimensional (2D) numerical model based on statistical mesoscopic damage mechanics is introduced to depict the micro-crack propagation of ceramic sheet under water quenching. The effects of grain size on critical thermal shock temperature, crack characteristics and residual strength are studied. And the microscopic mechanism of the influence of grain size on thermal shock resistance of ceramic is discussed based on the crack propagation path obtained from experimental and simulation results. The qualitative effect and evolution change of grain size on thermal shock property of alumina ceramic will be summarized.