High Temperature Plastic Deformation Behavior and Brittle Mechanism of S450EW Steel

High-temperature tensile tests were conducted for high corrosion resistant weathering steel S450EW.The morphologies of fracture microstructures,dislocations and precipitates were investigated by field emission scanning electron microscopy and transmission electron microscopy.The high-temperature plastic deformation behavior and brittle mechanism of S450EW steel were also studied.The experimental results show that the ductility troughs appear at 700-850℃ and 650-900℃ when the strain rates are 3×10^(-3)and 1.5×10^(-2)s^(-1),respectively.With the increase of strain rates,the ductility trough moves to the lower temperature side.The hot ductility is best when the cooling rate is 5℃/s before deformation at 750℃and the area reduction rate reaches 60.56%.Fine second phase particles and inclusions precipitated before and during deformation provide effective core positions for microcracks or microvoids formation during deformation process.It is also easy to cause stress concentration which results in microcracks or microvoids between grains during deformation and ultimately causes damage along the grain boundaries.The precipitated particles inhibit austenite dynamic recrystallization and therefore enhance intergranular fracture along austenite grain boundaries.The deformation induced proeutectoid ferrite films distribute along the austenite grain boundaries hinder the dynamic recrystallization.The deformation concentrated on network ferrite films produces damage of grain boundaries.

Validity of continuous-failure-state unloading triaxial tests as a means to estimate the residual strength of rocks

The residual strength of rocks and rock masses is an important parameter to be constrained for analysis and design purposes in many rock engineering applications.A residual strength envelope in principal stress space is typically developed using residual strength data obtained from compression tests on many different specimens of the same rock type.In this study,we examined the potential for use of the continuous-failure-state testing concept as a means to constrain the residual strength envelope using a limited number of specimens.Specifically,cylindrical specimens of three rock types(granodiorite,diabase,and Stanstead granite)were unloaded at the residual state such that a full residual strength envelope for each individual specimen was obtained.Using a residual strength model that introduces a single new strength parameter(the residual strength index,or RSI),the results of the continuous-failurestate unloading tests were compared to conventionally obtained residual strength envelopes.Overall,the continuous-failure-state residual strength data were found to be consistent with the conventional residual strength data.However,it was identified that the primary factor limiting an accurate characterization of the residual strength for a given rock type is not the amount of data for a given specimen,but the variety of specimens available to characterize the inherent variability of the rock unit of interest.Accordingly,the use of continuous-failure-state testing for estimation of the residual strength of a rock unit is only recommended when the number of specimens available for testing is very limited(i.e.<5).

Hydraulic properties of dune sand-bentonite mixtures of insulation barriers for hazardous waste facilities

The material and elastic properties of rocks are utilized for predicting and evaluating hard rock brittleness using artificial neural networks（ANN）. Herein hard rock brittleness is defined using Yagiz＇method. A predictive model is developed using a comprehensive database compiled from 30 years＇ worth of rock tests at the Earth Mechanics Institute（EMI）, Colorado School of Mines. The model is sensitive to density, elastic properties, and P- and S-wave velocities. The results show that the model is a better predictor of rock brittleness than conventional destructive strength-test based models and multiple regression techniques. While the findings have direct implications on intact rock, the methodology can be extrapolated to rock mass problems in both tunneling and underground mining where rock brittleness is an important control.