Water‑immersion softening mechanism of coal rock mass based on split Hopkinson pressure bar experiment

The coal mining process is afected by various water sources such as groundwater and coal seam water injection.Understanding the dynamic mechanical parameters of water-immersed coal is helpful for coalmine safe production.The impact compression tests were performed on coal with diferent moisture contents by using theϕ50 mm Split Hopkinson Pressure Bar(SHPB)experimental system,and the dynamic characteristics and energy loss laws of water-immersed coal with diferent compositions and water contents were analyzed.Through analysis and discussion,it is found that:(1)When the moisture content of the coal sample is 0%,30%,60%,the stress,strain rate and energy frst increase and then decrease with time.(2)When the moisture content of the coal sample increases from 30%to 60%,the stress“plateau”of the coal sample becomes more obvious,resulting in an increase in the compressive stress stage and a decrease in the expansion stress stage.(3)The increase of moisture content of the coal sample will afect its impact deformation and failure mode.When the moisture content is 60%,the incident rod end and the transmission rod end of the coal sample will have obvious compression failure,and the middle part of the coal sample will also experience expansion and deformation.(4)The coal composition ratio suitable for the coal immersion softening impact experiment is optimized.

Revealing the Pressure-Induced Softening/Weakening Mechanism in Representative Covalent Materials

Diamond, cubic boron nitride(c-BN), silicon(Si), and germanium(Ge), as examples of typical strong covalent materials, have been extensively investigated in recent decades, owing to their fundamental importance in material science and industry. However, an in-depth analysis of the character of these materials' mechanical behaviors under harsh service environments, such as high pressure, has yet to be conducted. Based on several mechanical criteria, the effect of pressure on the mechanical properties of these materials is comprehensively investigated.It is demonstrated that, with respect to their intrinsic brittleness/ductile nature, all these materials exhibit ubiquitous pressure-enhanced ductility. By analyzing the strength variation under uniform deformation, together with the corresponding electronic structures, we reveal for the first time that the pressure-induced mechanical softening/weakening exhibits distinct characteristics between diamond and c-BN, owing to the differences in their abnormal charge-depletion evolution under applied strain, whereas a monotonous weakening phenomenon is observed in Si and Ge. Further investigation into dislocation-mediated plastic resistance indicates that the pressure-induced shuffle-set plane softening in diamond(c-BN), and weakening in Si(Ge), can be attributed to the reduction of antibonding states below the Fermi level, and an enhanced metallization, corresponding to the weakening of the bonds around the slipped plane with increasing pressure, respectively. These findings not only reveal the physical mechanism of pressure-induced softening/weakening in covalent materials, but also highlights the necessity of exploring strain-tunable electronic structures to emphasize the mechanical response in such covalent materials.

Mechanism of hot-rolling crack formation in lean duplex stainless steel 2101

The thermoplasticity of duplex stainless steel 2205（DSS2205） is better than that of lean duplex steel 2101（LDX2101）, which undergoes severe cracking during hot rolling. The microstructure, microhardness, phase ratio, and recrystallization dependence of the deformation compatibility of LDX2101 and DSS2205 were investigated using optical microscopy（OM）, electron backscatter diffraction（EBSD）, Thermo-Calc software, and transmission electron microscopy（TEM）. The results showed that the phase-ratio transformations of LDX2101 and DSS2205 were almost equal under the condition of increasing solution temperature. Thus, the phase transformation was not the main cause for the hot plasticity difference of these two steels. The grain size of LDX2101 was substantially greater than that of DSS2205, and the microhardness difference of LDX2101 was larger than that of DSS2205. This difference hinders the transfer of strain from ferrite to austenite. In the rolling process, the ferrite grains of LDX2101 underwent continuous softening and were substantially refined. However, although little recrystallization occurred at the boundaries of austenite, serious deformation accumulated in the interior of austenite, leading to a substantial increase in hardness. The main cause of crack formation is the microhardness difference between ferrite and austenite.

Microstructure Evolution and Dynamic Softening Mechanism of 00Cr22Ni5Mo3N DSS During Hot Compression

Through plane strain compression, hot ductility of 00Cr22Ni5Mo3N DSS is studied under plane strain con dition, and the dynamic softening mechanism is investigated through microstructure observation under TEM. The results show that the deformation temperature can markedly influence the peak stress of 00Cr22NiSMo3N specimens. And being different from DSS softening mechanism generally reported, ferrite can be softened through dynamic re eovery and recrystallization, hut austenite can he softened only through dynamic recovery during hot deformation. The unfavourable effect of N on softening capacity of austenite is greater than that of Ni.

Hot Deformation Behavior of Ni_3Al-based Alloy MX246A

The hot deformation behavior of homogenized Nia Al-based alloy MX246A has been characterized on the basis of its flow stress variation obtained by isothermal constant true strain rate compression testing on the MTS 810 machine in the temperature range of 1 150--1225 ℃ and strain rate range of 0. 001-0.1 s-1. Microstructural obser- vation revealed striped secondary γ＇ phase which was vertical to compression axis, and precipitation of fine ternary γ＂ phase. The amount of striped secondary γ＇ phase reduced and that of fine ternary γ＇ phase increased with increasing temperature and decreasing strain rate. The material exhibited peak stress followed by flow softening, but no obvious steady-state flow behavior. Microstructural investigations have shown no dynamic recrystallization happened. TEM studies indicated that the flow softening was controUed by dynamic recovery mechanism.

Multi-scale crystal plasticity finite element simulations of the microstructural evolution and formation mechanism of adiabatic shear bands in dual-phase Ti20C alloy under complex dynamic loading

A dynamic compression test was performed on α+β dual-phase titanium alloy Ti20C using a split Hopkinson pressure bar.The formation of adiabatic shear bands generated during the compression process was studied by combining the proposed multi-scale crystal plasticity finite element method with experimental measurements.The complex local micro region load was progressively extracted from the simulation results of a macro model and applied to an established three-dimensional multi-grain microstructure model.Subsequently,the evolution histories of the grain shape,size,and orientation inside the adiabatic shear band were quantitatively simulated.The results corresponded closely to the experimental results obtained via transmission electron microscopy and precession electron diffraction.Furthermore,by calculating the grain rotation and temperature rise inside the adiabatic shear band,the microstructural softening and thermal softening effects of typical heavily-deformed α grains were successfully decoupled.The results revealed that the microstructural softening stress was triggered and then stabilized(in general)at a relatively high value.This indicated that the mechanical strength was lowered mainly by the grain orientation evolution or dynamic recrystallization occurring during early plastic deformation.Subsequently,thermal softening increased linearly and became the main softening mechanism.Noticeably,in the final stage,the thermal softening stress accounted for 78.4% of the total softening stress due to the sharp temperature increase,which inevitably leads to the stress collapse and potential failure of the alloy.