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This paper develops a three-dimensional (3D) thermal-structure coupling model, implements transient stress analysis of thermoelastic contact of disk brakes with a frictional heat variation and identifies the source of the thermal fatigue. This thermostructure model allows the analysis of the effects of the moving heat source (the pad) with a variable speed and integrates the heat flux coupling between the sliding surfaces. To obtain the transient stress/temperature fields of the brake under an emergency braking, the thermoelastic problem under this 3D model is solved by the finite element method. The numerical results from the analysis and simulation show the temperature/stress of the disk presenting periodic sharp fluctuation due to the continuous cyclic loading; its varying frequency corresponds to the rotated cycle times of the braking disk. The results demonstrate that the maximum surface equivalent stress may exceed the material yield strength during an emergency braking, which may cause a plastic damage accumulation in a brake disk, while a residual tensile hoop stress is incurred on cooling. These results are validated by experimental observation results available in the literature. Based on these numerical results, some suggestions for avoiding fatigue fracture propagation are further presented. [DOI: 10.1115/1.2736437] Keywords: transient stress analysis, heat flux coupling, thermal fatigue fracture, frictional heat, disk brakes

Different chip separation criteria for the FEM simulation of machining were examined. Criterion based on distance between the tool tip and the node located immediately ahead, criterion based on maximum shear stress in the element ahead of the tool tip, criterion based on average maximum shear stress in the shear plane, and criterion based on a combination of distance and stress were investigated. Under conditions of smooth separation of chip from workpiece, simulation results showed that, during steady-state cutting, the type of chip separation criteria did not greatly affect chip geometry, nor distributions of stress and strain. The magnitude of the chip separation criteria also did not significantly affect chip geometry and distributions of stress in the chip but it did affect the chip separation process, distributions of stress in the machined surface, and distributions of effective plastic strain both in the chip and in the machined surface. During the initiation of cutting, neither the geometrical nor physical criteria simulate the machining process correctly. A combination of geometric and physical criteria was also recommended in this study.

Abstract: Nickel has shown to be a good catalyst for improving the hydrolysis performances of hydrogenated Mg3RE (abbreviated as H-Mg3RE, RE = La, Ce, Pr, Nd) through investigating the hydrolysis kinetics and mechanisms of hydrogenated Mg3RENi0.1 (abbreviated as H-Mg3RENi0.1). A material with superior hydrolysis performance, namely H-Mg3CeNi0.1 has been identified, which can generate 276 mL g−1 min−1 hydrogen in the first 1.5 min and achieve a total yield of 1088 mL g−1. Ni served to modify the hydrolysis mechanisms of H-Mg3La, H-Mg3Pr, and H-Mg3Nd to improve their hydrolysis properties. Moreover, Ni has been found to promote the direct hydrolysis of CeH3 to CeO2 to liberate more hydrogen and to render the stable Nd2H5 reactive towards water. Therefore, the studied H-Mg3RE systems all show improved hydrolysis properties after the addition of Ni. [Copyright &y& Elsevier]

Abstract: Hydrogenated Mg3Mm (where Mm denotes La-rich and Ce-rich misch metal) has been identified as an excellent hydrolysis material for the first time. Hydrolysis of the as-hydrogenated Mg3Mm produced 695 mL g−1 hydrogen in 5 min, 828 mL g−1 hydrogen in 15 min, and the highest hydrogen yield reached 9.79 wt.% at 298 K. Compared to hydrogenated Mg3La, Mg3Ce, Mg3Pr, and Mg3Nd, hydrogenated Mg3Mm displayed the fastest hydrolysis rate due to mutual catalysis of LaH3, CeH3, PrH3, and Nd2H5. The hydrolysis mechanisms of hydrogenated Mg3RE are also discussed herein. [Copyright &y& Elsevier]

Abstract: In this work, the hydrogenation mechanism for LaMg12 alloy is revealed by introducing hydrogen step by step and the hydrogenation degree is found to affect the hydrolysis properties of LaMg12H x hydrides. Because the hydrolysis performance of Mg is affected limitedly while MgH2 is accelerated significantly by LaH3, the hydrolysis rate is controllable through adjusting the ratio of LaH3 to MgH2. The synergistic effect of LaH3 and MgH2 leads to the complete hydrolysis of the fully hydrogenated LaMg12H27, yielding 9.73 wt. % (1089.55mlg−1) hydrogen. [Copyright &y& Elsevier]

The trapping dynamics of photogenerated electrons and holes in MoS[sub 2] nanoclusters has been studied using time resolved emission polarization and absorption spectroscopies. These results are compared to absorption kinetics obtained on MoS[sub 2] nanoclusters with adsorbed DTDCI (DTDCI≡diethylthiodicarbocyanine iodide) dye. The results indicate that emission from the MoS[sub 2] band edge state is polarized, while emission from trapped electrons and holes is unpolarized. This polarization difference is used to obtain the electron and hole trapping times and values of 275 ps and 42 ps, respectively, are obtained. Decays having the same time constants are observed in the transient absorption results. The results obtained on the MoS[sub 2]/DTDCI system show that electron injection occurs with a time constant of 12 ps. These kinetics also show a 225 ps decay component which is assigned to electron trapping and reverse electron transfer. The 225 ps decay time along with the 275 ps trapping time indicates that reverse electron transfer from the conduction band has a time constant of about 1.2 ns. Following trapping, reverse electron transfer from deep traps is slow. © 2000 American Institute of Physics. [ABSTRACT FROM AUTHOR]