A damage-based constitutive model for rock under impacting load

Using the Splitting Hopkinson Pressure Bar （SHPB） experimental system, investigations were made into the dynamic mechanical performances of underground soft rocks. The experiments proved that the measured stress-strain curves display the characteristics of plastic deformation. By making use of a revised overstress constitutive formula for the stress model and by taking into account that the strain rate and strain are a function of I - E（t）/Eo, a revised overstress constitutive formula for the stress model was simplified by applying dimensional analysis and consequently, a simplified overstress formula was obtained for the stress model. Then, by taking into consideration the effects of damage under a dynamic load on the dynamic loading strength of the rock, the continuous damage theory and the statistical strength theory were introduced into the development of the simplified overstress constitutive formula for the stress model. Hence, a damage-based constitutive formula for an overstress model, which can be appropriately applied to the analysis of full dynamic stress-strain curves, was developed. By using the simplified damage-based constitutive formula for an overstress model, the actually measured curves are fitted, indicating that the fitting curves and those actually measured are in good agreement.

LED灯具芯片的趋额电应力防护设计

LED灯具设计师一般对过电应力冲击防护设计的重要性认识不足。设计工程师必须考虑在满足灯具尺寸、成本、照明效果等要求的前提下,为LED灯具提供有效的过电应力冲击保护设计。本文就LED灯具过电应力冲击的故障现象、产生原因、解决办法予以简单论述。

Study of the Formation Conditions of Aluminum Oxide Nanoparticles in an Overstressed Nanosecond Discharge Between Aluminum Electrodes in a Mixture of Nitrogen and Oxygen

The results of the study of oscillograms of voltage,current,pulsed electric power and energy input into the plasma of an overstressed nanosecond discharge between aluminum electrodes in argon and mixtures of nitrogen with oxygen(100-1)at pressures in the range of 13.3-103.3 kPa are presented,the emission plasma spectra are studied.It is shown that in mixtures of nitrogen with oxygen at atmospheric pressure,nanoparticles of aluminum oxide(Al2O3)are formed,the luminescence of which manifests itself in the spectral range of 200-600 nm and which is associated with the formation of F-,F+-centers and more complex aggregate formations based on oxygen vacancies.Calculations of the electron-kinetic coefficients of plasma,transport characteristics,such as mean electron energies in the range 5.116-13.41 eV,are given.The electron concentration was 1.6∙10^(20)m^(-3)-1.1∙10^(20)m^(-3)at a current density of 5.1∙10^(6)A/m^(2)and l.02∙10^(7)A/m^(2)on the surface of the electrode of the radiation source(0.196·10^(-4)m^(2)).Also drift velocities,temperatures and concentrations of electrons,specific losses of the discharge power for elastic and inelastic processes of collisions of electrons per unit of the total concentration of the mixture from the reduced electric field strength(E/N)for a mixture of aluminum,nitrogen,oxygen,rate constants of collisions of electrons with aluminum atoms on the E/N parameter in plasma on a mixture of aluminum vapor,oxygen and nitrogen=30:1000:100000 Pa at a total mixture pressure of P=101030 Pa are given.

AN ENDOCHRONIC CONSTITUTIVE MODEL FOR RATE-DEPENDENT AND NONPROPORTIONAL CYCLIC PLASTICITY

The total stress response of material is decomposed into a sum of an equilibrium stress response and a non-equilibrium overstress response. Correspondingly, the rate-independent intrinsic time and the rate-dependent intrinsic time are defined respectively. Additional hardening functions for describing the isotropic and anisotropic nonproportional effects are assumed to be related to the accumulation of plastic strain component along the normal of equilibrium stress trajectory, in which the effects of geometry of the loading path are included. An endochronic constitutive model for rate-dependent, nonproportional cyclic plasticity is formulated and applied to simulate the stress responses of stainless steel XCrNil8. 9 for some typical loading programs at different loading rates. A comparison between predicted results and experimental ones by Haupt and Lion shows that the former are in agreement with the latter.

Degradation of light emitting diodes:a proposed methodology

Due to their long lifetime and high efficacy, light emitting diodes have the potential to revolutionize the illumination industry. However, self heat and high environmental temperature which will lead to increased junction temperature and degradation due to electrical overstress can shorten the life of the light emitting diode. In this research, a methodology to investigate the degradation of the LED emitter has been proposed. The epoxy lens of the emitter can be modelled using simplified Eyring methods whereas an equation has been proposed for describing the degradation of the LED emitters.