Statistical formulation and experimental determination of growth rate of micrometre cracks under impact loading

A new statistical formulation and a relevant experimental approach to determine the growth rate of microcracks were proposed. The method consists of experimental measurements and a statistical analysis' on the basis of the conservation law of number density of microcracks in phase space. As a practical example of the method, the growth rate of microcracks appearing in an aluminium alloy subjected to planar impact loading was determined to be ca. 10 mu m/mu s under a tensile stress of 1470 MPa and load duration between 0.26 mu s and 0.80 mu s.

A stochastic jump and deterministic dynamics model of impact failure evolution with rate effect

Motivated by the observation of the rate effect on material failure, a model of nonlinear and nonlocal evolution is developed, that includes both stochastic and dynamic effects. In phase space a transitional region prevails, which distinguishes the failure behavior from a globally stable one to that of catastrophic. Several probability functions are found to characterize the distinctive features of evolution due to different degrees of nucleation, growth and coalescence rates. The results may provide a better understanding of material failure.

Combustion tube studies of dust flame acceleration

A new pneumatic dispersion system for obtaining a good quality uniform dust suspension in a horizontal dust combustion tube was developed. The effect of three different dispersion techniques on self-sustained dust flame acceleration in such a combustion tube was examined. The importance of the dispersion quality in the test tube for maintaining a self-sustained dust flame acceleration was demonstrated. A combustion tube for studies of flame acceleration in fine aluminum dust-air mixture and its transition to detonation under industrial ignition conditions was constructed in the course of the present study. It consists mainly of an initiation section and a test section. The initiation section must be equipped in a well-developed dispersion system for creating a good dispersion condition in the test tube. The length of this section is 3 meters. The test tube requires only to distribute uniformly the dust over the bottom of the tube prior to the experiment. The aluminum dust spherical in shape with 6 mu m in diameter was used for tests. Experimental results demonstrated that the increase in flame velocity is roughly linear through the entire length of the test tube. The highest flame propagation velocity in fine aluminum dust-air mixture approaches some 1200m/s at a distance of 4.8m from the ignition plate.

Fatigue crack-growth rate in ferrite martensite dual-phase steel

An empirical study is made on the fatigue crack growth rate in ferrite-martensite dual-phase (FMDP) steel. Particular attention is given to the effect of ferrite content in the range of 24.2% to 41.5% where good fatigue resistance was found at 33.8%. Variations in ferrite content did not affect the crack growth rate when plotted against the effective stress intensity factor range  which was assumed to follow a linear relation with the crack tip stress intensity factor range ΔK. A high  corresponds to uniformly distributed small size ferrite and martensite. No other appreciable correlation could be ralated to the microstructure morphology of the FMDP steel. The closure stress intensity factor , however, is affected by the ferrite content with  reaching a maximum value of 0.7. In general, crack growth followed the interphase between the martensite and ferrite.

Dividing the fatigue crack growth process into Stage I and II where the former would be highly sensitive to changes in ΔK and the latter would increase with ΔK depending on the  ratio. The same data when correlated with the strain energy density factor range ΔS showed negligible dependence on mean stress or R ratio for Stage I crack growth. A parameter α involving the ratio of ultimate stress to yield stress, percent reduction of area and R is introduced for Stage II crack growth so that the  data for different R would collapse onto a single curve with a narrow scatter band when plotted against αΔS.

Coflow jets with large velocity difference - a new aerodynamic principle of flame stabilization and combustion intensification

A new aerodynamic principle of flame stabilization and combustion intensification, the coflow jets with large velocity difference, is described. One or more small high-velocity jets of air or steam, injected off the axis and in the same direction as the low-velocity main fuel-air flow into the combustor, create a large recirculation zone of high turbulence intensity in which the combustibles and high temperature gases are effectively mixed, so that stable and intensive combustion can be maintained even for fuels with poor ignition. A pulverized coal combustor based on the principle mentioned above is shown to be characteristic of excellent combustoom and a simple structure. A number of precombustors of this type are in operation at some power stations and industrial boilers of China. Using such precombustor, successtul startups and part-load operation of the boilers have become available under conditions of unpreheated air and low-grade coal with volatiles as low as 15% and ash content as high as 30%. This principle shows good promise as an attractive new technology of combustion.

Diffusion sliding rate in the binary gaseous mixture

radiation incident upon a test cell filled with gaseous SF6 has