The law of strength damage and deterioration of jointed sandstone after dry-wet cycles

Under the periodic rise and fall of the water level in the Three Gorges Reservoir in China,the rock mass in the ebb and flow zone of the slope is always in a state of a dry-wet cycle.In order to explore the influence of dry-wet cycle on mechanical properties of jointed sandstone,the triaxial and uniaxial compression tests of dry-wet cycle of jointed sandstone were carried out.For the experiment,four groups of samples with different numbers of joints were set up,and the jointed rock samples were subjected to 20 dry-wet cycles.Using both the triaxial compression test and the Mohr-Coulomb(M-C)rock fracture criterion,the strength envelope of the sandstone samples was fitted,and their strength degradation was further analyzed and studied.The results show that:(1)The peak intensity and elastic modulus of the sandstone samples decrease with increased number of dry-wet cycles.(2)The total deterioration of mechanical properties of intact rock samples is bigger than that of jointed sandstone samples as the number of dry-wet cycles increases.(3)With the increase of confining pressure,the peak intensity of intact sandstone samples increases much more than that of jointed sandstone samples,which indicates that joints and their numbers have obvious influence.(4)Joints and their numbers play an important role in guiding the damage effects of sandstone samples,which weaken the damage caused by dry-wet cycles.Therefore,the envelope of the M-C strength criterion of intact sandstone samples moves more than that of jointed sandstone samples.

Anisotropy of crack initiation strength and damage strength of coal reservoirs

The crack volume strain method and acoustic emission(AE)method are used to analyze the anisotropy of the crack initiation strength,damage strength,the failure mode and the AE characteristics of coal reservoir.The results show that coal reservoirs show obvious anisotropic characteristics in compressive strength,cracking initiation strength and damage strength.The compressive strength of coal reservoirs decreases with the increase of bedding angle,but the reservoirs with bedding angles of 450 and 900 differ little in compressive strength.The crack initiation strength and damage strength decrease first and then increase with the increase of bedding angle.The crack initiation strength and damage strength are the highest,at the bedding angle of 0°,moderate at the bedding angle of 90°,and lowest at the bedding angle of 45°.When the bedding angle is 0°,the failure of the coal reservoirs is mainly steady propagation of large-scale fractures.When the bedding angle is 45°,one type of failure is caused by steady propagation of small-scale fractures,and the other type of failure is due to a sudden instability of large-scale fractures.When the bedding angle is 90°,the failure is mainly demonstrated by a sudden-instability of small-scale fractures.Compared with the cumulative count method of the AE,the cumulative energy method is more suitable for determining crack initiation strength and damage strength of coal reservoirs.

Damage and Fracture Strength Behavior of Jointed Rockmass

The strength of rockmass from two aspects is analyzed.Firstly,the strength of the rockmass is mainly controlled by the critical stress value of rock,and the contribution of joints is to increase the effective stresses of rock and to decrease the damage strength of rockmass according to the macro damage mechanics of rockmass.Secondly,the strength of rockmass is mainly controlled by the fracture strength of joints.Based on the comprehensive analysis and comparison for the damage strength of rockmass and the fracture strength of joints,a composite damage theory of rockmass may be established.

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.

Experimental Investigation of the Effect of the Material Damage Induced in Sheet Metal Forming Process on the Service Performance of 22MnB5 Steel

The use of ultra-high strength steels through sheet metal forming process offers a practical solution to the lightweight design of vehicles.However,sheet metal forming process not only produces desirable changes in material properties but also causes material damage that may adversely influence the service performance of the material formed.Thus,an investigation is conducted to experimentally quantify such influence for a commonly used steel（the 22MnB5 steel） based on the hot and cold forming processes.For each process,a number of samples are used to conduct a uniaxial tensile test to simulate the forming process.After that,some of the samples are trimmed into a standard shape and then uniaxially extended until fracture to simulate the service stage.Finally,a microstructure test is conducted to analyze the microdefects of the remaining samples.Based on the results of the first two tests,the effect of material damage on the service performance of 22MnB5 steel is analyzed.It is found that the material damages of both the hot and cold forming processes cause reductions in the service performance,such as the failure strain,the ultimate stress,the capacity of energy absorption and the ratio of residual strain.The reductions are generally lower and non-linear in the former process but higher and linear in the latter process.Additionally,it is found from the microstructure analysis that the difference in the reductions of the service performance of 22MnB5 by the two forming processes is driven by the difference in the micro damage mechanisms of the two processes.The findings of this research provide a useful reference in terms of the selection of sheet metal forming processes and the determination of forming parameters for 22MnB5.

Fragility of disconnect switch subjected to random earthquake ground motions

A fragility calculation scheme is estabtished in this paper for porcelain-type equipments subjected to random earthquake ground motions. All steps of the method are illustrated by the seismic damage analysis of GW4-110 disconnect switch. The model of the equipment is built applying the finite element method with flexible joints, and the seismic response of the equipment is analyzed using elastic time history method. On the base, according to the strength damage index and Monte-Carlo Method, the seismic damage ratios are counted and the seismic fragility curves are presented. Then the seismic damage of GW4-110 disconnect switch can be predicted.

Damage characteristics and constitutive modeling of the 2D C/Si C composite: Part I – Experiment and analysis

This paper reports an experimental investigation on the macroscopic mechanical behaviors and damage mechanisms of the plain-woven（2D） C/Si C composite under in-plane on- and offaxis loading conditions. Specimens with 15, 30, and 45 off-axis angles were prepared and tested under monotonic and incremental cyclic tension and compression loads. The obtained results were compared with those of uniaxial tension, compression, and shear specimens. The relationships between the damage modes and the stress state were analyzed based on scanning electronic microscopy（SEM） observations and acoustic emission（AE） data. The test results reveal the remarkable axial anisotropy and unilateral behavior of the material. The off-axis tension test results show that the material is fiber-dominant and the evolution rate of damage and inelastic strain is accelerated under the corresponding combined biaxial tension and shear loads. Due to the damage impediment effect of compression stress, compression specimens show higher mechanical properties and lower damage evolution rates than tension specimens with the same off-axis angle. Under cyclic tension–compression loadings, both on-axis and off-axis specimens exhibit progressive damage deactivation behaviors in the compression range, but with different deactivation rates.