Crack evolution behavior of rocks under confining pressures and its propagation model before peak stress

The understanding of crack propagation characteristics and law of rocks during the loading process is of great significance for the exploitation and support of rock engineering.In this study,the crack propagation behavior of rocks in triaxial compression tests was investigated in detail.The main conclusions were as follows:1)According to the evolution characteristics of crack axial strain,the differential stress?strain curve of rocks under triaxial compressive condition can be divided into three phases which are linear elastic phase,crack propagation phase,post peak phase,respectively;2)The proposed models are applied to comparison with the test data of rocks under triaxial compressive condition and different temperatures.The theoretical data calculated by the models are in good agreement with the laboratory data,indicating that the proposed model can be applied to describing the crack propagation behavior and the nonlinear properties of rocks under triaxial compressive condition;3)The inelastic compliance and crack initiation strain in the proposed model have a decrease trend with the increase of confining pressure and temperature.Peak crack axial strain increases nonlinearly with the inelastic compliance and the increase rate increases gradually.Crack initiation strain has a linear relation with peak crack axial strain.

Laboratory determination of fracture aperture, permeability and stress repationships

It is well known that the formation permeability is not a constant but a function of the in situ stress environment. This study has been primarily carried out numerically, and to a certain extent, in the field. However, since the rock properties are generally tested in the laboratory, this last situation needs to be modeled to maintain consistent scales in the analysis. In this paper, concepts and techniques of laboratory experiments are presented to determine relationships between fracture aperture and external loading in simulated rocks (concrete).

Relationship among Parameters Evaluating Stress Corrosion Cracking

The threshold stress, σc, for sulfide stress corrosion cracking (SCC) of seven pipeline steels and five other steels, the critical stress, Sc, for seven pipeline steels and two drill rod steels with various strengths and the susceptibility to SCC, IRA or σf(SCC)/σf, for four pipeline steels, two drill rod steels and five other steels were measured. The results showed that there are no definite relationships among σc, Sc and IRA or σf(SCC)/σf.The threshold stress for hydrogen induced cracking (HlC) during charging with loading in the H2S04 solution, σc(H), decreased linearly with logarithm of the concentration of diffusible hydrogen c0, i.e., σc(H)=A-B Inco for four pipeline steels. σc(H) obtained with a special cathodic current ic, which was corresponding to the diffusible hydrogen concentration during immersing in the H2S solution, were consistent with /c for sulfide SCC for four pipeline steels. Therefore, σc for sulfide SCC can be measured using dynamically charging in the H2SO4 solution with the special cathodic current ic.

Influence of Carbonation on Mechanical Properties of Concrete

As one of the most important factors that determine the lifespan of a reinforced concrete structure, carbonation not only corrodes the reinforcing steel, but also changes the mechanical properties of concrete. For better understanding the performance of carbonated concrete structure, it is necessary to study the mechanical properties of carbonated concrete. The strees strain relationship of carbonated concrete was analyzed on the basis of experiments. The specimens were made by means of accelerated carbonation and then compressed on the testing machine. Some very important characteristics of carbonated concrete were revealed by the testing results. In addition, a useful constitutive model of carbonated concrete, which proved to be suitable for analyzing carbonated concrete members, was established in this research.

A new modeling approach for stress-strain relationship taking into account strain hardening and stored energy by compacted graphite iron evolution

Compacted graphite iron(CGI)is considered to be an ideal diesel engine material with excellent physical and mechanical properties,which meet the requirements of energy conservation and emission reduction.However,knowledge of the microstructure evolution of CGI and its impact on flow stress remains limited.In this study,a new modeling approach for the stress–strain relationship is proposed by considering the strain hardening effect and stored energy caused by the microstructure evolution of CGI.The effects of strain,strain rate,and deformation temperature on the microstructure of CGI during compression deformation are examined,including the evolution of graphite morphology and the microstructure of the pearlite matrix.The roundness and fractal dimension of graphite particles under different deformation conditions are measured.Combined with finite element simulation models,the influence of graphite particles on the flow stress of CGI is determined.The distributions of grain boundary and geometrically necessary dislocations(GNDs)density in the pearlite matrix of CGI under different strains,strain rates,and deformation temperatures are analyzed by electron backscatter diffraction technology,and the stored energy under each deformation condition is calculated.Results show that the proportion and amount of low-angle grain boundaries and the average GNDs density increase with the increase of strain and strain rate and decreased first and then increased with an increase in deformation temperature.The increase in strain and strain rate and the decrease in deformation temperature contribute to the accumulation of stored energy,which show similar variation trends to those of GNDs density.The parameters in the stress–strain relationship model are solved according to the stored energy under different deformation conditions.The consistency between the predicted results from the proposed stress–strain relationship and the experimental results shows that the evolution of stored energy can accurately predict the stress–strain relationship of CGI.

Semi-analytical model for flat indentation of metal materials and its applications

The increasing use of small material components in a wide range of industrial fields necessitates the development of an accurate and robust indentation testing method.To this end,this paper proposes an Energy-density-equivalence for a Flat Indentation(E-FI)model based on the energy density equivalent principle.The proposed model describes the relationships among the material parameters of Hollomon's power law(H-law),flat indenter diameter,energy,and indentation displacement.An E-FI Method(E-FIM)that determines the H-law parameters of materials through the indentation test is also developed.The energy-displacement curves forward-predicted by the E-FI model(based on known H-law parameters of materials)and the H-law parameters of materials given by the E-FIM(based on known energy-displacement curves)are consistent with the results of Finite Element Analysis(FEA)and the H-law parameters of materials used as the input for FEA,respectively.Using E-FIM,the goodness of fit for both stress–strain curves with H-law,predicted based on the displacement with 2%signal interference,and that for stress–strain curves without interference is more than 0.98.The stress–strain relations predicted by E-FIM were consistent with the results obtained via uniaxial tensile tests of ten ductile materials.

Theoretical model and testing method for ball indentation based on the proportional superposition of energy in pure elasticity and pure plasticity

For a homogeneous,continuous,and isotropic material whose constitutive relationships meets with the Ramberg-Osgood law(R-O law),the energy in the elastoplastic indentation with a ball indenter was theoretically analyzed,and the proportional superposition of energy in pure elasticity and pure plasticity during indentation was considered based on the equivalence of energy density.Subsequently,a Proportional Superposition-based Elasto Plastic Model(PS-EPM)was developed to describe the relationships between the displacement and the load during the ball indentation.Furthermore,a new test method of Ball Indentation based on Elastoplastic Proportional Superposition(BI-EPS)was developed to obtain the constitutive relationships of R-O law materials.The load–displacement curves predicted using the PS-EPM model were found to agree closely with the Finite Element Analysis(FEA)results.Moreover,the stress vs.strain curves predicted using the BI-EPS method were in better agreement with those obtained by FEA.Additionally,ball indentation was performed on eleven types of metal materials including five types of aluminum alloys and six types of steel.The test results showed that the stress vs.strain relationships and the tensile strength values predicted using the proposed BI-EPS method agreed well with the results obtained using conventional uniaxial tensile tests.

Confinement properties of circular concrete columns wrapped with prefabricated textile-reinforced fine concrete shells

This paper proposes an innovative column composed of a core column(including both reinforced concrete(RC)and plain concrete(PC)columns)and a prefabricated textile-reinforced fine concrete(TRC)shell.To study the confinement properties of TRC shells on this novel type of concrete column,20 circular specimens,including 12 PC columns and 8 RC columns,were prepared for axial compressive tests.Four key parameters,including the column size,reinforcing ratio of the carbon textile,concrete strength,and stirrup spacing,were evaluated.The results indicated that the compressive properties of the columns were improved by increasing the reinforcing ratio of the textile layers.In the case of TRC-confined PC columns,the maximum improvement in the peak load was 56.3%,and for TRC-confined RC columns,the maximum improvement was 60.2%.Based on the test results,an analytical model that can be used to calculate the stress–strain curves of prefabricated TRC shell-confined concrete columns has been proposed.The calculated curves predicted by the proposed model agreed well with the test results.