Fracture behaviors of columnar jointed rock mass using interface mechanics theorem

For a special geological structure of columnar jointed rock mass(CJRM),its mechanical properties are strongly affected by the columnar joints.To describe the fracture behaviors of CJRM using the basic theories of interface mechanics for composite materials,the interface stresses of the vertical and horizontal joints,which are the two primary joints in the CJRM under triaxial compression,are studied,and their mathematical expressions are derived based on the superposition principle.Based on the obtained interface stresses of the vertical and horizontal joints in the CJRM,the crack initiation of the joint interface in the CJRM is studied using the maximum circumferential stress theory in fracture mechanics.Moreover,based on this investigation,the fracture behaviors of CJRM are analyzed.According to the results of similar material physical model tests for the CJRM,the theoretical study is verified.Finally,the influence of the mechanical parameters of the CJRM on the joint interface stress is discussed comprehensively.

Improving flexural strength of Ti_(2)C-Ti cermet by hot compressive deformation:Microstructure evolution and fracture behaviors

In order to improve the microstructure and mechanical properties,the hot compressive deformation with 50%height reduction at 1100℃was conducted on a Ti_(2)C-Ti cermet.The results showed that the lamellar Ti precipitates in Ti_(2)C grains were transformed to bimodal size distribution,which was approximately 290 nm and 5.8μm in diameter,respectively,after the hot deformation.The bimodal Ti precipitates suppressed{011}cleavage surfaces of Ti_(2)C during flexural fracture,which resulted in an 18.5%increment of strength.This phenomenon can be attributed to the bimodal Ti precipitates that decreased the average crack driving force due to their gentle variation in elastic modulus compared with the monolithic lamellar Ti precipitates.The present work can guide further deformation and mechanical property improvement of Ti_(2)C cermets.

Fracture behaviors of commercially pure titanium under biaxial tension:Experiment and modeling

The fracture behaviors and associated mechanisms of metallic materials under biaxial stress are vital for their manufacturability and service performance.In this work,the fracture behaviors of commercially pure titanium(CP-Ti)under quasi-uniaxial and equi-biaxial tension were investigated by using the digital image correlation technique and finite element modeling.The fracture behaviors under quasi-uniaxial tension were characterized by a general normal fracture.In contrast,normal fracture firstly occurred per-pendicular to the rolling direction(RD)under equi-biaxial tension,followed by secondary shear fracture along the 45°direction relative to the RD.The normal fracture was attributed to the lower strain hard-ening ability in RD compared to the transverse direction(TD)induced by the TD-split type basal texture.The different hardening abilities introduced large shear stress in the 45°direction,which contributed sig-nificantly to the secondary shear fracture.An anisotropy parameter K(△S_(s)/σ_(s)),defined as the ratio of the equivalent effective traction stress to the yield strength,was proposed for the first time,to predict the fracture path with the impact of crystallographic preferred orientation.

Effect of Particle Size on Mechanical Properties and Fracture Behaviors of Age-Hardening SiC/Al–Zn–Mg–Cu Composites

15 vol.% SiC/Al-6.5Zn-2.8 Mg-1.7 Cu(wt%) composites with varying particle sizes(3.5, 7.0, 14 and 20 μm), i.e., C-3.5, C-7.0, C-14, and C-20, respectively, were fabricated by powder metallurgy(PM) method and subjected to microstructural examination. The effect of particle size on mechanical properties and fracture behaviors of the T6-treated composites was revealed and analyzed in detail. Element distribution and precipitates variations in the composites with varying particle sizes were emphatically considered. Results indicated that both tensile strength and plasticity of the T6-treated composites increased first and then decreased with particle size decreasing. The C-7.0 composite simultaneously exhibited the highest ultimate tensile strength(UTS) of 686 MPa and best elongation(El.) of 3.1%. The smaller-sized SiC particle would introduce more oxide impurities, which would react with the alloying element in the matrix to cause Mg segregation and depletion. According to strengthening mechanism analysis, the weakening of precipitation strengthening in the T6-treated C-3.5 composite was the main cause of the lower tensile strength. Additionally, the larger SiC particle, the more likely to fracture, especially in the composites with high yield strength. For the T6-treated C-20 composites, more than 75% SiC particles were broken up, resulting in the lowest plasticity. As decreasing particle size, the fracture behaviors of the T6-treated composites would change from particle fracture to matrix alloy fracture gradually.

Thermal effects on prediction accuracy of dense granite mechanical behaviors using modified maximum tangential stress criterion

Thermally-induced changes in the fracture properties of geological reservoir rocks can influence their stability,transport characteristics,and performance related to various deep subsurface energy projects.The modified maximum tangential stress(MMTS)criterion is a classical theory for predicting the fracture instability of rocks.However,there is a lack of research on the accuracy of MMTS theory when rocks are subjected to different temperatures.In this study,mechanical theoretical analysis and failure and fracture mechanics experiments of granite under the influence of temperatures ranging from 20℃to 600℃are carried out.The results showed that the theoretical estimated value of MMTS differs significantly from the experimental data at 20℃-600℃.The Keff/KIC ratio is less than the experimental test value due to the critical crack growth radius(rc)estimated by the conventional method being larger than the critical crack growth radius(rce)derived from the experimental data.Varied temperatures affect the fracture process zone size of fine-grained,compact granite,and the MMTS theoretical estimation results.Therefore,it is essential to modify the critical crack growth radius for MMTS theory to accurately predict the fracture characteristics of thermally damaged rocks.In addition,the variation of the rock’s me-chanical properties with temperature and its causes are obtained.Between 20℃and 600℃,the mode-Ⅰ,mode-Ⅱ,and mixed-mode(a-30℃and 45℃)fracture toughness and Brazilian splitting strength of the granite decrease by 80%and 73%,respectively.When the rock is heated above 400℃,its deterioration is mainly caused by a widening of its original cracks.

Experimental Investigation on Fracturing Behaviors after Liquid Nitrogen Pre-Injection in High-Temperature Sandstone

The fracturing process of sandstone is inherently complex due to its loose internal structure and deformation adaptability.Liquid nitrogen pre-injection has emerged as a promising approach to damage reservoir rocks,effectively reducing fracture pressure and establishing intricate fracture networks,thus offering a potential solution for reservoir reconstruction.To unravel the fundamental mechanisms governing sandstone fracturing behaviors following liquid nitrogen pre-injection,sandstone fracturing experiments were conducted under varying durations of liquid nitrogen injection,rock temperature,and in-situ stress conditions.The experiments showcased the evolution of injection pressure and fracture characteristics under different testing conditions,complemented by electron microscope analysis to elucidate the factors driving the complex fracture characteristics of sandstone.The findings revealed a significant decrease in fracture pressure after liquid nitrogen pre-injection,accompanied by a notable increase in the complexity of the fracture network and the roughness of the fracture surface.Moreover,prolonging the duration of liquid nitrogen injection and elevating reservoir temperature further contributed to reducing fracture pressure,consequently enhancing fracture complexity and surface roughness.Conversely,the application of confining pressure amplified fracture pressure while intensifying the degree of fracturing.Notably,the investigation highlighted the increased presence of microcracks in sandstone resulting from liquid nitrogen preinjection,facilitating fluid diffusion during fracturing and yielding lower fracture pressures,thereby enhancing the effectiveness of sandstone reservoir reformation.The research results can provide theoretical guidance for geothermal reservoir reconstruction.

Enhancing the mechanical properties of casting eutectic high -entropy alloys via W addition

The effect of W element on the microstructure evolution and mechanical properties of Al_(1.25)CoCrFeNi3 eutectic high-entropy alloy and Al_(1.25)CoCrFeNi_(3-x)W_(x)(x=0,0.05,0.1,0.3,and 0.5;atomic ratio)high-entropy alloys(HEAs)were explored.Results show that the Al_(1.25)CoCrFeNi_(3-x)W_(x) HEAs are composed of face-centered cubic and body-centered cubic(BCC)phases.As W content increases,the microstructure changes from eutectic to dendritic.The addition of W lowers the nucleation barrier of the BCC phase,decreases the valence electron concentration of the HEAs,and replaces Al in the BCC phase,thus facilitating the nucleation of the BCC phase.Tensile results show that the addition of W greatly improves the mechanical properties,and solid-solution,heterogeneous-interface,and second-phase strengthening are the main strengthening mechanisms.The yield strength,tensile strength,and elongation of the Al_(1.25)CoCrFeNi2.95W0.05 HEA are 601.44 MPa,1132.26 MPa,and 15.94%,respectively,realizing a balance between strength and plasti-city.The fracture mode of the Al_(1.25)CoCrFeNi_(3-x)W_(x) HEAs is ductile–brittle mixed fracture,and the crack propagates and initiates in the BCC phase.The eutectic lamellar structure impedes crack propagation and maintains plasticity.

Fracture Behavior of CrN Coatings Under Indentation and Dynamic Cycle Impact

Fracture behavior of CrN coatings deposited on the surface of silicon and AISI52100 steel by different energy ion beam assisted magnetron sputtering technique （IBAMS） was studied using indentation and dynamic cycle impact. It is found that, for the coatings on silicon substrate, the cracks form in the indentation corners and then propagate outward under Vickers indentation. The coating prepared using ion assisted energy of 800 eV shows the highest fracture resistance due to its compact structure. Under Rockwell indentation, only finer radial cracks are found in the CrN coating on AISI 52100 steel without ion assisting while in the condition of ion assisting energy of 800 eV, radial, lateral cracks and spalling appear in the vicinity of indentation. The fracture of CrN coatings under dynamic cycle impact is similar to fatigue. The impact fracture resistance of CrN coatings increases with the increase of ion assisting energy.

The transient fracture behavior for a functionally graded layered structure subjected to an in-plane impact load

The transient fracture behavior of a functionally graded layered structure subjected to an in-plane impact load is investigated. The studied structure is composed of two homogeneous layers and a functionally gradedinterlayer with a crack perpendicular to the boundaries. The impact load is applied on the face of the crack. Fourier transform and Laplace transform methods are used to formulate the present problem in terms of a singular integral equation in Laplace transform domain. Considering variations of parameters such as the nonhomogeneity constant, the thickness ratio and the crack length, the dynamic stress intensity factors （DSIFs） in time domain are studied and some meaningful conclusions are obtained.

Effect of tool plunge depth on the microstructure and fracture behavior of refill friction stir spot welded AZ91 magnesium alloy joints

We used refill friction stir spot welding(RFSSW)to join 2-mm-thick AZ91D-H24 magnesium alloy sheets,and we investigated in detail the effect of tool plunge depth on the microstructure and fracture behavior of the joints.A sound joint surface can be obtained using plunge depths of 2.0 and 2.5 mm.Plunge depth was found to significantly affect the height of the hook,with greater plunge depths corresponding to more severe upward bending of the hook,which compromised the tensile-shear properties of the joints.The hardness reached a minimum at the thermo-mechanically affected zone due to the precipitation phases of this zone as it dissolved into theα-matrix during the welding process.The fracture modes of RFSSW joints can be divided into three types:shear fracture,plug fracture,and shear–plug fracture.Of these,the joint with a shear–plug fracture exhibited the best tensile-shear load of 6400 N.

Influence of silicide on fracture behavior of a fully lamellar Ti-46Al-0.5W-0.5Si alloy

The fracture behavior of fully lamellar binaryγ-TiAl alloys is extremely anisotropic with respect to the lamellar orientation.For the fully lamellar Ti-46Al-0.5W-0.5Si alloy,the existence of silicide clusters plays a critical role on the fracture behavior.In the present study,tensile test and three point bending test were performed at room temperature with the loading axis parallel and perpendicular to the lamellar orientation,respectively.To investigate the influence of silicide clusters on the initiation and propagation of cracks,the fracture surface and the cracks adjacent to the fracture zone of the specimens have been analyzed.Results show that the fracture process is related to the morphology and distribution of the silicide clusters.Crack preferentially initiates at and propagates along the interface of silicide andα 2 /γlamellar with the loading axis perpendicular to the length direction of silicide.While the silicide can prevent the propagation of cracks from running across with the crack growth direction perpendicular to the length direction of silicide.

A sophisticated simulation for the fracture behavior of concrete material using XFEM

The development of a powerful numerical model to simulate the fracture behavior of concrete material has long been one of the dominant research areas in earthquake engineering. A reliable model should be able to adequately represent the discontinuous characteristics of cracks and simulate various failure behaviors under complicated loading conditions. In this paper, a numerical formulation, which incorporates a sophisticated rigid-plastic interface constitutive model coupling cohesion softening, contact, friction and shear dilatation into the XFEM, is proposed to describe various crack behaviors of concrete material. An effective numerical integration scheme for accurately assembling the contribution to the weak form on both sides of the discontinuity is introduced. The effectiveness of the proposed method has been assessed by simulating several well-known experimental tests. It is concluded that the numerical method can successfully capture the crack paths and accurately predict the fracture behavior of concrete structures. The influence ofmode-Ⅱ parameters on the mixed-mode fracture behavior is further investigated to better determine these parameters.

Influence of columnar grain boundary on fracture behavior of as-cast fully lamellar Ti-46Al-0.5W-0.5Si alloy

The fracture behavior of fully lamellar γ-TiAl alloys depends on the angle between the lamellar orientation and loading axis,but the role of the presentation of grain boundary cannot be ignored.To investigate the influence of the grain boundary on the initiation and propagation of cracks,the tensile test of the alloy was conducted at room temperature with loading axis parallel and perpendicular to the lamellar orientation,respectively.The cracks adjacent to the fracture zone of the tensile specimens have been investigated to analyze the fracture behavior.Results show that the grain boundary has dual influences on the fracture behavior.When the loading axis is parallel to the lamellar orientation,cracks are preferentially initiated at and propagate along the grain boundaries.When the loading axis is perpendicular to the lamellar orientation,the grain boundaries can prevent the propagation of cracks from running across.Additionally,serrated-shape grain boundaries have a better inhibiting effect on the propagation of cracks than planar boundaries.

FRACTURE MECHANISM AND MICROMECHANICAL ANALYSIS OF POLYSYNTHETICALLY TWINNED CRYSTALS OF γTiAl ALLOYS

The fracture behavior and mechanism of PST crystals of a Ti 49%(mole fraction)Al alloy have been studied by using in situ straining and micromechanical calculation. The three dimensional micromechanical model representing the structure of PST crystal has been built, and the stress distribution ahead of the sharp and blunt crack tips either parallel to lamellar interface or perpendicular to the lamellae has been calculated by using finite element method based on linear elasticity of PST crystals. The experimental results show that the fracture behaviors and mechanisms are strongly dependent on the angle of loading axis to the lamellae. The calculation indicates that nucleation and propagation of microcrack along the interfaces are controlled by the normal stress and translamellar microcrack is controlled by shear stress ahead of crack tip.

Fracture toughness and fracture behavior of SA508-Ⅲ steel at different temperatures

The fracture toughness of SA508-Ⅲ steel was studied in the temperature range from room temperature to 320℃ using the J-integral method. The fracture behavior of the steel was also investigated. It was found that the conditional fracture toughness （JQ） of the steel first decreased and then increased with increasing test temperature. The maximum and minimum values of do were 517.4 kJ/m^2 at 25℃ and 304.5 kJ/m^2 at 180℃, respectively. Dynamic strain aging （DSA） was also observed to occur when the temperature exceeded 260℃ with a certain strain rate. Both the dislocation density and the number of small dislocation cells effectively increased because of the occurrence of DSA; as a consequence, crack propagation was more strongly inhibited in the steel. Simultaneously, an increasing number of fine carbides precipitated under high stress at temperatures greater than 260℃. Thus, the deformation resistance of the steel was improved and the Jo was enhanced.

INVESTIGATION ON FRACTURE BEHAVIOR FOR WELDED JOINT OF NEW GENERATION FINE GRAINED STEEL SS400

The fracture behavior for welded joint of new generation fine grained steel SS400 was investigated and assessed on the basis of fitness for purpose philosophy. The actual critical defect sizes for the SS400 base metal and its weld HAZ (heat affected zone) defined by the gross yielding criterium have been determined directly by means of wide plate tests. It has been shown that although the HAZ grain growth occurs due to the welding heat, the resistance to fracture is not deteriorated. The deformation behavior of wide plate specimen was also studied by finite, element (FE) analysis. The deformation of weld HAZ is protected by the high strength weld metal, so it is easier to get the general yielding for the welded joint specimen.

High Temperature Tensile Property and Fracture Behavior of Directionally Solidified Fe-Al-Ta Eutectic Composites

Fe-Al-Ta eutectic composites with solidification rates of 6,20,30,80 and 200μm/s were obtained by a modified Bridgman directional solidification technique and alloying.Moreover,tensile property and fracture behavior of Fe-Al-Ta eutectic composites were studied at 600℃.The relationship between mechanical property and microstructure at high temperature was studied.Microstructure of Fe-Al-Ta eutectic is composed of Fe_(2)Ta(Al)Laves phase and Fe(Al,Ta)matrix phase.In addition,the tensile strength at high temperatures is higher than that at room temperature.The tensile strength is increased with the increase of solidification rate.Moreover,fracture morphology transforms from cleavage fracture to dimple fracture as the solidification rate is increased at high temperatures.

EVALUATION OF EMBRITTLEMENT BEHAVIOR DUETO SENSITIZATION IN A CRYOGENIC AUSTENITICSTAINLESS STEEL BY MEANS OF SMALL PUNCHAND FRACTURE TOUGHNESS TESTS

In order to evaluate the tendency of mechanical properties degrudation due to weld-ing and other processing in materials used for supporting coils in super conducting rnaguets utilized in thermonuclear jusion reactore, a small punch (SP) test was used.This test, which was originally developed to study irradiation damage using miniatursized specimens was performed at 77 and 4 K for solution treated and sensitized JN1 austenitic stainless steel, a candidate cryogenic structural material. The area under the load-deflection curve up to the maximum applied load in SP test was defined as the SP enerpy, to characterize the resistance to fracture. Although solution treated material exhibited ductile fracture mode with high SP enerpy, embrittlement behavior due to sensitization at 650-800°for 1-5 h was shown clearlg by SP test with brittle intergranular fracture and decreased SP enerpy. Comparison of the results obtained by SP test with those by fracture toughness test showed the usefulness of SP test for evaluation of sensitization induced embrittlement at cryogenic temperature. The re-sults obtained in this study can be very usefol in predicting the degradation due to welding and other processing in cryogenic materials.

A MICROMECHANICS METHOD TO STUDY THE EFFECT OF DOMAIN SWITCHING ON FRACTURE BEHAVIOR OF POLYCRYSTALLINE FERROELECTRIC CERAMICS

The effect of domain switching on anisotropic fracture behavior of polycrystalline ferroelectric ceramics was revealed on the basis of the micromechanics method. Firstly, the electroelastic field inside and outside an inclusion in an infinite ferroelectric ceramics is carried out by the way of Eshelby-Mori-Tanaka's theory and a statistical model, which accounts for the influence of domain switching. Further, the crack extension force (energy-release rate) G(ext) for a penny-shape crack inside an effective polycrystalline ferroelectric ceramics is derived to estimate the averaged effect of domain switching on the fracture behavior of polycrystalline ferroelectric ceramics. The simulations of the crack extension force for a crack in a BaTiO3 ceramics are shown that the effect of domain switching must be taken into consideration while analyzing the fracture behavior of polycrystalline ferroelectric ceramics. These results also demonstrate that the influence of the applied electric field on the crack propagation is more profound at smaller mechanical loading and the applied electric field may enhance the crack extension in a sense, which are consistent with the experimental results.