Numerical investigation on the tensile fracturing behavior of rock-shotcrete interface based on discrete element method

Four groups of numerical models of Brazilian tests on rock-shotcrete interfaces were successfully conducted by PFC2D. The tensile strength and Young’s modulus of shotcrete were considered. Six different undulations of rock-shotcrete interface were set up. The influences of multiple parameters on the bearing characteristics of the rock-shotcrete interface were studied. The results showed that a better support performance can be obtained by increasing the Young’s modulus of shotcrete rather than the tensile strength of shotcrete. For different tensile strength and Young’s modulus, the increase of sawtooth height has different effects on the support performance. The failure mechanism of the rock-shotcrete interfaces was analysed in detail. The stress shielding effect and stress concentration effect caused by the shape characteristics of rock-shotcrete interface were observed. The influence of these parameters on the overall support performance should be fully considered in a reasonable support design.

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.

Microstructure evolution,mechanical properties and fracture behavior of Al-xSi/AZ91D bimetallic composites prepared by a compound casting

In this paper,the effect of the Si content on microstructure evolution,mechanical properties,and fracture behavior of the Al-xSi/AZ91D bimetallic composites prepared by compound casting was investigated systematically.The obtained results showed that all the Al-xSi/AZ91D bimetallic composites had a metallurgical reaction layer(MRL),whose thickness increased with increasing Si content for the hypoeutectic Al-Si/AZ91D composites,while the hypereutectic Al-Si/AZ91D composites were opposite.The MRL included eutectic layer(E layer),intermetallic compound layer(IMC layer)and transition region layer(T layer).In the IMC layer,the hypereutectic Al-Si/AZ91D composites contained some Si solid solution and flocculent Mg_(2)Si+Al-Mg IMCs phases not presented in the hypoeutectic Al-Si/AZ91D composites.Besides,increasing Si content,the thickness proportion of the T layer increased,forming an inconsistent preferred orientation of the MRL.The shear strengths of the Al-xSi/AZ91D bimetallic composites enhanced with increasing Si content,and the Al-15Si/AZ91D composite obtained a maximum shear strength of 58.6 MPa,which was 73.4% higher than the Al-6Si/AZ91D composite.The fractures of the Al-xSi/AZ91D bimetallic composites transformed from the T layer into the E layer with the increase of the Si content.The improvement of the shear strength of the Al-xSi/AZ91D bimetallic composites was attributed to the synergistic action of the Mg_(2)Si particle reinforcement,the reduction of oxidizing inclusions and the ratio of Al-Mg IMCs as well as the orientation change of the MRL.

Regulation of Pore Structure and Hightemperature Fracture Behavior of CACbonded Alumina-Spinel Castables Based on Hydration Design

The lamellar hydrates of CAC were designed with the introduction of nano CaCO_(3)or Mg-Al hydrotalcite(M-A-H),and the effects on the green strength,pore structures,and high-temperature fracture behavior of alumina-spinel castables were investigated.The results show that nano CaCO_(3)or M-A-H stimulates rapidly the hydration of CAC and the formation of lamellar C_(4)AcH_(11)or coexistence of C_(2)AH_(8)and C_(4)AcH_(11)at 25℃.The formation of lamellar hydrates can contribute to a more complicated pore structure,especially in the range of 400-2000 nm.Meanwhile,the incorporation of well-distributed CaO or MgO sources from nano CaCO_(3)or M-A-H also regulates the distribution of CA_(6)and spinel(pre-formed and in-situ).Consequently,the optimized microstructure and complicated pore structure can induce the deflection and bridging of cracks,thus facilitating the consumption of fracture energy and enhancing the resistance to thermal stress damage.

Effects of temperature on fracture behavior of Al-based in-situ composites reinforced with Mg_2Si and Si particles fabricated by centrifugal casting

An aluminum-based in-situ composites reinforced with Mg2Si and Si particles were produced by centrifugal casting A1-20Si-5Mg alloy. The microstructure of the composites was examined, and the effects of temperature on fracture behavior of the composite were investigated. The results show that the average fraction of primary Si and Mg2Si particles in the composites is as high as 38%, and ultimate tensile strengths （UTS） of the composites first increase then decrease with the increase of test temperature. Microstructures of broken specimens show that both the particle fracture and the interface debonding affect the fracture behavior of the composites, and the interface debonding becomes the dominant fracture mechanism with increasing test temperature. Comparative results indicate that rich particles in the composites and excellent interface strength play great roles in enhancing tensile property by preventing the movement of dislocations.

Effects of plastic deformation on precipitation behavior and tensile fracture behavior of Mg-Gd-Y-Zr alloy

The effects of plastic deformation on precipitation behavior and tensile fracture behavior of Mg-10Gd-3Y-0.6Zr alloy were investigated.The results indicate that more precipitation cores can be provided by the crystal defects caused by the plastic deformation,as well as increasing the amount of β＇ phases,and the formation of precipitations at grain boundaries and interfaces between the twins and matrix.Because of an increase in precipitations,the dislocation slipping during deformation process is effectively hindered and the matrix is strengthened,especially for the 2% deformed alloy which can achieve a good combination of strength and ductility.With increasing the plastic deformation,the microcracks occur at the interface between grain boundary precipitations and matrix,and then propagate intergranularly.When intergranular fracture combines with the formation of smoothing facets on the fracture surface,the tensile properties decrease.

Fracture behavior of double-pass TIG welded 2219-T8 aluminum alloy joints under transverse tensile test

2219-T8 aluminum alloys were butt welded by the double-pass tungsten inert gas （TIG） arc welding process. The transverse tensile test of the joint showed that the fracture mainly occurred in the partially melted zone （PMZ）. Effects of the PMZ on the fracture behavior were systematically studied. Continuous intergranular eutectics were observed in the PMZ close to the fusion line. Away from the fusion line, the intergranular eutectics in the PMZ became discontinuous. The fracture morphology and the microhardness distribution of the joint showed that the PMZ was gradient material with different mechanical properties, which strongly affected the fracture process. It was observed that the crack initiated in the PMZ near the front weld toe, and propagated in the PMZ away from the fusion line. Then, the crack tip was blunt when it propagated into the PMZ with higher plasticity. Finally, the rest part of the joint was shear fractured.

Strength and fatigue fracture behavior of Al-Zn-Mg-Cu-Zr(-Sn) alloys

The strength and fatigue fracture behavior of A1-Zn-Mg-Cu-Zr（-Sn） alloys were studied by performing tensile tests and fatigue crack propagation （FCP） tests. The microstructures of the experimental alloys were further analyzed using optical microscopy （OM）, scanning electron microscopy （SEM）, and transmission electron microscopy （TEM）; phase analysis of these alloys was conducted with an X-ray diffraction （XRD）. The results show that when Sn is included, growth of the recrystallization grains in the solution-treated A1-Zn-Mg-Cu-Zr alloy is obstructed, the precipitation-free zone （PFZ） of the overaged A1-Zn-Mg-Cu-Zr-Sn alloy becomes narrow, and the grain boundary precipitates are smaller. Consequently, the FCP resistance is higher. In addition, the overaged Sn-containing alloy has considerably higher tensile strength than the alloy without Sn.

Fracture behavior and microstructure of as-cast NiTi shape memory alloy

The as-cast ingot of equiatomic nickel-titanium shape memory alloy （NiTi SMA） was prepared by vacuum consumable arc melting. The tensile tests and the compressive tests with respect to as-cast NiTi SMA were performed to study its mechanical properties of fracture. The microanalysis of as-cast NiTi SMA as well as its fractured samples was performed so as to better understand microstructure evolution and fracture behavior of NiTi SMA. Under tensile loading, the as-cast NiTi SMA shows higher plasticity and is characterized by ductile fracture at 750℃, but it demonstrates poorer plasticity and is characterized by cleavage fracture as well as transcrystalline fracture at room temperature and -100 ℃. Under compressive loading at -100 ~C, the as-cast NiTi SMA is characterized by shear fracture where the normal to the shearing fracture surface inclines about 45° to the compressive axis, and belongs to cleavage fracture where the cracks exoand via transcrvstalline fracture.

Variation of the second-phase morphology and its influence on fracture behavior of Al-Si alloy

Using an optical microscope and scanning electron microscope (SEM), the variation of eutectic Si morphology of Al-Si alloy in solution treatment was observed to study its influence on mechanical properties and fracture behavior. The results show that eutectic Si undergoes stubbing, necking, fragmentation, and growth in the initial stage (250 min); in the middle solution stage (250 to 400 min), the eutectic Si morphology has no significant change, only the degree of spheroidizing becomes higher; after 600 min, the growth of eutectic Si is a coarsening process controlled by diffusion and follows the Liftshitz-Slyozov-Wangner (LSW) model, and the eutectic Si morphology deteriorates due to the occurrence of facets and lap. Based on the quantitative measure and regression analysis, the eutectic Si morphology has a remarkable influence on mechanical properties and fracture behavior.

Fracture behavior of high strength Mg-Gd-Y-Zr magnesium alloy

The fracture behavior of a permanent mould casting Mg-8.57Gd-3.72Y-0.54Zr(mass fraction,%)(GW94) alloy was investigated under different thermal conditions,including as-cast,solution-treated,peak-aged,and over-aged states.Scanning electron microscopy(SEM) and optical microscopy(OM) were employed to examine the crack nucleation and fracture model.The results indicate that the GW94 alloy shows different behaviors of crack initiation and fracture under different thermal conditions. During tensile test at room temperature,the fracture model of the as-cast GW94 alloy is quasi-cleavage,while that of the solution-treated alloy is transgranular cleavage.It is a mixed pattern of transgranular and intergranular fracture for both the aged conditions.Large cavities formed at grain boundaries are observed in the peak-aged sample tested at 300℃,corresponding to the intergranular fracture.Localized plastic deformation at grain boundaries is also observed and corresponds to the high elongation at 300℃.

Hot ductility behavior of a Fe-0.3C-9Mn-2Al medium Mn steel

The hot ductility of a Fe-0.3C-9Mn-2Al medium Mn steel was investigated using a Gleeble3800 thermo-mechanical simulator.Hot tensile tests were conducted at different temperatures(600-1300℃)under a constant strain rate of 4×10^(−3)s^(−1).The fracture behavior and mechanism of hot ductility evolution were discussed.Results showed that the hot ductility decreased as the temperature was decreased from 1000℃.The reduction of area(RA)decreased rapidly in the specimens tested below 700℃,whereas that in the specimen tested at 650℃was lower than 65%.Mixed brittle-ductile fracture feature is reflected by the coexistence of cleavage step,intergranular facet,and dimple at the surface.The fracture belonged to ductile failure in the specimens tested between 720-1000℃.Large and deep dimples could delay crack propagation.The change in average width of the dimples was in positive proportion with the change in RA.The wide austenite-ferrite intercritical temperature range was crucial for the hot ductility of medium Mn steel.The formation of ferrite film on austenite grain boundaries led to strain concentration.Yield point elongation occurred at the austenite-ferrite intercritical temperature range during the hot tensile test.

Rock damage and fracturing induced by high static stress and slightly dynamic disturbance with acoustic emission and digital image correlation techniques

A series of coupled static-dynamic loading tests is carried out in this study to understand the effect of slightly dynamic disturbance on the rocks under high static stress.The acoustic emission(AE)and digital image correlation(DIC)techniques are combined to quantitatively characterize the damage and fracturing behaviors of rocks.The effects of three influencing factors,i.e.initial static stress,disturbance amplitude,and disturbance frequency,on the damage and fracturing evolution are analyzed.The experimental results reveal the great differences in AE characteristics and fracturing behaviors of rocks under static loads and coupled static-dynamic loads.Both the Kaiser effect and Felicity effect are observed during the disturbance loading process.The crack initiation,stable and unstable propagation in the highly-stressed rocks can be triggered by cyclic disturbance loads,and more local tensile splitting cracks are found in the rocks subjected to coupled static-dynamic loads.The damage and fracturing evolution of rocks during cyclic disturbances can be divided into two stages,i.e.steady and accelerated stages,and the increase rate and proportion of each stage are greatly affected by these influencing factors.High initial static stress,low disturbance frequency,and high disturbance amplitude are considered to be adverse factors to the stability of the rocks,which would induce a high increase rate of the steady stage and a high proportion of the accelerated stage within the whole disturbance period.Based on the two-stage damage evolution trend,a linear-exponential damage model is employed to predict the instability of the rocks under coupled static-dynamic loads.

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 binary γ-TiAI 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 and a2/7 lamellar 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.

Failure behavior of horseshoe-shaped tunnel in hard rock under high stress:Phenomenon and mechanisms

A particle flow code(PFC) was first applied to examining the mechanical response of a horseshoe-shaped opening in prismatic rock models under biaxial compression. Next, an improved complex variable method was proposed to derive the stress distribution around the opening. Lastly, a case study of tunnel failure caused by rock burst in Jinping Ⅱ Hydropower Station was further analyzed and discussed. The results manifest that a total of four types of cracks occur around the opening under low lateral confining stress, namely, the primary-tensile cracks on the roof-floor, sidewall cracks on the sidewalls, secondary-tensile cracks on the corners and shear cracks along the diagonals. As the confining stress increases, the tensile cracks gradually disappear whilst the spalling failure becomes severe. Overall, the failure phenomenon of the modelled tunnel agrees well with that of the practical headrace tunnel, and the crack initiation mechanisms can be clearly clarified by the analytical stress distribution.

Squeeze casting of aluminum alloy A380: Microstructure and tensile behavior

A380 alloy with a relatively thick cross-section of 25 mm was squeeze cast using a hydraulic press with an applied pressure of 90 MPa. Microstructure and tensile properties of the squeeze cast A380 were characterized and evaluated in comparison with the die cast counterpart. Results show that the squeeze cast A380 possesses a porosity level much lower than the die cast alloy, which is disclosed by both optical microscopy and the density measurement technique. The results of tensile testing indicate the improved tensile properties, specifically ultimate tensile strength(UTS: 215.9 MPa) and elongation(Ef: 5.4%), for the squeeze cast samples over those of the conventional high-pressure die cast part(UTS: 173.7 MPa, Ef: 1.0%). The analysis of tensile behavior shows that the squeeze cast A380 exhibits a high tensile toughness(8.5 MJ·m-3) and resilience(179.3 k J·m-3) compared with the die cast alloy(toughness: 1.4 MJ·m-3, resilience: 140.6 k J·m-3), despite that, during the onset of plastic deformation, the strain-hardening rate of the die cast specimen is higher than that of the squeeze cast specimens. The microstructure analyzed by the scanning electron microscopy(SEM) shows that both the squeeze and die cast specimens contain the primary α-Al, Al2 Cu, Al5 Fe Si phase and the eutectic Si phase. But, the Al2 Cu phase present in the squeeze cast alloy is relatively large in size and quantity. The SEM fractography evidently reveals the ductile fracture features of the squeeze cast A380 alloy.

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.