Damage and fracture mechanism of 6063 aluminum alloy under three kinds of stress states

To study the damage and fracture mechanism of 6063 aluminum alloy under different stress states,three kinds of representative triaxial stress states have been adopted,namely smooth tensile,notch tensile,and pure shear.The results of the study indicate the following.During the notch tensile test,a relatively higher stress triaxiality appears in the root of the notch.With the applied loading increasing,the volume fraction of microvoids in the root of the notch increases continuously.When it reaches the critical volume fraction of microvoids,the specimen fractures.During the pure shear test,the stress triaxiality almost equals to zero,and there is almost no microvoids but a shear band at the center of the butterfly specimen.The shear band results from nonuniform deformation constantly under the shear stress.With stress concentration,cracks are produced within the shear band and are later coalesced.When the equivalent plastic strain reaches the critical value（equivalent plastic fracture strain）,the butterfly specimen fractures.During the smooth tensile test,the stress triaxiality in the gauge of the specimen remains constant at 0.33.Thus,the volume of microvoids of the smooth tensile test is less than that of the notch tensile test and the smooth specimen fractures due to shearing between microvoids.The G-T-N damage model and Johnson-Cook model are used to simulate the notch tensile and shear test,respectively.The simulated engineering stress-strain curves fit the measured engineering stress-strain curves very well.In addition,the empirical damage evolution equation for the notch specimen is obtained from the experimental data and FEM simulations.

Fracture mechanism of a cylindrical shell cut by circumferential detonation collision

The failure mechanism of a cylindrical shell cut into fragments by circumferential detonation collision was experimentally and numerically investigated.A self-designed detonation wave regulator was used to control the detonation and cut the shell.It was found that the self-designed regulator controlled the fragment shape.The macrostructure and micro-characteristics of fragments revealed that shear fracture was a prior mechanism,the shell fractured not only at the position of detonation collision,but the crack also penetrated the shell at the first contact position of the Chapmen-Jouguet(C-J)wave.The effects of groove number and outer layer thickness on the fracture behavior were tested by simulations.When the thickness of the outer layer was 5e18 mm,it has little effect on fragmentation of the shell,and shells all fractured at similar positions.The increase of the groove number reduced the fracture possibility of the first contact position of the C-J wave.When the groove number reached 7 with a 10 mm outer layer(1/4 model),the fracture only occurred at the position of detonation collision and the fragment width rebounded.

Mechanical Properties and Fracture Mechanism of a Glass Fiber Reinforced Polypropylene Composites

To study the effect of some parameters, such as, length and fraction of glass fiber (GF), and the fraction of maleic anhydride grafted polypropylene (PP-g-MAH), on the mechanical properties of glass fiber reinforced polypropylene (GF/PP) composites, tensile tests, bending tests and impact tests were conducted. Scanning electron microscope (SEM) was used to characterize the fracture mechanisms of the composites. The results show that, compared with 3 mm GF, 9 mm GF can significantly improve the strength of the composite better. Addition of PP-g-MAH, a kind of grafting agent, into the PP-30% LGF composite can result in a better mechanical properties because of the strengthening of the bonding interface between the matrix and the fiber. When the mass fraction of GF is 30% and the PP-g-MAH fraction is 6%, the mechanical properties of the composite are the best.

Mechanical Properties and Fracture Mechanism of Boron Carbide-Lanthanum Boride Composite

The properties of boron carbide-lanthanum boride composite material prepared by hot pressed sintering method was tested, and lanthanum boride as a sintering aid for boron carbide was investigated. The result shows that the hardness of boron carbide-lanthanum boride increases with the increasing content of lanthanum boride. When the content of the lanthanum boride is 6%, the hardness reaches its supreme value of 31.83 GPa, and its hardness is improved nearly 20.52% compared to monolithic boron carbide. The content of the lanthanum boride does not greatly affect flexibility strength, however, it gives much effect on fracture toughness. The curve of fracture toughness likes the form of saw-toothed wave as the content of lanthanum boride increases in the test. When the content of the lanthanum boride is 6%, the fracture toughness reaches its supreme value of 5.14 MPa·m 1/2, which is improved nearly 39.67% compared with monolithic boron carbide materials. The fracture scanning electric microscope analysis of boron carbide-lanthanum boride composite material shows that, with the increase of the content of lanthanum boride, the interior station of monolithic boron carbide is changed. The crystallite arrangement is so compact that pores disappear gradually. The main fracture way of boron carbide-lanthanum boride composite material is intercrystalline rupture, while the transcrystalline rupture is minor, which is in accordance with fracture mechanism of ceramic material. It indicates that this change of fracture mode by the addition of lanthanum boride gives rise to the improvement of the fracture toughness.

Effects of the Solution Treatment on Microstructural Evolution, Mechanical Properties, and Fracture Mechanism of Nickel-Based GH4099 Superalloy

Grain growth, mechanical properties, and fracture mechanism of nickel-based GH4099 superalloy are investigated using heat treatments, tensile tests, optical microscopy (OM), and scanning electron microscopy (SEM) with energy dispersive X-ray spectroscopy (EDS). The OM observation shows that the matrix grains (γ-grains) undergo an apparent growth during the solution treatment. The grain size diameter increases from 100 to 174 μm when the solution temperature rises from 1100℃ to 1160℃ for 30 min. When the holding time increases from 15 to 60 min at 1140℃, the grain size diameter increases from 140 to 176 μm, indicating that the γ-grain growth is more sensitive to temperature than time. Standard deviation, Sv, and the grain size distribution are utilized to characterize the microstructural uniformity. To predict the grain size more accurately, we develop the grain growth kinetics and find that the growth index is close to 5. The yield strength (Rp0.2), tensile strength (Rm), and ductility (Af) are also measured. It is found that the effect decreases in the order cooling rate, solution temperature, time. Rp0.2 reduces by 47% with the increase in the cooling rate from 1℃ to 8000℃/min, while both strength and ductility exhibit little changes with time. The SEM results show that the fracture surfaces have typical mixed brittle and ductile characteristics when specimens are subjected to water quenching and air cooling. However, a complete brittle fracture occurs under furnace cooling conditions. The EDS analysis indicates that the brittle γ' (Ni3Ti) phase precipitates around the γ-grain boundary during the slow cooling process, which is the main factor yielding the complete brittle fracture. Finally, the optimal solution treatment scheme for the GH4099 superalloy is proposed—a temperature of 1140℃ for 30 min followed by air cooling.

Fracture Mechanism of 3D, Five-directional Braided (SiO_2)_f /SiO_2 Composites Prepared by Silicasol-infiltration-sintering Method

Three-dimensional （3D） five-directional braided （SiO2）/SiO2 composites were prepared by silicasol-infiltration-sintering （SIS） method. The flexural properties and microstructures were studied. The flexural strength and flexural elastic modulus were found to be 73 MPa and 12 GPa, respectively. The results of stress vs deflection curve and SEM examinations revealed that the fracture mechanism of 3D, five-directional braided （SiO2）/SiO2 composite was a mixture mode of ductile and brittle. The ductile mode was attributed to the weak bonding strength of fiber/matrix at low temperature. The brittle fracture might be caused by the propagation of micro defect or crack, which existed in the as-prepared composites for the ten-cycle process.

SUPERPLASTICAL DEFORMATION AND FRACTURE MECHANISM 0F A SiC_p/2024Al COMPOSITE

Soperplastic tensions on an IM SiCp/2024Al composite were conducted. The microstrvcture and fmcture sudece of the composite under the optimum saperplastic deformation condition were examined. The eoperimental results show that as the increase of strain during superpldstic deformation, grains fundarnentally remained equiaxed structure, and dislocation density increases gradually and its structure changes hem intererossed into nets each other to tangled and cellular structure,and the amount of liquid phase at the intedeces or gruin boundaries increases gradually. Mcrostructure examination revealed that failure took place by damage accumulation of the pmpressive decohesion of the SiC particle-matrix until a critical volume fruction was reached.

Fracture mechanism of inclusions in thick plate center under tensile loading

Segregation of solute atoms in the center of thick plates of the tempered steel can cause an inhomogeneous structural transformation and generate micron-sized inclusions,which leads to lamellar tearing of thick plate and decreases the plasticity and toughness.The formation and fragmentation mechanisms of micron-sized inclusions,like MnS and(Nb,Ti)C,in the center of thick plates were investigated by using thermodynamic calculations,finite element simulations,and electron backscatter diffraction characterization techniques.The results show that micron-sized inclusions nucleate and grow in the liquid phase,and under tensile loading,they exhibit three fragmentation mechanisms.The local stress during the fragmentation of inclusions is lower than the critical fracture stress of adjacent grains,and phase boundaries can effectively impede crack propagation into the matrix.The existence of a low proportion of high-angle grain boundaries(58.1%)and high Kernel average misorientation value(0.534°)in the segregation band promotes inclusions fragmentation and crack propagation.The difference in crack initiation and propagation direction caused by the morphology of inclusions and physical properties,as well as different matrix arrest abilities,is the main reasons for the diversity of inclusion fragmentation.

Microdynamic mechanical properties and fracture evolution mechanism of monzogabbro with a true triaxial multilevel disturbance method

The far-field microdynamic disturbance caused by the excavation of deep mineral resources and underground engineering can induce surrounding rock damage in high-stress conditions and even lead to disasters.However,the mechanical properties and damage/fracture evolution mechanisms of deep rock induced by microdynamic disturbance under three-dimensional stress states are unclear.Therefore,a true triaxial multilevel disturbance test method is proposed,which can completely simulate natural geostress,excavation stress redistribution(such as stress unloading,concentration and rotation),and subsequently the microdynamic disturbance triggering damaged rock failure.Based on a dynamic true triaxial test platform,true triaxial microdynamic disturbance tests under different frequency and amplitudes were carried out on monzogabbro.The results show that increasing amplitude or decreasing frequency diminishes the failure strength of monzogabbro.Deformation modulus gradually decreases during disturbance failure.As frequency and amplitude increase,the degradation rate of deformation modulus decreases slightly,disturbance dissipated energy increases significantly,and disturbance deformation anisotropy strengthens obviously.A damage model has been proposed to quantitatively characterize the disturbance-induced damage evolution at different frequency and amplitude under true triaxial stress.Before disturbance failure,the micro-tensile crack mechanism is dominant,and the micro-shear crack mechanism increases significantly at failure.With the increase of amplitude and frequency,the micro-shear crack mechanism increases.When approaching disturbance failure,the acoustic emission fractal dimension changes from a stable value to local large oscillation,and finally increases sharply to a high value at failure.Finally,the disturbance-induced failure mechanism of surrounding rock in deep engineering is clearly elucidated.

Fracture mechanism and separation conditions of pineapple fruit-stem and calibration of physical characteristic parameters

At present,the research on the physical composition and properties of pineapple plants is scarce,and the uncertainty of fruit picking method is the key bottleneck factor hindering the research and development of pineapple harvesting machinery.Based on the statistics of survey data from many places,this paper analyzes the fruit-stem fracture mechanism and the theoretical conditions for optimal separation through structural modeling,mechanical behavior analysis and function judgment.On this basis,the“pineapple plant fixation bench”and“fruit-stem bending separation torque test equipment”were developed,and large-size,small-size tests and random optimization tests were carried out successively.The test results showed that the larger of the stem-stalk fixation distance,the more torque and fracture starting angle required for fruit fracture would increase,and the change range was small when the stem-stalk fixation distance was within 50 mm,and the probability of brittle fracture and complete separation was very high.When the space between the fracture section and the fruit-stem connecting point is about 5mm,the range of bending moment value required for the fruit-stem fracture is 1.88 to 2.77 N·m,the range of fracture starting angle is 12.2°to 18.1°,and the angular travel range during the separation process is 82.9°to 87.5°.When the stem-stalk fixation distance is about 15 mm,it is the best fruit-stem separation condition and the breaking torque measured in the verification test is about 2.76 N·m.The fracture starting angle is about 13.8°,the maximum prediction error is 13.1%,and the elastic modulus near the fruit-stem joint ranges from 16.1 to 23.9 MPa.This conclusion can provide an important design basis for the research and development of pineapple field picking robot and harvesting equipment.

Correlation between Porosity and Fracture Mechanism in High Pressure Die Casting of AM60B Alloy

X-ray tomography was used to characterize the porosity in high pressure die casting of AM60B alloy. In situ tensile deformation was performed to observe the change of porosities and their influences on crack initiation, propagation and subsequent fracture of specimen. Results showed that four types of porosi- ties, including gas-shrinkage pore, gas pore, net-shrinkage and island-shrinkage, could be identified according to the formation mechanism and morphology characterization. During tensile deformation, it was shown that the gas-shrinkage pore and net-shrinkage, rather than gas pore or island-shrinkage, were the main sources for crack initiation. In addition, the crack propagated by interconnecting the po- rosities at the cross section with minimum efficient force bearing area. At these locations where externally solidified crystals （ESCs） were present, the crack would propagate along the ESC boundaries in an inter- granular mode, while at these locations without ESCs, the crack would propagate roughly along the direction perpendicular to the tensile stress in a combination of trans-granular and inter-granular modes.

Fatigue Performance of Microalloyed High-strength Rebar and Analysis of Fracture Mechanism

Fatigue performance of hot-rolled ribbed-steel bar with the yield strength of 500 MPa （HRB500） was stud- ied with bend-rotating fatigue test at a stress ratio of R = -1. It is determined by staircase method that its fatigue strength for 107 cycles is 451 MPa, which is higher than that of common carbon structural steel. This should be at- tributed to the fine-grain strengthening resulting from the high content of alloy element V and Thermo-Mechanical Control Process （TMCP）. The S-N curve function is also obtained by nonlinear regression with three parameters power function. The fatigue fractures of the specimen were further analyzed with Scanning Electron Microscopy （SEM） and Energy Disperse Spectroscopy （EDS） to study the fracture mechanism. Taking into account microstruc- ture, hardness and cleanliness of the material, it implies that the fatigue fractures of HRB500 rebar all arise from surface substrates in which many brittle inclusions are contained, and that the fatigue crack propagation is principally based on the mechanism of quasi-cleavage fracture, because of the intracrystalline hard spots leading to stress con- centration and thus to the cracks. Moreover, the transient breaking area exhibits microvoid coalescence of ductile fracture due to the existing abundant inclusions.

Microstructural evolution and fracture mechanism of a new AlZn-Mg-Cu-Zr alloy processed by equal-channel angular pressing

Equal-channel angular pressing(ECAP) is considered to be one of the most effective processing procedures to produce ultrafine-grained(UFG) materials.A new AI-ZnMg-Cu-Zr alloy was subjected to ECAP experiment in Route B_(C)(i.e.,rotated 90° in the same sense between each pass).The effect of ECAP on the microstructural evolution and fracture mechanism of the alloy was analyzed by optical microscope(OM),X-ray diffraction(XRD),tensile test(at room temperature),and scanning electron microscope(SEM).The results show that the main strengthening phase of the alloy is MgZn_(2),whose content increases apparently after ECAP,the dislocation density increases by one order of magnitude,the hardness value of the 1 pass ECAPed sample increases by 53 %,the ultimate tensile strength(UTS)increases by 29 %,and the true strain increases by 15.4 %.But the further increase in the strength with the passes increasing would cost a slight drop in the true strain.The morphology of the tensile dimples is converted from elongated one to equiaxed one with a uniform distribution of size and depth after pressing.

Evaluation on Fatigue Performance and Fracture Mechanism of Laser Welded TWIP Steel Joint Based on Evolution of Microstructure and Micromechanical Properties

The fatigue performance and fracture mechanism of laser welded twinning induced plasticity（TWIP）steel joint were investigated experimentally based on the evolution of microstructure and micromechanical properties.The optical microscopy was used to analyze the evolution of microstructure.The variation of composition and phase structure of fusion zone were detected by energy dispersive X-ray and X-ray diffraction spectrometers.The micromechanical behaviors of the various zones were characterized using nanoindentation.The static tensile test and high cycle fatigue test were performed to evaluate the mechanical properties of welded joint and base metal.The microstructures,tensile properties and fatigue strength of base metal as well as welded metal were analyzed.The fatigue fracture surfaces of base metal and welded joint were observed by means of scanning electron microscopy,in order to identify fatigue crack initiation sites and propagation mechanisms.Moreover,the fatigue fracture characteristics and mechanisms for the laser welded TWIP steel joints were analyzed.

ANALYSIS ON MECHANISM OF FRACTURE FOR THE SURFACE-HARDENING STEEL

The tensile characteristic of nitrocarburized 45 steel treated by different cooling rates was studied in this work. We had observed a new kind zigzig tensile curve and it showed that the tensile curve and the characterastics of the failure surface related to the cooling rate.

Size effects on the tensile strength and fracture toughness of granitic rock in different tests

This study investigates the tensile failure mechanisms in granitic rock samples at different scales by means of different types of tests.To do that,we have selected a granitic rock type and obtained samples of different sizes with the diameter ranging from 30 mm to 84 mm.The samples have been subjected to direct tensile strength(DTS)tests,indirect Brazilian tensile strength(BTS)tests and to two fracture toughness testing approaches.Whereas DTS and fracture toughness were found to consistently grow with sample size,this trend was not clearly identified for BTS,where after an initial grow,a plateau of results was observed.This is a rather complete database of tensile related properties of a single rock type.Even if similar databases are rare,the obtained trends are generally consistent with previous scatter and partial experimental programs.However,different observations apply to different types of rocks and experimental approaches.The differences in variability and mean values of the measured parameters at different scales are critically analysed based on the heterogeneity,granular structure and fracture mechanics approaches.Some potential relations between parameters are revised and an indication is given on potential sample sizes for obtaining reliable results.Extending this database with different types of rocks is thought to be convenient to advance towards a better understanding of the tensile strength of rock materials.

A Study on Propagation Mechanism of Fracture Systems in Rock Masses by Discontinuity Displacement Method

The main task of fracture mechanics of rock masses is the study on the propagating mechanism of fractures in rock masses , which can be efficiently conducted by discontinuty displacement (DD) numerical evaluation . Firstly ,the element stress and displacement are analysed and the principle and steps of the numerical calculation of stress intensity factor and fracture extension force are introduced .The numerical results of parallel and echelon fracture systems ,which are compared with real field fractures .are presented. Finally . a simple engineering application example is presented .

Influence of gas transport mechanisms on the productivity of multi-stage fractured horizontal wells in shale gas reservoirs

In order to investigate the influence on shale gas well productivity caused by gas transport in nanometer- size pores, a mathematical model of multi-stage fractured horizontal wells in shale gas reservoirs is built, which considers the influence of viscous flow, Knudsen diffusion, surface diffusion, and adsorption layer thickness. A dis- crete-fracture model is used to simplify the fracture mod- cling, and a finite element method is applied to solve the model. The numerical simulation results indicate that with a decrease in the intrinsic matrix permeability, Knudsen diffusion and surface diffusion contributions to production become large and cannot be ignored. The existence of an adsorption layer on the nanopore surfaces reduces the effective pore radius and the effective porosity, resulting in low production from fractured horizontal wells. With a decrease in the pore radius, considering the adsorption layer, the production reduction rate increases. When the pore radius is less than 10 nm, because of the combined impacts of Knudsen diffusion, surface diffusion, and adsorption layers, the production of multi-stage fractured horizontal wells increases with a decrease in the pore pressure. When the pore pressure is lower than 30 MPa, the rate of production increase becomes larger with a decrease in pore pressure.

Relationship between fracture spacing and bed thickness in sedimentary rocks:Approach by means of Michaelis-Menten equation

Fractures occur in nearly all rocks at the Earth’s surface and exert essential control on the mechanical strengths of rock masses and permeability.The fractures strongly impact the stability of geological or man-made structures and flow of water and hydrocarbons,CO_(2) and storing waste.For this,the dependence of opening mode fracture spacing(s)on bed thickness(t)in sedimentary basins(reservoirs)is studied in this context.This paper shows that the MichaeliseMenten equation can provide an algebraic expression for the nonlinear s-t relationship.The two parameters have clear geological meanings:a is the maximum fracture spacing which can no longer increase with increasing t,and b is the characteristic bed thickness when s=0.5a.The tensile fracture strength(C)of the brittle beds during the formation of tensile fractures can be estimated from the two parameters.For sandstones of 16 areas reported in the literature,C ranges from 2.7 MPa to 15.7 MPa with a mean value of 8 MPa,which lies reasonably within the range of tensile strengths determined experimentally.This field-based approach by means of MichaeliseMenten equation provides a new method for estimating the tensile fracture strength of rock layers under natural conditions.

A generalized nonlinear three-dimensional failure criterion based on fracture mechanics

Based on fracture mechanics theory and wing crack model,a three-dimensional strength criterion for hard rock was developed in detail in this paper.Although the basic expression is derived from initiation and propagation of a single crack,it can be extended to microcrack cluster so as to reflect the macroscopic failure characteristic.Besides,it can be derived as HoekeBrown criterion when the intermediate principal stress σ_(2) is equal to the minimum principal stress σ_(3)(Zuo et al.,2015).In addition,the opening direction of the microcrack cluster decreases with the increase of the intermediate principal stress coefficient,which could be described by an empirical function and verified by 10 kinds of hard rocks.Rock strength is influenced by the coupled effect of stress level and the opening direction of the microcrack clusters related to the stress level.As the effects of these two factors on the strength are opposite,the intermediate principal stress effect is induced.