Effect of Relative Stress on Post-Peak Uniaxial Compression Fracture Energy of Concrete

Fracture energy in strain softening regime was investigated analytically by considering microstructures interaction and interplay.Based on gradient-dependent plasticity, the thickness of localized band was determined completely and strictly by characteristic length in relation to average grain diameter.After obtaining the plastic shear displacement of the band,the formula on axial response of concrete was proposed and the analytical post-peak fracture energy was deduced.A comparison between present theoretical results and earlier experimental results was carried out and the analytical result is reasonable and has a plausible foundation as considering the localized failure theoretically.Decreasing the relative stress leads to increasing the fracture energy non-linearly.The larger the shear elastic modulus and shear softening modulus,the lower the post-peak fracture energy.A larger fracture energy is caused by a larger thickness of shear band or a larger characteristic length of concrete material.If the inclination angle of the shear band and the compressive strength are not concerned with structural size of specimen,the post-peak fracture energy is size independent.

Fracture energy evaluation on 7075-T651 aluminum alloy welds determined by instrumented impact pendulum

By using an instrumented impact pendulum, the force versus time curves of 7075-T651 aluminum welds were obtained from standard Charpy-V samples. Considering the force-time curves and constant impact velocity, the fracture energies for different zones were quantified. A fracture energy improvement for the HAZ(33.6 J) was observed in comparison with the weld metal(7.88 J), and base metal(5.37 J and 7.37 J for longitudinal and transverse directions, respectively). This toughness increment was attributed to the microstructural transformation caused by the thermodynamic instability of η′ precipitates during the welding. Fracture energy for weld metal was higher than that for base metal, probably due to pores created during solidification. Regarding the dynamic yielding force obtained from the force-time curves, an approximation to the dynamic yield strength for weld, HAZ and base metal was determined. Fracture surfaces revealed an intergranular failure for base metal in longitudinal direction, whereas a predominately brittle failure(cleavage) with some insights of ductile characteristics was observed for the transverse direction. In contrast, a ductile failure was observed for weld metal and HAZ.

Specimen size effect on the fracture energy of architected stretchable materials

Architected stretchable materials with well-organized microarchitectures evolve very rapidly due to their potential in customizing mechanical properties and achieving exotic functions.In many applications,the architected stretchable materials are required to sustain large deformation,and their fracture is size-dependent.However,the size effect on the fracture of architected stretchable materials is still elusive.Here,we study this issue by experiment and finite element calculation.It is found that the fracture energy of architected stretchable materials increases with the specimen size ratio,H/h,within a range.When H/h reaches a transition ratio,Rt,the fracture energy approaches a plateau.This transition ratio differentiates the size-dependent and size-independent fracture behavior of architected stretchable materials.The mechanical properties of constituent material only have a minor effect on the transition ratio.The degree of constraint and stress concentration at the node,which are affected by the geometry of the unit-cell,dominate the specimen size effect.The result gives a practical guidance in choosing the specimen size to measure the steady state fracture energy of this class of materials.This work provides insights into the fracture of architected stretchable materials and design for fractureresistant architected stretchable devices.

Size effect of fracture characteristics for anisotropic quasi-brittle geomaterials

Understanding the size effect exhibited by the fracture mechanism of anisotropic geomaterials is important for engineering practice. In this study, the anisotropic features of the nominal strength, apparent fracture toughness, effective fracture energy and fracture process zone(FPZ) size of geomaterials were first analyzed by systematic size effect fracture experiments. The results showed that the nominal strength and the apparent fracture toughness decreased with increasing bedding plane inclination angle.The larger the specimen size was, the smaller the nominal strength and the larger the apparent fracture toughness was. When the bedding inclination angle increased from 0° to 90°, the effective fracture energy and the effective FPZ size both first decreased and then increased within two complex variation stages that were bounded by the 45° bedding angle. Regardless of the inherent anisotropy of geomaterials,the nominal strength and apparent fracture toughness can be predicted by the energy-based size effect law, which demonstrates that geomaterials have obvious quasi-brittle characteristics. Theoretical analysis indicated that the true fracture toughness and energy dissipation can be calculated by linear elastic fracture mechanics only when the brittleness number is higher than 10;otherwise, size effect tests should be adopted to determine the fracture parameters.

Research on the Bending Impact Resistance and Transverse Fracture Characteristics of Bamboo under the Action of Falling Weight

Drop weight impact tester was used to accurately measure the bending impact resistance of various parts of Phyllostachys edulis,commonly known as moso bamboo,with a growth cycle of 3–8 years.Cellulose crystallinity in the bottom(B),middle(M)and top(T)of bamboo at different ages was calculated using peak height analysis in X-ray diffraction.Heatmap of Spearman correlation analysis was used to represent the correlation between chemical composition and impact mechanics.The breaking load(BL),fracture energy(FE)and impact deflection(ID)of 3–8-year-old bamboo were found to be in the range of~670–2120 N,~5.17–15.55 J,and~3.60–~17.76 mm,respectively.As the growth period of bamboo rises,the cellulose crystallinity at the B and T decreases first and then increases,while that for the M increases first and then remains stable.Similarly,the bending impact performance of bamboo was found to become stable with its growth and age.The flexural impact and toughness of the 4-year-old bamboo base material were better than other specimens.The enhancement in the bending impact properties of bamboo at different growth periods was influenced by the lignin content,while the value of FE was mainly positively correlated with ash,cold and hot water extracts and benzyl alcohol content.However the content of holocellulose and pentosan,air-dry density and,base density negatively influenced the FE.With the change in the height of the bamboo,the correlation between its impact mechanical properties and chemical composition gradually decreased.This study provides data support and theoretical basis for the age-appropriate thinning and application of moso bamboo.

Microstructure and fracture toughness of a TiAl-Nb composite consolidated by spark plasma sintering

A TiAl-Nb composite was prepared by spark plasma sintering （SPS） at 1250 °C and 50 MPa for 5 min from prealloyed TiAl powder and elemental Nb powder in a molar ratio of 9：1 for improving the fracture toughness of TiAl alloy at room temperature. The microstructure, phase constitute, fracture surface and fracture toughness were determined by X-ray diffractometry, electron probe micro-analysis, scanning and transmission electron microscopy and mechanical testing. The results show that the sintered samples mainly consist of γ phase, O phase, niobium solid solution （Nbss） phase and B2 phase. The fracture toughness is as high as 28.7 MPa？m1/2 at room temperature. The ductile Nbss phase plays an important role in absorbing the fracture energy in front of the cracks. Moreover, B2 phase can branch the propagation of the cracks. The microhardness of each phase of the composite was also tested.

Effects of freeze-thaw cycles on fracture behavior of epoxy asphalt concrete

According to the winter temperature of Peking,the freeze-thaw（FT） condition in laboratory was determined.Seven groups of epoxy asphalt concrete（EAC） specimen were exposed to different FT cycles.The flexural modulus and fracture energy（G_F） of EAC exposed to different FT cycles were obtained through the 3-point bending test.Meanwhile,the plane strain fracture toughness（K_（IC）） of EAC was obtained through numerical simulation.The results show that the flexural modulus of the FT conditioned EAC samples decreases with the increase of FT cycles.The FT damage of flexural modulus is 60%after 30 FT cycles.Nevertheless,with the increase of FT cycles,the G_F and K_（IC） of EAC decrease first and then increase after 15 FT cycles.

Effect of Mineral Admixture on Fracture Behavior of Concrete under Freeze-Thaw Cycles

According to GBJ82-85 test method for frost resistance of concrete,four-point bending tests were used to examine the effects of mineral admixture(silica fume and fly ash)on fracture toughness,fracture energy and dynamic elastic modulus of concrete subjected to rapid freeze-thaw cycles The microstructure of the concrete was also analyzed.The results show that with the increase of the number of freeze-thaw cycles,the fracture toughness of concrete,and the loss of fracture energy as well as the l0ss of relative dynamic elastic modulus showed a downward trend.The air content of fresh concrete is the most important parameter to improve frost resistance of concrete There existed distinct difference for silica fume and fly ash to enhance fracture performance and microstructure of the concrete under freeze-thaw cycles.The l0ss of fracture energy and the loss of relative dynamic elastic modulus kept a good linear relationship.

Understanding the Function of Rays and Wood Density on Transverse Fracture Behaviour of Green Wood in Three Species

In recent years, there has been an increasing interest in wood properties, because wood is a commonly used and advanced building material. In this paper, the effect of anatomical characters on the transverse fracture properties of green wood was investigated. The specific fracture energy （Gf J/m2） of ash （Fraxinus excelsior）, cherry （Prunus avium） and birch （Betula pendula） was evaluated using double edge notched tensile tests. The tests were performed on both earlywood （EW） and latewood （LW） zones in both the radial-tangential （RT） and the tangential-radial （TR） crack propagation systems. Wood anatomy and the failure patterns of each species were also investigated using environmental scanning electron microscopy （ESEM） and light microscopy （LMC）. The results showed that the Gfof RT fracture systems was around 1.5 times greater than in the TR one, whereas there were no significant differences between EW and LW zones. ESEM micrographs showed that the RT fracture system had a rougher fracture surface, while the TR had a nearly smooth and fiat fracture surface. In particular, the wood ofF. excelsior was the toughest, because of its greater percentage of rays and homogenous distribution of ray cells, while P. avium and B. pendula showed a lower Gf due to their smaller percentage of rays with a distinctive arrangement of ray cells.

The energy release rate for hygrothermal coupling elastic materials

In the fracture problems of hydrophilic elastic materials under coupling effects of heat conduction, moisture diffusion and mechanical deformation, the conventional J-integral is no longer path independent. The value of J is unequal to the energy release rate in hygrothermal coupling cases. In the present paper, we derived a general form of the energy release rate for hygrothermal fracture problems of the hydrophilic elastic materials on the basis of energy balance equation in cracked areas. By introducing the constitutive relations and the essential equations of irreversible thermodynamics, a specific expression of the energy release rate was obtained, and the expression can be reformmulated as path independent integrals, which is equivalent to the energy release rate of the fracture body. The path independence of the integrals is then verified numerically.

An Integrated Optimization Method for CO_(2) Pre-Injection during Hydraulic Fracturing in Heavy Oil Reservoirs

CO_(2) pre-injection during hydraulic fracturing is an important method for the development of medium to deep heavy oil reservoirs.It reduces the interfacial tension and viscosity of crude oil,enhances its flowability,maintains reservoir pressure,and increases reservoir drainage capacity.Taking the Badaowan Formation as an example,in this study a detailed three-dimensional geomechanical model based on static data from well logging interpretations is elaborated,which can take into account both vertical and horizontal geological variations and mechanical characteristics.A comprehensive analysis of the impact of key construction parameters on Pre-CO_(2) based fracturing(such as cluster spacing and injection volume),is therefore conducted.Thereafter,using optimized construction parameters,a non-structured grid for dynamic development prediction is introduced,and the capacity variations of different production scenarios are assessed.On the basis of the simulation results,reasonable fracturing parameters are finally determined,including cluster spacing,fracturing fluid volume,proppant concentration,and well spacing.

A technique for enhancing tight oil recovery by multi-field reconstruction and combined displacement and imbibition

A seepage-geomechanical coupled embedded fracture flow model has been established for multi-field coupled simulation in tight oil reservoirs,revealing the patterns of change in pressure field,seepage field,and stress field after long-term water injection in tight oil reservoirs.Based on this,a technique for enhanced oil recovery(EOR)combining multi-field reconstruction and combination of displacement and imbibition in tight oil reservoirs has been proposed.The study shows that after long-term water flooding for tight oil development,the pressure diffusion range is limited,making it difficult to establish an effective displacement system.The variation in geostress exhibits diversity,with the change in horizontal minimum principal stress being greater than that in horizontal maximum principal stress,and the variation around the injection wells being more significant than that around the production wells.The deflection of geostress direction around injection wells is also large.The technology for EOR through multi-field reconstruction and combination of displacement and imbibition employs water injection wells converted to production and large-scale fracturing techniques to restructure the artificial fracture network system.Through a full lifecycle energy replenishment method of pre-fracturing energy supplementation,energy increase during fracturing,well soaking for energy storage,and combination of displacement and imbibition,it effectively addresses the issue of easy channeling of the injection medium and difficult energy replenishment after large-scale fracturing.By intensifying the imbibition effect through the coordination of multiple wells,it reconstructs the combined system of displacement and imbibition under a complex fracture network,transitioning from avoiding fractures to utilizing them,thereby improving microscopic sweep and oil displacement efficiencies.Field application in Block Yuan 284 of the Huaqing Oilfield in the Ordos Basin has demonstrated that this technology increases the recovery factor by 12 percentage points,enabling large scale and efficient development of tight oil.

A brief discussion on the relationship between apparent stress and slip-weakening law based on the energy partition criteria

According to the representation theorem of seismic energy radiation, we know that, at any point on the fault, the instantaneous seismic radiation energy, Es, the seismic moment M0, and the apparent stress σa=μEs /M0 （μ is the shear modulus on the fault plane） should be positive values at any time during an earthquake faulting. However, we have noticed that, in recent source parameter inversion scheme for deriving the critical slip-weakening distance, the apparent stress used as a constraint condition on the fault plane could be less than zero or negative value, and the negative part was considered as dissipation energy and incorporated to the fracture energy. Although the mathematical formula in such case has no influence to the final resolution, however, the earthquake dynamic source process violates obviously the basic physical law, which could results in the overestimating of radiated seismic energy. In this study, we have proposed an alternative way to take account of the apparent stress expression based on the earthquake energy partition principle, and at the same time, we have also suggested that an additional constraint regarding to the radiation energy conservation on the fault could be added into the source parameter inversion in order to estimate the critical slip-weakening distance Dc.

Experimental Investigation on Fracture Performance of Short Basalt Fiber Bundle Reinforced Concrete

In this paper,a notched three-point bending test is used to study the fracture performance of the short basalt fiber bundle reinforced concrete(SBFBRC).To compare and analyze the enhancement effect of different diameters and different content of basalt fiber bundles on the fracture performance of concrete,some groups are set up,and the P-CMOD curves of each group of specimens are measured,and the fracture toughness and fracture energy of each control group are calculated.The fracture toughness and fracture energy are two important fracture performance parameters to study the effect and law of the new basalt fiber bundles on the fracture performance of concrete.The research results show that the diameter and content of the new basalt fiber bundles have a certain effect on the fracture performance of concrete.With the increase of the content of basalt fiber bundles,the peak load,crack initiation toughness,instability toughness and fracture energy of SBFBRC are greatly improved compared with the benchmark group.When the fiber bundle diameter is 0.2 mm,the peak load increases by 69.5%compared with the reference group.The instability toughness reaches its maximum value at 0.2 mm diameter,which is 59.7%higher than the benchmark.

High Energy Gas Fracturing Succeeds in Changqing Oilfield

High energy gas fracturing provides a new way for economic and high efficient development of the low permeability reservoir.By the end of 1993,it is tested and used in more:than 50 oil production and water injection wells in Changqing Oilfield with treatment success rate of more than 92%,and treatment effective rate of 86%,with average increase of oil production 3.4 times and somewhat increase.of water iniectivity in the water wells,cumulative increment of oil produc-tion and water injection had reached 11660 tons and 39220 cubic meters respectively.The test was undertaken by 0il Drilling&Production Tech-nology Research Institute in collaboration with Oil Production Plant,both are subordinated to Changqing Petroleum Exploration Bureau.

Experimental and numerical investigation on the dynamic shear failure mechanism of sandstone using short beam compression specimen

In this study,a novel testing method is proposed to characterize the dynamic shear property and failure mechanism of rocks by introducing the short beam compression(SBC)specimen into the split Hopkinson pressure bar(SHPB)system.Firstly,the stress distribution of SBC specimen is comprehensively analyzed by finite element method(FEM),and the results show that the optimal notch separation ratio of SBC specimen is C/H?0.2 to achieve successful dynamic simple-shear tests.Then,dynamic shear tests are conducted on sandstone using the SBC-SHPB method.Via careful pulse shaping technique,the dynamic force balance is guaranteed for SBC specimens,and the testing results show that the dynamic shear strength of sandstone is significantly rate-dependent.Combining the results of dynamic compression and tension tests,the failure envelopes of sandstone under different loading rates are obtained in the principle stress plane.It is found that the failure envelope of sandstone constantly expands outwards with increasing loading rate.Moreover,the energy partition of SBC specimen is quantified by virtue of high-speed digital image correlation(DIC)technique.The results show that the kinetic energy portion is non-negligible,and the shear fracture energy increases with increasing loading rate.In addition,the microscopic shear cracking mechanism of SBC specimen is analyzed by the thin section observation:the intra-granular(TG)fracture of minerals dominates the dynamic shear failure of sandstone,and the portion of TG fracture increases with increasing loading rate.This study provides a convenient and reliable method to investigate the dynamic shear property and failure mechanism of rocks.

Static and dynamic mechanical behaviour of ECO-RPC

Ecological reactive powder concrete (ECO-RPC) with small sized and differentvolume fraction steel fibers was prepared by substitution of ultra-fine industrial waste powder for50% to 60% cement by weight and replacement of ground fine quartz sand with natural fine aggregate.The effect of steel fiber volume fraction and curing ages on the static mechanical behaviour ofECO-RPC was studied. Using the split Hopkinson pressure bar technique, the dynamic mechanicalbehaviour of ECO-RPC was investigated under different strain rates. The results show that the staticmechanical behaviour of ECO-RPC increases with the increase of steel fiber volume fraction andcuring ages. The type of ECO-RPC with the substitution of 25% ultra-fine slag, 25% ultra-fine flyash and 10% silica fume is better than the others with compressive strength, flexural strength, andfracture energy more than 200 MPa, 60 MPa and 30 kJ/m^2, respectively. ECO-RPC has excellent strainrate stiffening effects under dynamic load. Its peak stress, peak strain and the area understrain-stress curve increase with the increase of strain rate. Its fracture pattern changes frombrittleness to toughness under high strain rates.

Effect of Coarse Aggregate Size on and Crack Opening in Normal and Relationship between Stress High Strength Concretes

Fine and coarse aggregates play an important role in the fracture of concrete. However, quantitative information available on the effect of the coarse aggregate size on the fracture properties of concrete is still limited. In the present paper, the effect of coarse aggregate size （single grade of 5～10, 10～16, 16～20 and 20～25 mm） on stress-crack opening （σ-w） relation in normal and high strength concretes （compressive strength of 40 and 80 MPa, respectively） was studied. The investigation was based on three-point bending tests implemented by fictitious crack analysis. The result shows that coarse aggregate size and cement matrix strength significantly influence the shape of σ-w curve. For a given total aggregate content, in normal strength concrete, smaller size of aggregate leads to a high tensile strength and a sharp stress drop after the peak stress. The smaller the coarse aggregate, the steeper the σ-w curve. By contrast, in high strength concrete, the effect of aggregate size on σ-w relation almost vanishes. A similar σ-w relation is obtained for the concrete except for the case of 20～25 mm coarse aggregate size. The stress drop after the peak stress is more significant for high strength concrete than that for normal strength concrete. Meanwhile, the smaller the coarse aggregate size, the higher the flexural strength. Fracture energy and characteristic length increase with increasing coarse aggregate size in both normal and high strength concretes.

Determination of key parameters of Al–Li alloy adhesively bonded joints using cohesive zone model

The key parameters of the adhesive layer of a reinforcing patch are of great significance and affect the ability to suppress crack propagation in an Al–Li alloy patch-reinforced structure.This paper proposes a method to determine the key parameters of the adhesive layer of adhesively bonded joints in the Al–Li alloy patch-reinforced structure.A zero-thickness cohesive zone model(CZM)was selected to simulate the adhesive layer’s fracture process,and an orthogonal simulation was designed to compare against the test results.A three-dimensional progressive damage model of an Al–Li alloy patch-reinforced structure with single-lap adhesively bonded joints was developed.The simulation’s results closely agree with the test results,demonstrating that this method of determining the key parameters is likely accurate.The results also verify the correctness of the cohesive strength and fracture energy,the two key parameters of the cohesive zone model.The model can accurately predict the strength and fracture process of adhesively bonded joints,and can be used in research to suppress crack propagation in Al–Li alloy patch-reinforced structures.

Peeling behavior and spalling resistance of CFRP sheets bonded to bent concrete surfaces

In this paper, the peeling behavior and the spalling resistance effect of carbon fiber reinforced polymer （CFRP） sheets externally bonded to bent concrete surfaces are firstly investigated experimentally. Twenty one curved specimens and seven plane specimens are studied in the paper, in which curved specimens with bonded CFRP sheets can simulate the concrete spalling in tunnel, culvert, arch bridge etc., whereas plane specimens with bonded CFRP sheets can simulate the concrete spalling in beam bridge, slab bridge and pedestrian bridge. Three kinds of curved specimens with different radii of curvature are chosen by referring to practical tunnel structures, and plane specimens are used for comparison with curved ones. A peeling load is applied on the FRP sheet by loading a circular steel tube placed into the central notch of beam to debond CFRP sheets from the bent concrete surface, meanwhile full-range load-deflection curves are recorded by a MTS 831.10 Elastomer Test System. Based on the experimental results, a theoretical analysis is also conducted for the specimens. Both theoretical and experimental results show that only two material parameters, the interfacial fracture energy of CFRP-concrete interface and the tensile stiffness of CFRP sheets, are needed for describing the interfacial spalling behavior. It is found that the radius of curvature has remarkable influence on peeling load-deflection curves. The test methods and test results given in the paper are helpful and available for reference to the designer of tunnel strengthening.