Tension-shear Failure Research Articles

Experimental study on in-plane behavior of composite frame slabs and the influence of out-of-plane effect

This paper experimentally investigated the in-plane behavior of a steel–concrete composite frame slab under cyclic loading by considering the influence of the out-of-plane effect. A pure in-plane loading test and two in-plane loading tests of frame slabs with different out-of-plane loads were designed and conducted. Subsequently, the test observations, load–displacement relationships, and shear and flexural deformation components were discussed to investigate the in-plane load resistance behaviors and failure mechanisms of the composite frame slabs with different out-of-plane loads. The experimental results of the pure in-plane loading test demonstrated an evident shear cracking concentration behavior and a pinching hysteretic curve associated with a typical shear-tension failure mode of the composite frame slab. Further, the in-plane loading test of the composite frame slab with small out-of-plane loads also exhibited a typical shear-tension failure mode, and thus had a load–displacement relationship similar to that of the pure in-plane loading test. In contrast, the in-plane loading test with large out-of-plane loads exhibited a horizontal shear failure mode, and thus exhibited a load–displacement curve with a more obvious pinching effect and significantly decreased ultimate load capacity. In addition, the influence of the out-of-plane loads on the in-plane stiffness of the composite frame slab was analyzed.

The effect of CFRP strip stirrups on the shear strength of SCC high strength lightweight concrete beams

The development and applications of structural high-strength lightweight self-consolidating concrete (HLWSCC) have increased significantly in recent years owing to its better mechanical properties and economic benefits over conventional normal weight concrete (NWC). This paper presents the results of an experimental and analytical study to evaluate the shear performance of HLWSCC beams internally reinforced with CFRP sheet stirrups. In addition to one control specimen, seven beams were cast and reinforced with CFRP sheet stirrups. The variables studied in the experimental program were the number of layers, strip spacing, strip (sheet) width, and strip configuration of CFRP stirrups. All the beams were subjected to four-point bending, and the experimental results include load–deflection response curves and strain values along with the failure modes. Experimental results showed that all the tested beams failed due to shear tension failure. However, some of the specimens also failed due to the rupture of CFRP sheet stirrups. The addition of CFRP sheet stirrups has substantially increased the shear capacity of HLWSCC beams and the increase in shear strength ranged from 64% to 140% over the control specimens. Additionally, the nominal shear strength was predicted using various Fiber Reinforced Polymer (FRP) design standards, including ACI 440.1R-15, CSA S806-12, CSA S6-14, CNR-DT (2013), JSCE, and Fib Bulletin 40. Comparing the predicted and experimental shear strength has shown that current design standards can be used safely to predict the ultimate shear strength of HLWSCC beams reinforced with newly innovative CFRP sheet stirrups.

Simulation analysis on seismic dynamic response of pile supported tunnels in deep backfill area of soil-rock mixture

To reveal the seismic dynamic response of the pile-supported tunnel group in the soil-rock mixture deep backfill region, a three-dimensional finite element model was established based on the engineering conditions of the subway section and three tunnels with close access lines. Subsequently, the seismic dynamic response of the tunnel lining structure was studied. The results show that: Under the action of seismic, the soil-rock mixture stratum presents nonlinear characteristics with shear failure and plastic deformation. In addition, the acceleration and earth pressure of the soil-rock mixture stratum is in a “saturated” state; The seismic dynamic response of the three tunnels influences each other. The bending moments in the X and Y directions of the tunnel lining cross-section are distributed in “X” and inverted “V” shapes, respectively. Meanwhile, the tensile stress and shear stress are distributed in an “X” shape; Under the action of seismic, the main failure form of tunnel lining is tension shear failure, and the most vulnerable position is the left and right arch foot, followed by the left and right arch shoulder; The bending moment of the pile body changes nonlinearly in the height direction. The most significant bending moment value appears at the top 1/5 of the pile length and the junction of different strata. Furthermore, the most significant horizontal displacement of the lining structure occurs at the tunnel vault.

Study of mechanical behavior and cracking mechanism of prefabricated fracture cemented paste backfill under different loading rates from the perspective of energy evolution

Loading rate (LR) and initial defects are the vital factors influencing the mechanical properties of cemented paste backfill (CPB). Under the action of external load, the essence of the deformation and damage of the filling body is the evolutionary process of progressive damage driven by energy, which is the comprehensive performance of crack closure, development, expansion, and convergence. In this paper, uniaxial compressive strength (UCS) tests with different levels of LRs were conducted on 0°, 45°, 90° prefabricated fracture CPBs (PFCPB), and CPB. The results show that UCS and elastic modulus of the filling body were positively correlated with LR. The mechanical properties of the filling body were degraded by PF. During loading, the top and bottom surfaces of 0° and 45° PFs were closed, and the stress–strain curves showed stress drop phenomenon. The stress drop of the curve corresponded to the rapid release of elastic energy and the rapid increase of dissipated energy. The dissipation energy curve corresponded to crack opening and expansion, which could be divided into slowly growing, steadily growing, and rapidly growing stages. The steadily growing stage of the dissipative energy curve for higher LRs was beneath the other curves, and a decelerating rising period occurred in the rapidly growing stage. Meanwhile, the dissipation energy curves of 0° and 45° PFCPBs suffered from an accelerated rise, slow rise, and rapid rise periods in the rapidly growing stage. The cracks after the failure of PFCPB mainly included resistance to tensile cracks, wing cracks, coplanar and non-coplanar secondary cracks. The failure mode of PFCPB was the occurrence of mixed shear-tension failure, and CPB exhibited shear failure. The progressive damage law and cracking mechanism of the filled body were investigated with the help of AE energy rate (cumulative AE energy)-energy-time (strain) curves and microscopic morphology. The results could provide a theoretical basis for the stability assessment of the filling body.

Crack Coalescence Behavior of Rock-Like Specimens Containing Two Circular Embedded Flaws

Abstract Experimental research on the growth of internal flaws has rarely been reported due to the fact that it is difficult to cut internal flaws in specimens and cannot capture the initiation and propagation processes of internal flaws through direct observations. This paper proposed a method for creating internal flaws in specimens by utilizing the volatilization of camphor. A series of compression tests were performed on rock-like specimens including two embedded circular flaws, and CT techniques were used to investigate the internal damage behavior of flawed specimens. Experimental results illustrate that the strength and deformation properties of flawed specimens increase nonlinearly with the confining pressure as well as flaw inclination angle. Crack coalescence patterns and failure modes of flawed specimens depend on not only the confining pressure but also the flaw inclination angle. The crack coalescence pattern varies from wing crack coalescence to mixed tension-shear crack coalescence and then to the shear crack coalescence as the crack inclination angle increases. Confining pressure contributes to shear crack growth and has an inhibiting effect on the propagation of tension cracks. For specimens with the same flaw inclination angle, the failure mode changed from tension failure to mixed shear-tension failure or from mixed shear-tension failure to pure shear failure with the increase of confining pressure.

3D morphology and formation mechanism of fractures developed by true triaxial stress

As main part of underground rock mass, the three-dimensional (3D) morphology of natural fractures plays an important role in rock mass stability. Based on previous studies on 3D morphology, this study probes into the law and mechanism regarding the influence of the confining pressure constraints on 3D morphological features of natural fractures. First, fracture surfaces were obtained by true triaxial compression test and 3D laser scanning. Then 3D morphological parameters of fractures were calculated by using Grasselli’s model. The results show that the failure mode of granites developed by true triaxial stress can be categorized into tension failure and shear failure. Based on the spatial position of fractures, they can be divided into tension fracture surface, S-1 shear fracture surface, and S-2 shear fracture surface. Micro-failure of the tension fracture surface is dominated by mainly intergranular fracture; the maximum height of asperities on the fracture surface and the 3D roughness of fracture surfaces are influenced by σ 3 only and they are greater than those of shear fracture surfaces, a lower overall uniformity than tension fracture surface. S-1 shear fracture surface and S-2 shear fracture surface are dominated by intragranular and intergranular coupling fracture. The maximum height of asperities on the fracture surface and 3D roughness of fracture surface are affected by σ 1 , σ 2 , and σ 3 . With the increase of σ 2 or σ 3 , the cutting off of asperities on the fracture surface becomes more common, the maximum height of asperities and 3D roughness of fracture surface further decrease, and the overall uniformity gets further improved. The experimental results are favorable for selecting technical parameters of enhanced geothermal development and the safety of underground mine engineering.

Experimental study on influence of host rock strength on shear performance of Micro-NPR steel bolted rock joints

The development of deep underground engineering has brought new challenges to bolt materials. Micro-Negative Poisson's Ratio (NPR) steel is a new type of bolt material with high strength, high toughness, and super energy absorption. The effect of host rock strength on the shear performance of micro-NPR steel anchoring rock joints was studied by performing an indoor direct shear test . Ordinary Q235 steel bolted joints and unbolted joints were set as the test control groups. The shear force-shear displacement curves, surface failure characteristics, bolt deformation, fracture characteristics , shear strength, and shear energy absorption were all compared and analyzed. The study found that the bolted rock joints have significant ductile failure characteristics under shear load , which effectively improves the brittle failure characteristics of the unbolted rock joints. The ability of the rock to resist the crushing damage of the bolt is continuously enhanced with an increase in the host rock strength, while the damaged area of the grout and rock mass at the opening of the joint surface gradually decreases. The failure area of the Q235 steel bolted joint surface is significantly smaller than that of the micro-NPR steel. Additionally, the equivalent internal friction angle , equivalent cohesion, and peak shear strength of the micro-NPR steel are significantly higher than those of Q235 steel. The higher the strength of the host rock, the greater the bolt peak shear strength of the joint. The bolts undergo local shear deformation near the joint surface, and the bolt deforms into an ‘S’ shape after shearing. The improvement of the host rock strength limits the development of the bolt's plastic hinge to a position far from the joint surface. The shear fracture displacement and shear energy absorption of the bolt decrease with an increase of the host rock strength. The shear energy absorption capacity is at least 3.7 times higher than that of Q235 steel. When the host rock strength is low, the bolt yields in tension and bends at the plastic hinge, at which time tension-bending failure occurs. With an increase of the host rock strength, the bolt undergoes tension-shear failure. The high strength and high ductility of traditional steel are well balanced by the micro-NPR steel, with the tensile and shear resistance of the steel bolt being greatly improved, which allows for large deformation and super energy absorption in the bolt material, which allows for a wide range of applications at deep engineering sites.

Combined Tension-Shear Failure of Ductile Materials

Failure of ductile materials is usually expressed in terms of effective plastic strain. Ductile materials can fail by two different failure modes, shear failure and tensile failure. Under dynamic loading, shear failure has to do with shear localization and formation of adiabatic shear bands. In these bands plastic strain rate is very high, dissipative heating is extensive, and shear strength is lost. Shear localization starts at a certain value of effective plastic strain, when thermal softening overcomes strain hardening. Shear failure is therefore represented in terms of effective plastic strain. On the other hand, tensile failure comes about by void growth under tension. For voids in a tension field there is a threshold state of the remote field for which voids grow spontaneously (cavitation) and the material there fails. Cavitation depends on the remote field stress components and on the flow stress. In this way failure in tension is related to shear strength and to failure in shear. Here, we first evaluate the cavitation threshold for different remote field situations, using 2D numerical simulations with a hydrocode. We then use the results to compute examples of rate dependent tension-shear failure of a ductile material.

Study on the tensile-shear mechanical behavior of sandstone using a simple auxiliary apparatus

Mixed failure of rock by tension and shear occurs frequently in underground openings, rock slopes, and transtension faults. In this study, a simple auxiliary device (which consists of an indenter and a hollowed base) is proposed to implement tension-shear tests of rock samples combined with a servo-controlled uniaxial compression machine. Experimental and numerical tension-shear tests are carried out on red sandstones using the proposed auxiliary device and particle flow code. The effects of sample height H and base hollow diameter D on the mechanical behaviors are analyzed in detail, including the peak load, fracture characteristics, and AE characteristics. The results show that peak load increases with the sample height while decreases with hollow diameter due to the change of inclination angle of the failure surface. The initial crack and dominant cracks changed from shear cracks to tensile cracks as D increase from 2 cm to 4 cm, which suggests the transition from shear failure to hybrid tension-shear failure.

Experimental Study on Deformation Behavior and Permeability Evolution of Sandstone Responding to Mining Stress

Mining-induced hydromechanical behavior of sandstone is critical to mining safety and disaster prevention. To investigate the evolution behavior of the mechanical and permeability properties of sandstone, mining-induced stress was imitated by increasing axial stress and decreasing confining stress, and a set of hydromechanical experiments were further performed, incorporating the effect of in situ stress, pore pressure, and mining stress. The results show the similar variation tendencies of the deformation and permeability of sandstone under different loading paths of in situ stress and pore pressure. Most sandstone samples maintain a compression state for the peak stress condition. The failure mode evolved from shear failure to shear–tension failure with the increase in in situ stress. The stress-relief effect significantly effects the permeability, since the permeability of sandstone increases exponentially with decreasing effective confining stress. The growth rate of permeability in Stage II is significantly greater than that in Stage I. One order of magnitude of permeability was presented at the peak stress situation. A fitting exponential model based on the alteration of effective confining stress was proposed to describe the permeability evolution dominated by the stress-relief effect, and the discovered permeability model can accurately describe the experimental results. The research results provide significant guidance for understanding the hydromechanical behavior and water hazard prevention for underground coal mines.

Experimental Study on the Shear Behavior of Reinforced Highly Ductile Fiber-Reinforced Concrete Beams with Stirrups

The aim of this study is to improve the shear behavior of reinforced concrete (RC) beams with stirrups by using highly ductile fiber-reinforced concrete (HDC), which is a fiber-reinforced cement-based composite material with tensile-strain-hardening properties. Twelve reinforced HDC (RHDC) beams and three RC beams with stirrups were tested under a concentrated load. The experimental parameters involved the shear span to effective depth ratio, stirrup ratio, and longitudinal reinforcement ratio. The results revealed that the mode of failure of RHDC beams, which exhibited better ductility than RC beams, included diagonal compression, shear compression, diagonal tension, and flexural shear failure. RHDC beams exhibited stable multiple crack propagation behavior and satisfactory integrity, thus showing that HDC effectively restricted the development of shear cracks and improved the damage resistance of beams. Compared with RC beams, the shear strength, displacement ductility factor, and deflection-clear span ratios corresponding to the peak load and ultimate deflection increased by up to 30.5%, 44.9%, 150.0%, and 148.0%, respectively. RHDC beams exhibited higher residual strength and deformation capacity than RC beams, thus indicating that HDC significantly improved the brittle shear failure mode. Specimens H-1 and H-2 exhibited the largest improvement in shear strength and displacement ductility factor, respectively, compared with RC beams. The shear strength of RHDC beams increased as the shear span to effective depth ratio decreased. For RHDC beams with the same shear span to effective depth ratio, the shear strength increased with the increase in the longitudinal reinforcement ratio and stirrup ratio under shear compression failure.

Study on hydraulic fracturing initiation model based on nonlinear constitutive equation

In this article, the nonlinear characteristics of rock deformation were characterized based on the Power-enhanced constitutive equation, and the piecewise nonlinear constitutive equation was established by using the piecewise approximation method. Based on the constitutive equation, an elastic-plastic stress field model around wells was established to analyze the stress field around wells during hydraulic fracturing. The initiation mode of elastic-plastic formation was analyzed and optimized. The results showed that the plastic yield of rock was determined by the in-situ stress field and the initial yield stress. The smaller the yield stress was, the greater the in-situ stress was, and the more easily the plastic yield occurred. Under the combined of rock mechanics parameters and in-situ stress, the tension failure occurred when the wellbore was under elastic state, and there were two failure modes of plastic tension failure and plastic shear failure after plastic state. The initiation directions of elastic tension, plastic tension, and plastic shear failure were the direction of maximum horizontal principal stress, while there was a failure angle between the fracture plane and the maximum horizontal principal stress under the shear failure.

The Tension-Shear and Compression-Shear Joint Strength Model for Unsaturated Clay and Its Application to Slopes

The capillary component and adsorptive component of matric suction differently impact the soil strength. Due to the cavitation effects of pore water, the adsorption effect dominates the behavior of soil when matric suction exceeds the cavitation tension. Based on the binary medium theory, a compression-shear strength model for unsaturated soils considering both capillary effect and adsorption effect is established. Compared with test data, the proposed compression-shear strength model has better prediction performance on the compression-shear strength of soil over a range of wide suction. The soil failure depends both on tension-shear stress and compression-shear stress. The tension-shear coupling mechanism in the soil is first investigated. A concept of closed stress point is introduced to divide the two zones of tension-shear coupling stress and compression-shear stress. According to the compression-shear strength model and tension-shear failure mechanism, the tension-shear and compression-shear joint strength model applicable to plane stress conditions is then established. Compared with test data, the proposed model in this article can better predict the nonlinear strength characteristics of clays and has better applicability. Finally, using the user material subroutine (UMAT), the secondary development of the joint strength model is conducted in ABAQUS and then applied to the slope stability analysis. The calculation results show that the established strength model presents a reasonable description of the development of the tension-shear coupling plastic zone in slope and gives an accurate safety factor.

Field Detection and Numerical Simulation Study on Roof Asymmetric Fracture Mechanism of Large-Span Soft Rock Roadway with Discontinuity Surface

Existence of roof discontinuity surface is one of the extremely important factors causing the asymmetric fracture of roadway roof, especially for large-span soft rock roadway. In this paper, a crack density coefficient ( D ) is firstly defined by local thresholding-microcell segmentation method and used to analyze the evolution law of D with roof drilling depth ( d ) (the lower and upper of roof discontinuity are defined as regions I and II, respectively). Then, UDEC numerical simulation is used to investigate the asymmetry evolution law of roof total displacement, maximum principal stress, and crack density with stress release coefficient ( α ) considering the effect of discontinuity surface. Research results indicate that (1) the roof parameters (D) in regions I and II both show a negative logarithmic function decreasing trend with the increase of drilling depth. When d < 4.5 m, the parameter ( D ) in region II is about 1.5 times that in region I; when d ≥ 4.5 m, the parameter ( D ) in region I is almost zero, while the parameter ( D ) in region II maintains a slight decreasing trend. Roof failure presents asymmetric distribution characteristics along both sides of the discontinuous surface. (2) In the initial stage of open-off cutting excavation, the top left and right corners of roof as well as the bottom of discontinuous surface first occurred the tension-shear failure, and then as the α increases, the two side cracks gradually shift to the middle of roof in regions I and II with the appearance of stress concentration in two top corners. Meanwhile, the direction of maximum principal stress is transformed from “direction approximately parallel to the excavation surface” to “direction perpendicular to the discontinuous surface,” of which the location transfers to the deep anisotropically, forming an asymmetric stress loosening zones in the roof of regions I and II. The range of stress loosening zone in region II is significantly larger than that in region I. When the surrounding rock stress is completely released, the roof cracks in region I gradually transition from nonconnected to connected state and form a “quasi-right triangle” loosening zone. In addition, an “isosceles triangle-like” high crack density loosening zone with the roof middle as the axis is also formed in region II. The roadway roof presents a markedly asymmetric caving feature. (3) With the increase of discontinuity surface angles, the roof fracture range gradually decreases in region I and increases in region II as well as the structural features of roof pressure-bearing arch transform from “left-low right-high continuous asymmetric structure” (30°-60°) to “left-low right-high discontinuous asymmetric structure” (90°) to “unilateral partial pressure-bearing arch structure” (120°-150°). Research results can provide an extremely important reference for the optimization and design of support scheme with discontinuity-thick soft rock roof-large span roadways.

Study on in-plane shear failure mode of cross-laminated timber panel

To explore in-plane shear failure mode of cross-laminated timber (CLT) panel, this paper carried out relevant research work from the perspective of stress analysis and combined with the crack morphology of the specimen after planar shear. In this study, the load–displacement curve of the hemlock [Tsuga canadensis (L.) Carrière] CLT specimen was obtained by a three-point bending test or an improved planar shear test, the crack morphology of the CLT vertical layer and the azimuth angle of the crack surface were observed and recorded synchronously. The shear strength values of CLT specimens under the two tests were obtained by corresponding calculation. Then the stress analysis of the CLT vertical layer was combined with the azimuth angle of the crack surface to discuss the failure mode of the CLT vertical layer in planar shear. The results showed that the planar shear strength measured by the three-point bending test and the improved planar shear test was in good agreement, and the results measured by the improved planar shear test were more dispersed than those measured by the three-point bending test; Considering the approximation that the in-plane shear of the CLT vertical layer could be treated as pure shear, the three-point bending test was better than the improved planar shear test; For the vertical layer of 63.3% CLT specimens, the azimuth of the crack surface was near the azimuth of the first principal plane obtained by stress analysis; There were two failure modes in the CLT vertical layer in-plane shear: tension failure and shear failure.

Experimental Study on Uniaxial Compression Fracture Process of Marble Specimens with Circular Holes with Different Diameters

After a long period of geological tectonic action, the rock mass develops defects such as pores and fissures of different sizes and scales. The mechanical properties of the rock are largely affected by these discontinuous structures, and there will be obvious stress concentration at the edge of the hole inside the rock. Under the action of concentrated stress, cracks initiate, expand, and penetrate from the edge of the hole to form the final fracture surface and cause instability and failure of the rock mass. In practical engineering, due to the limitation of various factors, it is impossible to conduct uniaxial compressive strength test for each geotechnical engineering. Therefore, the rock in the weak part of the project can be selected as the test specimen before the indoor physical tests. In this paper, the static uniaxial compression test of specimens with circular holes with different diameters is carried out. Digital Image Correlation (DIC) monitoring system is used to observe the test process in real time; then, the data from uniaxial compression test and DIC monitoring test are obtained. At the same time, the macroscopic failure characteristics and crack propagation law of rock specimens are obtained, and the evolution law of strain field during the failure process of rock specimens is also obtained. Meanwhile, Realistic Failure Process Analysis (RFPA) is carried out to verify the accuracy of static uniaxial compression physical tests numerically. Physical tests show that the uniaxial compressive strength of marble specimens with circular holes and the dispersion of measured peak stress data decrease with the increase of hole diameter. The main failure type of marble specimens with circular holes is tension shear failure; the macroscopic failure zone of the sample, the strain field localization zone, and the maximum shear stress field of the numerical test are distributed in a semi-“X” shape with the hole as the center. The localization zone of the strain field on the specimen is usually the region of crack initiation, propagation, and penetration on the surface of the marble specimens. It also shows that specimen breaks in the radial direction along the y -axis of the circular hole, and cracks connecting both ends of the circular hole and the specimen are generated on the specimen. It is of great theoretical value and practical significance to the influence of the cavities in the rock specimens on the stress and strain characteristics and the initiation, propagation, and penetration modes.

Experimental Investigation on Long-Term Strength and Acoustic Emission Characteristics of Coal Pillar under Inclined Compression Loading

Pillar rheological failure is one of the important reasons to induce earthquake, surface collapse, and water inrush disaster in underground mining engineering. Pillar is generally present under an inclined state and is significantly influenced by combining compression and shear loading. However, many scholars regard the long-term strength of coal or rock mass under pure uniaxial compression loading as the main evolution index of pillar strength, which is not consistent with engineering practice. In this paper, a new inclined uniaxial compression strength (IUCS) test system was developed and then used to carry out the IUCS test and creep test of coal specimens combined with an acoustic emission (AE) technology at various inclination angles (0°, 5°, 10°, 15°, and 20°). The variation of time-dependent deformation, peak strength, long-term strength (LTS), creep fracture model, and AE behavior of the coal with the inclination angels were discussed in detail. The results indicated that the peak strength and LTS of the coal nonlinearly decreased with the inclination angle increasing. The proportion of the peak strength to the LTS had remained constant between 58.5% and 62.9%, which can be considered as the inherent properties of coal rock. The creep failure model of the coal was transformed from tension-splitting failure at the inclination angle of 0° and 5° to tension-shear failure at the inclination angle of 10°-20°, which revealed that the inclination angles were favorable to the initiation and propagation of shear cracks. No matter any inclination angles, AE events can be divided into quiet period, low amplitude rising period, and high amplitude rising period with the periodic mutation of multistage loading points. Moreover, the cumulative AE energy gradually decreases with the increase of the inclination angles, which indicated that the shear stress caused by the specimen inclination can make crack initiation and propagation with less energy absorption. The research results will assist in the long-term strength design and time-dependent stability assessment of the coal pillar.

Evaluation of shear strength of pre-stressed hollow core slab

Pre-stressed hollow core slabs have been largely used in the precast concrete industry. They have also been extensively studied, especially regarding the characterisation of strength against web shear failure. However, in most of the studies reported in the literature, the pre-stressing force acting on the tested slabs was not known and torsional effects influenced the results. This work presents a simple and economical methodology that reduces the torsional effects in the tests, to evaluate the shear strength of pre-stressed hollow core slabs. From longitudinally-cut segments of 210 mm deep hollow core slabs with greased strands, the tensile strength of the concrete used in casting the hollow core slab was determined. From longitudinally-cut slab segments, the effective pre-stressing force was also determined at the time of carrying out the shear strength characterisation test. From these tests, the contribution of the concrete and pre-stressing force to the shear strength of the hollow core slab make up by slipformer machine was isolated and determined. Two failure modes were experimentally characterised from the segments, that is, flexural shear and tension shear failure. From the experimental results, an analytical model is proposed to determine the flexural shear strength of 210 mm hollow core slabs and the tension shear strength of non-pre-stressed concrete. A pre-stressed hollow core slab was also tested to validate the proposed analytical model. Additionally, the pre-stressing force transfer length in the hollow core slab was determined, obtaining a value of 65ϕ. Three databases of experimentally tested hollow core slabs, with depths ranging from 160 mm to 320 mm and a/h ratio equal to 2.5, were used to validate the proposed analytical model. A good estimate of the flexural shear strength of these slabs was obtained.

Experimental Study on Mechanical Damage and Fracture Characteristics of Granite under Different Water Cooling Conditions

In order to investigate the mechanical property deterioration and fracture characteristics of granite under different temperature drop and thermal cycle conditions, the evolution laws of mechanical properties, acoustic emission event distribution, and macro and micro failure characteristics of granite under different temperature changes were studied and analyzed by the servo loading, acoustic emission monitoring, and scanning electron microscope systems. The following conclusions were gained from the test results. (1) The peak stress and elasticity modulus of the three temperature drop treatments all decreased with the increase of the number of thermal cycles. In terms of magnitude, the following relationship was satisfied: 10°C > 15°C > 20°C. After 8 cycles, the peak stress and elasticity modulus tended to be stable for 15°C and 20°C temperature drops. (2) At a temperature drop of 20°C, the heterogeneity first increased and then tended to be stable; when the temperature was dropped by 15°C at each cycle, however, the heterogeneity first decreased and then became stable; as for the case of 10°C, the heterogeneity showed an overall decreasing trend. After 4 cycles, the heterogeneities were ranked as 15°C > 20°C > 10°C. After 8 cycles, 20°C > 15°C > 10°C. (3) With the decrease of temperature drop amplitude or the increase of cycles, the connectivity of microcracks in granite improved on the whole, the aperture and shape factor of microcracks increased, the damage of granite intensified, and the duration of the quiet period in the acoustic emission ringing count rate prolonged. (4) The tensile failure dominated at a temperature drop amplitude of 10°C. When the temperature drop was 15°C, the failure mode transitioned from hybrid tension-shear failure to tensile failure as the cycle times increased, whereas the hybrid tension-shear failure dominated with a temperature drop of 20°C.

Crack evolution law and failure mode of red sandstone under fatigue–creep interaction

AbstractTo explore the fracture characteristics of rock under the interaction of fatigue load and creep load, fatigue–creep interactive loading experiments were performed on red sandstone with prefabricated cracks. The crack evolution process and failure mode were analyzed using acoustic emission technology and digital image correlation. The results showed that crack growth mainly occurred in the fatigue loading stage; the crack evolution of the sample could be divided into three stages: nucleation and initiation ( ), stable expansion ( ), and unstable fracture ( ). There were distinct differences in the crack propagation modes of the rock samples with different prefabricated crack angles. The relationship between the crack initiation angle and prefabricated crack angle was analyzed based on the maximum circumferential stress theory. Moreover, with an increase in the prefabricated crack angle, the rock sample gradually changed from compression–shear failure to tension–shear failure.