Cyclic shear behavior of en-echelon joints under constant normal stiffness conditions

To reveal the mechanism of shear failure of en-echelon joints under cyclic loading,such as during earthquakes,we conducted a series of cyclic shear tests of en-echelon joints under constant normal stiffness(CNS)conditions.We analyzed the evolution of shear stress,normal stress,stress path,dilatancy characteristics,and friction coefficient and revealed the failure mechanisms of en-echelon joints at different angles.The results show that the cyclic shear behavior of the en-echelon joints is closely related to the joint angle,with the shear strength at a positive angle exceeding that at a negative angle during shear cycles.As the number of cycles increases,the shear strength decreases rapidly,and the difference between the varying angles gradually decreases.Dilation occurs in the early shear cycles(1 and 2),while contraction is the main feature in later cycles(310).The friction coefficient decreases with the number of cycles and exhibits a more significant sensitivity to joint angles than shear cycles.The joint angle determines the asperities on the rupture surfaces and the block size,and thus determines the subsequent shear failure mode(block crushing and asperity degradation).At positive angles,block size is more greater and asperities on the rupture surface are smaller than at nonpositive angles.Therefore,the cyclic shear behavior is controlled by block crushing at positive angles and asperity degradation at negative angles.

Sliding modes of fault activation under constant normal stiffness conditions

Fault activation has been the focus of research community for years.However,the studies of fault activation remain immature,such as the fault activation mode and its major factors under constant normal stiffness(CNS)conditions associated with large thickness of fault surrounding rock mass.In this study,the rock friction experiments were conducted to understand the fault activation modes under the CNS conditions.Two major parameters,i.e.the initial normal stress and loading rate,were considered and calibrated in the tests.To reveal the response mechanism of fault activation,the local strains near the fault plane were recorded,and the macroscopic stresses and displacements were analyzed.The testing results show that the effect of displacement-controlled loading rate is more pronounced under the CNS conditions than that under constant normal load(CNL)conditions.Both the normal and shear stresses drop suddenly when the stick-slip occurs.The decrease and increase of the normal stress are synchronous with the shear stress in the regular stick-slip scenario,but mismatch with the shear stress during the chaotic stick-slip process.The results are helpful for understanding the fault sliding mode and the prediction and prevention of fault slip.

Experimental Studies on Cyclic Shear Behavior of Steel-Clay Interface Under Constant Normal Load

The degradation of the shear stress between pile-clay interface caused by undrained cyclic jacking affects the jacking force.A series of large displacement monotonic shear,cyclic shear and post-cyclic monotonic steel plate-clay interface shear te sts were performed under the constant normal load(CNL)condition to inve stigate the effects of normal stre ss,cyclic amplitude,and number of cycles on a steel plate-clay interface using the GDS multi-function interface shear tester.Based on the experimental results,in monotonic shear tests,change of shear stress took place in the specimen,the shear stress rapidly reached the peak value at shear displacement of 1 mm,and then abruptly decreased to the residual value.In cyclic shear te sts,accumulated displacement was a better parameter to describe the soil degradation characteristics,and the degradation degree of shear stress became greater with the increasing of normal stress and accumulated displacement.Shear stress in post-cyclic monotonic shear tests did not generate a peak value and was lower than that in monotonic shear tests under the same normal stress.The soil was completely disturbed and reached the residual strength when the cumulative displacement approached 6 m.An empirical equation to evaluate shear stress degradation mechanism was formulated and the procedure of parameter identification was presented.

Laboratory investigation into effect of bolt profiles on shear behaviors of bolt-grout interface under constant normal stiffness (CNS) conditions

Rock bolts have been widely used for stabilizing rock mass in geotechnical engineering.It is acknowledged that the bolt profiles have a sound influence on the support effect of the rock bolting system.Previous studies have proposed some optimal rib parameters(e.g.rib spacing);unfortunately,the interface shear behaviors are generally ignored.Therefore,determination of radial stress and radial displacement on the bolt-grout interface using traditional pull-out tests is not possible.The load-bearing capacity and deformation capacity vary as bolt profiles differ,suggesting that the support effect of the bolting system can be enhanced by optimizing bolt profiles.The aim of this study is to investigate the effects of bolt profiles(with/without ribs,rib spacing,and rib height)on the shear behaviors between the rock bolt and grout material using direct shear tests.Thereby,systematic interfacial shear tests with different bolt profiles were performed under both constant normal load(CNL)and constant normal stiffness(CNS)boundary conditions.The results suggested that rib spacing has a more marked influence on the interface shear behavior than rib height does,in particular at the post-yield stage.The results could facilitate our understanding of bolt-grout interface shear behavior under CNS conditions,and optimize selection of rock bolts under in situ rock conditions.

Microscratch of copper by a Rockwell C diamond indenter under a constant load

The scratch test is used for quality control mostly in phenomenological ways,and whether fracture toughness can be obtained from this test is still a matter of debate requiring further elucidation.In this paper,values of the fracture toughness of copper obtained by different scratch-based approaches are compared in order to examine the applicability of scratch-based methodologies to characterize the fracture toughness of soft metals.The scratch response of copper to a Rockwell C diamond indenter is studied under a constant normal load condition.The variations of penetration depth,residual depth,and residual scratch width with applied normal load are quantified from spherical to sphero-conical contact regimes by piecewise functions.A newly proposed size effect law is found to be the most suitable for scratch-based approaches to characterizing the fracture toughness of soft metallic materials with significant plasticity.A simple expression relating the nominal stress to the penetration depth is proposed for the spherical contact regime and gives almost the same value of fracture toughness.The residual scratch width provides useful information on pile-up of material and on the spherical tip radius of the indenter.It is found that the values of the fracture toughness obtained from the microscratch test are influenced by the data range for analysis.

Effect of bolt inclination angle on shear behavior ofbolted joints under CNL and CNS conditions

Rock bolts are widely used in rock engineering projects to improve the shear capacity of the jointed rock mass.The bolt inclination angle with respect to the shear plane has a remarkable influence on the bolting performance.In this study,a new artificial molding method based on 3D scanning and printing technology was first proposed to prepare bolted joints with an inclined bolt.Then,the effects of the bolt inclination angle and boundary conditions on the shear behavior and failure characteristic of bolted joints were addressed by conducting direct shear tests under both CNL and CNS conditions.Results indicated that rock bolt could significantly improve the shear behavior of rock joints,especially in the post-yield deformation region.With the increase of bolt inclination angle,both the maximum shear stress and the maximum friction coefficient increased first and then decreased,while the maximum normal displacement decreased monotonously.Compared with CNL conditions,the maximum shear stress was larger,whereas the maximum normal displacement and friction coefficient were smaller under the CNS conditions.Furthermore,more asperity damage was observed under the CNS conditions due to the increased normal stress on the shear plane.

Degradation of joint surface morphology, shear behavior and closure characteristics during cyclic loading

In order to investigate the failure mechanism of rock joint,a series of laboratory tests including cyclic direct shear tests under constant normal load(CNL)conditions were conducted.Morphology parameters of the rock joint surface were precisely calculated by means of a three-dimensional laser scanning machine.All test results were analyzed to investigate the shear behavior and normal displacement behavior of rock joints under CNL conditions.Degradation of rock joint surface during cyclic shear tests was also analyzed.The comparison results of the height parameters and the hybrid parameters of the joint surface during cyclic tests show that the degradation of the surface mostly happens in the first shear and the constant normal loads imposed on the joints have significant promotion effects on the morphology degradation.During cyclic shear tests,joints surfaces evolve from rough state to smooth state but keep an overall undulation.Dilatancy of rock joints degrades with the degradation of joint surface and the increase of normal loads.The closure deformation of joint is larger than that of the intact rock,and the normal stiffness increases with the increase of shearing times.

Experimental and modeling study of kinetics for methane hydrate formation in a crude oil-in-water emulsion

A low-viscosity emulsion of crude oil in water can be believed to be the bulk of a flow regime in a pipeline.To differentiate which crude oil would and which would not counter the blockage of flow due to gas hydrate formation in flow channels,varying amount of crude oil in water emulsion without any other extraneous additives has undergone methane gas hydrate formation in an autoclave cell.Crude oil was able to thermodynamically inhibit the gas hydrate formation as observed from its hydrate stability zone.The normalized rate of hydrate formation in the emulsion has been calculated from an illustrative chemical affinity model,which showed a decrease in the methane consumption（decreased normalized rate constant） with an increase in the oil content in the emulsion.Fourier transform infrared spectroscopy（FTIR） of the emulsion and characteristic properties of the crude oil have been used to find the chemical component that could be pivotal in selfinhibitory characteristic of the crude oil collected from Ankleshwar,India,against a situation of clogged flow due to formation of gas hydrate and establish flow assurance.

DELIBERATlON TO THE DEFINITION OF TOOTH THICKNESS FOR INVOLUTE HELICAL GEAR IN INTERNATIONAL STANDARD ISO1122／1

The definitions of the normal chordal tooth thichness and the constant chord in interna-tional standard ISO 1122/1-1983(E) are analyzed according to the basic principle of involute cy-lindrical gear geometry. It is pointed out that in the case of spur gears, the two definitions arerespectively identical with generally recognized traditional formulas, but in the case of helical gears,they are respectively contradictory with their traditional formulas. The general principle of themethod of the two-point measuring for the tooth thickness of gears is analysized, and the calcula-tion formulas of the two-point measuring for the tooth thickness of the involute helical gear ispresented. It is proved theoretically that a constant chord can be measured with the method of thetwo-point measuring (with the tooth thickness caliper) while normal chordal tooth thickness cannot be measured with the tooth thickness caliper (the method of the two-point measuring). The is-sues of the two definitions of the tooth thickness in the international standard are analysed outagain in measurng.

Effects of joint persistence and testing conditions on cyclic shear behavior of en-echelon joints under CNS conditions

Cyclic shear tests on rock joints serve as a practical strategy for understanding the shear behavior of jointed rock masses under seismic conditions.We explored the cyclic shear behavior of en-echelon and how joint persistence and test conditions(initial normal stress,normal stiffness,shear velocity,and cyclic distance)influence it through cyclic shear tests under CNS conditions.The results revealed a through-going shear zone induced by cyclic loads,characterized by abrasive rupture surfaces and brecciated material.Key findings included that increased joint persistence enlarged and smoothened the shear zone,while increased initial normal stress and cyclic distance,and decreased normal stiffness and shear velocity,diminished and roughened the brecciated material.Shear strength decreased across shear cycles,with the most significant reduction in the initial shear cycle.After ten cycles,the shear strength damage factor D varied from 0.785 to 0.909.Shear strength degradation was particularly sensitive to normal stiffness and cyclic distance.Low joint persistence,high initial normal stress,high normal stiffness,slow shear velocity,and large cyclic distance were the most destabilizing combinations.Cyclic loads significantly compressed en-echelon joints,with compressibility highly dependent on normal stress and stiffness.The frictional coefficient initially declined and then increased under a rising cycle number.This work provides crucial insights for understanding and predicting the mechanical response of en-echelon joints under seismic conditions.