On the calibration and verification of Voronoi-based discontinuous deformation analysis for modeling rock fracture

Since its introduction,discontinuous deformation analysis(DDA)has been widely used in different areas of rock mechanics.By dividing large blocks into subblocks and introducing artificial joints,DDA can be applied to rock fracture simulation.However,parameter calibration,a fundamental issue in discontinuum methods,has not received enough attention in DDA.In this study,the parameter calibration of DDA for intact rock is carefully studied.To this end,a subblock DDA with Voronoi tessellation is presented first.Then,a modified contact constitutive law is introduced,in which the tensile and shear meso-strengths are modified to be independent of the bond lengths.This improvement can prevent the unjustified preferential failure of short edges.A method for imposing confining pressure is also introduced.Thereafter,sensitivity analysis is performed to investigate the influence of the calculated parameters and meso-parameters on the mechanical properties of modeled rock.Based on the sensitivity analysis,a unified calibration procedure is suggested for both cases with and without confining pressure.Finally,the calibration procedure is applied to two examples,including a biaxial compression test.The results show that the proposed Voronoi-based DDA can simulate rock fracture with and without confining pressure very well after careful parameter calibration.

Numerical Modeling of the Undertow Structure and Sandbar Migration in the Surfzone

A process-based 3D numerical model for surfzone hydrodynamics and beach evolution was established. Comparisons between the experimental data and model results proved that the model could effectively describe the hydrodynamics, sediment transport feature and sandbar migration process in the surfzone with satisfactory precision. A series of numerical simulations on the wave breaking and shoaling up to a barred beach were carried out based on the model system. Analyzed from the model results, the wave-induced current system in the surfzone consists of two major processes, which are the phase-averaged undertow caused by wave breaking and the net drift caused by both of the nonlinear wave motion and surface roller effect. When storm waves come to the barred beach, the strong offshore undertow along the beach suppresses the onshore net drift, making the initial sandbar migrate to the seaside. Under the condition of calm wave environment, both the undertow and net drift flow to the shoreline at the offshore side of the sandbar, and then push the initial sandbar to the shoreline. The consideration of surface roller has significant impact on the modeling results of the sandbar migration. As the roller transfer rate increases, the sandbar moves onshore especially under the storm wave condition.

Drivability of Large Diameter Steel Cylinders During Hammer-Group Vibratory Installation for the Hong Kong–Zhuhai–Macao Bridge

The Hong Kong–Zhuhai–Macao Bridge(HZMB)involved the installation of 120 mega-cylinders with a diameter of 22 m,weights up to 513 t,and penetration depths up to 33 m using an eight-vibratory hammer group.Due to the lack of engineering experience on the drivability of large-diameter cylinders under multiple vibratory hammers,predicting the penetration rate and time of steel cylinders is an open challenge that has a considerable impact on the construction control of the HZMB.In this study,the vibratory penetration of large-diameter steel cylinders in the HZMB is investigated based on geological surveys,field monitoring,and drivability analysis.The vibratory penetration rate,installation accuracy,and dynamic responses of the steel cylinders at both the eastern and western artificial islands are analyzed.The dynamic soil resistance has a great influence on the cylinder drivability.However,the current design methods for estimating the vibratory driving soil resistance are proven inaccurate without considering the scale effects.Therefore,a modified method with a normalized effective area ratio A_(r,eff)is proposed in this study to calculate the vibratory soil resistance for open-ended thin-wall cylinders under unplugged conditions.Considering the scale effects on the vibratory driving soil resistance,the proposed method leads to closer results to the measured data,providing a reference for future engineering practice.

Boundary element method of hydrodynamic characteristics of lubricant in three-leaf dislocated floating-ring bearing

The problem of hydrodynamics of the three-leaf dislocated floating-ring bearing was studied by means of boundary element method.The law including the distribution of pressure on boundary surface(axial,bearing and floating-ring)and its friction loss in different eccentricities was obtained.The results show that the inner friction of three-leaf dislocated bearing increases from 390.875to 1 091.65,and the inner friction of three-leaf dislocated floating-ring bearing increases from 94.2523to 114.5069with eccentricity varying from 0to 0.075in nondimensional.So changing the pressure and flow field of bearing by adding floating-ring is more stability and less wasted work of friction than three-leaf dislocated bearing.

Deformation twinning in equiaxed-grained Fe-6.5 wt.%Si alloy after rotary swaging

Tensile behavior of an equiaxed-grained Fe-6.5 wt.%Si alloy,which was deformed intoφ6 mm bar by hot rotary swaging,was investigated at various temperatures(300–400℃)and stretching rates(0.42–1 mm/min).The results revealed an enhancement in the intermediate-temperature tensile ductility after heat treatments.Deformation twinning was found in the equiaxed-grained Fe-6.5 wt.%Si bars during the tensile test,and heat treatments can enhance the deformation twinning.More twins can be observed in the necking areas than other regions.The high Schmid factor values above 0.4 after heat treatments demonstrated that deformation twinning can easily occur in the equiaxed-grained Fe-6.5 wt.%Si alloy.Higher deformation temperatures,higher strain rates,and larger degree of order suppressed the formation of deformation twinning,while the grain sizes had little effect on the deformation twinning.The twinning stress of the Fe-6.5 wt.%Si alloy increased with the increasing grain size,which did not agree with the Hall–Petch type relationship.The deformation twinning resulted in the improved ductility of the Fe-6.5 wt.%Si alloy.