Heterogeneities of grain boundary contact for simulation of laboratoryscale mechanical behavior of granitic rocks

From a practical point of view,grain structure heterogeneities are key parameters that control the rock response and still remains a challenge to incorporate in a quantitative manner.One of the less discussed topics in the context of the grain-based model(GBM)in the particle flow code(PFC)is the contact heterogeneities and the appropriate contact model to mimic the grain boundary behavior.Generally,the smooth joint(SJ)model and linear parallel bond(LPB)model are used to simulate the grain boundary behavior.However,the literature does not document the suitability of different models for specific problems.Another challenge in implementing GBM in PFC is that only a single bonding parameter is used at the grain boundaries.The aim of this study is to investigate the responses of a laboratory-scale specimen with SJ and LPB models,considering grain boundary heterogeneous and homogeneous contact parameters.Uniaxial and biaxial compression tests are performed to calibrate the response of Creighton granite.The stressestrain curves,volumetric dilation,inter-crack(crack in the grain boundary),and intra-crack(crack within the grain)development,and failure patterns associated with different contact models are examined.It was found that both the SJ and LPB models can reproduce the pre-peak behavior observed for a granitic rock type.However,the LPB model is unable to reproduce the post-peak behavior.Due to the large interlocking effect originating from the balls in contact and the ball size in the LPB model,local dilation is induced at the grain boundaries.This overestimates the volumetric dilation and residual shear strength.The LPB model tends to result in discontinuous inter-cracks and stress localization in the rock specimen,resulting in fine fragments at the rock surface during failure.

Mechanic model of water-based boundary lubricated contact based on surface force effects

In water-based boundary lubrication regime,the contact gaps(or boundary lubricated film thickness)and surface pressure distribution must be determined to really understand the boundary lubricated contact mechanism.However,the accurate determination of these parameters is limited.In this study,a mechanical model based on boundary lubricated contact involving surface force effects is developed.The surface force distribution characteristics,normal force vs.central film thickness curve,and macroscale water-based boundary lubricated contact are investigated numerically.The results show that hydration directly affects surface force interaction.The accurate boundary lubricated film thickness and surface pressure distribution can be obtained using this model in point contact.Furthermore,the mechanism of macroscale water-based liquid boundary lubricated contact is investigated,in which a water-based boundary lubricated film is formed under appropriate operating conditions based on surface force effects during running-in.This study can reveal the water-base boundary lubricated contact behavior and the carrying capacity of the surface force effect,and provides important design guidance for the surface force effect to achieve liquid superlubricity in water-based boundary lubricated contacts.

CALCULATION OF W-N GEAR DRIVES’RUNNING-IN PROCEDURE

A new calculating method of the running in procedure of W-N gear drives according to the criterion of frictional work consumed in meshing cycles is presented.Combining with the amount of clearances between two teeth flanks,the running-in procedure of W-N gear drives is reappeared more authentically. The erroneous conclusion that considering that the contact pressure uniformly distribute along the tooth depth after full running-in is corrected. So a foundation for the calculation of the contact pressure of W-N gear drives is provided.

Calculation of Electrostatic/Magnetic Adhesion Force Between Adjacent Objects Considering Thin Gap Effect

This paper presents a new approach to evaluating the electrostatic/magnetic adhesion force between two adjacent objects separated by a thin gap.In this approach,instead of generating mesh for the gap,a contact boundary is introduced in the finite element modeling to obtain a reasonable field distribution;then the field in the gap is approximated based on the continuity condition at their interface,so that the adhesion force can be properly calculated.Moreover,a simple equivalent circuit model is introduced to explain how the thin gap influences the adhesion force significantly.Numerical experiments are given to demonstrate the validity of the proposed method and the significance of the thin gap.