Evolution of mechanical parameters of Shuangjiangkou granite under different loading cycles and stress paths

Surrounding rocks at different locations are generally subjected to different stress paths during the process of deep hard rock excavation.In this study,to reveal the mechanical parameters of deep surrounding rock under different stress paths,a new cyclic loading and unloading test method for controlled true triaxial loading and unloading and principal stress direction interchange was proposed,and the evolution of mechanical parameters of Shuangjiangkou granite under different stress paths was studied,including the deformation modulus,elastic deformation increment ratios,fracture degree,cohesion and internal friction angle.Additionally,stress path coefficient was defined to characterize different stress paths,and the functional relationships among the stress path coefficient,rock fracture degree difference coefficient,cohesion and internal friction angle were obtained.The results show that during the true triaxial cyclic loading and unloading process,the deformation modulus and cohesion gradually decrease,while the internal friction angle gradually increases with increasing equivalent crack strain.The stress path coefficient is exponentially related to the rock fracture degree difference coefficient.As the stress path coefficient increases,the degrees of cohesion weakening and internal friction angle strengthening decrease linearly.During cyclic loading and unloading under true triaxial principal stress direction interchange,the direction of crack development changes,and the deformation modulus increases,while the cohesion and internal friction angle decrease slightly,indicating that the principal stress direction interchange has a strengthening effect on the surrounding rocks.Finally,the influences of the principal stress interchange direction on the stabilities of deep engineering excavation projects are discussed.

Enhanced micro/nano-tribological performance in partially crystallized 60NiTi film

The microstructure,mechanical and micro/nano-tribological properties of the 60NiTi film annealed at different temperature were investigated.The results reveal that annealing as-deposited 60NiTi film at 300,375,and 600℃ for 1 h leads to structural relaxation,partial crystallization and full crystallization,respectively.Compared with the structurally relaxed structure,the partially crystallized structure exhibits increased hardness but decreased elastic modulus.This is because that the elastic modulus is reduced by Voigt model while the hardness is improved by composite effect.Due to the highest hardness and ratio of hardness to elastic modulus(H/E),the partially crystallized 60NiTi film has the lowest penetration depth and residual depth(i.e.,groove depth).Besides,the results also reveal that ductile plowing is the dominant wear mechanism for all the annealed 60NiTi films.Under the condition of the ductile plowing,coefficient of friction and wear resistance are related to penetration depth and residual depth,respectively.Therefore,the partially crystallized 60NiTi film shows the best tribological performance at the micro/nano-scale.The current work not only highlights the important roles of hardness and H/E in improving the micro/nano-tribological properties but also concludes an efficient and simple method for simultaneously increasing hardness and H/E.