Frictional shear stress of ZnO nanowires on natural and pyrolytic graphite substrates

The friction behaviour of ZnO nanowires on natural graphite(NG)and highly oriented pyrolytic graphite(HOPG)substrates was tested in ambient conditions by use of optical microscopy based nanomanipulation.Nanowires on the step-free and waviness-free NG substrate exhibit a diameter-independent nominal frictional shear stress of 0.48 MPa,and this provides a benchmark for studying how the surface topography of graphite influences nanowire friction.Nanowires on the HOPG substrate present a significant diameter-dependent frictional shear stress,increasing from 0.25 to 2.78 MPa with the decrease of nanowire diameter from 485 to 142 nm.The waviness of HOPG has a limited effect on the nanowire friction,as a nanowire can fully conform to the substrate.The surface steps on the HOPG can significantly enhance the nanowire friction and lead to a much higher frictional shear stress than that on NG due to mechanical blocking and the presence of a Schwoebel barrier at step edges.The surface steps,however,can also generate small wedge-shaped gaps between a nanowire and substrate,and thus reduce the nanowire friction.With the decrease in nanowire diameter,the capacity for the nanowire to better conform to the substrate reduces the length of the wedge-shaped gaps,leading to the observed increase in nanowire friction.The results have improved our understanding of the unique friction behaviour of nanowires.Such an improved understanding is expected to benefit the design and operation of nanowire-friction-based devices,including bio-inspired fibrillar adhesives,soft grippers,rotary nanomotors,and triboelectric nanogenerators.

An improved wall shear stress measurement technique using sandwiched hot-film sensors

In this letter we present a novel wall shear stress measurement technique for a turbulent boundary layer using sandwiched hot-film sensors. Under certain conditions, satisfactory results can be obtained using only the heat generated by one of the hot-film and a calibration of the sensors is not required. Two thin Nickel films with the same size were used in this study, separated by an electrical insulating layer. The upper film served as a sensor and the bottom one served as a guard heater. The two Nickel films were operated at a same temperature, so that the Joule heat flux generated by the sensor film transferred to the air with a minimum loss or gain depending on the uncertainties in the film temperature measurements. Analytical solution of the shear stress based on the aforementioned heat flux was obtained. The preliminary results were promising and the estimated wall shear stresses agreed reasonablywell with the directly measured values （with errors less than 20%） in a fully developed turbulent pipe flow. The proposed technique can be improved to further increase precisions.

Shear strength criteria for rock,rock joints,rockfill and rock masses:Problems and some solutions

Although many intact rock types can be very strong,a critical confining pressure can eventually be reached in triaxial testing,such that the Mohr shear strength envelope becomes horizontal.This critical state has recently been better defined,and correct curvature or correct deviation from linear Mohr-Coulomb（MC） has finally been found.Standard shear testing procedures for rock joints,using multiple testing of the same sample,in case of insufficient samples,can be shown to exaggerate apparent cohesion.Even rough joints do not have any cohesion,but instead have very high friction angles at low stress,due to strong dilation.Rock masses,implying problems of large-scale interaction with engineering structures,may have both cohesive and frictional strength components.However,it is not correct to add these,following linear M-C or nonlinear Hoek-Brown（H-B） standard routines.Cohesion is broken at small strain,while friction is mobilized at larger strain and remains to the end of the shear deformation.The criterion ＇c then σn tan φ＇ should replace ＇c plus σn tan φ＇ for improved fit to reality.Transformation of principal stresses to a shear plane seems to ignore mobilized dilation,and caused great experimental difficulties until understood.There seems to be plenty of room for continued research,so that errors of judgement of the last 50 years can be corrected.

Interfacial bonding mechanism of Cu/Al composite plate produced by corrugated cold roll bonding

Corrugated cold roll bonding(CCRB) produces metal composite plate with improved mechanical properties compared with conventional methods,but the interfacial mechanism is not fully understood.Here,Cu/Al composite plate with good plate shape was produced by CCRB,and the bonding mechanism and strength along the corrugated interface were studied by experiments and finite element simulations.The results showed that the average bonding strength of Cu/Al composite plate produced by CCRB was nearly twice that of conventional composite plate at an average reduction of 40% during rolling.Strong friction shear stresses occurred at the interface of the corrugated composite plate,which promoted the plastic deformation of the metals and accelerated the rupture of the brittle interfacial layer.Electron backscattered diffraction analysis showed that higher degrees of grain elongation and refinement occurred in the matrices at the front waist and trough due to the stronger normal and shear stresses.Energy-dispersive spectroscopy line scans showed that the thickest atomic diffusion layer occurred at the front waist.The present combination of experimental and computational analyses provides insights into the underlying mechanism of mechanically improved metal composites prepared by CCRB.