T.H. Zhang

Interfacial stress transfer in a graphene nanosheet toughened hydroxyapatite composite

In recent years, graphene has emerged as potential reinforcing nanofiller in the composites for structural engineering due to its extraordinary high elastic modulus and mechanical strength. As recognized, the transfer of stress from a low modulus matrix to a high-modulus reinforcing graphene and the interfacial behavior at a graphene-matrix interface is the fundamental issue in these composites. In the case of graphene nanosheet (GNS) reinforced hydroxyapatite (HA) composite, this research presented analytical models and simulated that the number of graphene layers of GNSs has little effect on the maximum axial stress (∼0.35 GPa) and the maximum shear stress (∼0.14 GPa) at a GNS-HA interface, and the energy dissipation by GNS pull-out decreases with increasing the number of graphene layers due to weak bonding between them. Also, GNS-HA interfacial delamination and/or GNS rupture were also indentified to be the two key failure mechanisms. The computed results are expected to facilitate a better understandi...

A novel torsion testing technique for micro-scale specimens based on electromagnetism

A novel torsion apparatus for micro-scale specimens is developed based on electromagnetism, in which a coil-magnet component is used for actuating and torque measuring. When the current gets through the coil, the torque, produced by Ampere force, can be easily measured by recording the current. A laser displacement sensor is applied to measure the rotation angle. The torque is calibrated using Sartorius BP211D balance. The calibration results demonstrate there is a perfect linear relationship between the torque and the current. The torque capacity is 4.0 × 10(-4) N m with noise-floor of less than 10(-8) N m. The rotation angle capacity is 60° with noise-floor of less than 0.02°. Two sets of copper wire specimens, with diameter of 100 μm and 140 μm, are tested using this apparatus. Experimental results, with good resolution and repeatability, successfully demonstrate the effectiveness of the torsion testing technique for micro-scale specimens.

Effect of Applied Stress on the Mechanical Properties of a Zr-Cu-Ag-Al Bulk Metallic Glass with Two Different Structure States

In order to investigate the effect of applied stress on mechanical properties in metallic glasses, nanoindentation tests were conducted on elastically bent Zr-Cu-Ag-Al metallic glasses with two different structure states. From spherical P-h curves, elastic modulus was found to be independent on applied stress. Hardness decreased by ~8% and ~14% with the application of 1.5% tensile strain for as-cast and 650 K annealed specimens, while it was slightly increased at the compressive side. Yield stress could be obtained from the contact pressure at first pop-in position with a conversion coefficient. The experimental result showed a symmetrical effect of applied stress on strengthening and a reduction of the contact pressure at compressive and tensile sides. It was observed that the applied stress plays a negligible effect on creep deformation in as-cast specimen. While for the annealed specimen, creep deformation was facilitated by applied tensile stress and suppressed by applied compressive stress. Strain rate sensitivities (SRS) were calculated from steady-state creep, which were constant for as-cast specimen and strongly correlated with applied stress for the annealed one. The more pronounced effect of applied stress in the 650 K annealed metallic glass could be qualitatively explained through the variation of the shear transformation zone (STZ) size.

Nanoindentation Hardness Distribution and Strain Field and Fracture Evolution in Dissimilar Friction Stir-Welded AA 6061-AA 5A06 Aluminum Alloy Joints

Aluminum alloy AA 6061-T651 and 5A06-H112 rolled plates were successfully welded by friction stir welding (FSW) at three rotation speeds of 600, 900, and 1200 rpm with two transverse speeds of 100 and 150 mm/min. Mechanical properties and strain field evolution of FSW AA 6061-AA 5A06 were characterized by the uniaxial tension and digital image correlation (DIC) tests. Furthermore, the hardness distribution map of whole cross section was obtained via the nanoindentation method with 700 indents. Both DIC and nanoindentation results reveal that the heat-affected zone (HAZ) of AA 6061 alloy is the softest area in the weldment, and the fracture happens in this region. The microstructure evolution characterized by electron-backscatter diffraction (EBSD) indicates that the continuous dynamic recrystallization is the primary grain structure evolution in the stirring zone, and the grain refinement helps improve the mechanical properties. Analyses of the micro- and macrofeatures of the fracture surfaces via scanning electron microscopy (SEM) and optical microscope suggest that the increasing of heat input could enlarge the size of HAZ and reduce the slant angle of HAZ and thus lead the fracture angle to decrease and cause the dimples change from inclined ones to normal ones.

Measuring the mechanical responses of a jammed discontinuous shear-thickening fluid

Discontinuous shear thickening observed in many dense suspensions is rather complicated but takes place as a transient. Its details after jamming are still not clear. By using a modified rheometer with an analog-to-digital converter, we show that there are three amplitude regimes of shear strain after jamming. First, the shear modulus and the force chain angle increase with strain; the jammed state gradually deepens. Second, the jammed state is stable against a further increase in shear stress; the force chain angle is constant and the force network is stable. Third, the jammed state cannot resist a further increase in shear stress; with the increase in the force chain angle, the force chain network is broken.