J. C. M. Li

Impression creep; a new creep test

A new indentation creep test is introduced in which the indenter is a circular cylinder with a flat end. Unlike conventional indentation tests, a steady-state velocity is observed in this new test shortly after a transient period during which the indenter makes a shallow impression on the surface of the specimen; hence the name “impression creep”. This steady-state velocity is found to have the same stress and temperature dependences as the conventional undirectional creep tests using bulk specimens. Three possible mechanisms are analysed in detail, bulk diffusion, surface diffusion, and dislocation creep. They have different stress and indenter-size (radius) dependences. Experimental results on succinonitrile crystals are reported.

Impression creep and other localized tests

Impression creep and impression fatigue, both using cylindrical indenters, are reviewed in this paper. For impression creep, analytical solutions and computer simulations for different situations are presented. Materials tested include metals and alloys, superplastic materials, weldments, glasses, ceramics and polymers. Viscosity measurements using indentation techniques and impression creep of thin films are discussed also. For impression fatigue, a steady state per cycle is shown and the power law dependence of maximum stress is presented. Underloading and overloading effects, as well as delayed retardation, are described. Other localized tests, such as nanoindentation, stress relaxation, impression recovery and the adhesion energy determined by impression testing, are briefly discussed also.

High‐Angle Tilt Boundary—A Dislocation Core Model

The dislocation model of a symmetric tilt boundary is modified to take into consideration the stress field of the cores of the dislocations. It is found that such modification enables discussion of many properties of high‐angle tilt boundaries including porosity, strain energy, intergranular fracture, grain boundary shearing, void formation, preferential melting, preferential diffusion, boundary migration, segregation, precipitation, amorphous behavior, continuous and discontinuous transition, etc., in terms of the interaction among the cores of the dislocations and the properties of the cores themselves. This is a first attempt to devise a working model capable of interpreting all the properties of grain boundaries. Unlike other models which are designed to have a few particular properties in a limited angular range, this model has the advantage of reducing to the dislocation model at low angles and of providing for amorphous behavior at high angles. The characteristic of a grain boundary of behaving as ...

Impression test of 63Sn37Pb eutectic alloy

Abstract Impression creep and stress relaxation experiments on the SnPb eutectic alloy were carried out in the RSA II instrument modified to use a 0.5 mm diameter cylindrical punch under 1.5–47 MPa punching stress and within a temperature range of 25 °C to 110 °C. Based on a power law between the impression velocity and stress or between the stress rate and stress, the exponent increased with stress from 1 to 3.5 within the temperature range 80–110 °C and 2.5 to 6 within the range 25–65 °C. These exponents were generally comparable to those reported in the literature. Because of the change of stress exponent, several mechanisms have been proposed. However, the stress dependence was found to obey a hyperbolic sine function of stress for all the stresses and temperatures studied. Similarly before using the hyperbolic sine function, the activation energy was found to increase with stress, an abnormal behaviour. Fortunately, after using the hyperbolic sine function, a single activation energy, 55 kJ mol−1 was obtained. Based on the present data, a single mechanism of interfacial viscous shearing between the two eutectic phases is proposed for both creep and stress relaxation. In addition to the effect of stress and temperature, the impression velocity based on this model should be directly proportional to the punch radius and inversely proportional to the nth (n = 1–3) power of the size of phase particles. These predictions are consistent with available information in the literature.

Slip processes in the deformation of polystyrene

Two slip processes are characterized in the deformation of atactic polystyrene by compression. In the optical microscope, one appears as intensive shear bands and the other as diffuse shear zones. But in the electron microscope, the latter reveals itself in the form of two sets of numerous, fine, discontinuous shear bands intersecting at nearly right angles. In addition to their differences in appearance, the coarse slip band propagates fast along a localized path, inclines at less than 45‡ with the compression axis, and invariably produces shear fracture when it extends across the specimen. On the other hand, the fine slip bands spread slowly by multiplication mainly along the maximum shear stress direction, contribute to almost all the macroscopic strain and cause shape changes of the specimen. Hence the coarse band process is a brittle mode and the fine band process a ductile mode. The relative abundance of these bands depends on the thermal history of the specimen, the loading condition, and the deformation temperature. The average shear strain inside either band is about 1.5 and is recoverable upon annealing.

Edge dislocations emitted along inclined planes from a mode I crack

Abstract The stress field around the tip of a semi-infinite crack with an edge dislocation parallel to the tip but located in any nearby position has been obtained in closed form. Using this, the equilibrium positions under mode I loading of an array of edge dislocations emitted from the crack tip on a slip plane oriented at an angle to the crack plane were obtained numerically. A dislocation-free zone (DFZ) was found to exist if the slip plane was not saturated with dislocations. The DFZ disappeared at saturation. The case of two arrays emitted on two symmetrically oriented slip planes was studied also. The effects of the applied stress, the angle of inclination of the slip plane, the lattice friction for dislocation motion and the number of dislocations in the plastic zone are presented.

Insight into Sulfur Reactions in Li–S Batteries

Understanding and controlling the sulfur reduction species (Li2Sx, 1 ≤ x ≤ 8) under realistic battery conditions are essential for the development of advanced practical Li-S cells that can reach their full theoretical capacity. However, it has been a great challenge to probe the sulfur reduction intermediates and products because of the lack of methods. This work employed various ex situ and in situ methods to study the mechanism of the Li-S redox reactions and the properties of Li2Sx and Li2S. Synchrotron high-energy X-ray diffraction analysis used to characterize dry powder deposits from lithium polysulfide solution suggests that the new crystallite phase may be lithium polysulfides. The formation of Li2S crystallites with a polyhedral structure was observed in cells with both the conventional (LiTFSI) electrolyte and polysulfide-based electrolyte. In addition, an in situ transmission electron microscopy experiment observed that the lithium diffusion to sulfur during discharge preferentially occurred at the sulfur surface and formed a solid Li2S crust. This may be the reason for the capacity fade in Li-S cells (as also suggested by EIS experiment in Supporting Information ). The results can be a guide for future studies and control of the sulfur species and meanwhile a baseline for approaching the theoretical capacity of the Li-S battery.

Pressure and normal stress effects in shear yielding

The classical yielding theory of Tresca is generalized to include the effects of both hydrostatic pressure and normal stress in the slip plane. The three-parameter theory is the most general for polyhedral yield surfaces. The new theory is applied to the yielding of polystyrene. Two modes of slip: a brittle mode in the form of coarse slip bands, and a ductile mode in the form of fine slip bands, are studied separately. All three parameters are determined for each mode.

Diffusion-induced beam bending in hydrogen sensors

The diffusion-induced bending of both single-layer and bilayer beam structure is analyzed by using linear elastic beam theory and the Moutier theorem. A closed form solution of the radius of curvature due to diffusion is obtained. For the single-layer beam structure, the radius of curvature is inversely proportional to the bending moment created by nonuniform concentration distribution. For the bilayer beam structure, the curvature is a linear function of the mismatch strain between the two layers and the bending moment introduced by diffusion. The mismatch strain depends on the concentration and the partial molar volume of the diffusing component in both layers. Application to microelectromechanical systems hydrogen sensors with a layer of Pd is shown.

Micro and nano mechanical testing of materials and devices

Principles and Applications of Indentation.- Size Effects in Nanoindentation.- Indentation in Shape Memory Alloys.- Adhesive Contact of Solid Surfaces.- Nanomechanical Characterization of One-Dimensional Nanostructures.- Deformation Behavior of Nanoporous Metals.- Residual Stress Determination Using Nanoindentation Technique.- Piezoelectric Response in the Contact Deformation of Piezoelectric Materials.- Mechanics of Carbon Nanotubes and Their Composites.- Microbridge Tests.- Nanoscale Testing of One-Dimensional Nanostructures.- Metrologies for Mechanical Response of Micro- and Nanoscale Systems.- Mechanical Characterization of Low-Dimensional Structures Through On-Chip Tests.