Contribution of texture to the strengthening and fracture in hot-rolled magnesium-12.7 at % cadmium alloy

The grain size dependencies of the yield and fracture stresses in hot rolled Mg-12.7 at % Cd alloy have been measured in the temperature range 77 to 420 K and are found to be in accordance with HalI-Petch type of equations. In hot rolled Mg-12.7 Cd alloy, the HalI-Petch intercept a w is higher than that in hot rolled magnesium, while the slope ky is comparable. The fracture is intercrystalline at 77 K, mixed mode at 300 K and ductile at 420 K. The above flow and fracture behaviours are interpreted in terms of the complimentary effects of texture hardening and solid solution strengthening.

On global summer monsoonal drought mechanics

It is shown that within the framework of a linear five-level quasi-geostrophic steady state global model the middle latitude systems can always have significant influence on the Asian summer monsoonal system through the lower tropospheric monsoonal westerly window region around 80°E. It is hypothesized that quasistationarity of the middle latitude longwave systems results in stronger teleconnections through this window and the consequent monsoon breaks when the phase is right.

Numerical simulations of void growth near a notch tip in ductile single crystals

The objective of this work is to study the growth of a cylindrical void ahead of a notch tip in ductile FCC single crystals under mode I, plane strain, small scale yielding (SSY) conditions. To this end, finite element simulations are performed within crystal plasticity framework neglecting elastic anisotropy. Attention is focussed on the effects of crystal hardening, ratio of void diameter to spacing from the notch and crystal orientation on plastic flow localization in the ligament connecting the notch and the void as well as their growth. The results show strong interaction between shear bands emanating from the notch and angular sectors of single slip forming around the void leading to intense plastic strain development in the ligament. Further, the ductile fracture processes are retarded by increase in hardening of the single crystal and decrease in ratio of void diameter to spacing from the notch. Also, a strong influence of crystal orientation on near-tip void growth and plastic slip band development is observed. Finally, the synergistic, cooperative growth of multiple voids ahead of the notch tip is examined.

On a linear steady-state system of equations in microcontinuum fluid mechanics

Galerkin representations and integral representations are obtained for the linearized system of coupled differential equations governing steady incompressible flow of a micropolar fluid. The special case of 2-dimensional Stokes flows is then examined and further representation formulae as well as asymptotic expressions, are generated for both the microrotation and velocity vectors. With the aid of these formulae, the Stokes Paradox for micropolar fluids is established.

The formation of carbon nanostructures by in situ TEM mechanical nanoscale fatigue and fracture of carbon thin films

A technique to quantify in real time the microstructural changes occurring during mechanical nanoscale fatigue of ultrathin surface coatings has been developed. Cyclic nanoscale loading, with amplitudes less than 100 nm, is achieved with a mechanical probe miniaturized to fit inside a transmission electron microscope (TEM). The TEM tribological probe can be used for nanofriction and nanofatigue testing, with 3D control of the loading direction and simultaneous TEM imaging of the nano-objects. It is demonstrated that fracture of 10-20 nm thick amorphous carbon films on sharp gold asperities, by a single nanoscale shear impact, results in the formation of < 10 nm diameter amorphous carbon filaments. Failure of the same carbon films after cyclic nanofatigue, however, results in the formation of carbon nanostructures with a significant degree of graphitic ordering, including a carbon onion.

Fracture Behavior of Ferrocement Beams

An attempt is made to study the fracture behavior of ferrocement beams using J-integral and critical crack opening displacement approaches. Ferrocement beams with three different relative notch depths and different percentages of mesh reinforcement were tested in four-point bending (third-point loading). The experimental results were used to evaluate the apparent J-integral and CODc values. Results show that the apparent J-integral does not seem to follow any particular trend in variation with notch depth, but is sensitive to the increase of mesh reinforcement. Hence, the apparent J-integral appears to be a useful fracture criterion for ferrocement. The computed values of CODt are found to be dependent on the depth of notch and, thus, cannot possibly be considered as a suitable fracture criterion for ferrocement.

Observation of kink shear bands in an aluminium single crystal fracture specimen

The near-tip deformation field in a high-constraint three-point bend specimen of pure aluminium single crystal is studied using in situ electron back-scattered diffraction and optical metallography. The orientation considered has the notch lying on the (0 1 0) plane and the notch front along direction. Results clearly show the occurrence of a kink shear sector boundary at 90° to the notch line on the specimen free surface as predicted by the analytical model of Rice [J.R. Rice, Mech. Mater. 6 (1987) 317].

Influence of heat-treatment on the strength and fracture-behavior of fe-12cr-6al ferritic stainless-steel

The changes in the tensile properties and fracture mode brought about by heat treatment of Fe-12Cr-6Al ferritic stainless steel have been studied. A favourable combination of high strength and good ductility is obtained by heating the material at 1370 K for 2 h followed by a water quench. The high-temperature treatment results in carbide dissolution as well as an increase in the grain size. The mechanism of strengthening has been evaluated from the apparent activation energy (28 kJ mol–1) and is identified to be the unpinning of dislocations from the atmosphere of carbon atoms. As the heat-treatment temperature is increased, the fracture behaviour changes from ductile to brittle mode and this is related to the changes in grain size and friction stress.

Charged-soliton excitations in statistical mechanics

A method is developed for demonstrating how solitons with some internal periodic motion may emerge as elementary excitations in the statistical mechanics of field systems. The procedure is demonstrated in the context of complex scalar fields which can, for appropriate choices of the Lagrangian, yield charge-carrying solitons with such internal motion. The derivation uses the techniques of the steepest-descent method for functional integrals. It is shown that, despite the constraint of some fixed total charge, a gaslike excitation of such charged solitons does emerge.

Non-Abelian monopoles break color. I. Classical mechanics

Monopoles which are sources of non-Abelian magnetic flux are predicted by many models of grand unification. It has been argued elsewhere that a generic transformation of the "unbroken" symmetry group H cannot be globally implemented on such monopoles for reasons of topology. In this paper, we show that similar topological obstructions are encountered in the mechanics of a test particle in the field of these monopoles and that the transformations of H cannot all be globally implemented as canonical transformations. For the SU(5) model, if H is SU(3)C×U(1)em, a consequence is that color multiplets are not globally defined, while if H is SU(3)C×SU(2)WS×U(1)Y, the same is the case for both color and electroweak multiplets. There are, however, several subgroups KT, KT′,… of H which can be globally implemented, with the transformation laws of the observables differing from group to group in a novel way. For H=SU(3)C×U(1)em, a choice for KT is SU(2)C×U(1)em, while for H=SU(3)C×SU(2)WS×U(1)Y, a choice is SU(2)C×U(1)×U(1)×U(1). The paper also develops the differential geometry of monopoles in a form convenient for computations.

Non-Abelian monopoles break color. II. Field theory and quantum mechanics

Many grand unified theories (GUT's) predict non-Abelian monopoles which are sources of non-Abelian (and Abelian) magnetic flux. In the preceding paper, we discussed in detail the topological obstructions to the global implementation of the action of the "unbroken symmetry group" H on a classical test particle in the field of such a monopole. In this paper, the existence of similar topological obstructions to the definition of H action on the fields in such a monopole sector, as well as on the states of a quantum-mechanical test particle in the presence of such fields, are shown in detail. Some subgroups of H which can be globally realized as groups of automorphisms are identified. We also discuss the application of our analysis to the SU(5) GUT and show in particular that the non-Abelian monopoles of that theory break color and electroweak symmetries.

AE energy release during the fracture of HSC beams

An attempt has been made experimentally to investigate the acoustic emission (AE) energy release in high-strength concrete (HSC) beams subjected to monotonically increasing load. Acoustic emission energy release during the fracture process of the HSC beams is measured. Stress waves released during the fracture process in materials cause acoustic emissions. AE energy released during the fracture of a notched three-point bend plain concrete beam specimens having 28-day compressive strengths of 50.0 MPa, 69.0 MPa and 78.0 MPa and mortar (cement: sand (1: 4) by weight) specimens are studied. Mortar consists of one part cement and four parts sand by weight. The specimens were tested by a material testing system of 1200 kN capacity employing crack mouth opening displacement control at the rate of 0.0004 mm/s. The fracture energy and the AE energy released during the fracture process of all the tested TPB and mortar specimens are compared and discussed. The observations made in the present experimental study have some applications for monitoring the integrity of structures.