On the yield stress of nanocrystals

A generalization of the Hall-Petch relationship is proposed. The generalized relationship takes account of the contributions from intergrain sliding, generation of lattice dislocations, and influence of disclination-like defects. From this approach, a critical size corresponding to a maximum of the Hall-Petch size dependence is obtained. The value of the critical size essentially depends on the state of boundaries and it explains contradictory results for the microhardness of nanocrystals (NCs), since grain-boundary sliding is facilitated in unrelaxed nanocrystals and constrained in aged ones.     

Vibrations of jammed disk packings with Hertzian interactions

Contact breaking and Hertzian interactions between grains can both give rise to nonlinear vibrational response of static granular packings. We perform molecular dynamics simulations at constant energy in 2D of frictionless bidisperse disks that interact via Hertzian spring potentials as a function of energy and measure directly the vibrational response from the Fourier transform of the velocity autocorrelation function. We compare the measured vibrational response of static packings near jamming onset to that obtained from the eigenvalues of the dynamical matrix to determine the temperature above which the harmonic approximation breaks down. We compare packings that interact via single-sided (purely repulsive) and double-sided Hertzian spring interactions to disentangle the effects of the shape of the potential from contact breaking. Our studies show that while Hertzian interactions lead to weak nonlinearities in the vibrational behavior (e.g. the generation of harmonics of the eigenfrequencies of the dynamical matrix), the vibrational response of static packings with Hertzian contact interactions is dominated near jamming by contact breaking as found for systems with repulsive linear spring interactions.     

Properties of force networks in jammed granular media

Discrete element method simulations are conducted to investigate the effects of applied uniaxial compressive load, and bidisperse particle size distributions on force networks within jammed granular media. The differences between the strong and weak networks are examined through investigating the spatial correlation and distribution of contact angles, and emergence of chainlike structures. The simulation results show that the chainlike structures are more prevalent in the strong network due to the larger cumulative probabilities of contact angles, but not all the contacts belonging to the strong or weak networks are able to constitute the chainlike structures. Although the contacts of coarse-fine particles are dominant for the bidisperse systems, the contacts of coarse–coarse particles dominate the strong network, as well as the linear chainlike structures. Upon increasing the pressure from very low to high, the probability of contact orientations with respect to the compression direction in the strong network increases for contact orientation less than \(60^{\circ }\) and decreases for contact orientation greater than \(60^{\circ }\), while the opposite trends are observed in the weak network. The tails of normalized normal contact forces distributions are quantified by \(\hbox {P}(\hbox {f}) = \hbox {exp}(-\hbox {cf}^{\mathrm{n}})\), and it is found that the value of n depends on the applied pressure and particle size distribution. Statistical analysis shows that the degree of homogeneity of contact force increases with increasing pressure, which is also validated by participation number.     

The stress response and onset of yield of rotating FGM hollow shafts

Summary An analytical model is developed to study the stress response and the yielding of rotating functionally graded hollow shafts in the framework of generalized plane strain. The modulus of elasticity and the uniaxial yield limit of the shaft material are assumed to vary radially in nonlinear forms. It is shown that different modes of plasticization may take place. Analytical expressions for the critical values of the material parameters leading to different modes of plastic deformation and the corresponding elastic limit rotation speeds are derived by the use of Tresca's yield criterion.     

Observations on the definition of yield stress

Summary The difficulties of establishing an unambiguously physical definition for the yield stress property are discussed. Although too complex for routine application, a specific and completely physical definition of yield stress is developed and illustrated here, showing that the definition need not be of an arbitrary specification.     

Controlled assembly of jammed colloidal shells on fluid droplets

Assembly of colloidal particles on fluid interfaces is a promising technique for synthesizing two-dimensional microcrystalline materials useful in fields as diverse as biomedicine, materials science, mineral flotation and food processing. Current approaches rely on bulk emulsification methods, require further chemical and thermal treatments, and are restrictive with respect to the materials used. The development of methods that exploit the great potential of interfacial assembly for producing tailored materials have been hampered by the lack of understanding of the assembly process. Here we report a microfluidic method that allows direct visualization and understanding of the dynamics of colloidal crystal growth on curved interfaces. The crystals are periodically ejected to form stable jammed shells, which we refer to as colloidal armour. We propose that the energetic barriers to interfacial crystal growth and organization can be overcome by targeted delivery of colloidal particles through hydrodynamic flows. Our method allows an unprecedented degree of control over armour composition, size and stability.     

On the yield stress of frozen sucrose solutions

Measurement or prediction of the mechanical and fracture properties of foods is very important in the design, operation and optimization of processes, as well as for the control of quality of food products. This paper describes the measurement of yield stress of frozen sucrose solutions under indentation tests using a spherical indenter. Effects of composition, temperature and strain rate on yield stress of frozen sucrose solutions have also been investigated.     

Some aspects of fatigue relative to cyclic yield stress

This paper deals with cyclic fatigue behaviour in the regime between the classical LCF and the high cycle endurance limit. Plastic stress has been identified as a component of the stress range by investigating the relationship between the stress and plastic strain ranges. When the cyclic yield stress is incorporated into this relationship, it is found that: the plastic stress and plastic strain ranges can be correlated by the power rule, and; the cyclic yield stress is identical to the endurance limit. This paper proposes two methods for evaluating the cyclic yield stress from experimental data. Because of its relationship with the cyclic yield stress, the endurance limit can be well defined for practical applications.     

Stress relaxation from initial stress levels below yield

The problem of relaxation of a uniaxially stressed specimen, loaded initially to a level within the so-called elastic range and then held under load in the testing machine, is analyzed using the Johnston-Gilman concept of plastic flow. A new functional form for the dislocation density is postulated and it is shown that this form is consistent with existing experimental data within the ranges of stress and strain that are considered in the problem.     

Oscillating-cup technique for yield stress and density measurement

We discuss a possibility for simultaneous measurement of different viscometric properties by the oscillating-cup technique. The emphasis is on the following problems related to liquid metal viscometry: identification of non-Newtonian fluid flow and combined viscosity/density measurement. The cases of Bingham and Newtonian fluids are analysed in detail. The basic features of nonlinear fluid flow, viscometer oscillations, modelling of experiment and data processing are systematized and interpreted in terms of linear viscous fluids.     

An empirical relation between yield stress and general yield load for a charpy U-notch specimen

An empirical expression for obtaining the dynamic yield stress, σ_yd, from the general yield load, p_gy, of a Charpy U-notch specimen has been experimentally derived assuming: (i) σ_yd is the same for both Charpy U- and V-notch specimens; and (ii) the functional relations between P_GY and σ_yd for both the geometries are the same except for a dimensionless proportionality constant.     

The influence of the temperature dependence of the yield stress on the stress distribution in a thermally assembled elastic-plastic shrink fit

Summary The subject of the investigation is the thermal assembly of an elastic-plastic shrink fit with hollow inner component. Based on Tresca's yield condition and the flow rule associated with it, the transient stress distribution is studied. Special attention is paid to the influence of the reduction of the yield stress at elevated temperatures on the interface pressure. However, it turns out that this influence is not significant.Dedicated to Prof. Dr. Kurt Desoyer on the occasion of the 70^th anniversary of his birthday     

High-temperature anomaly in the yield stress of doped NaCl crystals

The yield stress of NaCl crystals doped with divalent metals was found to rise with temperature in the range 293-600 K, the effect being most pronounced around 573 K. At higher temperatures, the yield stress decreases substantially.