Nucleation kinetics: runaway nucleation

Abstract

An analysis of the kinetic critical nucleus size and of 'runaway' nucleation by Nishioka and Maksimov is shown to be flawed and an accurate version of their model is presented. The concept of runaway nucleation is valid only when the thermodynamic critical nucleus size is very small, containing only a few atoms/molecules, and in the regime where the capillarity model of a nucleus is highly suspect.     

Evaluation of chemical nucleation driving force for stoichiometric compounds with variable composition

Abstract

This work puts in evidence some inconsistencies in the method currently used in the literature for estimating the driving force for nucleation of stoichiometric phases with variable chemical composition, such as carbonitrides of Ti, Nb, etc. in steel. An alternative approach, properly accounting for the capillarity contribution to the Gibbs free energy change associated to the formation of the critical nucleus, is proposed. The chemical composition of the critical nucleus, which maximises the driving force for precipitation and allows the calculation of the critical radius, is found. Some examples of application are presented and the results are compared to those from the classical approach and discussed. The proposed method eliminates the contradiction encountered in previous approaches found in the literature and demonstrates that for these compounds the usual thermodynamic solution of the chemical equilibrium, which refers to flat surfaces, can not be directly extrapolated to describe the nucleation.     

A STUDY OF THE ACCURACY OF OPTICAL FRAUNHOFER DIFFRACTION SIZE ANALYZER

ABSTRACT

In this paper the discrepance of scattered light between Mie theory and Fraunhofer diffraction is showed at different refractive indexes and sizes of particle. The accuracy of the Malvern particle size analyzer is discussed when it is used to measure small particles and some advice is proposed for its use.     

Does vibration amplitude influence the evaluation of nonlocal small scale parameter of single layered graphene sheets?

ABSTRACT

This study aims to evaluate the nonlocal small scale parameter for large amplitude vibration of single layered graphene sheets (SLGSs) comparing nonlinear resonant frequencies obtained via nonlocal continuum and molecular dynamics (MD) simulations. Nonlinear governing equations of motion are numerically solved employing the pseudo-spectral method to obtain the frequency response. Results reveal that the calibrated small scale parameter decreases when the vibration amplitude increases. Also, from MD simulations it is seen that for all length sizes after an ultimate vibration amplitude around 31% length size, the graphene sheets start to fracture.     

Geometry dependent ductile to brittle transition of 10CrMoNbV ferritic–martensitic steel Manet II in terms of critical crack sizes

Abstract

The 10CrMoNbV Manet II cast has been selected as the reference ferritic–martensitic steel in the framework of the European Fusion Technology Programme. Charpy impact tests have been carried out in the ductile to brittle transition temperature range of this steel, such that a dynamic quasi-equilibrium has been achieved in the process zones of investigated specimens before brittle failure. This type of testing enables the evaluation of dynamic Weibull moduli and, consequently, dynamic Weibull master curves. Thus, Weibull parameters have been calculated for normal size and subsize Charpy impact specimens. The evaluated, geometry dependent dynamic Weibull master curves facilitate computation of the failure probability densities of the investigated steels as functions of scaled critical crack sizes or scaled initial defect sizes.     

Small sample reliability growth modeling using a grey systems model

ABSTRACT

When performing system-level developmental testing, time and expenses generally warrant a small sample size for failure data. Upon failure discovery, redesigns and/or corrective actions can be implemented to improve system reliability. Current methods for estimating reliability growth, namely the Crow (AMSAA) growth model, stipulate that parameter estimates have a great level of uncertainty when dealing with small sample sizes. For purposes of handling limited failure data, we propose the use of a modified GM(1,1) model to predict system reliability growth parameters and investigate how parameter estimates are affected by systems whose failures follow a poly-Weibull distribution. Monte-Carlo simulation is used to map the response surface of system reliability, and results are used to compare the accuracy of the modified GM(1,1) model to that of the AMSAA growth model. It is shown that with small sample sizes and multiple failure modes, the modified GM(1,1) model is more accurate than the AMSAA model for prediction of growth model parameters.     

Theoretical estimation of dwell and consolidation times in rotary tablet machines

Abstract

Knowledge of the time-dependent mechanical properties of pharmaceutical materials is critical to understanding of the compaction of tablets. In effort to determine the time-scale involved during compaction, equations for the dwell time and the consolidation time are proposed in this paper. Expressions for the consolidation time are obtained from generally accepted equations for punch displacement and separation between punch tips. These equations are deduced for rotary tablet machines with pressure rollers of the same and of different sizes. Specifically, the equations are applied to calculate the dwell and consolidation times for Manesty Betapress and Korsch Pharmapress Rotary tablet machines. The results are graphically presented.     

Influence of grain size on plastic anisotropy in low-carbon steels

Abstract

The plastic anisotropy of textured polycrystalline materials has often been described by the r-value – the ratio of the width and thickness strains. The measured –-values of low-carbon steels for larger grain sizes are found to agree reasonably well with values calculated by the authors on the basis of the Taylor theory. In steels with small grain sizes the absolute level of r-value is not predicted particularly well, but the variation with testing direction (planar anisotropy) is predicted well. Calculations showed that in all cases the instantaneous anisotropy parameter ρ as a function of strain, when extrapolated to zero strain, agreed with the value predicted from the measured texture. With small grain sizes a sudden drop in ρ was observed with increasing strain, this being complete within 2–3% of strain. The influence of grain size on the plastic anisotropy can be understood if it is assumed that the grain boundary resistance parameter K in the Hall–Petch relation depends on the contraction ratio – in a different way from the Taylor factor M(q). Measurements of the Hall–Petch constant in plane-strain compression support this view.

MST/84     

Effects of sizes of ferrite grains and carbide particles on toughness of notched and precracked specimens of low-alloy steels

Four point bending (4PB) tests of notched specimens and COD tests of precracked specimens were carried out on two steels; one steel was treated into two groups with the same ferrite grain size but different carbide sizes, the other steel with different ferrite grain sizes but similar carbide sizes. The results of the tests show that the toughness measured in notched specimens is mainly determined by the grain sizes, which define the local fracture stress _f; the size of carbide particle plays a minor role. However, on the contrary, in precracked specimens the toughness is sensitive to the carbide sizes, which affect the critical plastic strain _pc for initiating a crack nucleus; the effect of grain size is indistinct. By these inferences the behavioral discrepancy of large grain steel in improvement of crack fracture toughness while reducing the notch toughness is explained.     

Modeling the dependence of strength on grain sizes in nanocrystalline materials

Abstract

A model was developed to describe the grain size dependence of hardness (or strength) in nanocrystalline materials by combining the Hall–Petch relationship for larger grains with a coherent polycrystal model for nanoscale grains and introducing a log-normal distribution of grain sizes. The transition from the Hall–Petch relationship to the coherent polycrystal mechanism was shown to be a gradual process. The hardness in the nanoscale regime was observed to increase with decreasing grain boundary affected zone (or effective grain boundary thickness, Δ) in the form of Δ^?1/2. The critical grain size increased linearly with increasing Δ. The variation of the calculated hardness value with the grain size was observed to be in agreement with the experimental data reported in the literature.     

A general approach for testing the process capability index

This paper presents a generalized approach to the problem of testing with the process capability index C?_p. It allows for testing at any α-level and any sample size. For small sample sizes n < 30, n odd, tables are included with αs incremented by 0.025, which are generated by a computer program. This program is written in BASIC and appears in an Appendix. For different α levels (say α = 0·1) the program can be used to generate the appropriate value to use in determining the critical values. For larger sample sizes an approximation method is supplied. Additionally, the method to derive an associated operating characteristic curve is examined.     

Effect of dislocation splitting on γ′-hardening

Abstract

From an analysis of the local atomic arrangement in ordered γ′-particles it is deduced that the effect of dislocation splitting on the critical resolved shear stress of γ′-hardened materials is very small. This result is in striking contrast to the ones obtained for lattice misfit and modulus hardening.

MST/146     

An efficient genetic algorithm for TSK‐type neural fuzzy identifier design

Abstract

In this paper, an efficient genetic algorithm (EGA) of the Takagi‐Sugeno‐Kang (TSK) ‐type neural fuzzy identifier (TNFI) is proposed for solving various identification problems. For the proposed EGA method, the better chromosomes will be initially generated while the better mutation points will be determined to perform efficient mutation. The advantages of the proposed learning algorithm are that, first, it converges quickly and the obtained fuzzy rules are precise. Secondly, the proposed EGA method only requires a small population sizes.     

The Small Sample Behavior of Some Statistics Which Test the Equality of Several Means

Four statistics which may be used to test the equality of population means are com-pared with respect to their robustness under heteroscedasticity, their power, and the overlap of their critical regions. The four are: the ANOVA F-statistic; a modified F which has the same numerator as the ANOVA but an altered denominator; and two similar statistics proposed by Welch and James which differ primarily in their approximations for their critical values.

The critical values proposed by Welch are a better approximation for small sample sizes than that proposed by James. Both Welch's statistic and the modified F are robust under the inequality of variances. The choice between them depends upon the magnitude of the means and their standard errors. When the population variances are equal, the critical region of the modified F more closely approximates that of the ANOVA than does Welch's.     

LIMITATIONS OF LINEAR ELASTIC FRACTURE MECHANICS IN RESPECT OF SMALL FATIGUE CRACKS AND MICROSTRUCTURE

The growth characteristics of small fatigue cracks were investigated under rotary bending in a low alloy steel prepared with two prior austenite grain sizes of 15 μm (fine grain) and 91 μm (coarse grain). The influence of grain boundaries on crack growth rate and the crack aspect ratio was examined, and the critical crack length above which linear elastic fracture mechanics (LEFM) is applicable was evaluated for a growing small crack. When the surface crack length is shorter than three grain diameters (3d), crack growth rates decrease near the grain boundaries. Aspect ratios are also affected by the microstructure and thus vary widely. Cracks longer than 3d are not influenced by the microstructure, but they grow faster than would be expected based on LEFM until their lengths reach 3d+ 150 μm. This behaviour may be attributed to the difference in crack closure between small cracks and large cracks. If the contribution of crack closure to the growth of small cracks can be established experimentally or analytically, the critical crack length above which LEFM is applicable would be 3d. However, because it is difficult to evaluate crack closure, 3d+ 150 μm is considered to be the critical crack length for engineering applications.     

Processing and superplastic properties of fine grained Si3N4/Al–Mg–Si composites

Abstract

Composites with an Al–Mg–Si alloy matrix containing 20 vol.-% of either Si₃N₄ whiskers or Si₃N₄ particulates were extruded at 773 K with a reduction ratio of 100: 1, and tensile experiments were performed under conditions of constant true strain rate. Recrystallisation and dynamic precipitation occurred during hot extrusion so that very small grain sizes of less than ~ 3 ;amp;#x03BC;m were produced. The extruded composites showed superplastic behaviour at high strain rates (above 10^?1 S^?1). The high strain rate superplasticity of the composites is attributed to the very small grain sizes. Internal cavities developed during straining and density studies revealed that the rate of increase of the extent of cavitation was lower at a temperature slightly above the partial melting temperature than at a temperature lower than the partial melting temperature. It is concluded that the presence of a liquid phase restricts the development of cavities because the liquid phase serves to relax the stress concentrations.

MST/3139     

Determination of r‐curves of steel fiber reinforced concrete from size effect tests

Abstract

A developed size effect law for blunt fracture is used to determine the R‐Curves and related parameters of steel fiber reinforced concrete. Geometrically similar single‐edge notched beams of different sizes made of cementitious mixes at various fiber volume fractions and different maximum aggregate sizes were used for the tests. Fracture energy of concrete is identified by linear regression using the size effect law. From the experimentally calibrated size effect law, the R‐curve is obtained as the envelope of the family of fracture equilibrium curves for different specimen sizes.     

Feasibility of electrospray deposition for rapid screening of the cocrystal formation and single step,continuous production of pharmaceutical nanococrystals

Objectives: This study employed electrospray deposition (ESD) for simultaneous synthesis and particle engineering of cocrystals.

Significance: Exploring new methods for the efficient production of cocrystals with desired particle properties is an essential demand.

Methods: The possibility of cocrystal formation by ESD was examined for indomethacin-saccharin, indomethacin-nicotinamide, naproxen-nicotinamide, and naproxen-iso-nicotinamide cocrystals. Solutions of the drug and coformer at stoichiometric ratios were sprayed to a high electric field which caused rapid evaporation of the solvent and the formation of fine particles. The phase purity, size, and morphology of products were compared with reference cocrystals. Experiments were performed to evaluate the effects of stoichiometric ratio, concentration and solvent type on the cocrystal formation. Physical stability and dissolution properties of the electrosprayed cocrystals were also compared with reference cocrystals.

Results: ESD was found to be an efficient and rapid method to produce cocrystals for all studied systems other than indomethacin-nicotinamide. Pure cocrystals only formed at a specific drug:coformer ratio. The solvent type has a weak effect on the cocrystal formation and morphology. Electrosprayed cocrystals exhibited nano to micrometer sizes with distinct morphologies with comparable physical stability with reference cocrystals. Nanococrystals of indomethacin-saccharin with a mean size of 219?nm displayed a threefold higher dissolution rate than solvent evaporated cocrystal.

Conclusion: ESD successfully was utilized to produce pure cocrystals of poorly soluble drugs with different morphologies and sizes ranging from nano to micrometer sizes in one step. This study highlighted the usefulness of ESD for simultaneous preparation and particle engineering of pharmaceutical cocrystals.     

NON-DAMAGING NOTCHES IN FATIGUE*

The fact that very small notches (cavities, holes, scratches, etc.) have no effect on the fatigue limit of metallic materials is well known. This paper presents both a qualitative explanation for the existence of non-damaging notches and a quantitative derivation of their critical sizes. The condition for a notch (characterized by the stress concentration factor K_t and the notch root radius ρ) to be non-damaging in a metallic material (characterized by a critical crack size l₀) is (K²_t? 1)ρ≤ 4.5 l₀. The critical crack size can be expressed with good approximation in terms of the threshold stress intensity for fatigue crack growth and the plain fatigue limit. Therefore the above relation can be applied for an engineering evaluation of non-damaging notches. Test results obtained for copper and a pressure vessel steel demonstrate the applicability of the proposed method.     

Critical perturbations in a monostable active medium

An analysis is made of critical perturbations which initiate the evolution of instability in a monostable active medium described by a reaction-diffusion equation. A group-theoretical analysis of the problem yields an analytic expression for the energy of the critical perturbations. The results may be important for analyses of stability with respect to external perturbations of a wide range of active media. Pis’ma Zh. Tekh. Fiz. 24, 76–80 (November 26, 1998)