Solving some key failure analysis problems using advanced methods for materials testing

A dilemma encountered in engineering practice is a proliferation of newly designed (mostly high-strength and/or corrosion-resistant) steels and alloys that are unusable in industry as they are highly susceptible to failure under operating conditions including environmentally assisted cracking. The problem of materials failure has several sources, the most significant of which is how engineers select which material to use in which industry. As a rule, selection is based solely on assessing the mechanical properties of materials with little or no consideration of how these mechanical properties will interface with specific operating parameters found within different industries. The functional design, selection and use of materials aimed at preventing in-service failures depends, therefore, on finding testing methods, standards and approaches appropriate to real operating conditions guided by analysis of material performance under those conditions.     

Clustering of Floating Particles by Surface Waves

We study experimentally and theoretically how waves affect the distribution of particles floating on a liquid surface. According to the Archimedes’ law the weight of floating particle is equal to the weight of displaced liquid. This law is not quite precise for small floating objects. An additional force generating by surface tension pulls a hydrophilic particle deeper into the water so that the mass of the displaced liquid exceeds the particle mass. This mass mismatch makes the floating particle effectively inertial that may lead to the clustering of particles when they move by the surface waves. Here we report the results of our recent work showing that particles gather in the nodes or antinodes of a standing surface wave depending on the sign of capillarity effect. Experimentally measured rate of particles gathering is shown to be proportional to the square of the wave amplitude which agrees with the theoretical model.     

Shape-factor effect on melting in an elliptic capsule

An approximate mathematical model of contact melting of an unfixed material in an elliptical capsule is developed. The main characteristic scales and non-dimensional parameters which describe the principal features of the melting process are found. Choosing a special heat flux distribution on the wall of the capsule allows us to derive a closed-form evolution equation for the motion of the solid accounting for the energy convection in the liquid, expressed through the non-linearity of the temperature distribution across the molten layer. It is shown that the melting rate of the solid depends on the shape of the capsule. Generally, elliptical capsules show higher rate of melting than circular ones. Elongated capsules provide more effective melting than oblate ones, even though they have the same aspect ratios and vertical cross-sectional areas. This phenomenon is caused by the fact, that the pressure necessary to support the solid is larger for the elongated capsules than that for oblate ones, which leads to thinning of the molten layer along with the increase of the heat flux across it. The time required for complete melting can be achieved by the right choice of the shape of the capsule, which is specified by the value of the aspect ratio. The found influence of the capsule shape on the melting rate can be used for design and optimization of practical latent-heat-thermal-energy systems.     

Structure-dependent Optical Activity of Single-walled Carbon Nanotube Enantiomers

Single-walled carbon nanotube (SWCNT) samples contain a variety of distinct structural species labeled by (n,m) indices. Nearly all of these are helical structures that can exist as left-handed or right-handed optical isomers. Although SWCNT growth processes give equal (racemic) mixtures of these enantiomers, recently developed separation methods can now provide samples with high optical purity. We report quantitative circular dichroism measurements on both enantiomers of five different semiconducting (n,m) species: (6,4), (7,3), (6,5), (8,3), and (8,4), plus one enantiomer of (7,5). The sorted samples were prepared by nonlinear density gradient ultracentrifugation. Helicities are deduced from the signs of circular dichroism (CD) signals at the E₂₂ transition, with guidance from theoretical modeling. For each species, circular dichroism and absorption spectra are compared at the E₂₂ peak to obtain ellipticities normalized to absorbance. The normalized ellipticities show no apparent correlation with SWCNT roll-up angle but decrease smoothly with increasing diameter. These findings represent the first results on structure-dependent nanotube chiroptical properties.     

Controllable Broadband Absorption in the Mixed Phase of Metamagnets

Materials with broad absorption bands are highly desirable for electromagnetic filtering and processing applications, especially if the absorption can be externally controlled. Here, a new class of broadband‐absorption materials is introduced. Namely, layered metamagnets exhibit an electromagnetic excitation continuum in the magnetic‐field‐induced mixed ferro‐ and anti­ferromagnetic phase. Employing a series of complementary experimental techniques involving neutron scattering, muon spin relaxation, specific heat, ac and dc magnetization measurements, and electron magnetic resonance, a detailed magnetic phase diagram of Cu₃Bi(SeO₃)₂O₂Br is determined and it is found that the excitations in the mixed phase extend over at least ten decades of frequency. The results, which reveal a new dynamical aspect of the mixed phase in metamagnets, open up a novel approach to controllable microwave filtering.