On the instability of equilibrium of nonholonomic systems with nonhomogeneous constraints

The first Lyapunov method, extended by V. Kozlov to nonlinear mechanical systems, is applied to the study of the instability of the equilibrium position of a mechanical system moving in the field of potential and dissipative forces. The motion of the system is subject to the action of the ideal linear nonholonomic nonhomogeneous constraints. Five theorems on the instability of the equilibrium position of the above mentioned system are formulated. The theorem formulated in [V. V. Kozlov, On the asymptotic motions of systems with dissipation, J. Appl. Math. Mech. 58 (5) (1994) 787-792], which refers to the instability of the equilibrium position of the holonomic scleronomic mechanical system in the field of potential and dissipative forces, is generalized to the case of nonholonomic systems with linear nonhomogeneous constraints. In other theorems the algebraic criteria of the Kozlov type are transformed into a group of equations required only to have real solutions. The existence of such solutions enables the fulfillment of all conditions related to the initial algebraic criteria. Lastly, a theorem on instability has also been formulated in the case where the matrix of the dissipative function coefficients is singular in the equilibrium position. The results are illustrated by an example

New characterization based symmetry tests

Two new symmetry tests, of integral and Kolmogorov type, based on the characterization by squares of linear statistics are proposed. The test statistics are related to the family of degenerate U-statistics. Their asymptotic properties are explored. The maximal eigenvalue, needed for the derivation of their logarithmic tail behavior, was calculated or approximated using techniques from the theory of linear operators and the perturbation theory. The quality of the tests is assessed using the approximate Bahadur efficiency as well as the simulated powers. The tests are shown to be comparable with some recent and classical tests of symmetry

Physico-chemical Properties of Corn Extrudates Enriched with Tomato Powder and Ascorbic Acid

The aim of this research was to investigate the influence of the addition of tomato powder (TP) to corn grits at levels 4, 6 or 8 % and the addition of ascorbic acid (AA) at levels 0.5 and 1 %, on total polyphenol content (PF), and antioxidant activity of the extrudates. The hardness and the expansion ratio of the extruded products were also tested. Mathematical models that describe the influence of additives on the mentioned properties were also determined. Extrusion was performed at two temperature regimes: 135/170/170 °C and 100/150/150 °C. Lower temperature regime led to increased hardness and the expansion of extrudates. The addition of tomato and AA led to decreased hardness and the expansion at both temperature regimes. The addition of tomato increased PF and AA compared with pure corn extrudates. Greater degradation of PF and AA was at lower temperature regime. High correlation between PF and AA was demonstrated at both extrusion temperatures

Structural and Electrical Properties of Sintered Zinc-titanate Ceramics

Poster presented at the ELECTROCERAMICS X, June 18-22, 2006, Toledo, Spai

Dynamic Mechanical Properties of Aramid Fabrics Impregnated with Carbon Nanotube/Poly (Vinyl Butyral)/Ethanol Solution

In this study six samples of polyurethane/p-aramid multiaxial fabric forms (Colon fabrics) were coated with 10 wt.% poly (vinyl butyral) (PVB)/ethanol solution with the addition of multiwalled carbon nanotubes (MWCNT). The solution was impregnated on both sides of each of the fabrics. All composite samples consisted of four layers of the impregnated fabrics. The MWCNT/PVB content was 0, 0.1 and 1 wt.%. The three samples of the fabrics with different MWCNT/PVB content were coated with γ-aminopropyltriethoxysilane (AMEO silane)/ethanol solution due to the surface modification. The mechanical properties of the prepared composite samples were studied by dynamic mechanical analysis (DMA). The 60% increase in storage modulus was achieved by addition of MWCNT and impregnation of aramid fabrics with AMEO silane. The pristine multiwalled carbon nanotubes (MWCNT) were introduced in order to enhance additionally the mechanical properties of the materials for ballistic protection

JMASM 32: Multiple Imputation of Missing Multilevel, Longitudinal Data: A Case When Practical Considerations Trump Best Practices?

A pedagogical tool is presented for applied researchers dealing with incomplete multilevel, longitudinal data. It explains why such data pose special challenges regarding missingness. Syntax created to perform a multiply-imputed growth modeling procedure in Stata Version 11 (StataCorp, 2009) is also described

Improved Interyarn Friction, Impact Response, and Stab Resistance of Surface Fibrilized Aramid Fabric

Improvement of the ballistic performance of aramid fabric is an important topic in the study of soft body armors, especially with their increasing use in such applications over the past decades. To enhance and tailor the performance of fabrics, having control over one of its primary energy absorption mechanisms, interyarn friction, is required. Here, a recently reported surface fibrilization method is exploited and optimized to improve interyarn friction in aramid fabrics. Through tow pullout testing of fibrilized fabrics, the fibrilization treatment is shown to provide up to seven times higher pullout energy and six times higher peak load. To correlate the effects of the treatment on the ballistic response, impact tests are conducted on treated fabric targets using a gas gun setup. The fibrilized fabrics displayed a 10 m s‐1 increase in V50 velocity, compared to that of untreated fabrics, while retaining its original flexibility and mechanical strength. Similarly, the fibrilization treatment also resulted in 230% improvement in depth of penetration when dynamically stabbed using a spike impactor. The results demonstrate the potential of the proposed surface fibrilization treatment as a fast and cost‐effective technique to improve the ballistic and stab performance of aramid‐based soft body armors.This work shows improved interyarn, ballistic, and stab resistance properties in aramid fabric through a basic fibrilization treatment. The treated aramid fabrics display a maximum improvement of 665% in yarn pullout energy, a 10 m s−1 increase in V50 velocity, and 230% higher stab impact resistance, while maintaining its original tensile properties.Peer Reviewedhttps://deepblue.lib.umich.edu/bitstream/2027.42/151907/1/admi201900881.pdfhttps://deepblue.lib.umich.edu/bitstream/2027.42/151907/2/admi201900881_am.pd

Influence of mechanical activation on synthesis of zinc metatitanate

Investigations of a ZnO-TiO2 binary oxide mixture during mechanical treatment were mainly focused on obtaining orthotitanate Zn2TiO4 with a spinel structure. Due to the specific way of energy transfer during mechanical treatment using a high-energy ball mill, the system passes through low temperature ZnTiO3 metatitanate phase formation. Mechanical activation was performed on an equimolar ratio mixture of ZnO and TiO2. The anatase phase was previously submitted to heat treatment for achieving a starting mixture rich in a rutile phase. Milling conditions were preset for observing the formation of a low temperature ZnTiO3 phase with a perovskite structure. The powder microstructure was characterized using scanning electron microscopy. A nitrogen gas sorption analyzer with the BET method was used to determine the specific surface area and porosity, indicating changes of powder sample properties during mechanical activation. Also, X ray powder diffractometry was applied to obtain the phase composition. Powders were then pressed into pellets and their compressibility was observed through density changes. According to microstructures obtained by scanning electron microscopy analysis, the system underwent a primary and secondary agglomeration process. Specific surface area measurements supported that conclusion. Compressibility investigations established the difference between compressibility of the non-activated mixture and activated powders. X-ray diffraction analysis revealed that a perovskite structure forms simultaneously with a spinel phase during the process of mechanical activation

Structural characterization and electrical properties of sintered magnesium-titanate ceramics

In this article the influence of ball miling process on structure of MgO-TiO2 system, as well as the electrical properties of samples after sintering, was investigated. The mixtures of MgO-TiO2 powders were mechanically activated in a planetary ball mill for the time period from 0 to 120 min. The influence of mechanical activation and sintering on the lattice vibrational spectra was studied by Raman spectroscopy at room temperature. Structural investigations have been performed on produced powders. Nitrogen adsorption method was used to determine the BET specific surface area and pore size distribution. Unusual results have been obtained: specific surface area continuosly decreased up to 40 min of activation and increased after that, reaching its minimun value of 5.5 m(2)/g. The Raman spectra of activated powders have shown that anatase modes have been decreasing in intensity and broadening as the time of activation extended. Also, the additional modes attributed to TiO2 II, srilankite and rutile phases started to appear as a consequence of activation. The small differences noticed in the Raman spectra of sintered samples have been explained by structural modification of MgTiO3 phase due to the presence of defects. The effects of activation and sintering process on microstructure were investigated by scanning electron microscopy (SEM). The electrical measurements showed difference in dielectric constant (epsilon(r)), loss tangent (tg delta) and specific resistance (rho) as a function of time of mechanical treatment

The impedance analysis of sintered MgTiO3 ceramics

In the present study the effect of preparation conditions on microstructure and electrical properties of MgTiO3 ceramics was investigated. Mixture of MgO and TiO2 powders was subjected to mechanical activation for different time periods. Two types of sintering (two-step and HIP sintering) were performed on green bodies prepared by cold isostatic pressing. AC impedance spectroscopy in the low-frequency range (from 20 Hz to 1 MHz), at room and elevated temperatures, were performed in order to separate contribution of grains and grain boundaries to capacitance, resistance and conductivity of magnesium titanate ceramics. The special attention was directed to explanation of the influence of mechanical activation on electrical and morphological changes in sintered samples. The significant decrease of partial oxygen pressure in HP sintering atmosphere caused the change from insulating behavior of MgTiO3 samples after two-step sintering to semiconducting one