Stiffness and Energy Dissipation of Polymer Matrix Composites Containing Embedded Metallic Negative Stiffness Structures

Background

Conventional composites used in damping applications exhibit an undesirable tradeoff between stiffness and energy dissipation. Recent research demonstrates that it is possible to simultaneously achieve increased stiffness and energy dissipation for a configuration of a viscoelastic polymer matrix placed in parallel with a negative stiffness structure (NSS). This configuration resulted in energy dissipation equal to the sum of its components but is difficult to implement in practice.

Objective

In this paper, an alternative configuration is investigated in which the NSS is embedded simultaneously in series and parallel with the matrix. The main objectives are to examine the tradeoff between the stiffness and energy dissipation of the composite and to identify the mechanisms for enhanced energy dissipation.

Methods

To achieve this, FEA models were used to match the stiffness of a polymer matrix with that of a metallic NSS. Multiple specimens were manufactured and tested under quasi-static compressive loads to determine the force versus displacement curves and calculate the energy dissipation and stiffness.

Results

These tests demonstrate that the total energy dissipation of the composite can be greater than the sum of its components, while maintaining the benefit of increasing the stiffness and damping capacity simultaneously. The results also demonstrate that the applied strain rate plays a critical role in activating the NSS, which is essential to achieve the desired increase in energy dissipation.

Conclusions

The results indicate that localized strain and strain rate at the interface between the NSS and polymer matrix are the main contributors to achieving energy dissipation beyond the sum of its components. Furthermore, it was demonstrated that the strain rate affects the activation of the NSS and therefore composites containing mechanically activated NSS must be designed for the strain rate of interest.

     

Resonant Capture and Separatrix Crossing in Dual-Spin Spacecraft

We consider the rotational motion of a spacecraft composed of two bodies which are free to rotate relative to one another about a common shaft S. A motor on one of the bodies provides a small constant internal torque which influences the relative motion of the two bodies, and which may influence the orientation of their common shaft S. Resonant capture refers to the phenomenon that the spacecraft may end up in one of several possible orientations, including a nearly flat spin (transverse to S), in addition to the expected simple rotation about S.The method of averaging is used to treat the original equations of motion, and it is shown that the essential mathematical problem involves separatrix crossing in a problem with slowly moving separatrices. Energy changes represented by Melnikov integrals are used to supplement the averaged equations in the neighborhood of the heteroclinic motions. The method is used to predict which initial conditions lead to capture into each of three distinct capture regions. The asymptotic results are compared to those obtained by direct numerical integration of the equations of motion.     

A parameterized approach to modeling and forecasting mortality

A new method is proposed of constructing mortality forecasts. This parameterized approach utilizes Generalized Linear Models (GLMs), based on heteroscedastic Poisson (non-additive) error structures, and using an orthonormal polynomial design matrix. Principal Component (PC) analysis is then applied to the cross-sectional fitted parameters. The produced model can be viewed either as a one-factor parameterized model where the time series are the fitted parameters, or as a principal component model, namely a log-bilinear hierarchical statistical association model of Goodman [Goodman, L.A., 1991. Measures, models, and graphical displays in the analysis of cross-classified data. J. Amer. Statist. Assoc. 86(416), 1085-1111] or equivalently as a generalized Lee-Carter model with p interaction terms. Mortality forecasts are obtained by applying dynamic linear regression models to the PCs. Two applications are presented: Sweden (1751-2006) and Greece (1957-2006).     

Turbulent wall pressure reduction using suction control

A study of the fluctuating wall pressure beneath a 2-d turbulent boundary layer was conducted in a water tunnel with Reynolds numbers, based on momentum thickness, ranging between 2,100 and 4,300. The boundary layer was perturbed with steady mild suction to assess the effect of upstream suction on the fluctuating wall pressure measured downstream of the suction slit. Wall pressure signatures were captured using a custom-fabricated piezo-ceramic array with d ⁺ values ranging between 64 and 107. Likewise, the velocity field was measured with a laser Doppler velocimeter with l ⁺ values ranging between 4.0 and 6.7 for the lowest and highest Re _θ investigated. Estimates of the wall pressure spectra revealed a noticeable hydrodynamic peak that scaled reasonably well with outer variables and with an average convective speed of 75 % of the free stream velocity (based on unconditionally sampled pressure time series). Two boundary layer suction control cases were studied corresponding to suction rates of less then 30 % of the boundary layer momentum. The findings reveal how only modest amounts of suction are needed to reduce upwards 50–60 % of the hydrodynamic ridge.     

Nonlinear Cusped Caustics for Dispersive Waves

A multiple-scale perturbation analysis for slowly varying weakly nonlinear dispersive waves predicts that the wave number breaks or folds and becomes triple-valued. This theory has some difficulties, since the wave amplitude becomes infinite. Energy first focuses along a cusped caustic (an envelope of the rays or characteristics). The method of matched asymptotic expansions shows that a thin focusing region with relatively large wave amplitudes, valid near the cusped caustic, is described by the nonlinear Schrödinger equation (NSE). Solutions of the NSE are obtained from an asymptotic expansion of an equivalent linear singular integral equation related to a Riemann-Hilbert problem. In this way connection formulas before and after focusing are derived. We show that a slowly varying nearly monochromatic wave train evolves into a triple-phased slowly varying similarity solution of the NSE. Three weakly nonlinear waves are simultaneously superimposed after focusing, giving meaning to a triple-valued wave number. Nonlinear phase shifts are obtained which reduce to the linear phase shifts previously described by the asymptotic expansion of a Pearcey integral.     

New ideas in neutrino detection

What is new in the field of neutrino detection? In addition to new projects probing both the low and high ends of the neutrino energy scale, an inexpensive, effective technique is being developed to allow tagging of antineutrinos in water Cherenkov (WC) detectors via the addition to water of a solute with a large neutron cross-section and energetic γ daughters. Gadolinium is an excellent candidate since in recent years it has become very inexpensive, now less than $8 per kilogram in the form of commercially available gadolinium trichloride. This non-toxic, non-reactive substance is highly soluble in water. Neutron capture on gadolinium yields an 8.0 MeV gamma cascade easily seen in detectors like Super-Kamiokande. The uses of GdCl₃ as a possible upgrade for the Super-Kamiokande detector — with a view toward improving its performance as an antineutrino detector for supernova neutrinos and reactor neutrinos — are discussed, as are the ongoing R&;D efforts which aim to make this dream a reality within the next two years.     

COMPARISON OF THE EFFECTS OF UV-VISIBLE IRRADIATION OF MELANINS and MELANIN-HEMATOPORPHYRIN COMPLEXES FROM HUMAN BLACK and RED HAIR

Clinical evidence indicates that people with light skin and red hair have a higher incidence of UV radiation-induced diseases including cancer. It is not known whether this is because of the lower protection due to the smaller amounts of eumelanin present in the skin of these people or whether the presence of pheomelanin in their skin is responsible for the higher susceptibility to carcinogenesis. Irradiation of melanoproteins from red hair with UV-visible light has been reported to produce superoxide. Comparative studies on the formation of superoxide during the irradiation of the melanins isolated from human black and red hair (BHM and RHM, respectively) are reported in this paper. These showed that no superoxide formation could be detected in the case of the BHM. whereas the formation of superoxide during the irradiation of RHM could be definitely demonstrated. Irradiation of the RHM with NADH resulted in the oxidation of more NADH than the irradiation of NADH with BHM. The observation that RHM is an active photosensitizer indicates that this property of pheomelanin present in light skin may at least partly be responsible for the harmful effect of radiation on people with light skin and/or red hair. Administration of hematoporphyrin followed by visible irradiation is a currently used mode of therapy for cancer. The present studies have shown that hematoporphyrin is bound to both BHM and RHM. The binding of hematoporphyrin to the melanins increased the formation of superoxide by RHM and the oxidation of NADH by both the melanins. The binding of porphyrins to melanins may have an influence upon the photosensitivity in diseases such as porphyrias and may also affect the therapeutic efficacy of hematoporphyrin.     

有机硅活性中间体的研究 II. 一些含环丙基的有机硅化合物的合成及其结构

用锂有机物的方法合成了四种含有环丙基有机硅化合物,对上述每种化合物中可能存在着的立体异构进行了分离和和构型测定。     

Measurement of Longevity Risk Using Bootstrapping for Lee–Carter and Generalised Linear Poisson Models of Mortality

This paper provides a comparative investigation of simulation strategies for measuring the longevity risk associated with predictions of mortality rates and derived estimates of life expectancy. The study considers the Lee–Carter framework and a generalised linear Poisson model for representing the dynamics of mortality, as well as enhancements that allow for joint modelling of the dispersion and the effect of using a negative binomial rather than a Poisson assumption.