Dynamic modulus

Abstract: Various nylon 6-clay hybrids, such as molecular composites of nylon 6 and silicate layers of montmorillonite and saponite, NCH's and NCHP's, respectively, have been synthesized. To estimate the mechanical properties of these hybrids, tensile, flexural, impact, and heat distortion tests were carried out. NCH was found superior in strength and modulus and comparable in impact strength to nylon 6. The heat distortion temperature (HDT) of NCH (montmorillonite: 4.7 wt. %) was 152 °C, which was 87 °C higher than that of nylon 6. In NCHP, saponite had a smaller effect on the increase of these mechanical properties. The modulus and HDT of NCH and NCHP increased with an increase in the amount of clay minerals. It was found that these properties were well described by the contribution of the constrained region calculated from the storage and loss modulus at the glass transition temperature. According to the mixing law on elastic modulus, the following expression was obtained between the modulus E at 120 °C and the fraction of the constrained region C, En = Ecn = C, where the values of n and Ec (modulus of the constrained region) were 0.685 and 1.02 GPa, respectively.

Abstract: The dynamic mechanical analysis of banana fiber reinforced polyester composites was carried out with special reference to the effect of fiber loading, frequency and temperature. The intrinsic properties of the components, morphology of the system and the nature of interface between the phases determine the dynamic mechanical properties of the composite. At lower temperatures (in the glassy region), the E ′ values are maximum for the neat polyester whereas at temperatures above T g , the E ′ values are found to be maximum for composites with 40% fiber loading, indicating that the incorporation of banana fiber in polyester matrix induces reinforcing effects appreciably at higher temperatures. The loss modulus and damping peaks were found to be lowered by the incorporation of fiber. The height of the damping peaks depended on the fiber content. When higher fiber content of 40% was used, an additional peak in the tan δ curve, pointing to micro mechanical transitions due to the immobilized polymer layer was evident. The glass transition temperature associated with the damping peak was lowered up to a fiber content of 30%. The T g values were increased with higher fiber content. Cole–Cole analysis was made to understand the phase behavior of the composite samples. A master curve was constructed based on time–temperature super position principle, which allows the prediction of long-term effects. Apparent activation energy of the relaxation process of the composites was also analyzed. The value was found to be maximum for composites with 40% fiber content.

Abstract: Methods of quasi-static deformation and fracture analysis are developed for a class of nonlinear viscoelastic media and sample applications are given. Selection of the class of media is guided by actual rheological behavior of monolithic and composite materials as well as the need for simplicity to be able to understand the effect of primary material and continuum parameters on crack growth behavior. First, pertinent aspects of J integral and energy release rate theory for nonlinear elastic media are discussed. Nonlinear viscoelastic constitutive equations are then given, and correspondence principles which establish a simple relationship between mechanical states of elastic and viscoelastic media are developed. These principles provide the basis for the subsequent extension of J integral theory to crack growth in viscoelastic materials. Emphasis is on predicting mechanical work available at the crack tip for initiation and continuation of growth; some examples show how viscoelastic properties and the J integral affect growth behavior. Included is the problem of a crack in a thin layer having different viscoelastic properties than the surrounding continuum. The Appendix gives an apparently new constitutive theory for elastic and viscoelastic materials with changing microstructure (e.g. distributed damage) and indicates the conditions under which the fracture theory in the body of the paper is applicable.

Abstract: Lung epithelial cells are subjected to large cyclic forces from breathing. However, their response to dynamic stresses is poorly defined. We measured the complex shear modulus (G*(ω)) of human alveolar (A549) and bronchial (BEAS-2B) epithelial cells over three frequency decades (0.1–100 Hz) and at different loading forces (0.1–0.9 nN) with atomic force microscopy. G*(ω) was computed by correcting force-indentation oscillatory data for the tip-cell contact geometry and for the hydrodynamic viscous drag. Both cell types displayed similar viscoelastic properties. The storage modulus G′(ω) increased with frequency following a power law with exponent ∼0.2. The loss modulus G″(ω) was ∼2/3 lower and increased similarly to G′(ω) up to ∼10 Hz, but exhibited a steeper rise at higher frequencies. The cells showed a weak force dependence of G′(ω) and G″(ω). G*(ω) conformed to the power-law model with a structural damping coefficient of ∼0.3, indicating a coupling of elastic and dissipative processes within the cell. Power-law behavior implies a continuum distribution of stress relaxation time constants. This complex dynamics is consistent with the rheology of soft glassy materials close to a glass transition, thereby suggesting that structural disorder and metastability may be fundamental features of cell architecture.

Abstract: Preface. Introduction to Damping. Modeling the Dynamic Mechanical Behaviour of Viscoelastic Materials. The Effects of Temperature and Frequency on Complex Modulus Properties. Measurement of Complex Modulus Properties. Numerical Analysis of Measured Complex Modulus Data. The Complex Modulus Behaviour of Typical Polymeric Materials. Harmonic and Non-harmonic Response of Simple Viscoelastic Systems. Controlling Vibration using Viscoelastic Materials. References. Symbols for Chapter 8. Selected Computer Programmes. Units and Dimensions. Author Index. Subject Index.