Glasses stress behavior

Attention will now be turned to two important experimental aspects of glass transition behavior the temperature dependence of the modulus, and stress-relaxation studies. This will be followed by a brief discussion of mathematical models that describe polyblend glass transition behavior. 

This section will examine some of the characteristic features of IPN s from a physical and mechanical point of view. Emphasis will be on relating the glass transition behavior to corresponding aspects of morphology. The principal instrumentation employed in the studies discussed here includes a torsional tester for creep-type studies (Section 8.3.1) and a fixed-frequency vibrating unit for dynamic mechanical spectroscopy (see Section 8.3.2). In addition, stress-strain, tensile, and Charpy impact strength values will be briefly discussed. 

Figure 11.17 Measured ratio of yield stress to temperature as a function of the logarithm of strain rate for polycarbonate. The set of parallel straight lines is calculated from Equation (11.43). (Reproduced from Bauwens-Crowet, C., BauwensJ.C. and Homes, C. (1969) Tensile yield-stress behavior of poly (vinyl chloride) and polycarbonate in the glass transition region. J. Polymer Sci. A2, 7, 735. Copyright (1969) john Wiley Sons, Ltd.)...

Bauwens-Crowet, C., Bauwens, J.C. and Homes, G. (1969) Tensile yield-stress behavior of poly(vinyl chloride) and polycarbonate in the glass transition region. J. Polymer Sci. A2,7,735. 

Gla.ss Ca.pilla.ry Viscometers. The glass capillary viscometer is widely used to measure the viscosity of Newtonian fluids. The driving force is usually the hydrostatic head of the test Hquid. Kinematic viscosity is measured directly, and most of the viscometers are limited to low viscosity fluids, ca 0.4—16,000 mm /s. However, external pressure can be appHed to many glass viscometers to increase the range of measurement and enable the study of non-Newtonian behavior. Glass capillary viscometers are low shear stress instmments 1—15 Pa or 10—150 dyn/cm if operated by gravity only. The rate of shear can be as high as 20,000 based on a 200—800 s efflux time. 

Several experiments will now be described from which the foregoing basic stiffness and strength information can be obtained. For many, but not all, composite materials, the stress-strain behavior is linear from zero load to the ultimate or fracture load. Such linear behavior is typical for glass-epoxy composite materials and is quite reasonable for boron-epoxy and graphite-epoxy composite materials except for the shear behavior that is very nonlinear to fracture. 

Avoiding structural failure can depend in part on the ability to predict performance for all types of materials (plastics, metals, glass, and so on). When required designers have developed sophisticated computer methods for calculating stresses in complex structures using different materials. These computational methods have replaced the oversimplified models of materials behavior relied in the past. The result is early comprehensive analysis of the effects of temperature, loading rate, environment, and material defects on structural reliability. 

Chemically universally stabile is poly-tetrafluoroethylene (PTFE, Teflon ), but it is relatively expensive. A problem is the cold flow , that is, the polymer is slowly deformed under the mechanical stress of the pressure on the gaskets and a leakage of the cell can occur (PTFE compounds, e.g. with glass powder or graphite, show a better behavior.)... 

Panic disorder is characterized by the occurrence of panic attacks that occur spontaneously and lead to persistent worry about subsequent attacks and/or behavioral changes intended to minimize the likelihood of further attacks. Sporadic panic attacks are not limited, however, to those with syndromal panic disorder as they do occur occasionally in normal individuals and in those with other syndromal psychiatric disorders. The hallmark of panic disorder is that the panic attacks occur without warning in an unpredictable variety of settings, whereas panic attacks associated with other disorders typically occur in response to a predictable stimulus. For example, a person with acrophobia might experience a panic attack when on a glass elevator. A patient with obsessive-compulsive disorder (OCD) with contamination fears may have a panic attack when confronted with the sight of refuse, and a combat veteran with post-traumatic stress disorder (PTSD) may experience a panic attack when a helicopter flies overhead or an automobile backfires. 

A unified approach to the glass transition, viscoelastic response and yield behavior of crosslinking systems is presented by extending our statistical mechanical theory of physical aging. We have (1) explained the transition of a WLF dependence to an Arrhenius temperature dependence of the relaxation time in the vicinity of Tg, (2) derived the empirical Nielson equation for Tg, and (3) determined the Chasset and Thirion exponent (m) as a function of cross-link density instead of as a constant reported by others. In addition, the effect of crosslinks on yield stress is analyzed and compared with other kinetic effects — physical aging and strain rate. 

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