Stress dilatational response

A detailed atomistic approach was used to investigate the molecular segment kinematics of a glassy, atactic polypropylene system dilated by 30%. ° The microstructural stress—dilation response consists of smooth, reversible portions bounded by sudden, irreversible stress jumps. But compared to the micro-structural stress—strain curve of the shear simulation, the overall trend more closely resembles macroscopic stress—strain curves. The peak negative pressure was in the neighborhood of 12% dilatation, with a corresponding secondary maximum in the von Mises shear stress. The peak negative pressure was re-... 

Consider the relative change in interfacial area AA/A brought about by an external stress. Dilation leads to dilution of the monolayer which, in turn, results in a rise of the interfacial tension y. Conversely, reducing the interfacial area causes y to decrease. The response of the monolayer to the imposed deformation may be more or less elastic or viscous. Elastic behavior is expected for monolayers in which the amphiphilic molecules are interconnected forming a two-dimensional gel. Also, when the rate of deformation is too high to allow for relaxation back to equilibrium by, for example, adsorption or desorption of amphiphiles to or from the interface, or by reorientation and/or reconformation of the molecules in the monolayer (especially in the case of polymers and proteins), the monolayer responds partly elastically. 

Polymer rheology can respond nonllnearly to shear rates, as shown in Fig. 3.4. As discussed above, a Newtonian material has a linear relationship between shear stress and shear rate, and the slope of the response Is the shear viscosity. Many polymers at very low shear rates approach a Newtonian response. As the shear rate is increased most commercial polymers have a decrease in the rate of stress increase. That is, the extension of the shear stress function tends to have a lower local slope as the shear rate is increased. This Is an example of a pseudoplastic material, also known as a shear-thinning material. Pseudoplastic materials show a decrease in shear viscosity as the shear rate increases. Dilatant materials Increase in shear viscosity as the shear rate increases. Finally, a Bingham plastic requires an initial shear stress, to, before it will flow, and then it reacts to shear rate in the same manner as a Newtonian polymer. It thus appears as an elastic material until it begins to flow and then responds like a viscous fluid. All of these viscous responses may be observed when dealing with commercial and experimental polymers. 

An important example of PLP-dependent amino acid decarboxylation is the conversion of histidine into histamine. Histamine is often involved in human allergic responses, e.g. to insect bites or pollens. Stress stimulates the action of the enzyme histidine decarboxylase and histamine is released from mast cells. Topical antihistamine creams are valuable for pain relief, and oral antihistamines are widely prescribed for nasal allergies such as hay fever. Major effects of histamine include dilation of blood vessels, inflammation and swelling of tissues, and narrowing of airways. In serious cases, life-threatening anaphylactic shock may occur, caused by a dramatic fall in blood pressure. 

Predictions of the mechanical response of filled elastomers are further aggravated by the phenomenon of strain dilatation. As soon as dilatation commences, the tensile stress lag behind elongation, the degree of dilatation for a given composite being roughly a measure for the deviation from the expected mechanical response. Dilatation increases with particle size and volume fraction of filler—it decreases somewhat if the filler is bonded to the matrix. Farris (16,17) showed that dilatation can account well for the mechanical behavior of solid propellants and his equation ... 

A general description of the fundamental relationships governing the dynamic response of linear viscoelastic materials may be found in several sources (28, 37, 93). In general, sinusoidally applied strains (stresses) result in sinusoidal stresses (strains) that are out of phase. Measurements may be made under uniaxial, shear, or dilational loading conditions, and the resultant complex moduli or compliance and loss-phase angle are computed. Rotating radius vectors are usually taken to represent the... 

As discussed in Section 7.7, crystals, particularly organic crystals, usually exist in lower-symmetry nonorthogonal systems for them the off-diagonal terms of P become important. The response of a crystal to the stress tensor P is a series of fractional displacements, small compared to any dimension of the body these fractional displacements are called strains and are denoted by the strain (or dilatation) tensor s ... 

Crack tip blunting is attributed to localized yielding at the crack tip. Localized yielding may result from shear deformation, or normal stress deformation. Unlike shear deformation, which occurs at constant colume, normal stress deformation involves a volume dilatation and is considered to be responsible for the formation of crazes in thermoplastics. Since crazes are not observed in highly crosslinked epoxies, it is generally assumed that plastic deformation at the crack tip takes place via a shear yielding process. 

If the viscoelastic material is under the effect of an isotropic deformation (dilatation or compression), the diagonal components of both the stress and strain tensors differ from zero. In analogy with Eq. (4.92), the relationship between the excitation and the response is given by... 

By way of introduction, consider the dilational experiment of fig. 3.49. At t = t the monolayer is instantaneously subjected to a stress r° = r = r°, which is kept constant till t = t, after which it is suddenly removed. Panels (b) and (c) refer to the strain response AA/A for a purely elastic and a purely viscous monolayer, respectively, The elastic monolayer directly follows the applied stress after cessation it relaxes instantaneously. The viscous one starts to flow after cessation of the stress the flow stops. The height of the block in panel (b) is equal to r / K (compare [3.6,18]) whereas in panel (c) the interfacial (dilational) viscosity follows from the slope as... 

In the peripheral nervous system, norepinephrine is an important neurotransmitter in the sympathetic branch of the autonomic system. Sympathetic nerve transmission operates below the level of consciousness in controlling physiological function of many organs and tissues of the body. The sympathetic system plays a particularly important role in regulating cardiovascular function in response to postural, exertional, thermal, and mental stress. With sympathetic activation, the heart rate is increased, peripheral arterioles are constricted, skeletal arterioles are dilated, and the blood pressure is elevated. In addition, sympathetic nerve stimulation dilates pupils inhibits smooth muscles of the intestines, bronchi, and bladder and closes the sphincters. Sympathetic signals work in balance with the parasympathetic portion of the autonomic nervous system to maintain a stable internal environment. 

---