Spring back behavior

Figure 23.4 MSQ aerogel shows spring-back behavior in a uniaxial compression test...

The basic idea of the dynamic pressurization (DP) experiment is similar to the dynamic gas expansion (DGE) method. Both use a sudden pressure change around a gel specimen to initiate gas flow into or out of the sample, thus avoiding the delicate leakage problems typically encountered in static gas flow setups. While DGE monitors the gas pressure outside the gel as a function of time and deduces the permeability from its equilibration behavior (in principle a dynamic pycnometry experiment). DP utilizes the dynamics of the elastic deformation of the gel to deduce both elastic modulus and permeability. The deformation, or strain, is a consequence of the pressure difference between the interior and the exterior of the specimen. For example, after a sudden increase in pressure, the gas in the gel pores is initially only slightly compressed along with the elastic compression of the gel. After a characteristic time, the pressure equilibrates and the gel ideally springs back to its original dimensions. 

There is a special case when viscosity is low for homogeneous fiber spinning but higher than that required for jet breakup. In this case, beads are formed on fibers as droplets. At some places the liquid jet springs back and becomes twitched in specific locations because of the viscoelastic-behavior of the solution. A SEM micrograph of typical beaded fibers is shown in Figure 10.9 [79]. Usually, the charge density is relatively low. 

Plastics are often used to replace stamped steel because their viscoelastic behavior at lower strain rates enables them to spring back rather than to deform under suddenly applied loads. This characteristic does add a safety factor to a design, but it also means that a part can undergo large, nonlinear deflections. Such deflections, defined as being greater than the wall thickness of the part, are difficult to analyze accurately with standard engineering equations. 

Non-Newtonian fluids have both viscous and elastic properties, and they are called viscoelastic fluids. An example is so-called "silly putty," which is made from poly-dimethyl-siloxane (silicone). It flows like a liquid out of the container, but when it forms a ball, it behaves as elastic, i.e., it bounces back. The crucial factor determining the viscous and elastic behavior is the time period of the force applied short force pulse leads to elastic response, whereas long-lasting force causes flow. The viscoelasticity in polymers is due to shear-induced entanglements and nonlinear behavior of tire chains, coils. A well-known natural viscoelastic material is for, example, the egg white, which springs back when a shear force is released. A polymer resembles both liquid and solids. 

One of the fascinating properties of the elastomeric materials is their rubber-like elasticity— that is, they have the ability to be deformed to quite large deformations and then elastically spring back to their original form. This results from crosslinks in the polymer that provide a force to restore the chains to their undeformed conformations. Elastomeric behavior was probably first observed in natural rubber however, the past several decades have brought about the synthesis of a large number of elastomers with a wide variety of properties. Typical stress-strain characteristics of elastomeric materials are displayed in Figure 15.1, curve C. Their moduli of elasticity are quite small, and, they vary with strain because the stress-strain curve is nonlinear. 

Above Tg, amorphous polymers enter a robbery state in which they can support large deformations and still recover to nearly their original shape. Such recovery could pose a major problem for high-temperature de-molding, since embossed features may, under certain conditions, spring back when the load is removed. It is possible, however, to exploit the time-dependent viscous behavior of the polymer to produce permanent deformation. Proper... 

The behavior of a bead-spring chain immersed in a flowing solvent could be envisioned as the following under the influence of hydrodynamic drag forces (fH), each bead tends to move differently and to distort the equilibrium distance. It is pulled back, however, by the entropic need of the molecule to retain its coiled shape, represented by the restoring forces (fs) and materialized by the spring in the model. The random bombardment of the solvent molecules on the polymer beads is taken into account by time smoothed Brownian forces (fB). Finally inertial forces (f1) are introduced into the forces balance equation by the bead mass (m) times the acceleration ( ) of one bead relative to the others ... 

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