Stress elasticity

Fig. 1. Principal stresses acting on small element of cylinder waU at radius, r, when the cylinder is stressed elastically by internal pressure, (3).

Equations 1 to 3 enable the stresses which exist at any point across the wall thickness of a cylindrical shell to be calculated when the material is stressed elastically by applying an internal pressure. The principal stresses cannot be used to determine how thick a shell must be to withstand a particular pressure until a criterion of elastic failure is defined in terms of some limiting combination of the principal stresses. 

Gelation. A sol becomes a gel when it can support a stress elastically, defined as the gelation point or gelation time. tg. A sharp increase in viscosity accompanies gelation. A sol freezes in a particular polymer structure at the gelation point. This frozen-in structure may change appreciably with time, depending on the temperature, solvent, and pH conditions or on removal of solvent. 

All materials undergo a deformation, either noticeable or unnoticeable, whenever they are subjected to an external force. The deformation can be elastic, plastic, elastomeric, or viscoelastic. When an external force is applied on a body, the displacement of points of the body relative to neighboring points is measured as strain and the strength of the force applied to the local point is measured as stress. Elastic deformation such as rubber band stretch is recovered when the stress, or external force, is removed. Plastic deformation such as a dent on a metal car body is permanent and is not recoverable with the removal of the deforming stresses. 

In a typical indentation experiment the indenter is pressed onto the surface under investigation and the load is successively increased up to a certain maximum load. In the so-called compliance approach both load and indenter displacement are recorded and plotted as a load-displacement curve, the so-called compliance curve. If the experiment is exclusively run in the compressive load regime, the curve is also referred to as the load-penetration curve. Upon loading, elastic deformations occur succeeded by plastic ones. Upon releasing the imposed stress, elastic strain recovers immediately. 

Consider a cylindrical rod of cross-sectional area A stressed elastically in tension by a force F (Fig. 16-3). There is a stress (x, = FjA in the y direction but none in the x or z directions. (This stress is the only normal stress acting there are also shear stresses present, but these are not directly measurable by x-ray diffraction.) The stress Oy produces a strain , in the y direction given by... 

The fracture resistance of a material depends on all of the properties which have been discussed including tensile strength, yield stress, elastic modulus, flexural strength, and impact resistance, all of which depend, in part, on fillers. Fillers, consequently, are important determinants of fracture resis-Only those phenomena which are related... 

E22 = but the out-of-plane component E33 is immaterial for the present analysis. The in-plane macrostrain e is decomposed into the elastic and inelastic (plastic) parts, so that we write the increment de as de =de +ds =(5 +S )da. Here, 5 is the plane-stress elastic compliance, given by 5" = (C" in terms of the components of effective elastic... 

Elastic behavior. At least for a start, we shall also choose circumstances where elastic behavior can be neglected. A stressed elastic material differs from the same material when unstressed in respect of bond lengths and quantity of energy stored per gram or kilogram but we shall assume that these changes hardly affect the way the material flows or creeps—at least for a start. After discussion of the flow behavior, the question of elastic behavior will be taken up. 

EDPE/EPDM 1 X 4 X 20 RT air y r Fracture stress, yield stress, elastic modulus 27... 

The pressurized blister test is an excellent method to combine electrochemical reactions at polymer/metal interfaces with a mechanical load. It allows the application of a mechanical stress from a homogeneously pressurized electrolyte on the adhesive/metal interface in a sample geometry that is accessible for the HR-SKP [28]. Depending on the adjusted conditions, information on the synergy of mechanical stresses, elastic or inelastic deformations of the adhesive, transport processes, and corrosive reactions could be obtained with this method. 

Elastic strain is a transitory dimensional change that exists only while the initiating stress is applied and disappears immediately upon removal of the stress. Elastic strain is also called elastic deformation. The applied stresses cause the atoms in a crystal to move from their equilibrium position. All the atoms are displaced the same amount and still maintain their relative geometry. When the stresses are removed, all the atoms return to their original positions and no permanent deformation occurs. 

A fluid, which although exhibits predominandy viscous flow behavior, also exhibits some elastic recovery of the deformation on release of the stress. The term viscoelastic is reserved for solids showing both elastic and viscous behavior. Most plastic systems, both melts and solutions, are viscoelastic due to the molecules becoming oriented due to the shear action of the fluid, but regaining their equilibrium randomly coiled configuration on release of the stress. Elastic effects are developed during processing such as in die swell, melt fracture, and frozen-in orientation. 

Many properties of spider silks have been studied including the physicochem-istry, mechanical stress, elastic modulus and strain energy.The present author has investigated the thermal, optical, molecular, aging and mechanical properties of spider silks. 

Hory, P. J., and Spurr, O. K., Melting Equilibrium for Collagen Fibers imder Stress. "Elasticity in the Amorphous State" T. Amer. Chem. Soc.. 83,1308 (1961). 

---