Strain deformation caused

Stress relaxation. In a stress-relaxation test a plastic is deformed by a fixed amount and the stress required to maintain this deformation is measured over a period of time (Fig. 2-33) where (a) recovery after creep, (b) strain increment caused by a stress step function, and (c) strain with stress applied (1) continuously and (2) intermittently. The maximum stress occurs as soon as the deformation takes place and decreases gradually with time from this value. From a practical standpoint, creep measurements are generally considered more important than stress-relaxation tests and are also easier to conduct. 

The evaluation of viscosity is similar to the evaluation of elastic deformation except the stress in the element changes due to the local velocity gradient. The time variable is defined as the strain rate. The element changes its shape as a function of time, as long as the strain-rate-induced stress is present. Viscosity is the local slope of the function relating stress in the element to strain rate. The usual functionality is found in Fig. 3.3. The process can be visualized by a constant force on the top of the element that creates a strain rate throughout the element. This strain rate causes each molecular layer of the material to move relative to the adjacent layer continuously. Obviously the element is suspended in a continuum and material flows into and out of the geometric element. 

Stress-strain properties for unfilled and filled silicon rubbers are studied in the temperature range 150-473 K. In this range, the increase of the modulus with temperature is significantly lower than predicted by the simple statistical theory of rubber elasticity. A moderate increase of the modulus with increasing temperature can be explained by the decrease of the number of adsorption junctions in the elastomer matrix as well as by the decrease of the ability of filler particles to share deformation caused by a weakening of PDMS-Aerosil interactions at higher temperatures. 

Dynamic Mechanical (Low Strain Deformation). When a cyclic strain of small ampUtude is applied to a strip of material, a cyclic stress will be generated in response by the sample. If the material is ideal (Hookian) and stores all the input energy, the cyclic stress is in phase with the applied cyclic strain. Viscous components cause a finite phase lag or phase angle, 8, between the stress and strain. represents the elastic, real, or storage modulus while E" is the viscous, imaginary, or loss modulus. Tan 8 is equal to the ratio E /E" and is related to the molecular relaxations that occur in the sample. Transition temperatures and associated activation energy can be determined (72) by varying the frequency of oscillation at a fixed temperature or the temperature at a fixed frequency. 

Forces applied to a water-saturated porous medium will cause stresses which result in strain (deformation). The stress, strain and groundwater pressure in a water-saturated porous medium are coupled, as first recognized by Biot (1941). Under the assumed stress conditions, the vertical normal component of total stress (o ) that acts downwards on a horizontal plane at any depth is caused by the weight of the overlying water-saturated rock. This stress is born by the solid matrix of the porous medium (o ) and by the pressure of the groundwater in the pores (p ) (e.g. Hubbert andRubey, 1959)... 

The ratio of strain (deformation per unit length) caused by a stress (force per unit cross-sectional area) applied to a polymeric material is called Young s modulus. A sufficiently high Young s modulus is desired for a polymer when it is spun to hollow fibers. 

The nucleation of voids produce a decrease of the macroscopic response of the material, by continuing deformation, they growth and coalesh as a result of strain localization causing small increments, at every cycle, of crack tip extension. 

When a load is placed on a specimen, stress and strain result. Stress is the internal resistance to the load as the applied force. Strain is the amount of deformation caused by this stress, such as deflection in bending, contraction in compression, and elongation in tension. 

The internal energy is the strain energy caused by deformation of the body and can be written as ... 

During creep, a component is loaded under a constant stress, constantly straining until the material cannot withstand further deformation, causing it to rupture. 

Specially difficult systems are represented by fibers of macromolecules. All the topics treated in the prior sections must be considered under the additional aspect of the presence of crystal deformations caused by the drawing process (see Sect. 5.2.6 and 5.3.6) and possible strain retained in the amorphous areas (see Sect. 6.3.3), as well as the existence of sttain-induced mesophases (see Figs. 5.69-72 and 5.113-115). 

The liquid crystalline elastomers simultaneously exhibit properties associated with low molar mass LCs and standard elastomers. Therefore, the mechanical and optical properties of such networks are anisotropic below the clearing point Tc) and also are dependent upon stress/strain field caused by mechanical deformation. 

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