Elastic modulus plastics mechanical behavior

In the previous symposium, we reviewed mesophase mechanisms involved in the formation of petroleum coke ( 2 ). Since 1975, two significant developments have been the use of hot-stage microscopy to observe the dynamic behavior of the carbonaceous mesophase in its fluid state (3-6), and the emergence of carbon fibers spun from mesophase pitch (7-9) as effective competitors in applications in which high elastic modulus or good graphiticity is important. This paper focuses on mesophase carbon fibers as an example of how the plastic mesophase can be manipulated to produce fibers with intense preferred orientations and elastic moduli that approach the theoretical limit for the graphite crystal in the a-direction. 

Globally, carbon nanotubes have a positive effect on the mechanical properties of all the composites with PVA matrices described in the previous sections. However, the enhancement of mechanical properties differs substantially from a material to another, depending on the type of nanotubes, or on the process used to manufacture the composite. The Young s modulus and the strength are deduced from usual tensile experiments. As depicted in Figure 11.4, PVA/nanotube composites generally follow the same tensile behavior, with a short elastic regime on the first percent strain, followed by a more or less extended plastic behavior. 

Tensile and shear forces are not the only types of loads that can result in deformation. Compressive forces may as well. For example, if a body is subjected to hydrostatic pressure, which exists at any place in a body of fluid (e.g. air, water) owing to the weight of the fluid above, the elastic response of the body would be a change in volume, but not shape. This behavior is quantified by the bulk modulus, B, which is the resistance to volume change, or the specific incompressibihty, of a material. A related, but not identical property, is hardness, H, which is defined as the resistance offered by a material to external mechanical action (plastic deformation). A material may have a high bulk modulus but low hardness (tungsten carbide, B = 439 GPa, hardness = 30 GPa). 

Considering a mass of ceramic powder about to be molded or pressed into shape, the forces necessary and the speeds possible are determined by mechanical properties of the diy powder, paste, or suspension. For any material, the elastic moduli for tension (Young s modulus), shear, and bulk compression are the mechanical properties of interest. These mechanical properties are schematically shown in Figure 12.1 with their defining equations. These moduli are mechanical characteristics of elastic materials in general and are applicable at relatively low applied forces for ceramic powders. At higher applied forces, nonlinear behavior results, comprising the flow of the ceramic powder particles over one another, plastic deformation of the particles, and rupture of... 

Therefore, the main flow properties of plastics in the widest sense are influenced by the mean molar mass. These properties include melt viscosity, modulus of elasticity and shear modulus above the glass transition range, creep behavior, stress cracking behavior, strain at break, mechanical strength, solubility and swelling behavior, etc. 

High polymers show pronounced viscoelastic and viscous (plastic) behavior under normal mechanical loads compared to most other materials, meaning the deformations that occur are in some cases elastic (reversible), and in some cases viscous and thus plastic (irreversible). A result of this is that material parameters such as modulus of elasticity, shear modulus and other important related mechanical properties of high polymers depend not only on temperature, but rather - among other things - on load application times and rates as well. 

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