Solids, resisting deformation

Since the resistance of solids to deformation does not change very much with moderate changes of deformation rate, the power dissipation by the displacement of compressed priming mixt by a firing pin is nearly proportionate to its velocity (Ref 19)... 

Although not one of the most frequently discussed properties of solids, hardness is an important consideration in many instances, especially in the area of mineralogy. In essence, hardness is a measure of the ability of a solid to resist deformation or scratching. It is a difficult property to measure accurately, and for some materials a range of values is reported. Because of the nature of hardness, it is necessary to have some sort of reference so that comparisons can be made. The hardness scale most often used is that developed by Austrian mineralogist F. Mohs in 1824. The scale is appropriately known as the Mohs scale. Table 7.11 gives the fixed points on which the scale is based. 

When the carbon content is greater than about 2 wt% and less than about 5 wt% carbon, the material cannot be heated to give a homogeneous solid solution. At all temperatures below the eutectic temperature of 1148 °C the solid is a mixture of austenite and cementite or ferrite and cementite (FeaC). The effect of this is that the materials are hard, brittle and resist deformation. The material can be cast into the desired shape, and is referred to as cast iron. Commercial cast irons rarely contain much more than about 4.5 wt% carbon. 

In 1983, Thingstad and Pengeurd conducted photo-oxidation experiments and found that photo-oxidized oil formed emulsions (11). Nesterova et al. studied emulsion formation and concluded that it was strongly correlated with both the asphaltene and tar content of oil and also the salinity of the water with whieh it was formed (12). Mackay and Nowak studied emulsions and found that stable emulsions had low conductivity and therefore a continuous phase of oil (13,14). Stability was discussed and proposed to be a funetion of oil composition, particularly waxes as asphaltenes. It was proposed that a water droplet could be stabilized by waxes, asphaltenes, or a combination of both. The viscosity of the resulting emulsions was correlated with water content. Later work by the same group reported examination of Russian hypotheses that emulsions are stabilized by colloidal particles which gather at the oil— water interface and may combine to form a near-solid barrier that resists deformation and thus water-water coalescence (15). It was speculated that these particles could be mineral, wax crystals, aggregates of tar and asphaltenes, or mixtures of... 

Usually this criterion allows us to unambiguously classify a phase as either a solid or a fluid. Over a sufficiently long time period, however, detectable flow occurs in any material under shear stress of any magnitude. Thus, the distinction between solid and fluid actually depends on the time scale of observation. This fact is obvious when we observe the behavior of certain materials (such as Silly Putty, or a paste of water and cornstarch) that exhibit solidlike behavior over a short time period and fluid-like behavior over a longer period. Such materials, that resist deformation by a suddenly-applied shear stress but undergo flow over a longer time period, are called viscoelastic solids. 

Liquids are more complicated. Liquids can deform continuously when a force is applied, indicating that a liquid structure cannot completety negate an tq>plied force. However, like solids it is not common to find conditions in which a liquid is accelerating at 9.8 m s A liquid s structure can oppose an applied force, but often this structure does not negate the rq lied force. Just how easily a liquid resists deformation due to an applied force such as gravity is... 

In all materials (plastics, metals, wood, etc.) elementary mechanical theory demonstrates that some shapes resist deformation from external loads. This phenomenon stems from the basic physical fact that deformation in beam or sheet sections depends upon the mathematical product of the modulus of elasticity (E) and the moment of inertia (I), commonly expressed as El (Chapter 3, Stress-strain behavior). It is applied to all types of constructions such as solids, foams, and sandwich structures. In many applications plastics can lend themselves in the form of a sophisticated lightweight stiff structure and the requirements are such that the structure must be of plastics. In other instances, the economics of fabrication and erection of a plastics lightweight structure and the intrinsic appearance and other desirable properties make it preferable to other materials. 

When the contacts between the particles in a free-disperse system are established, the transition of the system into a connected-disperse state takes place. This transition is associated with the development of a spatial network of particles in which the cohesive forces between the particles forming a network are sufficiently strong to resist thermal motion and the action of external forces. As a result of the transition, the system acquires a set of new structnral-mechanical (rheological) properties that characterize the ability of the syston to resist deformation and separation into individual parts. That is, the system acquires mechanical strength, which is the principal and universal characteristic of all solid and solid-like materials. For many materials, their mechanical strength defines the conditions of their use. 

The concept of viscosity is credited to Newton, who observed that the resistance to deformation of a fluid is proportional to the rate of change of deformation (Macosko et al. 1994). Just as the elastic modulus is associated with the resistance of a solid to deformation, viscosity is a measure of a fluid s internal resistance to flow or deformation rate. Viscosity is defined as the ratio between the shear stress and the shear strain rate ... 

The mechanical properties of materials involve various concepts such as hardness, stiffness, and piezoelectric constants, Young s and bulk modulus, and yield strength. The solids are deformed under the effect of external forces and the deformation is described by the physical quantity strain. The internal mechanical force system that resists the deformation and tends to return the solid to its undeformed initial state is described by the physical quantity stress. Within the elastic limit, where a complete recoverability from strain is achieved with removal of stress, stress g is proportional to strain e. The generalized Hooke s law gives each of the stress tensor components as linear functions of the strain tensor components as... 

A liquid foam is a dispersion comprising a gas phase and a liquid phase shaving cream and the froth in the head of a beer are two familiar examples. The gas phase forms bubbles that are separated by thin liquid films. The gas is thus called the dispersed phase, while the liquid is the continuous phase. There is an energetic cost associated with any interface between phases and an associated interfacial tension. This tension is, ultimately, the reason a material made up of two fluids can display solid-like responses it allows the thin liquid films to resist deformation. 

A solid, by definition, is a portion of matter that is rigid and resists stress. Although the surface of a solid must, in principle, be characterized by surface free energy, it is evident that the usual methods of capillarity are not very useful since they depend on measurements of equilibrium surface properties given by Laplace s equation (Eq. II-7). Since a solid deforms in an elastic manner, its shape will be determined more by its past history than by surface tension forces. 

Particle surface characteristics Type of solid (in terms of internal liquid content) gel, flocculated, hard particle Strength of particle (resistance to deformation under pressure) compressibility over time expressed cake... 

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