Plastic yield point

TKPP Cone. Density Apparent Plastic Yield Point... 

Forming techniques used for clay-based ceramics require control of water content in the batch. Water content, in turn, affects the response of the clay during forming [27], As the water content of the batch increases, the yield point of the clay-water mixture, and thus the force required to form the desired shape, generally decreases [26], However, the relationship is complex and depends on the composition of the clay, its structure, additives to the batch, and other factors [14], One method for quantifying the behavior of clay-water pastes is to measure the plastic yield point as a function of water content [14], The water contents and maximum yield points in torsion are compared for several clays in Table 9. Kaolins and plastic fire clays require the least amount of water to develop their maximum plasticity, ball clays require an intermediate amount, and bentonite requires the most. 

Removing the load from a sample after various degrees of deformation, we find that for the value corresponding to the maximum stress, a permanent deformation appears. The material has undergone plastic deformation and the maximum corresponds to the plasticity threshold (or plastic yield point). This is characterised by a stress ay and a strain Sy. 

It is important to differentiate between brittie and plastic deformations within materials. With brittie materials, the behavior is predominantiy elastic until the yield point is reached, at which breakage occurs. When fracture occurs as a result of a time-dependent strain, the material behaves in an inelastic manner. Most materials tend to be inelastic. Figure 1 shows a typical stress—strain diagram. The section A—B is the elastic region where the material obeys Hooke s law, and the slope of the line is Young s modulus. C is the yield point, where plastic deformation begins. The difference in strain between the yield point C and the ultimate yield point D gives a measure of the brittieness of the material, ie, the less difference in strain, the more brittie the material. 

Yield point The point at which material extension is no longer proportional to tension applied. Up to this limit, elastic deformation occurs after this limit, plastic deformation occurs. 

In the region where the relationship between stress and strain is nonlinear, the material is said to be plastic. Elastic deformation is recoverable upon removal of the load, whereas plastic deformation is permanent. The stress at which the transition occurs, o, is called the yield strength or yield point of the material, and the maximum... 

For most practical purposes, the onset of plastic deformation constitutes failure. In an axially loaded part, the yield point is known from testing (see Tables 2-15 through 2-18), and failure prediction is no problem. However, it is often necessary to use uniaxial tensile data to predict yielding due to a multidimensional state of stress. Many failure theories have been developed for this purpose. For elastoplastic materials (steel, aluminum, brass, etc.), the maximum distortion energy theory or von Mises theory is in general application. With this theory the components of stress are combined into a single effective stress, denoted as uniaxial yielding. Tlie ratio of the measure yield stress to the effective stress is known as the factor of safety. 

Direct Indicating Viscometer. This is a rotational type instrument powered by an electric motor or by a hand crank. Mud is contained in the annular space between two cylinders. The outer cylinder or rotor sleeve is driven at a constant rotational velocity its rotation in the mud produces a torque on the inner cylinder or bob. A torsion spring restrains the movement. A dial attached to the bob indicates its displacement. Instrument constants have been so adjusted that plastic viscosity, apparent viscosity, and yield point are obtained by using readings from rotor sleeve speeds of 300 and 600 rpm. 

Plastic viscosity (PV) is centipoises equals the 600 rpm reading minus the 300 rpm reading. Yield point (YP) in pounds per 100 ft equals the 300-rpm reading minus plastic viscosity. Apparent viscosity in centipoises equals the 600-rpm reading, divided by two. The interpretations of PV and YP measurements are presented in Figure 4-107. 

Plastic Viscosity and Yield Point. Plastic viscosity and yield point measurements are obtained from a direct indicating viscometer. Due to the temperature effect on the flow properties of oil-base mud, the testing procedure is modified. The mud sample in the container is placed into a cup heater [23]. The heated viscometer cup provides flow property data under atmospheric pressure and bottomhole temperature. 

Figure 4-111. Suggested range of plastic viscosity and yield point for bentonite muds [29],...

KCl-polymer (potassium chloride-polymer) muds can be classified as low solids-polymer muds or as inhibitive muds, due to their application to drilling in water-sensitive, sloughing shales. The use of polymers and the concentration of potassium chloride provide inhibition of shales and clays for maximum hole stability. The inverted flow properties (high yield point, low plastic viscosity) achieved with polymers and prehydrated bentonite provide good hole cleaning with minimum hole erosion. 

Mud Weight ppg Plastic Viscosity cP Yield Point Ibs/IOOft Oil-Water Ratio Electrical Stability... 

As an example, for room-temperature applications most metals can be considered to be truly elastic. When stresses beyond the yield point are permitted in the design, permanent deformation is considered to be a function only of applied load and can be determined directly from the stress-strain diagram. The behavior of most plastics is much more dependent on the time of application of the load, the past history of loading, the current and past temperature cycles, and the environmental conditions. Ignorance of these conditions has resulted in the appearance on the market of plastic products that were improperly designed. Fortunately, product performance has been greatly improved as the amount of technical information on the mechanical properties of plastics has increased in the past half century. More importantly, designers have become more familiar with the behavior of plastics rather than... 

Test rate and property The test rate or cross-head rate is the speed at which the movable cross-member of a testing machine moves in relation to the fixed cross-member. The speed of such tests is typically reported in cm/min. (in./min.). An increase in strain rate typically results in an increase yield point and ultimate strength. Figure 2-14 provides examples of the different test rates and temperatures on basic tensile stress-strain behaviors of plastics where (a) is at different testing rates per ASTM D 638 for a polycarbonate, (b) is the effects of tensile test-... 

The stress-strain behavior of plastics in flexure generally follows from the behavior observed in tension and compression for either unreinforced or reinforced plastics. The flexural modulus of elasticity is nominally the average between the tension and compression moduli. The flexural yield point is generally that which is observed in tension, but this is not easily discerned, because the strain gradient in the flexural RP sample essentially eliminates any abrupt change in the flexural stress-strain relationship when the extreme fibers start to yield. 

It has been found that with many plastics the calculated flexing stress can far exceed the yield point stress, if the assembly occurs too rapidly. In other words, the flexing finger will just momentarily pass through its condition of maximum deflection or strain, and the material will not respond as if the yield stress had been greatly exceeded. 

The yield point is the first point on the stress-strain curve at which an increase in strain occurs without an increase in stress. The stress at which a material exhibits a special limiting deviation from the proportionality of stress-to-strain is the yield strength. A material whose stress-strain curve exhibits points of zero slope may be considered to have a yield point such as described in Fig. 2-11. The data sheets usually omit the yield strength when there is a zero slope point on the stress-strain curve in the yield region. In reinforced plastic materials, the values of the yield strength and the tensile strength are very close to each other. 

Elastic-plastic transition It is the changes from recoverable elastic behavior to non-recoverable plastic strain which occurs on stressing a material beyond its yield point. 

Plastic flow pectins build up a yield point without giving the product a slimy characteristic. 

The rheological characteristics of AB cements are complex. Mostly, the unset cement paste behaves as a plastic or plastoelastic body, rather than as a Newtonian or viscoelastic substance. In other words, it does not flow unless the applied stress exceeds a certain value known as the yield point. Below the yield point a plastoelastic body behaves as an elastic solid and above the yield point it behaves as a viscoelastic one (Andrade, 1947). This makes a mathematical treatment complicated, and although the theories of viscoelasticity are well developed, as are those of an ideal plastic (Bingham body), plastoelasticity has received much less attention. In many AB cements, yield stress appears to be more important than viscosity in determining the stiffness of a paste. 

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