Plasticity onset

It should be noted also that hardness values p and H may differ significantly. Let us consider for instance the elastic domain (before plastic onset). The indent recovers entirely so that = 0 and p is not defined. In contrast, H can be determined advantageously by means of an instrumented indentation machine. Also, plastic relaxation of the indents may yield differences between Ap and A and this phenomenon has been invoked sometimes to interpret the ISE. 

Viscosity has been replaced by a generahzed form of plastic deformation controlled by a yield stress which may be determined by compression e)meriments. Compare with Eq. (20-48). The critical shear rate describing complete granule rupture defines St , whereas the onset of deformation and the beginning of granule breakdown defines an additional critical value SVh... 

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. 

Crazing. This develops in such amorphous plastics as acrylics, PVCs, PS, and PCs as creep deformation enters the rupture phase. Crazes start sooner under high stress levels. Crazing occurs in crystalline plastics, but in those its onset is not readily visible. It also occurs in most fiber-reinforced plastics, at the time-dependent knee in the stress-strain curve. 

Alexander Graham Bell believed the photophone, not the telephone, was his greatest invention. His photophone carried the spoken voice by reflected sunbeams instead of wire, but did not find any practical application a century ago. Because light has 20,000 times shorter frequency than microwaves, it can carry 20,000 time more information. Only since the onset of ( plastic ) computers has this ability been needed. It would seem that Alex Bell was ahead of his time. 

The onset of plastic deformation in a material under load is called yielding [60]. In contrast to the experiments described in the previous sections, yielding causes a permanent deformation, i.e. a deformation that remains after the load is removed. The effects of crosslinks on the yield behaviour of polymers are demonstrated by three experiments ... 

In rodent stroke models, statin pretreatment has been shown to reduce infarct volumes and improve outcomes. Similarly, several clinical studies have shown that prior statin use reduced the severity of acute ischemic stroke and myocardial infarction. Recent studies indicate that beneftt can be achieved even when treatment is initiated after the onset of symptoms. In rodents, atorvastatin and simvastatin have been shown to reduce the growth of ischemic lesions, enhance functional outcome, and induce brain plasticity when administered after stroke onset. A retrospective analysis of the population-based Northern Manhattan Stroke Study (NOMASS) showed that patients using lipid-lowering agents at the time of ischemic stroke have a lower incidence of in-hospital stroke progression and reduced 90-day mortality rates. Retrospective analysis of data of the phase III citicoline trial showed... 

Assuming that the onset of plastic instability occurs at a fairly large value of plastic strain, one may consider the volume to remain essentially constant, so that A L = Aq L0. Differentiation gives... 

The ultimate tensile strength (UTS) of a material refers to the maximum nominal stress that can be sustained by it and corresponds to the maximum load in a tension test. It is given by the stress associated with the highest point in a nominal stress-nominal stress plot. The ultimate tensile strengths of a ductile and of a brittle material are schematically illustrated in Figure 1.11. In the case of the ductile material the nominal stress decreases after reaching its maximum value because of necking. For such materials the UTS defines the onset of plastic instability. 

Indeed, it has been observed that the onset of yielding of isotropic polymers is approximately constant, 0.02< [<0.025, which implies that 0.04shear yield strain, the plastic shear deformation of the domain satisfies a plastic shear law. For temperatures below the glass transition temperature, the continuous chain model enables the calculation of the tensile curve of a polymer fibre up to about 10% strain [6]. Figure 7 shows the observed stress-strain curves of PpPTA fibres with different moduli compared to the calculated curves. 

Plastic deformation is the permanent change in shape of a specimen due to applied stress. The onset of plastic deformation is seen as curvature in the stress—strain curve. Plastic deformation is important because it allows pharmaceutical excipients and drugs to establish large true areas of contact during compaction that can remain on decompression. In this way, good, intact, tablets can be prepared. 

A minor but promising line of investigation concerns the use of plasmas preformed from thin foil targets, via the exploding-foil technique. In this scheme, a thin foil of material, usually plastic of few micron thickness, is exploded in the interaction with the ASE pedestal. The main pulse interacts then with the preproduced plasma enabling the onset of the LWF mechanism. The choice of the thickness and material of the target, as well as... 

A typical time profile of the excited PMMA-Phe fluorescence intensity decay is shown in Figure 2. The MEK permeation commences at 24 sec. The SPR increases during the plasticization period until it becomes constant, the onset of the steady state. It is characterized by a linear relationship between the amount of solvent absorbed and time. It was determined from a linear regression analysis that the PMMA-Phe fluorescence intensity starts to deviate from linearity at 197 sec. This indicates a decrease in the SPR and/or the unquenched PMMA-Phe. The decrease in SPR is unexpected at this film thickness since the SPR in thicker PMMA-Phe films show no anomaly at 1 /tm. A more plausible explanation is the reduction in available PMMA-Phe, which is expected when the front end of the SCP reaches the substrate. 

Several groups tried to unravel whether the cavitation occurs before or after the onset of plastic deformation. Real-time small angle X-ray scattering [162] and light scattering [163,164] techniques were used to study the deformation of... 

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