Strain compression

An even wider range of wavelength, toward the infrared, can be covered with quantum well lasers. In the Al Ga As system, compressively strained wells of Ga In As are used. This ternary system is indicated in Figure 6 by the line joining GaAs and In As. In most cases the A1 fraction is quite small, X < 0.2. Such wells are under compressive strain and their thickness must be carefully controlled in order not to exceed the critical layer thickness. Lasers prepared in this way are characterized by unusually low threshold current density, as low as ca 50 A/cm (l )-... 

When an isotropic material is subjected to planar shock compression, it experiences a relatively large compressive strain in the direction of the shock propagation, but zero strain in the two lateral directions. Any real planar shock has a limited lateral extent, of course. Nevertheless, the finite lateral dimensions can affect the uniaxial strain nature of a planar shock only after the edge effects have had time to propagate from a lateral boundary to the point in question. Edge effects travel at the speed of sound in the compressed material. Measurements taken before the arrival of edge effects are the same as if the lateral dimensions were infinite, and such early measurements are crucial to shock-compression science. It is the independence of lateral dimensions which so greatly simplifies the translation of planar shock-wave experimental data into fundamental material property information. 

Fig. 25.9. The compressive stress-strain curve for a polymeric foam. Very large compressive strains ore possible, so the foam absorbs a lot of energy when it is crushed.

Dow and Rosen s results are plotted in another form, composite material strain at buckling versus fiber-volume fraction, in Figure 3-62. These results are Equation (3.137) for two values of the ratio of fiber Young s moduius to matrix shear modulus (Ef/Gm) at a matrix Poisson s ratio of. 25. As in the previous form of Dow and Rosen s results, the shear mode governs the composite material behavior for a wide range of fiber-volume fractions. Moreover, note that a factor of 2 change in the ratio Ef/G causes a factor of 2 change in the maximum composite material compressive strain. Thus, the importance of the matrix shear modulus reduction due to inelastic deformation is quite evident. 

Figure 3-62 Compressive Strain at Microbuckling for Fiber-Reinforced Composite Materials (After Dow and Rosen [3-28])...

Certain peculiarity was characteristic of each alloy studied. For example, any hydrogen content embrittled the non-alloyed titanium at room temperature. Other behavior was observed on the Ti-6A1-4.5V alloy at room temperature and low strain rates. The ultimate compressive strain of this alloy without hydrogen was about 8.5% at = 10 s while hydrogen alloying to r = 0.27 increased this value to about 20% and to 10 to 14% at other x. 

If a multidimensional state of stress exists, the Poisson s ratio effect causes the tensile and compressive strains to be dependent upon each of the components of stress. 

Oxide Powder liquid, gcm Wet compressive Strain at failure, % True porosity, %... 

To shed hght on the origin of the enhanced ORR activity, Xu and co-workers performed extensive DFT calculations to investigate the reactivity of the Pt skin [Xu et al., 2004], in particular how oxygen interacts in vacuum with the ordered PtsCo alloy and with a monolayer of Pt formed on the alloy as a model for Pt skin. Figure 9.10 identifies the various adsorption sites for O and O2. Experiments have shown that up to four layers of Pt could sustain a 2.5% compressive strain without creating any surface... 

The key parameters of the electronic structure of these surfaces are summarized in Table 9.3. The calculated rf-band vacancy of Pt shows no appreciable increase. Instead, there is a shght charge transfer from Co to Pt, which may be attributable to the difference in electronegativity of Pt and Co, in apparent contradiction with the substantial increase in Pt band vacancy previously reported [Mukerjee et al., 1995]. What does change systematically across these surfaces is the J-band center (s ) of Pt, which, as Fig. 9.12 demonstrates, systematically affects the reactivity of the surfaces. This correlation is consistent with the previous successes [Greeley et al., 2002 Mavrikakis et al., 1998] of the band model in describing the reactivity of various bimetallic surfaces and the effect of strain. Compressive strain lowers s, which, in turn, leads to weaker adsorbate-surface interaction, whereas expansive strain has the opposite effect. 

It hag been shown that transition of a backbone carbon from the sp to sp state is promoted by tensile stresses and inhibited by compressive strains (10,44). The acceleration of the process of ozone oxidation of the polymers under load is not associated with the changes in supramolecular structure or segmental mobility of the chain. The probably reason of this effect is a decreasing of the activation energy for hydrogen abstraction (44). The mechanism of initial stages of the reaction of ozone with PP can be represented as ... 

We prepare force versus compression plots in a similar fashion to force versus elongation plots. We generally perform compressive testing over a much more limited range of strain than tensile testing. Samples typically take the form of thick pads, which do not break in the same manner as tensile specimens. The limit of compressive strain can approach 100% for low density foams, but is much less for other samples. The most common property that we obtain from this mode of testing is compressive modulus. 

The layout of piping often provides inherent flexibility through changes in direction, so that displacements produce chiefly bending and torsional strains within prescribed limits. The amount of axial tension or compression strain (which produces large reactions) usually is small. 

The second-order fluctuating rate-of-strain tensor is real and symmetric. Thus, its three eigenvalues are real and, due to continuity, sum to zero. The latter implies that one eigenvalue (a) is always positive, and one eigenvalue (y) is always negative. In the turbulence literature (Pope 2000), y is referred to as the most compressive strain rate. 

Direct numerical simulation studies of turbulent mixing (e.g., Ashurst et al. 1987) have shown that the fluctuating scalar gradient is nearly always aligned with the eigenvector of the most compressive strain rate. For a fully developed scalar spectrum, the vortexstretching term can be expressed as... 

It was observed that the CC had the highest compressibility with the greatest compressive strains (70% at 10 MPa and 55% at 2.5 MPa) and the CFP showed two different behaviors depending on the pressure range. At low pressures, the thickness decreased rapidly and the compressive strain had a small slope that increased significantly after 3 MPa. In the second compression, the CFP behaved more like a hard stop this was also observed in the carbon felt. The compressive strains of the paper were 55% at 10 MPa and around 20% at 2.5 MPa. The residual strains after the first compression for both CFP and CC were very similar (30 vs. 25%). 

Among the three materials, the carbon felt had the most stable behavior, with the lowest compressive strain (25% at 10 MPa and 20% at 5 MPa) and almost no residual strain (<5% after first compression). It was also shown that CCs with hydrophobic treatment and CCs with MPFs improved the limit of compressibility by decreasing the slope of compressive strain versus compressive stress. 

Note Also referred to as volume compression, isotropic compression and bulk compressive strain. 

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