Stress-optical characteristics

An important phenomenon observed in amorphous plastics (also observed in optical glass) is the development of optical anisotropy due to stress. The stress-optical characteristic of transparent plastics is the basis of the important technique of photoelasticity by which stress and strain in complicated shapes, for which no analytical solution is readily available, can be determined experimentally and simply. 

The same stress-optical characteristic also permits examination of locked-in stresses in a molded plastic part. The part is examined under polarized light, and the amount of stress is indicated by the number of fringes or rings that become visible. Illumination with white light gives colorful patterns involving all the colors of the spectrum. Monochromatic light is, however, used for stress analysis because it permits more precise measurements. 

Instrumental measurement of whiteness has been the subject of much research. The parameters needed for unambiguous characterisation in the assessment of whiteness and tint of fluorescent substrates have been reviewed [21]. The importance of seeking good correlation between different instruments is stressed [20]. Various trials have demonstrated that it is possible to adjust modern instruments used to measure the optical characteristics of FBA-treated samples of paper so that the results agree with a standard deviation of the order of one CIE whiteness unit [22]. 

Birefringence induced by applied stress is caused by the two components of the refracted light traveling at different velocities. This generates interference which is characteristic of the material. The change in refractive index, An, produced by a stress S is often related by a factor C called the stress-optical coefficient as follows ... 

Analysis of Stress—Optical Data. The slight, if indeed real, improvement of the isotropic model over the Takayanagi model would be of little consequence were it not for a more pronounced difference between the two models in their ability to describe the stress-optical data. When the parameters obtained from the dynamic data (Table IV) are substituted into Equations 8 and 9, Equation 8 produces results which are uniformly too low. Equation 9 also underestimates the magnitude of Ka but only by an average 7% (Figure 14). For most blends the discrepancy is less than 5%, and all calculated values show the characteristic elevation of the birefringence attributed to the multiphase structure. 

LDPE is a highly branched materied, whose flow behavioTir exhibits some peculiarities, which constitute a good test for a numerical simulation. As presented in Section lll-l, birefringence patterns are perturbed around the re-entrant comer, which leads to the appearance of W-shaped fringes at the entry of the die land. It can be seen in Fig. 39 that the mPTT model ilows on to obtain these characteristic shapes (the computation is related to experiments carried out at 175 °C and 21 s, for an estimated value of the stress optical coefficient of... 

The use of photon-coimting techniques in combination with sinnsoidal variation of stress-induced polarization modulation imposes quantifiable lirtritatiorrs on the accnracy and precision of CPL measurements. We consider first the optical characteristics of the PEM. The phase difference q>) between the two orthogonal crystal axes of the PEM is related to the sinusoidally varying periodic stress (sin at) thronghthe following Bessel fnnction... 

Shape- and stress-recovery characteristics of the SMPU/SWCNT nanocomposite (0.57 vol.% or lwt%) are qualitatively depicted in Fig. 26. The SME of this nanocomposite was actuated thermally, optically, and electrically. Recovery of substantial deformation, in excess of 300%, was possible, as shown by the complete closing of a loose knot by heating the material to 55°C (Fig. 26a). Exposed to near-... 

Fig. 4. Strain-optical coefficient of polystyrene. The theoretical dependencies were calculated for polystyrene studied by Onogi et al [97] (see Fig. 3). The separate contributions from relaxation branches are shown by dashed curves 1 - conformational branch 2 -orientation or viscoelastic branch 3, 4, 5 - glassy branches. The values of parameters are B = 3000, E = 20, 000, x = 0.08, r = 5 x 10 s, nT = 1.7 x 10 Pa. The stress-optical coefficient is taken different for different relaxation branches Cl = C2 = — 1, C3 = C4 = C5 = 0.1. The characteristic features of the dependence of the strain-optical coefficient on frequency reminds us the empirical ones discovered by Inoue et al [123] for polystyrene with longer macromolecules...

In 1812, even before Maxwell, Kelvin and Boltzman, the Scottish scientist Sir David Brewster (1781-1868) discovered that certain transparent optically isotropic solids (e.g., glass) when loaded developed optical characteristics of natural crystals. That is, he found that such a solid when loaded exhibited birefringence or double refraction and thus behaved as a temporary crystal. His discovery was the beginning of the well-known photoelastic method by which it is possible to experimentally determine the state of stress or strain on the interior of a loaded elastic body using polarized light. Maxwell (as well as F. E. Neumann at an earlier date) also studied the technique and deduced the relationship between stress and the optic effect now known as the Maxwell-Neumann stress-optic law. The impor-... 

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