Photoelastic technique

The fiber fragment length can be measured using a conventional optical microscope for transparent matrix composites, notably those containing thermoset polymer matrices. The photoelastic technique along with polarized optical microscopy allows the spatial distribution of stresses to be evaluated in the matrix around the fiber and near its broken ends. 

We took advantage of the photoelasticity technique for studying in detail conditions under which inner stresses emerge or, vice versa, fuUy disappear in polymeric networks, depending on their swelling state. Three types of polystyrene networks have been chosen for comparison, namely a spherical conventional gel-type styrene-0.5% DVB copolymer, the hypercrosslinked network obtained by crosslinking this copolymer with MCDE to X = 100%, as well as the styrene-5% DVB copolymer prepared in the presence of 30% (by volume) of toluene. Table 7.6 presents their characteristics. 

The difference in thermal expansion between matrix and fibre reinforcement is an important parameter influencing the mechanical behaviour of the composites because it determines the residual stress distribution after fabrication [ 105]. In glass matrix composites, these residual stresses can be measured by photoelastic techniques [127]. 

The photoelastic technique relates to the Brewster s Constant law. It states that the index of refraction in a strained material becomes directional, and the change of the index is proportional to the magnitude of the strain present. Thus a polarized beam in a clear plastic splits into two wave fronts (X and Y directions) that contain vibrations oriented along the directions of principal strains. The index of refi action in these directions is different and the difference (or birefringence) is proportional to the stress level. Result is the colorful patterns seen when stressed plastic are placed between two polarized filters providing qualitative analysis. Observed colors correspond to different levels of retardation at that point, which in turn correspond to stress levels. 

S.9.8 Practice and Limitations of the Through-Transmission Photoelastic Techniques. 

It should be readily realized that only transparent glasses can be subjected to photoelastic techniques. Darkly colored glasses or sintered glasses are not amenable to these techniques. 

Another technique known as the photoelastic coating technique can be used to photoelastically stress-analyze opaque plastic parts. The part to be analyzed is coated with a photoelastic coating, service loads are applied to the part, and the strains on the surface of the part are transmitted to the coating, which assumes the same strain condition as the part. The coating is then illnminated by polarized light from the reflection polariscope. With a digital compensator attached to the polariscope, quantitative stress analysis can be easily performed (27). In summary, photoelastic techniques can be used successfully for failure analysis of a defective product. 

They established the quantitative limits for these assumptions from strain-energy considerations, which later work [53] actually suggested were somewhat conservative. For the first case, Sancaktar and Lawry [52] have recently used a photoelastic technique to examine the elastic tensile stresses and reported good... 

Experimental work [118-125], especially that using photoelastic techniques, has established the presence of residual stresses in joints but the results have often not been quantitative. For the simpler case of polymeric films coated onto metallic substrates, photoelastic techniques and a method based upon the bimetallic strip principle have often been employed [126-131]. Using the latter method, Danneberg [130] showed that for a wide range of epoxy-based coatings on an aluminium substrate thermal contraction was a major cause of internal stress and that the stresses generated were of the order of 0.08... 

Figure 7.12 Response for increasing amounts of spatial disorder (a) regular hexagonal packing of monodisperse photoelastic disks (b) polydisperse disk packing (c) packing of pentagons [62]. In this and the next several figures, the response is shown in color for an ensemble of multiple experiments where a point force was applied to the upper boundary of the indicated stacking of photoelastic particles. More details about photoelastic techniques are given at the end of this chapter.

This photoelastic stress analysis is a technique for the nondestructive determination of stress and strain components at any point in a stressed product by viewing a transparent plastic product. If not transparent, a plastic coating is used such as certain epoxy, polycarbonate, or acrylic plastics. This test method measures residual strains using an automated electro-optical system. 

The heart of the polarization-modulated nephelometer is a photoelastic modulator, developed by Kemp (1969) and by Jasperson and Schnatterly (1969). The latter used their instrument for ellipsometry of light reflected by solid surfaces (the application described here could be considered as ellipsometry of scattered light). Kemp first used the modulation technique in laboratory studies but soon found a fertile field of application in astrophysics the modulator, coupled with a telescope, allowed circular polarization from astronomical objects to be detected at much lower levels than previously possible. 

Relatively simple optical experimental techniques to study noncohesive particulate flow have also been developed, such as the polarized light probe system by Allersma (42,43). With this technique, the principal stress distribution and displacement of photoelastic granular material flowing in two-dimensional hoppers, with and without obstructions (distribution bars), can be obtained, as shown in Fig. 4.18. 

For the measurement of differential absorption spectra in the visible and ultraviolet spectral region, a light modulation technique was introduced over two decades ago [7]. The principle of this measurement also governs the measurement of infrared VCD. This technique involves the high frequency modulation of the exciting beam between left and right circular polarization states via a photoelastic modulator, and measuring the differential absorption with a lock-in amplifier tuned to the modulation frequency. 

Stress can be evaluated by using strain gages60 located in that part of the equipment or structure which is suspected to be critical. Using the measured values of strain in the component, together with the known mechanical properties the stress of the component or structure of interest can be evaluated using the stress-strain relationships. The locations where the strains are to be measured in complex structures are obtained by techniques involving photoelasticity or brittle coating techniques. The preliminary identification of the... 

All commercially available CD instrumentation uses the modulation technique introduced by Grosjean and Legrand. Light from a monochromator is linearly polarized, then passes through a modulating device, today a photoelastic modulator (PEM), which converts the plane-polar-... 

The main difficulty of the PM IRRAS technique for quantitative analysis of ultrathin films at metal electrodes stems from the broad-band background that is introduced to the spectra by the photoelastic modulator (PEM), the so called PEM response functions. Recently, Buffeteau et al. described methods to remove... 

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