Stress redistribution measurement

Figure 35. Strain measurements (in %) at beam midspan, taken at two loads of 5 kN and 30 kN. The two drawings at the top are for the reference beam (conventional GL24h) the strains are linearly distributed over the beam height. At the bottom, the results for a selected bipartite beam with adhesive layer 027 are shown the interrupted linear strain distribution confirms the theoretical prediction of stress redistribution.

The strain measurements confirmed the theoretical model of stress redistribution in the bipartite beam as compared to conventional glulam beam (Fig. 35). 

Tarantili, P.A., Andreopoulos, A.G., Galiotis, C. Retd-time micro-Rtiman measurements on stressed polyethylene fibers. 1. strain rate effects tmd molecular stress redistribution. Macromolecules 31, 6964-6976 (1998)... 

The optical transition moments for vibrational or electronic transitions between defect states have specific orientations with respect to the defect coordinates. The absorption strength of polarized light for each of the differently oriented centers is proportional to the square of the component of the transition moment that is along the polarization direction. Hence, a stress-induced redistribution of the defects among their different orientations will be detected as an anisotropy in the polarized optical absorption. A convenient measure of the anisotropy is the dichroic ratio, defined as... 

Since oxidative stress has been reported to produce subcellular redistribution and activation of NF-kB, we measured total NF-kB content in cytosolic and particulate fractions of the hearts subjected to ischemia/reperfusion. In another set of experiments, the activation of NF-kB protein was studied by measuring the total NF-kB and phospho-NF-KB in the heart homogenate. Elevated ratios of NF-kB in particulate versus cytosolic fraction and of phospho-NF-KB and total NF-kB in the hearts subjected to ischemia/reperfusion were reduced by selenium. As oxidative stress has been reported to produce subcellular redistribution and activation of NF-kB and antioxidants have been shown to prevent this alteration (Cargnoni et al. 2002), it is possible that our observation on selenium-induced NF-kB translocation might be due to its antioxidant action. It should be noted that since selenium treatment could reduce the tumor necrosis factor-cc (TNF-ct)-mediated activation of NF-kB (Kim and Stadtman 1997), it suggests a possible involvement of a reduction in the formation of TNF-tx in the hearts subjected to ischemia/reperfusion by selenium. 

Several experimental procedures can be used to measure the residual stresses. The three preferred methods involve diffraction (X-ray or neutron), beam deflection, and permanent strain determination. X-ray diffraction measurements have the limitation that the penetration depth is small, such that only near-surface information is obtained. Moreover, in composites, residual stresses are redistributed near surfaces.47 Consequently, a full stress analysis is needed to relate the measured strains to either q or a. ... 

Stress Relaxation Tests. Stress relaxation involves measurement of the stress required to hold a given deformation as a function of time. The deformation is initially applied as rapidly as possible to strain the sample to a certain degree. The modulus calculated from such an experiment decreases with increasing time, reflecting the polymer structure ability to redistribute the stress. 

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