Stresses, thin foils

Strain Gages. Strain gages are used to measure the strain or elongation caused by the stress on a material. They are usually made of a thin foil grid laid on a plastic support as shown in Figure 4-259. They are the size of a small postal stamp and are glued to the structure to be stressed. 

The Zero Creep Technique. The zero creep technique was developed by Udin, Shaler, and Wulff(8) to measure the surface energy of Cu wires. The technique was later extended for use with thin foils by Hondros(16). Very thin foils, approximating a surface, are readily available. When shaped into a cylinder, the sample will tolerate large loads without necking. Since necking does not occur, the stress can be considered constant throughout the experiment. Figure 1 shows a schematic of a foil and the associated stress under an applied load... 

Figure 7.42 Almen test configuration to measure the specimen curvature, (a) Mounting block and (b) a thin foil before (1) and after grit blasting (2). The convex curvature will be reduced by tensile stresses induced during coating (3) (SAE Handbook, 1977).

Tunstall, W. J., Hirsch, P. B., Steeds, J. (1964). Effects of surface stress relaxation on the electron microscope images of dislocations normal to thin metal foils. Phil. Mag., 9, 99-119. 

Following China National standards of GB 474-83 and to maximize sample size, the coal sample is prepared as a cylindrical shape with 50 mm in diameter and 100 mm in height, and is numbered as CZ-1. In order to avoid destroying for partly stress overburden and to smooth the sample, the procedure of filling the fractures and flattening the surface with plaster must be done first. After that, the sample needs to be put into a vacuum oven for several days to remove the moisture, measuring weight twice a day to make sure it is totally dry. Moreover, the sample needs to be wrapped with a thin lead foil then a rubber sleeve before it is installed in the cell. 

Membranes are thin flexible fibrous or foil materials which are stabilised solely by tension forces and have a constant stress rate over their entire thickness. In contrast to other materials like glass panes, wood, stone or concrete, membrane structures are extremely Ught because the ratio of material weight to tensile strength is excellent. The low weight of the membrane material also reduces the weight of the primary supporting structure. Because textile membranes are made of thin and flexible materials, usually a double curvature and biaxial pre-tension is needed to stabiUse the structure. 

Figure 8.5 Change in transmittance (upper part) and deflection angle variation (lower part) associated with galvanostatic (1= 21 pAcm" ) insertion-deinsertion cycles of a lithiated nickel oxide thin-film electrode. The vertical bar corresponds to a stress variation of 0.5 GPa. Sample thickness 2000A. Counter-electrode Li foil. Electrolyte iM LiC104-PC. From [26] by permission of Elsevier Science Publishers, Amsterdam.

In as-rolled foils of 20 and 100 p,m thickness that were not annealed prior to the fatigue tests, Judelewicz (1993) and Judelewicz et al. (1994) find a marked difference in fatigue life between samples that were cut parallel and perpendicular to the rolling direction (Figs. 37 and 38). For thin samples the rolling imprints were found to play an important role. Samples that were rolled perpendicular to the stress direction fail at a... 

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