Residual stresses and strains

Measurement of Residual Stress and Strain. The displacement of the 2 -value of a particular line in a diffraction pattern from its nominal, nonstressed position gives a measure of the amount of stress retained in the crystaUites during the crystallization process. Thus metals prepared in certain ways (eg, cold rolling) have stress in their polycrystalline form. Strain is a function of peak width, but the peak shape is different than that due to crystaUite size. Usually the two properties, crystaUite size and strain, are deterrnined together by a computer program. 

Regardless of matrix or fiber materials, the key process variables for filament winding are temperature, compaction pressure/fiber tension, and laydown rate. Typical measures of final cylinder quality include degree of cure/crystallinity, void volume fraction, fiber volume fraction, and residual stresses and strains. 

Extensive theoretical investigations devoted to calculation of residual stresses have been carried out for metals. The principal theme of this work is assumption that residual stresses and strains are the result of differences between pure elastic and elastic-plastic deformations under fixed loading.127 128 The same mechanism, i.e., the appearance of plastic deformed zones, is responsible for the residual stresses arising during crystallization of metals, which occurs on quenching from the melt or cooling after welding. 

Unfortunately, it was not practical to determine directly the residual stresses and strains in the dielectric films for these samples. However, measurements on similar films showed compressive stress values of the order of 100 MPa for the Si3N films, and 50 MPa for the 810 4.5 wt.% P films. The corresponding residual strains are thus of the order of 0.001. Although these strains are not negligible, they are substantially smaller than the cracking strains we observed, but are within the uncertainties of... 

Figure 2.22 Effects of residual stress and strain on diffraction peak position and shape (a) no strain (b) uniform strain and (c) non-uniform strain. (Reproduced with permission from R. Jenkins and R.L. Snyder, Introduction to X-ray Powder Diffractometry, John Wiley Sons Inc., New York. 1996 John Wiley Sons Inc.)...

The stress state, where the stress can be both applied and residual, and the associated strain influence many different material properties, which is especially important in engineering and technological applications. The residual stress and strain can be advantageous or, on the contrary, can provoke a faster failure of machine parts or other manufactured materials. There are different methods to determine the strain and stress in materials mechanical, acoustical, optical and the diffraction of X-ray and neutrons. The diffraction method is applicable for crystalline materials and is based on the measurements of the elastic strain effects on the diffraction lines. There are two kinds of such effects, a peak shift and a peak broadening. The strain modifies the interplanar distances d. In a polycrystalline specimen a peak shift is produced if the average of the interplanar distance modifications on the crystallites in reflection is different from zero. If the dispersion of interplanar distance modifications is different from zero, then a peak broadening occurs. The effect of the strain on the peak breadth is described in Chapter 13. Here we deal only with the peak shift effect caused by the macroscopic, or Type I strain/stress. There is a substantial amount of literature on this subject. The comprehensive... 

R. L. Williamson, B. H. Rabin and G. E. Byerly, "FEM Study of the Effects of Interlayers and Creep in Redueing Residual Stresses and Strains in Ceramic-Metal Joints," Composites Eng., 5 [7] 851-863 (1995). 

Measurements of residual stress and strain, Young s modulus and adhesive bonding strength of the hydroxyapatite coating after immersion in HBSS showed... 

Vessel Wall Inelasticity of the Vessel Wall Residual Stress and Strain... 

When residual stresses are considered, the stress distributions flatten considerably and become almost uniform at in situ length and pressure. Figure 57.10 shows the radial stress distributions computed for a vessel with /3 = 1 and /3 = 1.11. Takamizawa and Hayashi have even considered the case where the strain distribution is uniform in situ [9]. The physiologic imphcations are that vascular tissue is in a constant state of flux. New tissue is synthesized in a state of stress that allows it to redistribute the internal loads more uniformly. There probably is no stress-free reference state [7,8,17]. Continuous dissection of the tissue into smaller and smaller pieces would continue to relieve residual stresses and strains [ 10). 

Results from warpage simulation include the deformed shape of the part part shrinkage deflections in the x, y, and z directions and residual stresses and strains. The deformed shape and its accompanying stress distribution may be subsequently used for structural analysis. 

If part of the cross section is subjected, to Iastic strain, as ii dicaied in Fig. 2.5ay and the remainder of the section und, rj oes plastic deformation, as indicated in Fig. 2.5c, residual stresses, and strains will remain in the cross section upon removal of the load condition. Figure 2.5b shows the loading and unloading condition that results when only a portion of the crass section undergoes plastic deformation. 

If 8 = 1, there is no residual strain. If p 1, residual stresses and strains are present. If > 1, a longitudinal cut through the wall will cause the blood vessel to open up, and the new cross-section wiU format-shaped section of an annuluswithlargerintemalandexternal radii. If p < 1, the cylindrical shape is unstable, but a thin section will tend to overlap itself. In Choung and Fung s formulation, p = it/ o, where the angle o is half the angle spanned by the open annular section [6]. 

Blood vessel wall, 56-1-56-2 homogeneity, 57-1 incompressibility, 57-1-57-2 inelasticity, 57-2 residual stress and strain in, 57-2... 

Mechanical factors such as stress and strain, residual stress and strain, and strain rate are cmcial parameters for EAC and closely related to the specific alloy/environment combinations. The threshold conditions for SCC depend on the material, the environment, the load, or a combination of these. The threshold stress intensity factor is often used as a threshold parameter for SCC (e.g., ASTM E1681-95 [123]). Quantification of the effect of K is especially important in understanding the rate-controlling processes and in CGR disposition equations. Generally, CGR is expressed in terms of AT by an empirical power law equation ... 

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