Microstresses

Microstresses

 

internal stresses that arise in crystals in the absence of external forces and are equilibrated within volumes that are small compared to that of the entire body. They are caused by imperfections of the crystal structure, such as point defects and clusters of them, and also dislocations. With decreasing distance from a defect, the stresses in the crystal increase and may reach values of the order of the ultimate strength of the material. Microstresses determine a number of the physical properties of crystals, above all the mechanism of their plastic deformation and failure.

References in periodicals archive ?
Due to such design of the tool and the use of optimal parameters, the VCH provides the formation of a ferritic class UFGS (Figures 2 and 3) with a grain size on the surface up to 190 nm, a high density of dislocations, and microstresses in the lattice (Figure 4).
The Mo deposited with high RF power caused delamination of the absorber layer after the selenization process, and this can be due to the presence of microstresses that existed on the Mo layer.
As reported, at room temperature, the residual microstresses generate nonuniform strains (distortions) in the molecular structure and cause broadening of the diffraction peaks thus hiding the relatively sharp lines that would normally be generated by the crystalline regions [32].
To determine the properties of the surface layers of the of optical elements before and after electron beam treatment (microhardness of surface ([H.sub.v], MPa), the quantities of residual thermo stressed ([sigma], MPa) and thickness of reinforced layers ([DELTA], m) we used known methods of physical and chemical analysis (micro identification by Vickers method, methods of optical microscopy and microprobe analysis, which includes the raster and scanning microscopy (SEM) and transmission electron microscopy (TEM), diffractometers DRON-0.5, DRON-2.0, DRON-3.0 with special consoles for measuring the microstresses in the surface layers, etc.
The efficiency of nitriding depends on many interconnected factors such as technology, composition of steel, level of residual macro and microstresses, hardness, depth and structural composition of nitrided layer.
[5] is based on the hypothesis that microstresses are distributed by the normal law while fracture occurs when critical probability of exceeding by microstresses responsible for fracture is reached.
Microstresses in the random point of nonuniform micro-sized medium with elastic deformation can be found as the sum:
where [B.sup.k.sub.11] [equivalent to] [B.sup.kk.sub.11] = d([[bar.[xi]].sup.(k).sub.11]) is dispersions and [B.sup.km.sub.11] = cov ([[bar.[xi]].sup.(k).sub.11] [[bar.[xi]].sup.(m).sub.11]) = <[[bar.[xi]].sup.(k).sub.11] [[bar.[xi]].sup.(m).sub.11]> - <[[bar.[xi]].sup.(k).sub.11]> <[[bar.[xi]].sup.(m).sub.11]> is covariances (correlation moments) of microstresses caused by single macrostresses [[bar.[sigma]].sub.k] and [[bar.[sigma]].sub.m].
Dispersions [B.sup.k.sub.11] and microstresses covariances [B.sup.km.sub.11] arising from the elastic anisotropic grains interaction are defined analytically with use of a hypothesis of strains uniformity, and also numerically for polycrystal model by means of finite elements solution for elastic plane problem for cubic crystals [6-8].
Chamis, "Simplified composite micro-mechanics for predicting microstresses," in Proceedings of the 41st Annul Conference, Reinforced Plastics/Composites Institute, pp.
According to Roy's model of stress uniformity [25, 33], such difference indicates an essential contribution of oriented microstresses of the deformation mode, surveyed by the [alpha]-[sin.sup.2][Psi] method.