Stress reversal

To determine in the laboratory if a component survives in use, a test bogey is frequentiy estabUshed based on past experience. The test bogey is with the particular test used to dupUcate (or simulate) field conditions. The bogey can be stated in cycles, hours, revolutions, stress reversals, etc. of components are placed on test and each component either survives or faUs. The reUabiUty for this situation is estimated. 

Malberg, J.E., Duman, R.S. (2003). Cell proliferation in adult hippocampus is decreased by inescapable stress reversal by fluoxetine treatment. Neuropsychopharmacology, 28, 1562-71. 

High/low/changing temperature and pressure Stress concentrations, stress reversals, vibration, noise Structural damage or failure, falling objects, collapse Electrical shock and thermal effects, inadvertent activation, power source failure... 

Figure 3 presents the time evolution of tangential stresses at the surface and the core of the cylinder according to the viscoelastic and elastic models. The stress reverse can be explained as follows when the body dries, the drier surface attempts to shrink but is restrained by the wet core. The surface is stressed in tension and the core in compression and inelastic strain occurs. Later, under a surface with reduced shrinkage, the core dries and attempts to shrink causing the stress state to reverse [4],... 

The following conclusions can be drawn a) the AE occurrence rate descriptor is useful for determination of the periods at which the state of stress becomes destructive for the material b) three possible groups of AE occurrence rates may arise during drying first, during the heating of the material second, when the tensional stresses at the surface reach their maximum third, when the tensional stresses in the core reach their maximum after the stress reverse c) the descriptor of total energy reflects accurately the stress state in the material and indicates whether the material suffers the destruction or not. 

A. Salleo, R. A. Street, Light-induced bias stress reversal in polyfluorene fhin-film transistors, ... 

Najafbadi and Yip (18) have investigated the stress-strain relationship in iron under uniaxial loading by means of a MC simulation in the isostress isothermal ensemble. At finite temperatures, a reversible b.c.c. to f.c.c. transformation occurs with hysteresis. They found that the transformation takes place by the Bain mechanism and is accompanied by sudden and uniform changes in local strain. The critical values of stress required to transform from the b.c.c. to the f.c.c. structure or vice versa are lower than those obtained from static calculations. Parrinello and Rahman (14) investigated the behavior of a single crystal of Ni under uniform uniaxial compressive and tensile loads and found that for uniaxial tensile loads less than a critical value, the f.c.c. Ni crystal expanded along the axis of stress reversibly. 

Cycles to craze breakdown and to fatigue fracture increase significantly upon changing from complete stress reversal to cycling in a tensile mode at the same maximum stress. Buckling of the craze fibrils under compressive stress is conducive to early crack formation within the craze. 

G. Gu, personal communication. On the other hand, it has been reported that illumination can assist in recovery from bias-stress effects in polymer TFTs. See Salleo, A. and Street, R.A., Light-induced bias stress reversal in polyfluorene thin-film transistors, J. Appl. Phys., 94, 471 2003. 

Pressure and stress reversion below the overpressured zone because of increased shale permeability... 

Compilation summary of cyclic strength of clays, pure stress reversal. (From Chaney, R.C., and Fang, I-LY., Static and dynamic properties of marine sediments. Proceedings of the Symposium on Marine Geotechnology and Nearshore/Offshore Structures, Shanghai, China, ASTM STP 923, pp. 74-111, 1986. Reprinted with permission. Copyright ASTM.)... 

FIGURE 40.30 Stress reversal due to the memory effect of wood. (Adapted from Rerre, R, The numerical modeling of physical and mechanical phenomena involved in wood drying An excellent tool for assisting with the study of new processes, in Tutorial, Proceedings of the Fifth International lUFRO Wood Drying Conference, Montreal, Quebec, Canada, 1996, pp. 9-38.)... 

In the third test (T =80°C, =76°C), the difference in moisture content between surfece and core remains low. The first drying period lasts an important part of the total drying time. Due to the high value of EMC, the board temperature is always above the softening zone consequently, all stress levels remain very low. These conditions allow wood of good quality to be obtained relatively free of stress reversal with a moderate drying time (less than 150 h against 400 h for the low-temperature test). 

Retaining important possibilities of viscoelastic creep (mechanosorptive creep is always a source of stress reversal) such an effect is obtained at high temperatures, provided the moisture content is sufficiently high (Irvine, 1984)... 

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