Anisotropic Stress Field Research Articles

Microstructural study by XRD profile analysis and tem obser vations on hydrided recrystallized zircaloy-4

Zircaloy-4, used as cladding tube material in the nuclear reactors, may become brittle due to the precipitation of hydrides. During hydride formation, the anisotropic misfit strains between hydrides and the hexagonal-close-packed zirconium matrix results in a preferred orientation of the hydride platelets in the anisotropic stress field caused by non-relieved fabrication residual stresses and misfit stresses. To understand the mechanism of rupture and to predict the threshold stresses for hydride stress orientation, it is necessary to study the residual stresses, especially the microstrain caused by crystalline lattice misfit, in a hydrided specimen. The X-ray diffraction profile analysis is very sensitive to all the microstructure evolution in metallic materials. It is a non-destructive and voluminal technique compared with transmission electron microscope observation. The XRD peak broadening can be related closely with the microstrain in case of hydrided Zircaloy-4, because the hydride formation creates in general a great number of dislocations which contributes especially to the diminution of coherent domain size and to the increase of microstrain. To calibrate the internal microstrain due to precipitation effect of hydrided specimens, XRD profile analysis has also been realized on the non-hydrided specimens deformed by uniaxial tension. In this paper the authors restrict to analyzing more » the results about the recrystallized state, because more informations about the anisotropic elasticity, plasticity, thermal expansion, neutron diffraction measurement and the crystallographic texture results are available. « less

Application of the hybrid mongrel displacement finite method to the computation of stress intensity factors in anisotropic material

A conformable and convergent method is proposed for the calculation of stress intensity factors in anisotropic plates. This approach is an extension to anisotropic materials of the one developed by the authors for 2-D isotropic material which is based on the formulation of the hybrid mongrel displacement finite element concept. Using singular six-node triangular hybrid mongrel isoparametric elements which contain the anisotropic singular stress field and the singular strain field, the stress intensity factors are directly obtained by the stress unknown parameters or indirectly by newly developed formulae. Very good agreement is shown for the crack problem in an orthotropic plate whose solutions are known.

Micromechanical modeling of healed crack orientations as a paleostress indicator: Application to Precambrian granite from Illinois and Wisconsin

We have used a micromechanical, four‐grain model to simulate intragranular cracks produced during primary cooling of a granitic pluton in a horizontally anisotropic stress field. The orientations of vertical, intragranular cracks were obtained using the criterion that they are perpendicular to the maximum principal tensile stress when it exceeds the local tensile strength. The preferred orientation was always parallel to the maximum regional compressive stress direction. The sharpness of the distribution peak increases with the ratio of the maximum to minimum horizontal stress. We applied the model results to new and previous data on Precambrian granites from Wisconsin and Illinois. Our new data are healed crack orientations and fluid inclusion properties in unoriented granite core from Illinois borehole UPH‐2. The fluid inclusion measurements and isotopic age dates indicate that most healed cracks were formed during the primary cooling of the pluton. The fluid inclusion characteristics and shape of the crack orientation distribution were almost identical to those previously determined for UPH‐3, 1 km away (Kowallis et al., 1987), suggesting that the preferred orientation of healed cracks in granite from UPH‐2 is the same as for UPH‐3 (N25°W ± 5°). The orientation distribution from the micromechanical model for a stress ratio of 4 is very similar to the healed crack orientation distribution measured in several Precambrian granites from the midcontinent region. We conclude that healed crack orientations are useful indicators of the paleostress field and that they may be used as a tool for orienting core in stable, structurally simple regions.

Anisotropic plastic stress fields at a slowly propagating crack tip

Under the condition that any perfectly plastic stress components at a crack tip are nothing but the functions of 0 only making use of equilibrium equations. Hill anisotropic yield condition and unloading stress-strain relations, in this paper, we derive the general analytical expressions of anisotropic plastic stress fields at the slowly steady propagating tips of plane and anti-plane strain. Applying these general analytical expressions to the concrete cracks, the analytical expressions of anisotropic plastic stress fields at the-slowly steady propagating tips of Mode I and Mode III cracks are obtained. For the isotropic plastic material, the anisotropic plastic stress fields at a slowly propagating crack tip become the perfectly plastic stress fields.

Fundamental aspects of stress, deformation, and phase transitions in crystallizable polymers: Experiments with poly(ethylene terephthalate) in uniaxial stress fields

AbstractInteractions between rheology and fluid→solid transformations through crystallization are demonstrated through an experimental study of crystallization of polyethylene terephthalate in uniaxial stress fields. Effect of the strength of the stress field on the kinetics of crystallization is shown to diminish greatly beyond the initiation of crystallization. Consequences of crystallization in anisotropic stress fields in the generation of mechanical properties are also described. This experimental study points clearly to the deficiencies in current theoretical formulations of kinetics of crystallization.

Anisotropic plastic stress fields at a rapidly propagating crack-tip

Anisotropic plastic stress fields at a rapidly propagating crack-tip

Stress amplification and plastic flow for spheroidal shells: Applications to myopia

Amplification of effective stress and plastic strain rates after yielding are derived for the anisotropic stress fields of the prolate and oblate spheroidal shells as models of the myopic and hyperopic eyes. Dimensionless closed-form results are presented for arbitrary axis ratio with both constant shell thickness and variable shell thickness, using the constant scleral mass assumption. The results show that the myopic and hyperopic eyes are susceptible to failure by plastic yielding at the equator and pole, respectively, for high intraocular pressures. Experimental data from the equatorial zone of rabbit sclera shows scleral yielding and plastic flow for intraocular pressures greater than 32 mm Hg. Practical applications include glaucoma and pathological myopia.

Anisotropic plastic stress field near a singular point

On condition that any perfectly plastic stress component near a singular point is nothing but the function of θ only, making use of equilibrium equations and Hill anisotropic yield condition, we derive the general analytical expressions of the anisotropic plastic stress field near a singular point in both the cases of anti-plane and in-plane strains. Applying these general analytical expressions to the concrete cracks and the plane-strain bodies with a singular point, the anisotropic plastic stress fields at the tips of Mode I, Mode II, Mode III and mixed mode I–II cracks, and the limit loads of anisotropic plastic plane-strain bodies with a singular point are obtained.

Measurement of anisotropic stress field in a gasketed diamond-anvil cell

A new technique is proposed to evaluate the axial and lateral stress components, p z and p r , separately within the solid pressure medium of a gasketed diamond cell. The values of p z and p r are obtained from shifts of the R-line of two ruby plate fragments placed in a pressure chamber, one having crystal plane (11 2 0) and the other (0001) parallel to the anvil face.

Low-frequency propagation in almost fluid sediments: Role of gravity and other stresses

Relatively unconsolidated sediments of low rigidity μ are common in oceans. Shear modes of low velocity cs(cs≲100 ms−1) then exist in sea floor sediments, as well as slow boundary waves of velocity v<cs at the water–sediment interface (Stoneley modes). Both have been observed in shallow water experiments at frequencies in the 1–5 Hz band [Rauch, Bottom-Interacti ng Acoustics (Plenum, New York, 1980)]. In poorly consolidated sediments of this kind ( μ<109 dyn cm−2) gravity and other stresses introduce small corrections to the propagation velocities. The role of gravity is of two sorts. On the one hand it introduces buoyancy forces and internal gravity waves—these are negligible for frequencies above 10−1 Hz; on the other, it introduces hydrostatic stresses which primarily affect the velocity of shear modes in the sediment—leading to corrections of order ρgz/μ (for an overburden of height z in a sediment or sediment+water column of mean density ρ). While the latter measurably affects propagation speeds in sediments, it is an isotropic effect which, in practice, finds itself lumped with other mechanisms of velocity stratification. Interesting and in principle more easily diagnosed in experiments are the effects of anisotropic stress fields—e.g., tensions and compressions associated with gravity slumping mechanisms on slopes and, for deeper and somewhat more consolidated sediments, with forces of local origin such as island loading or tectonic stresses. These can introduce measurable anisotropies into the propagation of various low-frequency bottom associated modes.

Influence of gravity and other stresses on vlf sound in almost fluid sediments

Relatively unconsolidated sediments of low rigidity μ are common. Shear modes of low velocity (cs ≲ 102 ms−1) then exist, as well as slow boundary waves of phase velocity v < cs at the water‐sediment interface (Stoneley modes). In such cases gravity and other stresses may introduce small corrections to the propagation velocities. The role of gravity is twofold. On the one hand, it introduces buoyancy forces and internal gravity waves (negligible for frequencies above 10−1 Hz) and, on the other, it introduces hydrostatic stresses which affect the velocity of shear modes in the sediment—leading to corrections of the order of ρgz/μ (for an overburden of height z in a sediment or sediment‐water column of mean density ρ). While the latter measurably affects propagation speeds in sediments, it is an isotropic effect which will, in practice, find itself lumped with other mechanisms of velocity stratification. Interesting and perhaps more easily diagnosed in experiments are the effects of anisotropic stress fields—e.g., tensions or compressions associated with gravity slumping mechanisms on slopes and, for deeper and somewhat more consolidated sediments, with forces of local origin such as island loading or tectonic stresses. These may introduce measurable anisotropies into the propagation of low‐frequency, bottom‐associated modes. [This work was supported by ONR.]

The Influence of Some Tectonic Structures upon Vitrinite Reflectance

Data are reported for oriented coal samples taken on traverses oblique to two high angle normal faults in underground mine workings in the Southern Coalfield of the Sydney Basin, N.S.W. Apart from faulting and regional warping, relatively little tectonic deformation has occurred in this portion of the basin. Four oriented sections cut normal to bedding at known azimuth were prepared from each of twenty-one coal samples. Vitrinite mean maximum reflectance (), using oil immersion, was measured for each section and results indicate that the vitrinite in this medium volatile coal has biaxial optical properties. Using the azimuths and values of the four sections of each sample, the true maximum reflectance () and its orientation can be determined from the elliptical calculated bedding plane section of the indicating surface (CBPSIS). The biaxial nature of the vitrinite is thought to result from asymmetrical growth of its molecular structure and to be related to stress fields which developed contemporaneously with coalification. In the vicinity of the normal faults, elliptical CBPSIS's with similar orientations occur in a pattern which is consistent for the two faults. The results suggest that anisotropic stress fields may influence coalification and that it may be possible to detect some faults using vitrinite reflectance. Vitrinite reflectance may also provide information concerning stress regimes and stress regime history in proximity to some faults. Biaxial vitrinites appear to provide direct evidence of strain even in weakly deformed terrane where indicators of strain are scarce.

Soft Ferromagnetic properties of some amorphous alloys

In rapidly quenched amorphous alloys (Fe 1 − x Co x ) 80 P 13C 7 and (Fe 1 − x Co x 75Si 15B 10, the zero magnetostrictive alloys are remarkably soft magnetic materials, while the magnetostrictive alloys have rather large coercive forces [1]. To investigate the effect of magnetostriction on the properties of the BH loop, some fundamental magnetic properties of these alloys were studied. The magnetic domain pattern observed by Bitter's method shows a periodic domain structure consisting of 180° and maze domain walls in all the magnetostrictive alloys, suggesting the existence of a magnetic anisotropy with an easy axis that varies locally. The estimated value of the anisotropy energy varies with x similarly to the concentration dependence of the magnetostriction. The coercive force also varies with x and is nearly proportional to the ratio of the magnetostriction to magnetization. It is, therefore, considered that the anisotropy is caused by the anisotropic internal stress field, and that the stress fluctuation which inevitably exists is responsible for the large coercive force.

The influence of anisotropic stress on the electrical characteristics of GaAs 1- xPx diodes

Anisotropic stress has been applied on shallow light emitting GaAsP junctions with stylii of relatively small radius of curvature made of diamond and—for the purpose of extracting the emitted radiation—of glass. It was found that the induced current changes, which were restricted to the ( q/2 kT) slope region of the forward bias current voltage characteristic and which is dominated by the Generation Recombination (GR)_current in the junction, were reversible. The currents flowing in the ( q/ kT) forward bias—light emitting—region and reverse bias region were not affected by stress. No changes in the spectral output in the ( q/ kT) region could be observed. In sharp contrast the near uniaxial stresses caused by glass stylii with large radii of curvature to which large forces were applied resulted in measurable (reversible) band gap changes as detected by the emitted radiation yet, despite the much higher stresses throughout the diode area, did not affect the currents in the GR domain. Highly anisotropic stress fields, though not necessarily high stresses seem therefore essential in the creation and/or changes of GR centres. An empirical relation of the excess stress induced electric currents with force F derived from experimental data is utilised to extract a theoretical relationship between local concentration of stress induced GR centres and elastic deformation energy. The theory confirms the requirement of an anisotropic stress field for the creation of these excess GR centres in the junction.

Early joint formation in sediments

SummaryJointing in some ‘ironstone’ intraclasts in a sandstone from the Sydney Basin, New South Wales shows that the joints were developed prior to contemporaneous erosion. A number of other features observed in the Sydney Basin sequence support this conclusion and suggest that an anisotropic horizontal stress field developed in the sediments very soon after their deposition.