552 熱可塑性樹脂のレーザ溶着における接合条件の主応力方向への影響(GS 加工)

Inversion of borehole breakout orientation data : J Geophys Res V96, NB12, Nov 1991, P20093–20107

An analysis of tensile principal stress below an HQ core stub was carried out with a finite element method for eleven lengths of core in 25 stress conditions under which core discing is likely to occur. The direction of the semi-axial tensile principal stress, which has an inclination angle relative to the core axis of less than 45°, was analyzed in detail for the central part of the core. Based on the results, a method for estimating three-dimensional directions of in-situ stress from the height at the periphery of the end-surface of a disc was proposed. Main results obtained in this study are summarized as follows:1) The direction of the semi-axial tensile principal stress is fairly uniform in a wide area of the central part of the core and does not depend on the length of core appreciably. Accordingly, the direction of the semi-axial tensile principal stress is considered to govern the geometry of the end-surface of a disc.2) The azimuth of the semi-axial tensile principal stress in the central part of the core approximately coincides with that of the minimum principal stress, while the inclination relative to the core axis (φm) is smaller than that of the minimum principal stress (φ3). A method for estimating φ3 from φm was proposed, which gave an error of less than 25 %.3) When the difference between the normal stresses perpendicular to the core axis (σx-σy) is great, a saddle-shaped disc is formed and the shape becomes more distinct with the increase in the stress difference. However the saddle shape becomes less distinct as the length of core decreases.4) By assuming that the end-surface of a disc is inclined in the direction of the semi-axial tensile principal stress in the central part of the core, a method for estimating three-dimensional directions of in-situ stress from the height at the periphery of the end-surface of a disc was proposed.

The Suki–Kinari hydropower station is situated in a landform with high mountains and deep valleys in northwest Pakistan. Affected by some geological conditions such as topography and faults, the in situ stress distribution of the project area is highly complex. To determine the in situ stress associated with this project, hydraulic fracturing in situ stress tests are conducted in four representative boreholes. Generally, the relation of the three principal stresses is σ H > σ z > σ h . However, the measured stress directions associated with the headrace tunnel and the powerhouse area are clearly different. The direction of the maximum horizontal principal stress of the headrace tunnel is NNE–NEE, whereas that of the underground powerhouse area is NNW–NWW. To identify the mechanism responsible for this difference, the present study simulates the stress field in the project area based on the three-dimensional finite element regression analysis method. Two theories are proposed on the basis of the numerical simulation results. On the one hand, the direction of the maximum horizontal principal stress in the rock mass above the valley elevation is controlled mainly by topography. On the other hand, the fault will disturb the stress field, causing noticeable changes in the stress direction and value of the fault-affected zone. By comparing the characteristics of the stress direction distribution at three elevations, the direction of maximum horizontal principal stress of the shallow strata is determined to be affected mainly by the topography and faults, and topography may be the most important factor. Moreover, with an increase in strata depth, the influence of the topography and faults on the stress direction decreases gradually. After reaching a particular strata depth, the direction of maximum horizontal principal stress tends to be the same as that of the regional tectonic stress, i.e., NNE.

We have performed a laboratory study of cryogenic fracturing for improving oil/gas recovery from low-permeability shale and tight reservoirs. Our objective is to develop well stimulation techniques using cryogenic fluids, e.g. liquid nitrogen (LN) to increase permeability in a large reservoir volume surrounding wells. The new technology has the potential to reduce formation damage created by current stimulation methods as well as minimize or eliminate water usage and groundwater contamination. The concept of cryogenic fracturing is that sharp thermal gradient (thermal shock) created at the rock surface by applying cryogenic fluid can cause strong local tensile stress and initiate fractures. We developed a laboratory system for cryogenic fracturing under true triaxial loading, with a liquid nitrogen delivery/control and measurement system. The loading system simulates confining stresses by independently loading each axis up to about 5000 psi on 8"×8"×8" cubes. Both temperature in boreholes and block surfaces and fluid pressure in boreholes were continuously monitored. Acoustic and pressure-decay measurements are obtained before and at various stages of stimulations. Cubic blocks (8"×8"×8") of Niobrara shale, concrete, and sandstones have been tested, and stress levels and anisotropies are varied. Three schemes are considered: gas fracturing without cryo-stimulation, gas fracturing after low-pressure cryogen flow-through, gas fracturing after high-pressure flow-through. Pressure decay results show that liquid nitrogen stimulation clearly increases permeability, and repeated stimulations further increase the permeability. Acoustic velocities and amplitudes decreased significantly following cryo-stimulation indicating fracture creation. In the gas fracturing without the stimulation, breakdown (complete fracturing) occurs suddenly without any initial leaking, and major fracture planes form along the plane containing principal stress and intermediate stress directions as expected theoretically. However, in the gas fracturing after cryogenic stimulations, breakdown occurred gradually and with massive leaking due to thermal fractures created during stimulation. In addition, the major fracture direction does not necessarily follow the plane containing principal stress direction, esp. at low confining stress levels. In tests, we have observed that cryogenic stimulation seems to disrupt the internal stress field. The increase of borehole temperature after stimulation affects the permeability of the specimen. While a stimulated specimen is still cold, it keeps high permeability because fractures remain open and local thermal tension is maintained near the borehole. When the rock becomes warm again, fractures close and permeability decreases. In these tests, we have not used proppants. Overall, fractures are clearly generated by low and high-pressure thermal shocks. The added pressure of the high-pressure thermal shocks helps to further propagate cryogenic fractures generated by thermal shock. Breakdown pressure is significantly lowered by LN stimulation with breakdown pressure reductions up to about 40% observed.

Previous investigations have suggested that the direction of least principal stress in the western United States has shifted from northerly to easterly between 50 to 30 Ma. An effort is made to more tightly constrain the time of this shift using igneous dike‐host rock systems in which the isotopic age of both host and dike is known and differs by less than 10 Ma. In such systems, the possibility that magma intruded along old fractures unrelated to the state of stress at the time of intrusion is minimized and the axis of least principal stress is considered to be normal to the strike of the subvertical dike during the time interval between the ages of the dike and host rock. These systems indicate that a northerly least principal stress orientation persisted more recently than previously believed, until probably after 30 Ma, possibly about 26 Ma, and conceivably as recently as 18 Ma in some areas. The data are insufficient and too loosely bracketed in time to decide how fast the change in state of stress occurred in any one place and if by systematic rotation or abrupt shift. However, the change could be diachronous—earlier in the southeast and later in the northwest. Major easterly trending magmatic and tectonic elements of Tertiary age in the Great Basin of Utah and Nevada are compatible with the dike data. However, numerous metamorphic core complexes and detachment fault terranes seem to indicate more or less easterly extension throughout the time period represented by the dikes. This inconsistency is not clearly resolvable, but may reflect differences between the regional plate stress field and gravitational stresses in a crust overthickened by Mesozoic compression.

In total, 55 undrained cyclic torsional shear tests were conducted at a constant mean confining stress, σ0m, constant intermediate principal stress ratios, b = (σ2 − σ3)/(σ1 − σ3), and principal stress directions, α. The cyclic resistance of loose, isotropically consolidated specimens exhibited a dip in strength, with the lowest values for α being 45°, an angle at which α was oriented such that the potential shear band directions dovetailed with the plane having the weakest strength (i.e. horizontal plane). For a given α, the cyclic resistance decreased to the lowest values for b = 0·5, at which the value of the principal stresses was different in the three major directions. For dense specimens, the direction of the major principal stress had no significant influence on the cyclic resistance. For loose, anisotropically consolidated specimens, higher cyclic resistances were observed at the point of approaching the direction of the major principal stress during the consolidation and loading stages.

This paper studies two-scale concurrent topology optimization with multiple micro heterogeneous materials subject to volume constraints. Unlike the existing work on concurrent two-scale optimization where only one material with optimal microstructure is used or that with multiple micro material where each material is distributed in a number of prescribed geometrical domains, selection of micro heterogeneous materials in this work is based on direction of principal stresses in macro structure. For a structure composed of m micro materials, the macro elements are classified into m categories according to their principal stress direction and each category is assigned with a uniform micro material. The interpolation scheme for macro elements is based on Discrete Material Optimization (DMO), where each element is assigned with m macro design variables. The categorization process of the macro structure is achieved by proper modification of volume constraints, where the macro design variables are multiplied by penalty functions. The penalty functions make it uneconomical for the usage of micro materials, which do not correspond to the principal stress direction of their macro element. The macro structure and micro material are connected through effective property, which is calculated through novel numerical implementation of asymptotic homogenization method (NIAH). Both macro structure and micro materials are optimized concurrently and analytical sensitivities are calculated with adjoint method. One minimum compliance numerical example of an L-bracket subject to volume constraints, where one micro material correspond to macro domain with principal stress angle near 0 or 90 degrees and another correspond to that with principal stress angle near 45 or 135 degrees, is presented to show the potential of the proposed method.

The development of the experimental geothermal site at Soultz-sous-forets, has provided awealth of data on the regional stress field in the local granite basement, down to depths larger than 4 km. Borehole imaging logs have outlined the pervasiveness of drilling induced fractures which yield very accurate indication on the maximum horizontal principal stress (N175o ± 6o E). They outline the existence of both time dependent compression breakouts in the direction of the minimum principal stress and thermal borehole elongations in the direction of the maximum horizontal principal stress. The magnitude of the minimum principal stress has been derived from large scale injection tests during which induced seismicity was monitored. The growth of the seismic cloud indicates that the vertical stress profile is not controlled by the equilibrium of optimally oriented pre-existing faults or fractures but that the development of large scale failure processes in this rock mass satisfies a standard Coulomb criterion, with zero tensile strength.

An important prerequisite for achieving efficient exploitation of coalbed methane wells is through forming map cracking by hydraulic fracturing. In order to analyze the mechanical mechanism for forming map cracking of the coal bed with cleats, the mechanical conditions for forming map cracking during hydraulic fracturing process of coal bed was proposed using extensional faulting in elastic mechanics and the shear damage criterion, and the minimum net pressure calculation model for forming map cracking was established when the butt cleat and face cleat in coal opened at the same time. It can be concluded through using the calculation model that the net pressure value that needed for forming map cracking first decreased and then increased with the increasing of the angle between the face cleat and the direction of horizontal minimum principal stress. The cleats and fissures developed along the horizontal maximum principal stress were easy to open and extend under the effect of hydraulic fracturing. The variation of the internal friction coefficient variation of the face cleat had little effect on the minimum net pressure that was needed for forming map fracturing after the angle between the direction of face cleat and horizontal minimum principal stress is determined.

The stress state is the basis for evaluating soil deformation and strength. To study the 3-D stress state of the soil, a 3-D stress device based on fiber Bragg grating (FBG) and a microelectromechanical system (MEMS) was designed to measure the 3-D stress state, principal stress, and direction of principal stress. First, the FBG 1-D earth pressure cell is developed and calibrated. Based on the calculation relationship between the normal stress and the 3-D stress state, and considering the device rotation, the 3-D stress device was developed and verified in the large-scale triaxial test. The results show that the range of the 1-D earth pressure cell is 1 MPa, and the linearity is within 2.5% FS. The device can effectively measure soil’s 3-D stress, principal stress, and direction. The measured results are consistent with the reference and theoretical results. The device provides a new method for effectively measuring 3-D stress in soil.

To study the initiation, propagation, and mechanical behaviour characteristics of fractures on the wall of a single hole during hydraulic fracturing for in-situ stress measurement, and to improve the accuracy of stress direction discrimination in hydraulic fracturing in-situ stress measurement. This paper adopts methods such as theoretical analysis, numerical simulation, and experimental research. On the basis of obtaining basic mechanical parameters through rock mechanics tests, and using the Weibull distribution to describe the inhomogeneity of rocks, a model based on COMSOL software is constructed. It studies the laws and influencing factors of fracture propagation on the borehole wall under different stress environments. The simulation results show that under different stress conditions, the initiation direction of fractures on the borehole wall is mainly parallel to the direction of the maximum principal stress and perpendicular to the direction of the minimum principal stress. At the same time, the propagation and closure of fractures are jointly controlled by the mechanical properties of rocks and the in-situ stress field. In addition, natural cracks have little impact on the propagation of fractures on the borehole wall, and the propagation of hydraulic fractures is not affected by natural cracks. This research achievement provides a reliable theoretical basis for accurately identifying the direction of underground stress, and has important value in guiding tunnel support design, preventing and controlling rockburst and other engineering practices.

Experimental study on the anisotropy of the effective stress coefficient of sandstone under true triaxial stress

In fully mechanized top‐coal caving (FMTC), the effects of coal caving are closely related to the degree of damage to the top‐coal which is determined by the stress state therein. Therefore, research into the evolution of mining‐induced stress field is key to understanding failure mechanisms of top‐coal. By taking the 12 309 FMTC face in Wangjialing Coal Mine (China) as the engineering background and using field measurement and numerical simulation, we studied the evolution of the principal stress field in FMTC under high horizontal stress. The research results demonstrated that in situ stress in Wangjialing Coal Mine was a high horizontal stress and lateral pressure coefficients in the north‐south and east‐west directions were 1.52 and 0.45, respectively. Under the influence of mining, principal stresses were concentrated in front of the coal wall, with concentration factors of 2.26, 2.24, and 2.85, respectively. The minimum principal stress in unconstrained sides of the top‐coal in the roof‐control zone was tensile. The directions of intermediate and minimum principal stresses in the top‐coal were more sensitive to the influence of mining compared to the maximum principal stress. During rotation of the principal stress direction in top‐coal, the location (19 m in front of the coal wall) where the maximum and intermediate principal stresses were equal and the location of the coal wall are key positions.

Based on the comprehensive analysis of wellbore characteristics in a deep shale gas field, the in-situ stress state of the shale reservoir was assessed in this study for the Longmaxi formation in the Dingshan area, Southwestern China. The data obtained from hydraulic fracturing, drilling-induced fractures, and in-situ core testing were used to determine the magnitude and direction of the maximum principal horizontal stress in five wells. Besides, hydraulic fracturing and cross-multipole array acoustic logging (XMAC) were employed to determine the vertical variation of the in-situ stress. Based on the logging interpretation and mechanical test results, the spatial distribution of rock mechanical parameters in the Dingshan area was assessed by the amplitude variation versus offset (AVO) seismic inversion. A 3D heterogeneous mechanical inversion model was realized via the ANSYS and CATIA3D finite element software packages, providing the area in-situ stress field simulation. The depth, fault strike, and position change effects on the main stress, horizontal stress difference, and horizontal stress difference coefficient were numerically simulated. The results show that the maximum principal stress azimuth was mainly concentrated in the NE20°-NE80° sector. Moreover, the development zone of natural fractures was related to the area with the highest principal stress differences. The maximum principal stress variation in the study area was mainly in the compressive range from −135 to −45 MPa, gradually increasing from east to west and south to north. The stress type mainly depended on the depth, fault strike, and rock mechanical parameters, while the stress difference and stress difference coefficient near the fault structure were relatively small. This study’s findings are considered instrumental in improving the borehole stability, determining the casing setting point, and optimizing the well location in deep shale reservoirs with similar geological conditions.

Biobased and biodegradable polyhydroxyalkanoates (PHAs) have great potential as sustainable packaging materials. However, improvements in their processing and mechanical properties are necessary. In this work, the influence of melt processing conditions on the mechanical properties and microstructure of poly(3-hydroxybutyrate-co-3-hydroxyhexanoate) (PHBHHx) is examined using a full factorial design of experiments (DoE) approach. We have found that strict control over processing temperature, mold temperature, screw speed, and cooling time leads to highly increased elongation at break values, mainly under influence of higher mold temperatures at 80 °C. Increased elongation of the moldings is attributed to relaxation and decreased orientation of the polymer chains together with a homogeneous microstructure at slower cooling rates. Based on the statistically substantiated models to determine the optimal processing conditions and their effects on microstructure variation and mechanical properties of PHBHHx samples, we conclude that optimizing the processing of this biopolymer can improve the applicability of the material and extend its scope in the realm of flexible packaging applications.