用原子-有限元法研究铁中裂纹的低温脆性解理扩展

运用原子 -有限元法 (FEAt)研究了裂纹在低温加载条件下的脆性解理扩展。原子模拟结果表明 :在低温和一定外部载荷条件下 ,bcc- Fe中 { 10 0 } <0 11>边界 I型裂纹的扩展过程是一个裂尖原子键断裂与层错或孪晶扩展相伴随的过程。裂纹低温脆性解理扩展的最大速率可达到 112 5m /s,即约为 0 .6 VR。相应的有限元分析表明 ,裂尖存在较大的能量和应力集中 ,这是导致裂尖原子键断裂及弹性孪晶形成的原因。     

Shear Strength Behavior of Fiber-Reinforced Sand Considering Triaxial Tests under Distinct Stress Paths

The results of drained triaxial tests on fiber reinforced and nonreinforced sand (Osorio sand) specimens are presented in this work, considering effective stresses varying from 20 to 680?kPa and a variety of stress paths. The tests on nonreinforced samples yielded effective strength envelopes that were approximately linear and defined by a friction angle of 32.5° for the Osorio sand, with a cohesion intercept of zero. The failure envelope for sand when reinforced with fibers was distinctly nonlinear, with a well-defined kink point, so that it could be approximated by a bilinear envelope. The failure envelope of the fiber-reinforced sand was found to be independent of the stress path followed by the triaxial tests. The strength parameters for the lower-pressure part of the failure envelope, where failure is governed by both fiber stretching and slippage, were, respectively, a cohesion intercept of about 15?kPa and friction angle of 48.6?deg. The higher-pressure part of the failure envelope, governed by tensile yielding or stretching of the fibers, had a cohesion intercept of 124?kPa, and friction angle of 34.6?deg. No fiber breakage was measured and only fiber extension was observed. It is, therefore, believed that the fibers did not break because they are highly extensible, with a fiber strain at failure of 80%, and the necessary strain to cause fiber breakage was not reached under triaxial conditions at these stress and strain levels.     

Experimental and Numerical Study of Railway Ballast Behavior under Cyclic Loading

This paper presents the results of the influence of frequency on the permanent deformation and degradation behavior of ballast during cyclic loading. The behavior of ballast under numerous cycles was investigated through a series of large-scale cyclic triaxial tests. The tests were conducted at frequencies ranging from 10–40 Hz, which is equivalent to a train traveling from 73 km/h to 291 km/h over standard gauge tracks in Australia. The results showed that permanent deformation and degradation of ballast increased with the frequency of loading and number of cycles. Much of breakage occurs during the initial cycle; however, there exists a frequency zone of 20?Hz ? f ? 30?Hz where cyclic densification takes place without much additional breakage. An empirical relationship among axial strain, frequency and number of cycles has been proposed based on the experimental data. In addition, discrete-element method (DEM) simulations were carried out using PFC2D on an assembly of irregular shaped particles. A novel approach was used to model a two-dimensional (2D) projection of real ballast particles. Clusters of bonded circular particles were used to model a 2D projection of angular ballast particles. Degradation of the bonds within a cluster was considered to represent particle breakage. The results of DEM simulations captured the ballast behavior under cyclic loading in accordance with the experimental observations. Moreover, the evolution of micromechanical parameters such as a distribution of the contact force and bond force developed during cyclic loading was presented to explain the mechanism of particle breakage. It has been revealed that particle breakage is mainly due to the tensile stress developed during cyclic loading and is located mainly in the direction of the movement of ballast particles.     

Modeling Compression Behavior of Structured Geomaterials

The influence of the natural (or artificially induced) structure of a geomaterial on its compression behavior is investigated. An approach for modeling this influence for various structured geomaterials is proposed by using the disturbed state concept. An isotropic compression model is formulated on three basic assumptions. A special version of the proposed model is also described for situations where the compression is one-dimensional. The proposed compression model is used to simulate the behavior of a variety of structured geomaterials such as clays, sands, calcareous soils, clay-shale, soft rock, unsaturated soils, and soils artificially treated by adding chemical agents or mechanical reinforcement, and the model is evaluated on the basis of these simulations. A general discussion on the influence of the structure of geomaterials on their mechanical properties is also presented.     

Mechanical properties of single crystal V-Ti-O alloys

Single-crystal compression samples of various V-Ti-O alloys were tested over a large temperature range. The critical-resolved-shear-stress of macroscopic yielding and the activation parameters were measured to establish the effect of titanium and oxygen additions on the thermal component of the yield stress of vanadium. In addition, internal friction and transmission electron microscopy experiments were conducted to confirm the results obtained from the compression tests. It was found that by increasing the oxygen content, the effective stress increased. The most likely explanation for this increase is that TiO molecules and/or clusters affect the intrinsic lattice. The observable precipitates produce an increase of the long-range internal stress. An anomalous increase of the effective stress was observed in the high-temperature region of the V-0.32 at. pct Ti alloy. This phenomenon was explained as the stress-induced-ordering of TiO clusters by the stress fields of moving dislocations. This research was suppoted by the United States Energy Research and Development Administration under Contract No. AT(40-1)-3612.     

Experimental Study on Bond Behavior between Concrete and FRP Reinforcement

Bonding between fiber-reinforced polymer (FRP) sheets and concrete supports is essential in shear and flexural applications for transfer of stress between concrete structure and reinforcement. This paper aims at better understanding FRP–concrete bond behavior and at assessing some of the common formulations for effective bond length and bond–slip models (τ-s) by means of an extensive experimental program on 39 concrete specimens strengthened with various types and amounts of FRP strips and covering a wide range of FRP axial rigidities, subjected to both double-shear and bending tests. Effective bond length, maximum bond/shear stress, slip when bond stress peaks, and slip when bond stress falls to zero, were all experimentally measured. The influence of FRP stiffness on effective bond length and bond–slip behavior was observed. New expressions for (1) effective bond length; (2) maximum shear/bond stress; (3) slip at peak value of bond stress; and (4) slip at ultimate, taking into account the influence of FRP stiffness, are proposed.     

Mechanical properties of single crystal V-Ti-O alloys

Single-crystal compression samples of various V-Ti-O alloys were tested over a large temperature range. The critical-resolved-shear-stress of macroscopic yielding and the activation parameters were measured to establish the effect of titanium and oxygen additions on the thermal component of the yield stress of vanadium. In addition, internal friction and transmission electron microscopy experiments were conducted to confirm the results obtained from the compression tests. It was found that by increasing the oxygen content, the effective stress increased. The most likely explanation for this increase is that TiO molecules and/or clusters affect the intrinsic lattice. The observable precipitates produce an increase of the long-range internal stress. An anomalous increase of the effective stress was observed in the high-temperature region of the V-0.32 at. pct Ti alloy. This phenomenon was explained as the stress-induced-ordering of TiO clusters by the stress fields of moving dislocations.     

Structuration and Destructuration Behavior of Cement-Treated Singapore Marine Clay

This paper examines the structuration and destructuration characteristics of cement-treated Singapore marine clay and their relation to the observed microstructural behavior. The pozzolanic reaction is found to be very significant up to curing periods of 1?year, and thus the unconfined compressive strength increases notably leading to the formation of more structured treated clay. Due to the effect of structuration (existing of cementation bond), the yield stress increases resulting in an expansion of the yield surface and failure envelope under compression and shearing. The microstructural observation of treated clay structure at various stress levels from one-dimensional consolidation shows that destructuration (breaking of cementation bond) is progressive; the largest intercluster voids being the first affected. As the consolidation proceeds, both inter and intracluster voids are affected. Consolidated undrained triaxial results reveal that complete destructuration only takes place on the shear plane at which the clay–cement cluster crushes.     

New Failure Load Criterion for Large Diameter Bored Piles in Weathered Geomaterials

In the literature, various “failure criteria” or methods of estimating the failure load in pile loading tests have been proposed. The criteria, based on varying assumptions, were intended for different methods of pile testing and were verified on tests of a variety of pile types and sizes. Most of the criteria were not developed for slow maintained loading tests of large-diameter (greater than 0.6 m) and long bored piles. Piles of this kind have considerable resistance, and it is often impractical to reach failure load as defined by the various criteria. In this paper, a total of 38 large-diameter bored piles (drilled shafts) that were tested, ranging from 0.6 to 1.8 m in diameter, varying from 12 to 66 m in depth, and founded in weathered geomaterials (rocks and saprolites), are critically reviewed and studied. Among them, a selection of seven pile load tests is examined in detail by using different existing failure criteria and specifications. The tests were chosen for their high degree of mobilization of pile capacity and the availability of reliable load-movement relationships. Specific aspects of pile behavior, such as the mobilization of toe resistance and shaft shortening, are also investigated using 31 loading tests to develop a new failure load criterion. The writers were heavily involved with the construction, testing, and analysis of 15 of the 38 piles. From the results of the study, a new nonsubjective, semiempirical method is proposed for estimating the approximate interpreted failure loads for piles founded in weathered geomaterials. The method is based on a moderately conservative estimation of the movement required to mobilize toe resistance and incorporates observations of shaft shortening from pile loading tests. Generally, the new method may allow more effective and consistent designs for large-diameter bored piles in weathered geomaterials.     

Triaxial Compression of Sand Reinforced with Fibers

Results from drained triaxial compression tests on specimens of fiber-reinforced sand are reported. It is evident that the addition of a small amount of synthetic fibers increases the failure stress of the composite. This effect, however, is associated with a drop in initial stiffness and an increase in strain to failure. Steel fibers did not reduce initial stiffness of the composite. The increase in failure stress can be as much as 70% at a fiber concentration of 2% (by volume) and an aspect ratio of 85. The reinforcement benefit increases with an increase in fiber concentration and aspect ratio, but it also depends on the relative size of the grains and fiber length. A larger reinforcement effect in terms of the peak shear stress was found in fine sand, compared to coarse sand, when the fiber concentration was small (0.5%). This trend was reversed for a larger fiber concentration (1.5%). A model for prediction of the failure stress in triaxial compression was developed. The failure envelope has two segments: a linear part associated with fiber slip, and a nonlinear one related to yielding of the fiber material. The analysis indicates that yielding of fibers occurs well beyond the stress range encountered in practice. The concept of a macroscopic internal friction angle was introduced to describe the failure criterion of a fiber-reinforced sand. This concept is a straightforward way to include fiber reinforcement in stability analyses of earth structures.     

Bond between Carbon Fiber Reinforced Polymer Bars and Concrete. I: Experimental Study

The bond characteristics of four different types of carbon fiber reinforced polymer (CFRP) rebars (or tendons) with different surface deformations embedded in lightweight concrete were analyzed experimentally. In a first series of tests, local bond stress-slip data, as well as bond stress-radial deformation data, needed for interface modeling of the bond mechanics, were obtained for varying levels of confining pressure. In addition to bond stress and slip, radial stress and radial deformation were considered fundamental variables needed to provide for configuration-independent relationships. Each test specimen consisted of a CFRP rebar embedded in a 76-mm-(3 in.)-diam, 102-mm-(4 in.)-long, precracked lightweight concrete cylinder subjected to a constant level of pressure on the outer surface. Only 76 mm (3 in.) of contact were allowed between the rebar and the concrete. For each rebar type, bond stress-slip and bond stress-radial deformation relationships were obtained for four levels of confining axisymmetric radial pressure. It was found that small surface indentations were sufficient to yield bond strengths comparable to that of steel bars. It was also shown that radial pressure is an important parameter that can increase the bond strength almost threefold for the range studied. In a second series of tests, the rebars were pulled out from 152-mm-(6 in.)-diam, 610-mm-(24 in.)-long lightweight concrete specimens. These tests were conduced to provide preliminary data for development length assessment and model validation (Part II).     

General Strength Criterion for Geomaterials Including Anisotropic Effect

The strengths of geomaterials and their variation under different factors are investigated in this paper. First, a general isotropic variation of a strength criterion is proposed for describing the critical state and peak strengths of geomaterials. Second, the proposed criterion is extended to describe the effect of anisotropy on the peak strength. After an analysis of experimental data, the hypothesis is made that the failure of an element of geomaterial generally occurs in a particular plane when the applied shear stress in that plane reaches the shear resistance of the material. Therefore, the variation of the peak strength of anisotropic materials should be described in terms of the stress tensor applied, a vector parameter defining the position of the potential failure plane of the material, and the material properties. A general failure criterion for geomaterials with cross anisotropy is obtained then from the proposed isotropic strength criterion. The proposed criterion is demonstrated to well represent both the isotropic and anisotropic strengths of various geomaterials. Finally, a general anisotropic criterion is introduced.     

Influence of Clod-Size and Structure on Wetting-Induced Volume Change of Compacted Soil

Volume changes due to wetting may occur in naturally deposited soils as well as earthen construction (e.g., compacted fills or embankments). Depending on the stress level, some soils exhibit increase in volume upon wetting (swell) while others may exhibit decrease in volume upon wetting (collapse). The work described in this paper focused on wetting-induced volume changes in compacted soils. Motivation for this work stemmed from observations of earthen structures that exhibit problematic behavior under wetting conditions, even though soils were compacted to engineering specifications (i.e., at or above minimum density and within moisture content ranges). Not only is this problematic behavior a concern but also the laboratory tests used to predict settlement of constructed facilities may not properly model the actual behavior of soil compacted under field conditions. For example, settlements experienced by compacted fills may be different from settlement predictions based on one-dimensional oedometer tests. These differences are partly related to the variations in the soil structure in tested specimens that arise because soil clods compacted in the laboratory are smaller than soil clods compacted in the field. The term “soil structure” includes the combined effects of soil fabric and interparticle forces. “Fabric” generally refers to the geometric arrangement of particles, whereas interparticle forces include physical and physicochemical interactions between particles. The soil structure in this case is associated with specimen preparation methods and is influenced by several factors including soil composition (including pore water chemistry), compaction method, clod sizes, initial moisture condition of clods, dry density or void ratio, and compaction moisture content. A laboratory research study was conducted to investigate the influence of variations in clod-size and structure on one-dimensional volume change, with emphasis on wetting-induced volume change, for nine different fine-grained soils. The results of the study suggest that the influence of structure in one-dimensional oedometer tests depends on soil type and nature of the clods in the compacted soil. Clayey soils appear to be influenced more by differences in structure, whereas silts or clayey sands of low plasticity (PI<10) do not appear to suffer as much from structure effects in one-dimensional oedometer tests. This is attributed to more extensive clod development in clayey soils. Furthermore, the moisture condition of clods appears to have an important influence on volume change behavior.     

Scale Transition in Steel-Concrete Interaction. I: Model

This paper deals with the analysis of reinforced concrete (RC) structures with special emphasis on modeling of the interaction between concrete and reinforcement. A new mode for consideration of the response of the composite material at the member (structural) scale is proposed. It is obtained from extension of the fracture energy concept, originally developed for the simulation of cracking of plain concrete, to reinforced concrete. Hereby, the fracture energy related to the opening of primary cracks is increased in order to account for bond slip between steel and concrete. This increase is determined from the distribution of bond slip by means of a one-dimensional composite model introduced at the bar scale. The model consists of steel bars and the surrounding concrete. Between these two components, a nonlinear bond stress–bond slip relation is considered. The obtained results at the bar scale, such as the average crack spacing between adjacent cracks and the load-displacement response of the composite material, form the basis for determination of the increase of the fracture energy at the member scale. The performance of the proposed transition of the steel-concrete interaction from the bar scale to the member scale is assessed by means of reanalysis of experiments performed on RC bars. The application of the respective material model for reinforced concrete to real-life engineering structures is reported in Part II of this series.     

Possibility of Postliquefaction Flow Failure due to Seepage

This study examines the postliquefaction flow failure mechanism, in which shear strain develops due to seepage upward during the redistribution of excess pore water pressure after an earthquake. The mechanism is addressed as both a soil element and a boundary value problem. Triaxial tests that reproduce the stress state of a gentle slope subjected to upward pore water inflow were performed, with the results showing that shear strain can increase significantly after the stress state reaches the failure line. In addition, when subject to equivalent volumetric strain, shear strain is considerably larger in loose sand conditions than in dense sand. Compared with consolidated and drained test results, the dilatancy coefficient β, which indicates the rate of dilation, is the same as that obtained from pore water inflow tests. Torsional hollow cylinder tests were also performed to ascertain the limit of dilation of sand specimens. It was found that the β values are nonlinear in behavior. In addition, a postliquefaction flow failure mechanism based on one-dimensional consolidation theory and shear deformation behavior as a result of pore water inflow is proposed.     

Front and Side View Image Correlation Measurements on FRP to Concrete Pull-Off Bond Tests

Understanding the transfer of force by bond between externally bonded fiber-reinforced polymer (FRP) reinforcement and concrete is an important step in formulating good models for predicting debonding failures observed in externally bonded reinforcement strengthened systems. In this paper, a 3D optical displacement measurement system was used to capture the full-field displacements from the front and side view in pull-off bond specimens. The experiments were carried using six specimens with carbon FRP (CFRP) strips having different axial stiffnesses but a constant bond length to the concrete substrate. Using the optical measurements, it was possible to obtain the in-plane displacement or slip and the out-of-plane displacement or separation between the CFRP strip and the concrete. It was demonstrated, that the usual assumption of pure shear stresses in such pull-off tests is not true and that the bond behavior is a two-dimensional problem involving shear and peeling stresses. The bond behavior in CFRP strip to concrete pull-off tests was characterized by three stages: (1) the initiation of the first crack; (2) the initiation of debonding; and (3) failure by complete debonding. Based on the test results it was found that there was a dependency between the maximum bond shear stress, the maximum fracture energy of the FRP-concrete interface, and the stiffness of the FRP. However, the slip values after initiation of debonding (Stage 2) were independent of the FRP stiffness. The measured anchorage force and anchorage length were in good agreement with predictions from existing code equations.     

Bond Behavior of Near-Surface Mounted CFRP Laminate Strips under Monotonic and Cyclic Loading

Near-surface mounted (NSM) carbon fiber reinforced polymer (CFRP) laminate strips are used to increase the load-carrying capacity of concrete structures. This is done by inserting the CFRP strips into slits made in the concrete cover of the elements to be strengthened and gluing the strips to the concrete with an epoxy adhesive. In several cases the NSM technique has substantial advantages when compared with externally bonded laminates. To assess the bond behavior between the CFRP and concrete under monotonic and cyclic loading, an experimental program was carried out based on a series of pullout-bending tests. The influence of the bond length and loading history on the bond behavior was investigated. In this work the details of the tests are described and the obtained results discussed. Using the experimental data and an analytical-numerical strategy, a local bond stress-slip relationship was determined. A finite-element analysis was performed to evaluate the influence of the adhesive on the global response observed in the pullout-bending tests.     

A comparison of the stresses developed in tension, shear peel, and torsion strength testing of direct bonded orthodontic brackets

Strength testing of direct bonded orthodontic bracket systems is commonly performed with tension, shear peel, or torsion loads. In general, the results of these tests are reported as an average stress that is computed by dividing the experimentally measured force at failure by the area of the bracket base. The average value, obtained in this manner, implies an evenly distributed stress field. In this project, finite element model (FEM) calculations were used to determine the more realistic stress distributions generated within the cement. The results indicate that the three loading modes produce very different non-uniform stress field patterns. Furthermore, the calculated stress peaks and the stress component proportions depend on the loading method. It was therefore concluded that the manner of loading affects the strength measurements and that the average stress does not adequately characterize bond strength.     

Pullout Behavior of Geogrid by Test and Numerical Analysis

This paper presents a study of geogrid pullout behavior in laboratory pullout tests and finite element modeling of the laboratory pullout tests. The pullout tests and the finite element method (FEM) analyses were carried out on two geogrid types with different stiffness values in dense sand under different overburden pressures. The pullout test results show that the geogrid behavior can be categorized into three types based on the bond stress distributions. The FEM results show reasonable agreement not only with the pullout force against the geogrid displacement, but also with the distributions of geogrid displacements, strains, tensile forces, and bond stresses along the geogrid length during deformation. This research demonstrates that the deformation characteristics of geogrids play an important role in the pullout tests while the interface properties play a significant role in the FEM simulations of geogrid pullout behavior. A method to obtain suitable interface parameters for designing of actual reinforced structures from the laboratory pullout tests is provided.     

不同应力条件下硬岩强度与破裂特性试验研究

深部工程围岩内的岩石可能处于一维、二维和三维应力状态下,分别对应室内单轴压缩、双轴压缩和真三轴压缩试验中岩样的应力状态。通过开展单轴、双轴和真三轴压缩试验,系统研究了不同应力状态和水平下岩石非常规破坏的发生机制。不同高宽比和宽厚比岩样的单轴压缩试验结果表明：随着岩样厚度的增加,单轴抗压强度单调增加;随着岩样高度的增加,单轴抗压强度往往先增加后减小,且矮薄岩样更容易发生岩爆和板裂等非常规破坏。双轴或真三轴压缩试验中岩样的抗压强度均表现出明显的中间主应力效应。在相同最小主应力下,随着中间主应力的增加,岩样的双轴抗压强度和真三轴抗压强度均呈先增加后减小的变化趋势,双轴抗压强度增长率则呈先减小而后小幅增大的趋势。通过定义强度增量参数ν和中间主应力位置参数λ构建了指数岩石真三轴强度准则。低围压限制、非对称围压限制和短裂纹扩展路径是引起岩石非常规破坏的主要条件。