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Context: Micropiles are used as reinforcing or underpinning elements, a function that has been tested and analyzed by different authors. In Colombia, the use of this technique is limited to the geotechnics expert’s own concepts, which consists of empirical knowledge and successful experiences. Consequently, it is necessary to compile key variables and analytical and numerical methodologies for a better understanding of this technique. Method: This document presents a state of the art supported by articles and research aimed at analyzing and evaluating the behavior of micropiles as load capacity recovery elements in groups of piles with defects. Results: The findings identified in the consulted documents are synthesized as a theoretical support that will become a starting point for future research in this field of geotechnical engineering. Conclusions: It is possible to evidence that micropiles have a positive influence on the load recovery of existing foundations, thus demonstrating the importance of conducting field tests and analyses directly in Colombia, considering the specific strata of soils found in the country., Contexto: Los micropilotes son utilizados como elementos de refuerzo del suelo o de recalce, dicha función es comprobada y analizada por diferentes autores. En Colombia el uso de esta técnica está limitado a conceptos propios del geotecnista que involucran conocimientos empíricos y experiencias exitosas. En consecuencia, es necesaria la compilación de variables clave y metodologías analíticas y numéricas para la mejor comprensión de esta técnica. Método: En este documento se presenta un estado del arte sustentado en artículos e investigaciones que tienen como objeto el análisis y la evaluación del comportamiento de los micropilotes como elementos recuperadores de capacidad de carga en grupos de pilotes que presentan defectos. Resultados: Los hallazgos identificados en los documentos consultados logran ser sintetizados como un soporte teórico que se convierte en un punto de partida para futuras investigaciones en este campo de la ingeniería geotécnica. Conclusiones: Se logra evidenciar que los micropilotes tienen una influencia positiva en la recuperación de carga de fundaciones existentes, demostrando la importancia de efectuar ensayos y análisis de campo directamente en Colombia, considerando los estratos de suelos específicos que se encuentran en el país.

The accuracy of multiscale modeling approaches for the analysis of heterogeneous materials hinges on the representativeness of the micromodel. One of the issues that affects this representativeness is the application of appropriate boundary conditions. Periodic boundary conditions are the most common choice. However, when localization takes place, periodic boundary conditions tend to overconstrain the microscopic problem. Weakly periodic boundary conditions have been proposed to overcome this effect. In this study, the effectiveness of weakly periodic boundary conditions in restoring transverse isotropy of representative volume elements (RVE) for a fiber-reinforced composite with elastoplastic matrix is investigated. The formulation of weakly periodic boundary conditions is extended to allow for force-controlled simulations where a uniaxial stress can be applied. A series of simulations is performed where the orientation of applied stress is gradually varied and the influence of this orientation on the averaged response is examined. An original method is presented to test the correlation between the ultimate principal stress and average localization angle of shear bands within an RVE. It is concluded that weakly periodic boundary conditions alleviate anisotropy in the RVE response but do not remove it., Applied Mechanics

レール支持部に存在する力学的非線形性が,レールの共振周波数と軸力との関係に及ぼす影響について考察した.そのために,まくらぎ位置を節とするレールの共振モードを対象に,調和加振下での定常解を与える汎関数をLagrangianの時間平均より求めた.当該汎関数より定常解や共振条件を導出し,共振周波数-軸力関係の加振力に対する鋭敏性を議論した.また,レール加振を模擬した数値解析を通し,本理論の妥当性を検証した., The influence of mechanical nonlinearity in rail supports on the resonant frequency-rail axial load relationship is investigated. To achieve this, a harmonic load exciting the pinned-pinned resonance mode is considered, and a functional which derives a steady-state solution is formulated by time average of the Lagrangian. In the theoretical study, the nonlinearity is introduced in the rotational stiffness at each sleeper support. Based on the variations of the functional, the sensitivity of the above relationship to the load amplitude is revealed. Furthermore, through numerical analyses, validity of the theory is proved.

This thesis studied the effect of elevated temperature (600 C) on the fretting fatigue behavior of IN-100. First, fretting and plain fatigue S-N curves were determined over a large range of applied stress at an identical stress ratio of 0.03 at 600 C and for fretting tests, with a constant contact load. The partial slip condition was verified by using hysteresis between shear force and axial stress techniques. The contact width and the crack initiation location were observed for all of the fretting specimens. The contact widths were consistent with expected values and the crack initiation location was at the trailing edge of contact for all fretting specimens. This study compared its high temperature results to room temperature testing accomplished by this study as well as room temperature data from a previous study using IN-100 with an identical microstructure and another room temperature study of IN-100 with a coarser microstructure. It was found that fretting fatigue reduces the cycles to failure compared to plain fatigue at elevated temperature but a more pronounced reduction was noticed than with room temperature. It was also found that the elevated temperature environment increased the both the plain and fretting fatigue life as compared to similar stress levels at room temperature. The study found that elevated temperature does have an effect on crack initiation and crack propagation as compared to room temperature environment. The higher temperature allowed a longer initiation and crack propagation time which strongly suggests the fatigue life increases with temperature up to 600 C. The creation of oxide films and temperature induced softening or plasticity were both found to act as crack closure mechanisms in another nickel based superalloy, IN-718, when fatigued in the high temperature environment., The original document contains color images.

Necking is a strain localization phenomenon that can cause catastrophic fracture in metal forming. Therefore, the prediction of necking is important. In the present work, an elastoplastic constitutive equation accounting for isotropic hardening coupled with damage is implemented in the finite element code ABAQUS. A damage variable, that provides a mesoscopic description of material degradation, is used to quantify the degree of damage due to excessive strain. Using the implicit integration scheme, a cylindrical rod loaded under uniaxial tension is successfully simulated by the finite element code with a specially designed user-defined material subroutine (UMAT). In addition, the distribution of damage caused by the deformation is computed.

A numerical model that can simulate the fracture process of plain concrete under variety of plane stress conditions is introduced and used to simulate the fracture patterns of concrete specimens subjected to different combinations of biaxial loadings. In this numerical model, the concrete is assumed to be a three-phase composite composed of matrices, aggregates and interfaces between them. The finite element program is employed as the basic stress analysis tool when the elastic damage mechanics is used to describe the constitutive law of meso-level element and the maximum tensile strain criterion and Mohr-Coulomb criterion are utilised as damage threshold. The numerical simulation predicts well the strength envelope and cracking patterns of concrete specimens under different biaxial loading conditions.

In this paper, the cavitation behavior of a commercially available coarse-grained Al5052 alloy under hot uniaxial and equibiaxial tension was studied for the first time. Tensile specimens were strained to different strain levels at various initial strain rates and temperatures. The results revealed that there was no significant difference in the cavitation size distribution under different stress states, and the spread of cavity size and the total number of cavities were found to increase with increasing strain. In microstructural examination, some voids were observed at the interface between the intermetallic particles and the matrix, and they were growing continuously with increasing strain. Quantitative analysis revealed that the cavity growth was essentially plasticity-controlled. A lower cavitation growth rate was observed for the Al5052 alloy as compared to that of the superplastic Al5083 alloy, which could not explain the relatively low elongation value of 194% of Al5052. It was considered that the hot formability of Al5052 did not only relate to its cavitation behavior, but also its strain rate sensitivity value.

This study examined the constructibility, feasibility, and flexural behavior of a new and untested post-tensioned, dry-stacked concrete masonry wall system--know as Formwall --as proposed by the National Concrete Masonry Association. The evaluation was conducted through mathematical predictions, wall construction, and experimental testing. To substantiate the findings, comparisons were made between the dry-stacked Formwall system, a conventional nonreinforced (plain masonry) wall panel with Type S mortar, and a dry-stacked wall panel using conventional six-inch concrete masonry units. Based on the test results of this study, the proposed Formwall system has virtually no structural capacity due primarily to stability issues stemming from problems with the geometric shape of the face shells and lack of composite action between the face shells and ties. Notwithstanding, it is the author's opinion that there is merit in pursuing the development of a dry-stacked masonry system. And when perfected, post-tensioned dry-stacked masonry could result in significant savings for roadside barriers, landscaping elements, basement walls for low income housing, and temporary structures.

This report contains Appendices I through IX which consist of data obtained from soil tests. The cell pressure during K sub o consolidation was 620 kPa resulting in a vertical stress of 1240 kPa. The rebound was such that the excess vertical stress was removed and the cell pressure reduced by a factor of 3. This operation leads to an overconsolidation ratio of 4 when reference is made to the mean effective stress. For isotropic specimens consolidation and rebound were spherical in nature. The specimen was hydrostatically consolidated under 620 kPa and rebounded to 207 kPa., See also main report ADA250896.

The 'cyclic creep' of commercially pure titanium was determined as a function of cyclic torsional strain amplitude and constant axial stress. For a fixed axial stress, the axial strain is proportional to the square of the cyclic torsional strain. Previous experiments have indicated a linear proportionality, but these more expensive experiments show conclusively that there is a quadratic proportionality. For a fixed peak cyclic torsional strain amplitude the axial strain is linearly proportional to the axial stress. In these tests no point of instability in the cyclic strain is observed as either axial stress or cyclic torsional strain amplitude is increased. This indicates that no truly elastic response is observed in this alloy under these cyclic test conditions.

Soil tests were conducted to provide information on the influence of degree of saturation on high-pressure stress-strain relations of undisturbed and remolded soils and to provide input data for computer codes concerning the relation between stress and strain invariants at high pressures. The test results presented herein show that large strains do not develop at high pressures in fine-grained soils such as silt and clay. The test program consisted of 12 one-dimensional tests on 4 specimens each of undisturbed and remolded silty clay, and 4 specimens of remolded silty clay, and 4 specimens of remolded sandy silt. In all tests the radial strain was essentially zero. Axial and radial stresses and axial strain were measured. The tests were carried to an axial stress of 20,000 psi unless soil extrusion occurred at a lower stress.

The maximum strength of initially imperfect, axially compressed, circular cylindrical shells has been studied with the use of Reissner's variational principle, von Karman-Donnell shell theory, and a deformation theory of plasticity. The results of the present analysis reflect families of load-end shortening curves for long circular cylinders. For a given material, each curve relates not only to an imperfection parameter which provides a loading path into the inelastic range but also to the radius-to-wall thickness ratio. Significant maximum strength reductions are obtained, relative to predictions based on linear-elastic theory, for specific materials in the range of radius-to-thickness ratios of practical interest. (Author)

Analytical solutions that describe the elastic-plastic behavior of a beam-column loaded at the ends and the mid-span are presented. Much work has been done on the elastic solution, so the current effort is to extend this work to the inelastic range. Difficulties associated with the analysis are outlined, and the method of attack in overcoming these difficulties is described. The theoretical analysis of the six different cases which correspond to different stages of plastification within the beam-column is discussed. Interaction curves for combinations of axial force and lateral load that can be safely supported by the beam-column of rectangular and wide-flange cross sections are presented. The beam-column with an initial curvature is also discussed. (Author), Report on Space Frames with Biaxial Loading in Columns.

A program is identified for strength testing of hull girder models representative of longitudinally framed ship construction. The purpose of the tests is to generate data (for correlation with theory where available) to provide the basis for engineering design of the primary structure of the hull girder. The major loads are longitudinal compression induced by primary hull bending, normal pressure from the seam and athwartship compression induced by the horizontal pressure on the sidewalls. (Author)