Heat treatment, microstructure and properties of 75Cr1 steel, for use in heavy loaded elements

This study aims to optimize the heat treatment of tool steel 75Cr1 which is used for heavy loaded elements in transmissions. A salt bath was used to quench and temper the steel at different temperatures. Mechanical tests and microstructural characterization were done to define the heat treatment parameters corresponding to the optimal performance of the elements. Optical microscopy, electron back scatter diffraction and x-ray diffraction were used to characterize the microstructure, while tensile tests and toughness tests were employed to determine the mechanical properties after different heat treatments. It was found that the yield strength decreases with increasing annealing temperature and that the toughness decreases with increasing annealing time and temperature. The changes of the mechanical properties are discussed in relation with the thermal treatment and the corresponding microstructures

Seismic Analysis of Post-tensioned Gravity Dams using Scaled Boundary Finite Element Method

Dams are hydraulic structures built across rivers to create reservoirs, which provide essential services to society such as flood control, human water supply, and electricity generation. A dam shall be designed to ensure stability against overturning and sliding caused by the hydro-pressure of the reservoir. A common type of dam is the concrete gravity dam that mainly relies on its self-weight and resistance to sliding on the foundation to maintain its stability. Installing post-tensioned anchors (PTAs) is a practical and cost-effective technique in dam engineering. It provides an additional stabilizing force and improves the shear resistance at the dam-foundation interface. Seismic safety evaluation of post-tensioned concrete gravity dams is necessary for new dam designs or strengthened existing dams to guarantee that the structures will survive at specified seismic hazard levels. This thesis presents the development of an efficient numerical framework for the seismic analysis of post-tensioned concrete gravity dam-reservoir-foundation systems. This framework is realized by implementing the scaled boundary finite element method (SBFEM) in the well-known commercial FEM software ABAQUS as user elements (UEL). Polytope elements (polygonal elements in 2D and polyhedral elements in 3D) are as versatile as standard FEM solid elements, while they provide greater flexibility in mesh generation for bounded domains. Unbounded user elements (UEL) are derived to model wave propagation in far-fields. An unbounded UEL only requires discretization with a small number of faces at the near-field/far-field interface and can rigorously satisfy the radiation condition at infinity. The ABAQUS software enhanced with the UELs is employed for two-dimensional seismic analysis of gravity dams, overcoming the difficulties encountered in standard FEM, for example, local mesh refinement for geometrical features, generating matching interfacial meshes for weak joints, and simulation of anchor-structure interactions. The overall system consists of a near-field containing the dam body and its neighboring reservoir and foundation, and a far-field of the reservoir and foundation continua. The near-field dam and foundation are discretized as quadtree meshes assigned with polygonal UELs. Quadtree meshes allow rapid and smooth transitions in element size, which facilitates the local mesh refinement for dam lift joints, anchor boreholes, drainage systems, etc. An unbounded UEL represents the far-field foundation in terms of displacement unit-impulse response matrices. It captures free-field motions and transfers them as equivalent seismic inputs acting at the near-field/far-field interface. The reservoir is modeled by ABAQUS built-in acoustic elements. At the far end of the reservoir, a non-reflecting acoustic boundary embedded in ABAQUS is employed to satisfy the radiation condition of the unbounded reservoir. Comprehensive considerations have been taken in the numeral simulation of post-tensioned gravity dams, such as weak joint behaviors, anchor-structure interaction, and concrete damage. Weak joints in a concrete gravity dam, such as the dam-foundation interface and the dam lift joints, are the most likely places where the sliding and cracking occur. A cohesive-frictional contact scheme is utilized to simulate the non-linear behaviors of these weak joints. A PTA is usually grouted with the structure along a portion of the length, called bond length. At the grouting interface, the bond stress develops with the slippage between the anchor and structure, and then transfers the prestressing in the anchor to the structure. Cohesive elements connected with the anchor and structure are generated along the bond length to simulate the bond-slip interaction. A Mazars' damage evolution law for dynamic loading is applied to simulate the quasi-brittle behaviors of the concrete. To avoid mesh sensitivity, a partially regularized local damage model is introduced into this application. Automatic re-meshing algorithms to generate conforming interfacial meshes are developed for the sake of the simulation of interfacial problems. For the weak joints, the domains in contact are allowed to be discretized individually, and then the existing meshes at the interfaces are re-meshed to be node-to-node matching. The anchor is embedded automatically in the structure by inserting additional nodes into the existing structural meshes along the anchor layout. By duplicating the inserted nodes and connecting the duplicated nodes, beam elements conforming with structural meshes are formed naturally. These re-meshing procedures are easily operated on the polygonal meshes allowing arbitrary numbers of nodes and edges. Cohesive elements can be generated with the matching nodes at interfaces, and no constraints are required to connect them with the surrounding elements. The proposed approach is verified by performing seismic analysis of a post-tensioned gravity dam with simple geometry, and comparing the results obtained from the model using ABAQUS built-in elements. The advantages of the proposed approach in handling complex problems are demonstrated through dams with multiple inclined anchors. Applications of this method can be extended to three-dimensional cases, and composite materials with randomly spread fiber inclusions

Research in Structures and Dynamics, 1984

A symposium on advanced and trends in structures and dynamics was held to communicate new insights into physical behavior and to identify trends in the solution procedures for structures and dynamics problems. Pertinent areas of concern were (1) multiprocessors, parallel computation, and database management systems, (2) advances in finite element technology, (3) interactive computing and optimization, (4) mechanics of materials, (5) structural stability, (6) dynamic response of structures, and (7) advanced computer applications

Optimizing Powder Metallurgy Methods: Carbon Nanotube Metal Composites

Since their discovery in 1991, CNTs have revolutionized the composites industries and continue to show great promise as exotic reinforcers in a variety of materials. Extensive studies have been carried out by researchers to impart the unique physical properties of CNTs in metal systems and understand their interfacial interactions. However, the thermal, electrical and mechanical advantages of CNTs in metal systems are limited because of their tendency to aggregate together in rope-like bundles. The objective of this research is two-fold: 1) optimize carbon nanotube (CNT) dispersal mechanisms in the fabrication of aluminum carbon nanotube metal-matrix composites (Al-CNT/MMCs) via the powder metallurgy method (PM), 2) characterize the effects of CNT morphology on the physical properties of Al-CNT/MMCs

Index to 1983 NASA Tech Briefs, volume 8, numbers 1-4

Short announcements of new technology derived from the R&D activities of NASA are presented. These briefs emphasize information considered likely to be transferrable across industrial, regional, or disciplinary lines and are issued to encourage commercial application. This index for 1983 Tech Briefs contains abstracts and four indexes: subject, personal author, originating center, and Tech Brief Number. The following areas are covered: electronic components and circuits, electronic systems, physical sciences, materials, life sciences, mechanics, machinery, fabrication technology, and mathematics and information sciences