Temperature and composition profile during double-track laser cladding of H13 tool steel

Multi-track laser cladding is now applied commercially in a range of industries such as automotive, mining and aerospace due to its diversified potential for material processing. The knowledge of temperature, velocity and composition distribution history is essential for a better understanding of the process and subsequent microstructure evolution and properties. Numerical simulation not only helps to understand the complex physical phenomena and underlying principles involved in this process, but it can also be used in the process prediction and system control. The double-track coaxial laser cladding with H13 tool steel powder injection is simulated using a comprehensive three-dimensional model, based on the mass, momentum, energy conservation and solute transport equation. Some important physical phenomena, such as heat transfer, phase changes, mass addition and fluid flow, are taken into account in the calculation. The physical properties for a mixture of solid and liquid phase are defined by treating it as a continuum media. The velocity of the laser beam during the transition between two tracks is considered. The evolution of temperature and composition of different monitoring locations is simulated.

Report of the Inter-American Tropical Tuna Commission for the year 1959

ENGLISH: The Inter-American Tropical Tuna Commission was established under authority of a Convention, between the Governments of the Republic of Costa Rica and the United States of America, which entered into force in 1950. The Convention provides for the adherence, by exchange of correspondence with existing members, of other governments having an interest in the tuna and tuna-bait resources of the Eastern Tropical Pacific Ocean. The Republic of Panama adhered to the Convention in 1953. SPANISH: La Comisión Interamericana del Atún Tropical fue establecida en virtud de una Convención suscrita entre los Gobiernos de la Republica de Costa Rica y de los Estados Unidos de America, que entro en vigencia en 1950. La Convención esta abierta para que otros gobiernos interesados en los recursos de atún y de carnada para la pesca de este en el Océano Pacifico Oriental Tropical puedan adherirse a la misma mediante intercambio de correspondencia con los Gobiernos Miembros. La Republica de Panamá se adhirió a la Convención en 1953. (PDF contains 156 pages.)

Three-dimensional nonlinear inelastic analysis of steel moment-frame buildings damaged by earthquake excitations

The Northridge earthquake of January 17, 1994, highlighted the two previously known problems of premature fracturing of connections and the damaging capabilities of near-source ground motion pulses. Large ground motions had not been experienced in a city with tall steel moment-frame buildings before. Some steel buildings exhibited fracture of welded connections or other types of structural degradation.

A sophisticated three-dimensional nonlinear inelastic program is developed that can accurately model many nonlinear properties commonly ignored or approximated in other programs. The program can assess and predict severely inelastic response of steel buildings due to strong ground motions, including collapse.

Three-dimensional fiber and segment discretization of elements is presented in this work. This element and its two-dimensional counterpart are capable of modeling various geometric and material nonlinearities such as moment amplification, spread of plasticity and connection fracture. In addition to introducing a three-dimensional element discretization, this work presents three-dimensional constraints that limit the number of equations required to solve various three-dimensional problems consisting of intersecting planar frames.

Two buildings damaged in the Northridge earthquake are investigated to verify the ability of the program to match the level of response and the extent and location of damage measured. The program is used to predict response of larger near-source ground motions using the properties determined from the matched response.

A third building is studied to assess three-dimensional effects on a realistic irregular building in the inelastic range of response considering earthquake directivity. Damage levels are observed to be significantly affected by directivity and torsional response.

Several strong recorded ground motions clearly exceed code-based levels. Properly designed buildings can have drifts exceeding code specified levels due to these ground motions. The strongest ground motions caused collapse if fracture was included in the model. Near-source ground displacement pulses can cause columns to yield prior to weaker-designed beams. Damage in tall buildings correlates better with peak-to-peak displacements than with peak-to-peak accelerations.

Dynamic response of tall buildings shows that higher mode response can cause more damage than first mode response. Leaking of energy between modes in conjunction with damage can cause torsional behavior that is not anticipated.

Various response parameters are used for all three buildings to determine what correlations can be made for inelastic building response. Damage levels can be dramatically different based on the inelastic model used. Damage does not correlate well with several common response parameters.

Realistic modeling of material properties and structural behavior is of great value for understanding the performance of tall buildings due to earthquake excitations.

Earthquake response of steel braces and braced steel frames

This thesis consists of three parts. Chapter 2 deals with the dynamic buckling behavior of steel braces under cyclic axial end displacement. Braces under such a loading condition belong to a class of "acceleration magnifying" structural components, in which a small motion at the loading points can cause large internal acceleration and inertia. This member-level inertia is frequently ignored in current studies of braces and braced structures. This chapter shows that, under certain conditions, the inclusion of the member-level inertia can lead to brace behavior fundamentally different from that predicted by the quasi-static method. This result is to have significance in the correct use of the quasi-static, pseudo-dynamic and static condensation methods in the simulation of braces or braced structures under dynamic loading. The strain magnitude and distribution in the braces are also studied in this chapter.

Chapter 3 examines the effect of column uplift on the earthquake response of braced steel frames and explores the feasibility of flexible column-base anchoring. It is found that fully anchored braced-bay columns can induce extremely large internal forces in the braced-bay members and their connections, thus increasing the risk of failures observed in recent earthquakes. Flexible braced-bay column anchoring can significantly reduce the braced bay member force, but at the same time also introduces large story drift and column uplift. The pounding of an uplifting column with its support can result in very high compressive axial force.

Chapter 4 conducts a comparative study on the effectiveness of a proposed non-buckling bracing system and several conventional bracing systems. The non-buckling bracing system eliminates buckling and thus can be composed of small individual braces distributed widely in a structure to reduce bracing force concentration and increase redundancy. The elimination of buckling results in a significantly more effective bracing system compared with the conventional bracing systems. Among the conventional bracing systems, bracing configurations and end conditions for the bracing members affect the effectiveness.

The studies in Chapter 3 and Chapter 4 also indicate that code-designed conventionally braced steel frames can experience unacceptably severe response under the strong ground motions recorded during the recent Northridge and Kobe earthquakes.

A retrofitting framework for pre-Northridge steel moment-frame buildings

In the 1994 M_w 6.7 Northridge and 1995 M_w 6.9 Kobe earthquakes, steel moment-frame buildings were exposed to an unexpected flaw. The commonly utilized welded unreinforced flange, bolted web connections were observed to experience brittle fractures in a number of buildings, even at low levels of seismic demand. A majority of these buildings have not been retrofitted and may be susceptible to structural collapse in a major earthquake.

This dissertation presents a case study of retrofitting a 20-story pre-Northridge steel moment-frame building. Twelve retrofit schemes are developed that present some range in degree of intervention. Three retrofitting techniques are considered: upgrading the brittle beam-to-column moment resisting connections, and implementing either conventional or buckling-restrained brace elements within the existing moment-frame bays. The retrofit schemes include some that are designed to the basic safety objective of ASCE-41 Seismic Rehabilitation of Existing Buildings.

Detailed finite element models of the base line building and the retrofit schemes are constructed. The models include considerations of brittle beam-to-column moment resisting connection fractures, column splice fractures, column baseplate fractures, accidental contributions from ``simple'' non-moment resisting beam-to-column connections to the lateral force-resisting system, and composite actions of beams with the overlying floor system. In addition, foundation interaction is included through nonlinear translational springs underneath basement columns.

To investigate the effectiveness of the retrofit schemes, the building models are analyzed under ground motions from three large magnitude simulated earthquakes that cause intense shaking in the greater Los Angeles metropolitan area, and under recorded ground motions from actual earthquakes. It is found that retrofit schemes that convert the existing moment-frames into braced-frames by implementing either conventional or buckling-restrained braces are effective in limiting structural damage and mitigating structural collapse. In the three simulated earthquakes, a 20% chance of simulated collapse is realized at PGV of around 0.6 m/s for the base line model, but at PGV of around 1.8 m/s for some of the retrofit schemes. However, conventional braces are observed to deteriorate rapidly. Hence, if a braced-frame that employs conventional braces survives a large earthquake, it is questionable how much service the braces provide in potential aftershocks.

Discontinuous yielding and fracture initiation near a notch in mild steel

This thesis presents the results of an experimental investigation of the initiation of brittle fracture and the nature of discontinuous yielding in small plastic enclaves in an annealed mild steel. Upper and lower yield stress data have been obtained from unnotched specimens and nominal fracture stress data have been obtained from specimens of two scale factors and two grain sizes over a range of nominal stress rates from 10^2 to 10^7 lb/in.^2 sec at -111°F and -200°F. The size and shape of plastic enclaves near the notches were revealed by an etch technique.

A stress analysis utilizing slip-line field theory in the plastic region has been developed for the notched specimen geometry employed in this investigation. The yield stress of the material in the plastic enclaves near the notch root has been correlated with the lower yield stress measured on unnotched specimens through a consideration of the plastic boundary velocity under dynamic loading. A maximum tensile stress of about 122,000 lb/in.^2 at the instant of fracture initiation was calculated with the aid of the stress analysis for the large scale specimens of ASTM grain size 8 1/4.

The plastic strain state adjacent to a plastic-elastic interface has been shown to cause the maximum shear stress to have a larger value on the elastic than the plastic side of the interface. This characteristic of dis continuous yielding is instrumental in causing the plastic boundaries to be nearly parallel to the slip-line field where the plastic strain is of the order of the Lüder's strain.

The tensile fracture of mild steel

The fibrous and cleavage tensile fracture of an annealed mild steel was investigated. Round tensile specimens of two geometries, one straight and one with a circumferential notch, were pulled at temperatures between room temperature and liquid nitrogen temperature. Tensile fractures occurred at average strains from 0.02 to 0.87. The mechanism of fibrous fracture at room temperature was investigated metallographically. The stress-strain values at which fibrous and cleavage fractures are initiated were determined.

Many fine microcracks, which are associated with pearlite colonies and inclusion stringers, develop prior to fibrous fracture. The macrofracture, which leads to final separation of the tensile specimen, is initiated by the propagation of a microcrack beyond the microstructural feature with which it is associated. Thus, the fibrous fracture of mild steel does not develop by the gradual growth and coalescence of voids that are large enough to be visible in the optical microscope. When the microcracks begin to open and propagate, final fracture quickly follows. Axial cracks are a prominent feature of the macrofracture that forms in the interior of the specimen immediately before final fracture.

The Bridgman distribution of stresses is not valid in a notched tensile specimen. Fibrous and cleavage fractures occur at approximately the same value of maximum tensile stress. When the maximum tensile stress that is necessary for cleavage fracture is plotted against the corresponding maximum tensile strain, the result is an unique locus.

Tarjeta postal enviada por Werner Beinhauer a Justo Garate en 1959

1 tarjeta postal (manuscrita) ; 150x105mm

Quantifying Earthquake Collapse Risk of Tall Steel Braced Frame Buildings Using Rupture-to-Rafters Simulations

This thesis examines collapse risk of tall steel braced frame buildings using rupture-to-rafters simulations due to suite of San Andreas earthquakes. Two key advancements in this work are the development of (i) a rational methodology for assigning scenario earthquake probabilities and (ii) an artificial correction-free approach to broadband ground motion simulation. The work can be divided into the following sections: earthquake source modeling, earthquake probability calculations, ground motion simulations, building response, and performance analysis.

As a first step the kinematic source inversions of past earthquakes in the magnitude range of 6-8 are used to simulate 60 scenario earthquakes on the San Andreas fault. For each scenario earthquake a 30-year occurrence probability is calculated and we present a rational method to redistribute the forecast earthquake probabilities from UCERF to the simulated scenario earthquake. We illustrate the inner workings of the method through an example involving earthquakes on the San Andreas fault in southern California.

Next, three-component broadband ground motion histories are computed at 636 sites in the greater Los Angeles metropolitan area by superposing short-period (0.2~s-2.0~s) empirical Green's function synthetics on top of long-period ($>$ 2.0~s) spectral element synthetics. We superimpose these seismograms on low-frequency seismograms, computed from kinematic source models using the spectral element method, to produce broadband seismograms.

Using the ground motions at 636 sites for the 60 scenario earthquakes, 3-D nonlinear analysis of several variants of an 18-story steel braced frame building, designed for three soil types using the 1994 and 1997 Uniform Building Code provisions and subjected to these ground motions, are conducted. Model performance is classified into one of five performance levels: Immediate Occupancy, Life Safety, Collapse Prevention, Red-Tagged, and Model Collapse. The results are combined with the 30-year probability of occurrence of the San Andreas scenario earthquakes using the PEER performance based earthquake engineering framework to determine the probability of exceedance of these limit states over the next 30 years.

The predatory feeding of Acanthocyclops viridis (Jur.) (Copepoda, Cyclopoida). [Translation from: Trudy Inst.Biol.Vodokhranil. 2(5) 117-127, 1959. ]

The feeding of freshwater copepods, especially cyclopoida, has been poorly covered in research so far. The majority of existing special works on the feeding of cyclopoida illustrate this question only from the qualitative side. The food content of the nauplius of freshwater cyclops has not been studied at all, as also the feeding of adult entomostracans on bacteria. Moreover the question of the suitability of vegetable food for Cyclops is not clear enough. This article aims to elucidate as fully as possible the nutrition of Acanthocyclops viridis (Jur.) - a large cyclops, inhabiting the mass of demersal layers of the open parts of the Rybinsk reservoir and its foreshore. The present work is devoted only to the predatory feeding of A. viridis, and includes data from the content of the intestines of cyclops, collected in natural conditions, and also the results of experimental observations carried out in a laboratory during 1958.

Chilodonella cyprini (Moroff, 1902) [Translation from: Izv.gos.nauchno-issled.Inst.ozer.i rech.ryb.kh-va 49 25-28, 1959]

The ciliated protozoan Chilodonella cyprini is described. Chilodon parasitizes the body, gills and fins of fishes. The infusoria produce irritation of the integument and strengthened mucous secretion. The sparse knowledsge of the ecology of chilodon is summarised and some advice on the struggle against cholodoniasis in carp given.