913 research outputs found

    Lifetime evaluation of concrete structures under sustained post-peak loading

    Get PDF
    Experimental tests on crack propagation in concrete under constant post-peak loading are simulated using the finite element method and the cohesive crack model, in both Mode I and Mixed-mode conditions. The time-dependent behaviour of concrete in the process zone is due to the interaction and growth of microcracks, a phenomenon which, for high constant load levels, turns out to be predominant over linear viscoelastic creep in the bulk material. In mechanical systems based on this type of material behaviour (creep and strain-softening taking place simultaneously), the initial value problem is non-parabolic, i.e., the error at one time level is affected by the accumulation of errors introduced at earlier time levels. Despite these difficulties, the scatter in numerical failure lifetime vs. load level turns out to be negligible in Mode I conditions and practically acceptable in Mixed-mode conditions. Therefore the time-dependent behaviour of the process zone can be inferred solely from the results of direct tensile tests

    Corrosion of stressed components

    Get PDF
    Many engineering components operate under the combined influence of service stresses and deleterious environment. For better life prediction and to prevent failures, it is necessary to understand the mechanical behaviour of materials under conjoint action of stress and environment. Depending upon the metal-stress-environment system, the operating mechanism that would affect the life of the component may differ. Some of these mechanisms are discussed in this paper. Typically, the life limiting factor of engineering materials in corrosive environments have been identified as stress corrosion cracking, damage in components due to the presence of hydrogen, corrosion fatigue and fretting corrosion. In this paper, each of these phenomena has been discussed with respect to their mechanisms, methods to quantify the damage and ways to prevent or control them. Developments in fracture mechanics have been used as an effective tool in assessing the materials resistance to fracture. Application offracture mechanics techniques to predict the fracture behaviour of materials in aggressive environments has been discussed along with the conventional techniques

    Minimization of defects in aluminium alloy castings using SQC

    Get PDF
    In the present world with the increasing use of Aluminium alloy wheels in automotive industry the Aluminium foundry industry had to focus on the quality of the products. The quality of a foundry industry can be increased by minimizing the casting defects during production. Aim of the current study is to study the production line of an aluminum alloy wheel manufacturing industry and to improve the quality of production using quality control tools. This study shows the systematic approach to find the root cause of a major defects in aluminium castings using defect diagnostic approach as well as cause and effect diagram. Casting defect analysis is carried out using techniques like historical data analysis, cause-effect diagrams, design of experiments and root cause analysis. Data from X-ray inspection (Radiographic Inspection) have been collected along with the production parameter data. Using check sheets data has been collected and all the defects have been studied. Using Pareto chart major defects in the aluminium castings were noted. The major defects for the rejections during production were identified as shrinkages, inclusions, porosity/gas holes and cracks. Each defect is studied thoroughly and the possible causes for the defects are shown in Fishbone Diagrams (Cause Effect Diagrams). As the shrinkages mainly occur due to lack of feedability during the fluid flow the stalk changing frequency is noted along with the shrinkages defects and a relation is drawn between them. As hydrogen forms gas holes and porosity in the aluminium castings the amount of hydrogen present in the molten metal is studied by finding specific gravity of the samples collected. The molten metal temperature effects the amount of the hydrogen absorbed by it. .So the effect of molten metal temperature on the specific gravity of the sample collected have been shown in a graph and the optimum value for molten metal temperature was found out

    Advanced Small Rocket Chambers. Basic Program and Option 2: Fundamental Processes and Material Evaluation

    Get PDF
    Propellants, chamber materials, and processes for fabrication of small high performance radiation cooled liquid rocket engines were evaluated to determine candidates for eventual demonstration in flight-type thrusters. Both storable and cryogenic propellant systems were considered. The storable propellant systems chosen for further study were nitrogen tetroxide oxidizer with either hydrazine or monomethylhydrazine as fuel. The cryogenic propellants chosen were oxygen with either hydrogen or methane as fuel. Chamber material candidates were chemical vapor deposition (CVD) rhenium protected from oxidation by CVD iridium for the chamber hot section, and film cooled wrought platinum-rhodium or regeneratively cooled stainless steel for the front end section exposed to partially reacted propellants. Laser diagnostics of the combustion products near the hot chamber surface and measurements at the surface layer were performed in a collaborative program at Sandia National Laboratories, Livermore, CA. The Material Sample Test Apparatus, a laboratory system to simulate the combustion environment in terms of gas and material temperature, composition, and pressure up to 6 Atm, was developed for these studies. Rocket engine simulator studies were conducted to evaluate the materials under simulated combustor flow conditions, in the diagnostic test chamber. These tests used the exhaust species measurement system, a device developed to monitor optically species composition and concentration in the chamber and exhaust by emission and absorption measurements

    Effect of Heat Treatment on the Microstructure and Mechanical Properties of Stainless Steel 316L Coatings Produced by Cold Spray for Biomedical Applications

    Get PDF
    Abstract In this study, the effects of heat treatment on the microstructure and mechanical properties of cold sprayed stainless steel 316L coatings using N2 and He as propellant gases were investigated. Powder and coating characterizations, including coating microhardness, coating porosity, and XRD phase analysis were performed. It was found that heat treatment reduced porosity, improved inter-particle bonding, and increased ductility. XRD results confirmed that no phase transformation occurred during deposition. Significant increase in UTS and ductility was observed for the annealed specimens obtained with nitrogen propellant, whereas little changes were observed for the helium propellant produced specimen

    Investigation of the Rupture Initiation and Mechanical Performance of Alumina Port Covers for Integrated Rocket Ramjet Systems

    Get PDF
    Hypersonic flight is widely considered essential to ensure a competitive defensive capability in the United States. In hypersonic air breathing propulsion, cruise vehicles are a priority research and development area given the ease to which they can be implemented with existing DoD infrastructure. Advancements in ramjet engine systems, including integrated rocket ramjets, play a considerable role in military and space access hypersonic vehicle designs. By design, ramjets cannot produce static thrust. They first need to be brought to operational speeds with the help of a rocket booster. An integrated rocket ramjet (IRR) combines the booster phase and the ramjet phase in one single propulsion chain, further enhancing the systems efficiency. The key factor in IRR flight success is the separation of these two phases achieved by the use of a port cover, which will be discussed in detail throughout this paper. In support of the continued maturation of IRR technologies, the University of Tennessee Space Institute has collaborated with Air Force Research Labs, NASA Langley Research Center and the Defense Science and Technology Labs to investigate the viability of alumina port covers through a numerical and experimental campaign. Verified by finite element stress analysis, UTSI tested alumina port covers with a 45-degree dome angle and 6mm thickness that were fabricated and purchased from three external vendors. Flight conditions were simulated using a high-pressure chamber and a mechanical fracturing mechanism was used to initiate fracture on the pressurized port covers. High-speed cameras and retroreflective shadowgraphy techniques were used to obtain initial crack propagation patterns and average fragment velocity. The fragments were collected post-rupture and measured to estimate an average fragment size. It is imperative the fragment size is small enough to ensure no damage will occur to the internal structure of the flight vehicle during exit. Ultimately, the goal of this research is to determine if alumina port covers are suitable for vehicle use

    Aerothermal-Mechanical Health Monitoring And Diagnostics Or Turbo-Compressor Sets

    Get PDF
    PaperPg. 75-94.High speed turbomachinery plays a critical role in today's petrochemical industry. There are very high penalty costs associated with nonavailability and catastrophic failure of critical unspared trains. Additionally the fuel and maintenance costs over the life cycle of plant turbomachinery is very significant. Both of the above factors point to the need for health monitoring and diagnostic systems. The petrochemical industry has in the past placed a heavy emphasis on mechanical (vibration) analysis for both health monitoring and diagnostics. This paper presents a methodology in which both mechanical and aerothermal parameters are utilized for machinery health monitoring, prognosis and diagnosis

    Control and qualification of titanium welds

    Get PDF
    This thesis was submitted for the degree of Doctor of Philosophy and awarded by Brunel University.The study was aimed at controlling the weld geometry of thin-plate titanium and one of its alloys (Ti-6Al-4V) by ultrasonic means and qualiFying the metals in the as-welded condition in terms of their grain sizes and mechanical properties. The alignment and symmetry of the weld pools were successfully tested by using ultrasonic shear waves. The grain sizes at the weld fusion zone were found to be related to their ultrasonic attenuation by a mathematical relationship. The temperature effect in locating weld pool radii in titanium was found at temperatures up to 600 °C. The ultrasonic velocity decreased as the temperature increased and the square of temperature affected the rate of change of the ultrasonic velocity. After compensation for the temperature effect, the maximum location error of the weld pool radius was 17 % which was comparable to previous measurement using different techniques.A positive relationship was seen between weld geometry (penetration depth and weld width) and heat input. A welding spectrum for titanium and its alloys of different thicknesses was obtained. Back shielding gas was beneficial in obtaining good welds. Both heat input rate and cooling rate were found to affect the grain size of the weld, with the cooling rate being the dominant factor. The grain size exhibited a Hall-Petch effect on mechanical properties, such as the tensile properties and fracture toughness of the weld. The phase transformation positively contributed to better mechanical properties in most cases, whilst the presence of interstitials worsened tensile properties. A system was developed in this study to utilise the above information and data for possible real-time and closed-loop control of the TIG welding process to give a desirable weld. Specifically, a process control data base was built up using software and a knowledge-based system for acceptable welding parameters, which were determined by acceptable penetration depth, grain size and mechanical properties. An algorithm was successfully written which relates the ultrasonic signal to the penetration depth of the weld. A hardware control circuit was built which took in the ultrasonic signal and converted it to a driving signal to change the welding speed and thereby change cooling rate
    corecore