Combustion gas turbine components designed for application in power generation equipment are subject to periodic replacement as a result of cracking, damage, and mechanical property degeneration that render them unsafe for continued operation. In view of the significant costs associated with inspecting, servicing, and replacing damaged components, there has been much interest in developing models that not only predict service life, but also estimate the evolved state of the material. This thesis explains manifestations of microstructural damage mechanisms that facilitate fatigue crack nucleation in directionally-solidified (DS) Ni-base superalloy components exposed to elevated temperatures and high stresses. In this study, models were developed and validated for damage and life prediction using DS GTD-111 as the subject material. This material has a chemical composition and grain structure designed to withstand creep damage occurring in blades of gas-powered turbines. The service conditions in these components, which generally exceed 600C, facilitate the onset of one or more damage mechanisms related to fatigue, creep, or environment. The study was divided into an empirical phase, which consisted of experimentally simulating service conditions in fatigue specimens, and a modeling phase, which entailed numerically simulating the stress-strain response of the material. Tests have been carried out to simulate a variety of thermal, mechanical, and environmental operating conditions endured by longitudinally (L) and transversely (T) oriented DS GTD-111. In some cases, tests in extreme environments/temperatures were needed to isolate one or at most two of the mechanisms causing damage. Microstructural examinations were carried out via microscopy. A continuum crystal plasticity model was used to simulate the material behavior in the L and T orientations. The constitutive model was implemented in ABAQUS and a parameter estimation scheme was developed to obtain the material constants. A physically-based model was developed for correlating crack initiation life based on the experimental life data. Assuming a unique relationship between the damage fraction and cycle fraction with respect to cycles to crack initiation for each damage mode, total crack initiation life has been represented in terms of the individual damage components observed at the end state of crack initiation.

Thermal fatigue failure, due to the fracture of solder bumps which was cased by the coefficient of thermal expansion mismatch deformation, is frequently encountered in flip-chip package. Therefore, this thesis attempts to study the effects of geometric parameters variation on lead-free flip-chip package under temperature cycling test. First, we used the finite element method to simulate the thermal loading response of lead-free flip-chip. The accumulated equivalent creep strain and accumulated creep strain energy density of the lead-free solder bumps were calculated, and were used to predict the thermal fatigue life of lead-free flip-chip package. The Taguchi method is applied to obtain the optimal design parameters in order to enhance reliability of the lead-free flip-chip under temperature cycling loading. The analysis of variance (ANOVA) is also used for estimating the influence of the factors quantitatively. The obtained results can be adopted as references for the lead-free flip-chip package design.

The user programmable features (UPF) of the finite element code ANSYS® are used to generate a customized ANSYS-executable including a more general creep behaviour of materials and a damage module. The numerical approach for the creep behaviour is not restricted to a single creep law (e.g. strain hardening model) with parameters evaluated from a limited stress and temperature range. Instead of this strain rate - strain relations can be read from external creep data files for different temperature and stress levels. The damage module accumulates a damage measure based on the creep strain increment and plastic strain increment of the load step and the current fracture strains for creep and plasticity (depending on temperature and stress level). If the damage measure of an element exceeds a critical value this element is deactivated. Examples are given for illustration and verification of the new program modules.

A hybrid material design has been developed to improve creep performance in pressurized metallic pipes subjected to high temperatures. Metallic pipes were reinforced with various arrangements of external wires which have substantially greater creep resistance than the pipe material. This research was conducted to explore the field of reinforcement of piping for creep reduction, exploit the creep strength of refractory metals, and investigate structure-property relationships in architectured materials. Two basic wire reinforcement architectures were tested, simple helical windings and braided sleeves. By adjusting the architecture of the reinforcement, apparent tangential (hoop) and longitudinal stresses on the pipe are altered, thereby allowing multiaxial creep strains to be controlled. The utilization of a reinforcement layer in a hybrid layup, where it is not bonded or embedded in a matrix is a relatively unexplored field. Hybridization allows the most desirable properties to be extracted from each component and have them work together in parallel. The use of braided refractory reinforcement is also a particularly novel concept, with refractory materials for reinforcement purposes traditionally being utilized in particle, whisker and discontinuous fibre form. Rather than testing in a uniaxial stress state, the present approach to creep testing pressurized pipes at high temperature remains largely underutilized, and is especially relevant to industry applications where creep takes place in the complex, multiaxial stress state of a pressurized pipe. In a low-temperature reinforcement architecture optimization study of a brass-stainless steel system, designed for ease of fabrication and to negate oxidation issues, pipes were pressurized and creep rupture tested at 400°C. Even in an unoptimized state, braided reinforcement was observed to out-perform a simple iv helical wrap by at least 22%, giving a 10-times life extension without rupture, and a reduction in creep rate in excess of 45-times for reinforcement oriented at a 50°. A simple analytical model from reinforced pressure vessel theory predicts a neutral angle (θN) of 54.7°, at which point the reinforcement is oriented to act proportionally to the applied pressure stresses. An empirical model of effective creep rate with varying reinforcement angle was derived in the present study, and used to find that a braid angle of approximately 54.7±1.5° is optimal to minimize the effective multiaxial creep rate of a hybrid pipe under internal pressure, reducing it to the point of being negligible. The braided reinforcement was observed to be constantly shifting towards the equilibrium point of θN, but only for initial angles below θN. This concept of braid reorientation is generally associated with rapid elastic deformation or static reinforcement of systems at room temperature, and the gradual shift towards θN facilitated by creep deformation has not been reported previously. A relationship for -θ (i.e. creep rate for a given reinforcement angle) was derived, including the reduction in as θ tends to θN. Findings of this optimization study were applied to a high temperature system which served as an acceleration of reformer furnace operating conditions: 253MA pipes were reinforced with tungsten wire and creep rupture tested at 1030-1040°C. Using braided reinforcement oriented at 52.6±1.4° a life extension in excess of 700x was observed, with no signs of bulk deformation after a 309x life extension. These high temperature results were considered in light of the intended industry application, with a balance of life extension, weight reduction and increased operating temperature preferred over outright life extension for the reformer furnace application.

Η πόλη της Πάτρας βρίσκεται σε μία περιοχή του Ελληνικού χώρου, που χαρακτηρίζεται από πρόσφατες τεκτονικές δομές. Το πλήθος και το μέγεθος των ρηγμάτων που απαντούν στην ευρύτερη περιοχή, μαρτυρούν μια έντονη νεοτεκτονική δραστηριότητα που συνεχίζεται μέχρι σήμερα με την εκδήλωση σεισμικότητας. Οι ρηξιγενείς ζώνες και εξ’ ορισμού οι σεισμικές πηγές, έχουν επιπτώσεις στην τρωτότητα πυκνοκατοικημένων περιοχών, τόσο στην περιοχή της Πάτρας, όσο και στην ευρύτερη περιοχή. Το σύνολο των ρηγμάτων αυτών έχει αντίκτυπο στις κατασκευές προκαλώντας προβλήματα στην καθημερινότητα των πολιτών όταν διέρχονται από τον αστικό ιστό μιας πόλης. Τρία ενεργά ρήγματα εντοπίσθηκαν να επηρεάζουν τον αστικό ιστό της Πάτρας. Από Βορρά προς Νότο αυτά είναι το ρήγμα στη περιοχή της εστίας του Πανεπιστημίου Πατρών, το ρήγμα της οδού «Πέντε Πηγαδίων» στη περιοχή της Αγίας Σοφίας και το ρήγμα της Αγίας Τριάδας. Ο εντοπισμός των ρηγμάτων έχει γίνει με τον συνδυασμό διαφόρων πληροφοριών από παλαιότερες μελέτες και σημερινά στοιχεία. Περιγράφεται και αναλύεται η μορφολογία και τα χαρακτηριστικά τους ως προς τις μακροσεισμικές παρατηρήσεις γύρω τους, όπως τις επιφανειακές διαρρήξεις οι οποίες προκαλούν ζημιές στις κατασκευές καθώς και τα επίπεδα θορύβου του εδάφους, ενώ επιχειρείται συσχετισμός μεταξύ τους. Για την εργασία χρησιμοποιήθηκαν το «Impulse laser» για την αποτύπωση της μορφολογίας και η τεχνική HVSR για τη μέτρηση του εδαφικού θορύβου. / The city of Patras is an area of the Greek space, characterized by recent tectonic structures. The number and size of faults occurring in the region, indicate an intense neotectonic activity that continues until today with the event seismicity. The fault zones and default seismic sources, affect the vulnerability of densely populated areas, in the region of Patras, and in the wider region. The total of these faults in construction causing problems in the daily life of citizens when crossing the urban fabric of a city. Three active faults identified to affect the urban fabric of Patras. From north to south these are the fault of the University of Patras, the fault of the road "Five Wells" in the area of St. Sophia and the fault in the area of the Ag.Triadas. The detection of these are done by combining various information from past research and current information. Described and analyzed the morphology and their characteristics in terms of macroseismic observations around them, such as surface faulting which cause damage to structures and the ground noise and attempted correlation between them. Was used the «Impulse laser» for the description of morphology and technical HVSR to measure the ground noise.

No description available.

No description available.

The viscoelastic and creep properties of salt create challenges in the design of salt mines. Salt undergoes steady state creep for a long period of time, and the time of failure is not easily predicted. Developing functions for creep behavior is important in predicting the deformation of salt pillars. Through literature reviews, it was found that there are many relationships to determine the deformation rate of salt specimens through constitutive models. Mine panels have also been modeled to understand the stress and deformational behavior of the pillars. The purpose of this was project was to develop a relationship that determines the convergence rate from knowing the pillar width to pillar height ratio and thickness of the salt strata immediately above and below the mine. The third power law was adopted in the modeling of salt pillars, which is applicable to low stresses of less than 10 MPa that is typical of salt mine conditions. The finite difference software, FLAC3D was used for the simulations of salt pillar models. A square pillar was modeled using four pillar width to pillar height ratios from 1.5 to 4.6. In mining practices, the pillar width to pillar height ratios are designed to be 1.0 to 5.0. Three sets of pillar dimensions were used for each pillar width to pillar height ratio, this was done to determine whether different room and pillar dimensions for each pillar width to pillar height ratio resulted in different convergence rates. Eight salt thicknesses of 0 m to 26 m were modeled for each set of pillar dimensions, which was sufficient to determine the effect of salt thickness on convergence rate. From the modeled results, general trends among the various pillar width to pillar height ratios were observed. The convergence rate increased as the pillar width to pillar height ratio decreased. In addition, an exponential relationship was found between the convergence rate and the pillar width to pillar height ratio. There was a strong correlation between convergence values calculated from the developed function and the modeled values for the power law exponent of three. The developed expression can be used to estimate the convergence rate due to pillar compression and room convergence.

No description available.

No description available.