Welded steel pipes are an essential structural part of any power plant. Longer lifetimes and higher pressures in the pipes cause an increased probability of failure due to creep damage. To maintain safe operation, nondestructive evaluation techniques to detect creep damage are needed. Nonlinear acoustic techniques employing longitudinal waves have been proven to be sensitive to creep damage. The objective of this research is to develop a robust experimental procedure to reliably measure the acoustic nonlinearity parameter using longitudinal waves, and then to validate the procedure on samples of different materials and sizes. Finally the technique is applied to characterize creep damage levels around the weld of a welded steel pipe. While the experimental technique presented can only measure the relative nonlinearity, it is accurate enough to detect changes in nonlinearity due to creep damage. Measurements show an increase in nonlinearity in the heat affected zone (HAZ). Experiments after annealing the creep damaged specimen show a decrease in nonlinearity in accordance with a decrease in dislocation density. Measurements on an undamaged welded A36 steel component suggest that the heat itself is not responsible for the increase in nonlinearity.

<p>In-service inspections of high temperature pressureequipment show that weldments are prone to creep and fatiguedamage. It is not uncommon that severely damaged weldments arefound even before the design life of the component has beenreached. In order to improve this situation action has beentaken during the last decades, both from industry, universitiesand research institutes, aiming at an enhanced understanding ofthe weldment response.</p><p>The work presented in this thesis focuses on numericalsimulation of weldment creep response. For a more profoundunderstanding of the evolution of creep damage in mismatchedlow alloy weldments, simulations are performed using thecontinuum damage mechanics, CDM, concept. Both design and lifeassessment aspects are addressed. The possibility to assessseam welded pipes using results from tests of cross-weldspecimens taken out from the seam is investigated. It is foundthat the larger the cross-weld specimen the better thecorrelation. The advantage to use the CDM concept prior to aregular creep analysis is also pointed out. In order to developthe CDM analysis, a modified Kachanov-Rabotnov constitutivemodel is implemented into ABAQUS. Using this model, a secondredistribution of stresses is revealed as the tertiary creepstage is reached in the mismatched weldment.</p><p>Creep crack growth, CCG, in cross-weld compact tension, CT,specimens is investigated numerically where a fracturemechanics concept is developed in two steps. In the first one,the C^*value and an averaged constraint parameter areused for characterising the fields in the process zone, whilein the second step, the creep deformation rate perpendicular tothe crack plane and a constraint parameter ahead of the cracktip, are used as characterising parameters. The influence oftype and degree of mismatch, location of starter notch as wellas size of CT specimen, is investigated. Results show that notonly the material properties of the weldment constituentcontaining the crack, but also the deformation properties ofthe adjacent constituents, influence the CCG behaviour.Furthermore, the effect of size is influenced by the mismatchof the weldment constituents.</p><p>A circumferentially cracked girth weld with differentmismatch is assessed numerically by use of the fracturemechanics concept developed. The results show that type anddegree of mismatch have a great influence on the CCG behaviourand that C^*alone cannot characterise crack tip fields.Corresponding R5 assessments are also performed. Comparisonwith the numerical investigation shows that the assumption ofplane stress or plane strain conditions in the R5 analysis isessential for the agreement of the results. Assuming the formerresults in a relatively good agreement for the axial stressdominated cases while for the hoop stress dominated cases, R5predicts higher CCG rates by an order of magnitude.</p><p><b>Keywords:</b>ABAQUS, constraint effect, continuum damagemechanics, creep, creep crack growth, design, design code,finite element method, fracture mechanics, life assessment,mismatch, numerical simulation, weldment</p>

Made available in DSpace on 2015-03-26T13:54:49Z (GMT). No. of bitstreams: 1 texto completo.pdf: 419222 bytes, checksum: 99cf33d92e0c4efe3c2261ebeb42795f (MD5) Previous issue date: 2009-07-28 / Conselho Nacional de Desenvolvimento Científico e Tecnológico / Two experiments were conducted. The first work aimed to evaluate the performance of female calves in creep feeding receiving different supplementation strategies, milk production, intake and digestibility of pasture by Nellore or crossbred cow (Nellore x Holstein), on pasture of Brachiaria decumbens Stapf during the dry-rainy transition season. Forty four female beef suckling calves, with age between 90 and 150 days and average initial body weight of 117.7 + 4.3kg, and their dam (24 Nellore and 20 crossbred) with average initial body weight of 417.5 + 8.3 kg, were used. The treatments were: TMM (control) mineral mixture, ad libitum; Strategy 1 supplement provided from 112 days prior to weaning, 0.375 kg/animal/day. Strategy 2 - supplement provided from 112 days prior to weaning, in increasing amount, 0.15, 0.30, 0.45 and 0.60 kg/animal/day for the four sub-periods, in this order. Strategy 3 - supplement provided from 56 days prior to weaning, 0.750 kg/animal/day. The animals of strategy 1 had higher average daily gain (0.672 kg/day) than control animals (0.582 kg/day) (P<0.10) and better efficiency of supplement use. The strategy of supplementation of female calves did not influence weight gain, body condition score and the milk production of cows (P>0.10). The crossbred cows produced more milk (6.21 kg) than Nellore cows (5.37 kg/day) (P<0.10). The intake of dry matter (DM) were higher (P<0.10) in crossbred cows. However, there were not differences in the nutrients digestibility between the genetic groups. It can be concluded that supplementation strategies in which there are equitable distribution along the period prior to weaning, provides higher weight gain in female calves. Crossbred cows produce more milk production and have higher intake of DM than Nellore cows. There are not differences in the nutrients digestibility between cow Nellore and crossbred cow (Nellore x Holstein). The second work aimed to evaluate the performance, microbial protein synthesis, intake and digestibility of nutrients of beef heifers receiving increasing levels of multiple supplements or nitrogen salt, in self-fed, on Brachiaria decumbens Stapf. pasture, during the dry season. Thirty five beef heifers, with age and average body weight of 8 months and 203.4 + 4.5 kg, respectively, were used. The treatments were: MM (control) mineral mixture, ad libitum; SN (nitrogen salt) supplement with 75 % of CP, based on urea + mineral mixture, in proportion of 1:1 (50%), and corn (50%); Q1, Q2, Q3 multiple supplements with 40 % of CP, using different proportions of intake controller mixture (urea + mineral mixture, in proportion of 1:1), corn and soybean meal. The consumptions observed were: 115, 173, 572 e 1214 g/animal/day for the treatments SN, Q1, Q2 and Q3, respectively. The levels of urea + salt resulted in quadratic effect on consumption of supplement (P<0.10). The supplemented animals had higher (P<0.10) average daily gain (ADG). The supply of multiple supplement resulted on positive linear effect on ADG (P<0.10). However, on average, there was not significant effect (P>0.10) on ADG of animals that received multiple supplements or nitrogen salt. The supplementation increased (P<0.10) the intake and digestibility of nutrients, with exception of digestibility of the neutral detergent fiber (NDF), although the intake of digested FDN had increased (P<0.10). The supplementation increased (P<0.10) the production of microbial nitrogen as well as nitrogen losses in urine, although the quantity of assimilated nitrogen by bacteria, on proportion to intake, was higher on the higher levels of multiple supplements. It can be conclude that urea + salt control the intake of supplement on nonlinear way. The levels of multiple supplement result on positive linear effect on ADG and, on average, have similar gain to nitrogen salt. The supplementation increases the intake, digestibility, microbial nitrogen production and the ADG. / Realizaram-se dois experimentos. No primeiro, objetivou-se avaliar o desempenho produtivo de bezerras em creep-feeding recebendo diferentes estratégias de suplementação e a produção de leite, o consumo e a digestibilidade do pasto por vacas Nelore ou mestiças (Nelore x Holandês), em pastagens de Brachiaria decumbens Stapf, durante o período de transição seca-águas. Utilizaram-se 44 bezerras de corte, lactentes, com idade entre 90 e 150 dias e peso médio inicial de 117,7 + 4,3 kg, e suas mães (24 Nelore e 20 mestiças) com peso inicial médio de 417,5 + 8,3 kg. Os tratamentos foram: TMM (controle) - mistura mineral, ad libitum; Estratégia 1 - suplemento fornecido a partir de 112 dias pré-desmama, de forma constante, correspondente a 0,375 kg/animal/dia; Estratégia 2 - suplemento fornecido a partir de 112 dias pré-desmama, de forma crescente, correspondente a 0,15, 0,30, 0,45 e 0,60 kg/animal/dia para os quatro sub-períodos (28 dias cada), nesta ordem; Estratégia 3 suplemento fornecido a partir de 56 dias pré-desmame, correspondente a 0,75 kg/animal/dia. Os animais submetidos à Estratégia 1 apresentaram maior ganho de peso (0,672 kg/dia) em relação aos animais controle (0,582kg/dia) (P<0,10) e melhor eficiência de uso do suplemento. A estratégia de alimentação das bezerras não influenciou o ganho de peso, o escore de condição corporal e a produção de leite das vacas (P>0,10). As vacas mestiças apresentaram maior produção de leite (6,21 kg/dia) em relação às vacas da raça Nelore (5,37 kg/dia) (P<0,10). O consumo de matéria seca (MS) foi maior (P<0,10) em vacas mestiças. Contudo, não houveram diferenças na digestibilidade dos nutrientes entre os grupos genéticos. Conclui-se que estratégia de suplementação em que se distribui de forma equitativa o suplemento, ao longo do período pré-desmama, propicia maior ganho de peso de bezerras. Vacas mestiças apresentam maior produção de leite e consumo de MS. Não há diferenças na digestibilidade dos nutrientes entre vacas Nelore e mestiças (Nelore x Holandês). No segundo experimento, objetivou-se avaliar o desempenho produtivo, produção de proteína microbiana, o consumo e a digestibilidade dos nutrientes ingeridos por novilhas de corte em fase de recria, recebendo níveis de suplementação múltipla ou sal nitrogenado, em auto-controle de consumo, em pastagens de Brachiaria decumbens Stapf, durante a seca. Foram utilizadas 35 novilhas de corte com idade média de 8 meses e peso corporal médio inicial de 203,4 + 4,5 kg. Os tratamentos foram: MM (controle) - mistura mineral; SN (sal nitrogenado) - suplemento com 75 % PB, à base uréia + mistura mineral (50%), na proporção 1:1, e milho (50%); Q1, Q2, Q3 suplementos múltiplos com 40 % de PB, utilizando diferentes percentagens de mistura controladora de consumo (uréia + mistura mineral, na proporção 1:1), milho e farelo de soja. Os consumos observados de suplementos foram: 115, 173, 572 e 1214 g/animal/dia para os tratamentos SN, Q1, Q2, Q3, respectivamente. Os níveis de uréia + sal apresentaram efeito quadrático sobre o consumo de suplemento (P<0,10). Os animais suplementados apresentaram maior (P<0,10) ganho médio diário (GMD). Os níveis de suplementação múltipla apresentaram efeito linear positivo (P<0,10) sobre o GMD. Entretanto, em média, não houve diferenças significativas (P>0,10) entre o GMD de animais que receberam suplementação múltipla ou sal nitrogenado. A suplementação elevou (P<0,10) o consumo e a digestibilidade dos nutrientes, com exceção da digestibilidade da fibra em detergente neutro (FDN), contudo o consumo de fibra em detergente neutro igerida elevou-se (P<0,10). A suplementação elevou (P<0,10) a produção de nitrogênio microbiano, assim como as perdas de N na urina, embora a quantidade de nitrogênio assimilado pelas bactérias, proporcionalmente ao ingerido, foi maior nos maiores níveis de suplementação múltipla. Conclui-se que uréia e sal mineral controlam o consumo de suplemento de forma não linear. Os níveis de suplementação múltipla apresentaram efeito linear positivo (P<0,10) sobre o GMD e, em média, apresentam ganhos similares ao sal nitrogenado. A suplementação incrementa o consumo, a digestibilidade, a produção de nitrogênio microbiano e o GMD.

Student Number : 9800022T - PhD thesis - School of Mechanical, Industrial and Aeronautical Engineering - Faculty of Engineering and the Built Environment / For many applications, it may be useful to be able to estimate creep properties of a material from simpler testing procedures such as tensile tests than the conventional creep testing procedures. Most alloys used for creep service conditions are in a hardened condition and thus tertiary creep, controlled by micro structural degradation, is dominant. The object of the study was to investigate a reasonably simple method for estimating the creep behavior of a low alloy 1% Cr, 0.25 % Mo steel from tensile yield data. The study involved performing of series of investigations, including age hardening, tensile and creep tests. Microstructural degradation was monitored from specimens held in a furnace for different times and temperatures, which were then tested in tension at room temperatures. Tensile tests were carried out at different temperatures and strain rates and the data used to determine material parameters for use in kinetic equations describing deformation. For comparison, creep curves were obtained from both creep tests and tensile tests results. Tests on furnace aged specimens were used to quantify softening due to material degradation and formulate a structure evolution and kinetic expressions used to determine creep curves. The modified equation by Dorn was used to determine the material parameters and to predict flow characteristics. Two sets of mechanisms were observed. At low temperature and high stress (above 550MPa) dislocation by glide mechanism was investigated. At higher temperatures and low stress (below 550MPa), some form of power law creep was observed. Glide mechanism was investigated and material parameters σ ) , n and activation volume v, were calculated. The calculated value of σ ) was assumed for both plastic deformation and the softening kinetics. A reasonably good estimate of the creep behavior of the low alloy steel used in this investigation in which tertiary creep dominates can be calculated from tensile yield stress values. Furthermore, the creep rate and recovery have similar stress dependences, with the stress and temperature dependence similar to that predicted by recovery theory. The value of activation energy observed for creep for this alloy is in line with the processes which could be related to self diffusion. In order to justify the significance of this study, four existing empirical models are discussed, highlighting their merits and demerits with respect to the models used in this study. These are θ-Projection, Damage Mechanics, Estrin-Mecking and the Internal Stress Methods. Generally, in this class of alloys, recovery process occurs under an effective stress (i.e. an applied stress less the internal stress). Thus the possibility of using tensile data obtained in this study in the internals stress model was explored. The model could replicate the one used in this study if the change in internal stress value o σ is assumed to be negligible. This could be assumed to be true for tensile data at high stresses and low temperature especially during secondary creep rate when the internal stress approximates to the applied stress and at short test durations.

The objective of this study was to develop a model to predict the viscoelastic material functions of a vinyl ester (VE) polymer (Derakane 441-400, Ashland Co.,) with variations in its material properties. Short-term tensile creep/creep recovery experiments were conducted at two stress levels and at four temperatures below the glass transition temperature of the VE polymer, with 10 replicates for each test configuration. Experimental strains in both the longitudinal and transverse directions were measured using a digital image correlation technique. The measured creep strain versus time responses were subsequently used to determine the creep compliances using the generalized viscoelastic constitutive equation with a Prony series representation. The variation in the creep compliances of Derakane 441-400 was described by formulating the probability density functions (PDFs) and the corresponding cumulative distribution functions (CDFs) of the creep compliances using the two-parameter Weibull and log-normal distributions. The maximum likelihood estimation technique was used to obtain the Weibull shape and its scale parameters and the log-normal location and its scale parameters. The goodness-ofit of the distributions was determined by performing Kolmogorov-Smirnov (K-S) hypothesis tests. Based on the K-S test results, the Weibull distribution is a better representation of the creep compliances of Derakane 441-400 when compared to the log-normal distribution. Additionally, the Weibull scale and shape parameters of the creep compliance distributions were shown to be time and temperature dependent. Therefore, two-dimensional quadratic Lagrange interpolation functions were used to characterize the Weibull parameters to obtain the PDFs and subsequently the CDFs of the creep compliances for the complete design temperature range during steady state creep. At each test temperature, creep compliance curves were obtained for CDF values of 0.05, 0.50 and 0.95 and compared with the experimentally obtained lowest, mean and highest creep compliances, respectively. The predicted creep compliances of Derakane 441-400 in the design space are in good agreement with the experimental data.

No description available.

The effect of thermoreversible physical aging on the linear viscoelastic creep properties of a thermoplastic composite was investigated. Radel X/IM7, an amorphous composite material considered for use in the next generation high speed transport aircraft, was studied. The operating environment for the aircraft material will be near 188°C (370°F) with a service life in excess of 60,000 hours at temperature. Accurate predictions of the viscoelastic properties of the material are essential to insure that design strength and stiffness requirements are met for the entire service life. The effect of physical aging on the creep response was studied using momentary tensile creep tests conducted at increasing aging times following a rapid quench from above the glass transition temperature (Tg) to a sub-Tg aging temperature. As the aging time increased, the creep response of the material significantly decreased. The tensile creep compliance data for each aging time were fit to the empirical equation for the creep compliance D(t): D(t)-Dₒ<i>e</i> ^{t/tₒ)m} where Dₒ, tₒ,and m are fitting parameters determined using a nonlinear fitting program based on the Levenberg-Marquardt finite difference algorithm. The short-term creep compliance curves, obtained at various aging times, were then shifted to form a momentary master compliance curve. The double logarithmic aging shift rate μ and its dependence on sub-T_g aging temperature were determined. The aging characterization process was conducted on unidirectional specimens with 0, 90, and 45 degree fiber direction orientations. This permitted the calculation of the complete principal compliance matrix for the composite material. The effect of physical aging becomes more apparent during long-term tests when creep and aging occur simultaneously. This results in a gradual stiffening and decrease in the creep response with increased time. Predictions based solely on the Time-Temperature Superposition Principle would significantly over-predict the creep response if physical aging effects were ignored. Theoretical predictions for long-term creep compliance were made using an effective time theory and compared to long-term experimental data for each fiber orientation. Finally, experimental results of a long-term test of a 30 degree fiber angle orientation specimen were compared to theoretical predictions obtained by transforming the principal compliance matrix to the 30 degree orientation. / Ph. D.

This investigation addresses the interaction mechanisms of time dependent material behavior and cyclic damage during fatigue loading of fiber reinforced composite laminates. A new term 'damorheology' has been coined to describe such physical behavior. The lamina has been chosen as the building block and a cross ply laminate configuration was the selected test case. The chosen material system is the Radel X/T65-42 thermoplastic composite by Amoco. The fatigue performance at the lamina level is represented by the dynamic stiffness, residual strength and fatigue life of unidirectional laminates. The time dependent behavior is represented at the lamina level by a Pseudo-Analog Mechanical model. The thermo-rheological characterization procedure combines mechanical (creep) and thermal (dynamic mechanical analysis) techniques. / Ph. D.

Concrete experiences volume changes throughout its service life. When loaded, concrete experiences an instantaneous recoverable elastic deformation and a slow inelastic deformation called creep. Creep of concrete is composed of two components, basic creep, or deformation under load without moisture loss and drying creep, or deformation under drying conditions only. Deformation of concrete in the absence of applied load is often called shrinkage. The deformation due to creep is attributed to the movement of water between the different phases of the concrete. When an external load is applied, it changes the attraction forces between the cement gel particles. This change in the forces causes an imbalance in the attractive and disjoining forces. However, the imbalance is gradually eliminated by the transfer of moisture into the pores in cases of compression, and away from the pores in cases of tension. Designs typically use one of the two code models to estimate creep and shrinkage strain in concrete, ACI 209 model recommended by the American Concrete Institute or the CEB 90 Eurocode 2 model recommended by the Euro-International Committee. The ASSHTO LRFD is based on the ACI 209 model. Three other models are the B3 model, developed by Bazant; the GZ model, developed by Gardner; and the SAK model developed by Sakata. The development of concrete performance specifications that limit the amount of compressive creep of concrete mixtures used by the Virginia Department of Transportation, specifically concrete mixtures used for prestressed members (A-5 Concrete) were assessed, along with determining the accuracy and precision of the creep models presented in the literature. The CEB 90 Eurocode 2 model for creep and shrinkage is the most precise and accurate predictor. The total strain for the VDOT portland cement concrete mixtures discussed in this study were found to be between 1200 ± 110 microstrain at 28 days, and 1600 ± 110 microstrain at 97 days, at a five percent significant level. / Master of Science

Increasing energy demand leads to two crucial problems for the whole society. One is the economic cost and the other is the pollution of the environment, especially CO2 emissions. Despite efforts to adopt renewable energy sources, fossil fuels will continue to dominate. The temperature and stress are planned to be raised to 700 °C and 35 MPa respectively in the advanced ultra-supercritical (AUSC) power plants to improve the operating efficiency. However, the life of the components is limited by the properties of the materials. The aim of this thesis is to investigate the high temperature properties of materials used for future power plants. This thesis contains two parts. The first part is about developing creep rupture models for austenitic stainless steels. Grain boundary sliding (GBS) models have been proposed that can predict experimental results. Creep cavities are assumed to be generated at intersection of subboundaries with subboundary corners or particles on a sliding grain boundary, the so called double ledge model. For the first time a quantitative prediction of cavity nucleation for different types of commercial austenitic stainless steels has been made. For growth of creep cavities a new model for the interaction between the shape change of cavities and creep deformation has been proposed. In this constrained growth model, the affected zone around the cavities has been calculated with the help of FEM simulation. The new growth model can reproduce experimental cavity growth behavior quantitatively for different kinds of austenitic stainless steels. Based on the cavity nucleation models and the new growth models, the brittle creep rupture of austenitic stainless steels has been determined. By combing the brittle creep rupture with the ductile creep rupture models, the creep rupture strength of austenitic stainless steels has been predicted quantitatively. The accuracy of the creep rupture prediction can be improved significantly with combination of the two models. The second part of the thesis is on the fatigue properties of austenitic stainless steels and nickel based superalloys. Firstly, creep, low cycle fatigue (LCF) and creep-fatigue tests have been conducted for a modified HR3C (25Cr20NiNbN) austenitic stainless steel. The modified HR3C shows good LCF properties, but lower creep and creep-fatigue properties which may due to the low ductility of the material. Secondly, LCF properties of a nickel based superalloy Haynes 282 have been studied. Tests have been performed for a large ingot. The LCF properties of the core and rim positions did not show evident differences. Better LCF properties were observed when compared with two other low γ’ volume fraction nickel based superalloys. Metallography study results demonstrated that the failure mode of the material was transgranular. Both the initiation and growth of the fatigue cracks were transgranular. / <p>QC 20160905</p>