Rozprawy doktorskie na temat „Bending-creep”

This PhD research project mainly examined the important yet incompletely understood field of the mechano-sorptive creep of softwood. It included the acquisition of accurate creep data during constant and cyclic moisture conditions. The study only concerns the loading of softwood in the longitudinal, parallel to the grain direction in bending mode. Mechano-sorptive creep deformation takes place when the wood is subjected to load in varying humidity conditions, where the creep deformation is substantially higher than the creep deformation under constant humidity conditions. Visco-elastic creep is defined as the creep strain that is primarily dependent on time. Further analysis on the pseudo-creep, which is reversible with moisture changes, was also investigated. The Young's Modulus (E) and density (p) of the samples used in this research were measured and the ratio of (E/p) was used to characterise the quality of the wood specimen. A program was written in Quick-Basic for the computer to take readings from the creep machines at all times during all creep tests for acquiring maximum creep data possible for obtaining better results. A 'steaming' technique was implemented to accelerate creep recovery, in order to allow the re-use of the same sample for repeated creep testing, so reducing the effects of variability. Measurements of visco-elastic creep, mechano-sorptive creep limit and pseudo-creep rate as functions of percentage relative humidity and E/p were investigated and analysed. The mechano-sorptive creep limit was successfully reached by the load-reduction method. The pseudo-creep rate was similar for samples with the same values of E/p at different relative humidity. The higher the relative humidity, the larger the magnitude of visco-elastic and the 'reference' creep at the mechano-sorptive creep limit. A higher value of E/p produced lower pseudocreep rate, visco-elastic creep and mechano-sorptive creep limit. Equations were derived from the one week visco-elastic creep experimental data in order to extrapolate visco-elastic creep to longer time durations, (medium, long and permanent periods, and compared to the new European code of practice for timber design, namely the Draft Eurocode 5), and at two critical relative humidity conditions, (63% and 90%rh), which correspond to the Service Classes 1 and 2 in the Draft Eurocode 5. Also, an equation to predict the mechano-sorptive creep limit at various values of E/p for the two critical relative humidity conditions, (63% and 90%rh), was acquired. The prediction of creep at various relative humidities along the pseudo creep line was also quantified. The extrapolated visco-elastic creep along with the total upper-bound creep values were compared to the creep allowances of the Draft Eurocode 5 for both Service Classes 1 and 2. Moreover, a design scheme for estimating the upper-bound creep, consisting of the addition of visco-elastic, mechano-sorptive and pseudo creep for Draft Eurocode 5 Service Classes 1 and 2 was achieved. A study of the degree of interaction between visco-elastic and mechanosorptive creep was further investigated. Two different experiments were performed to reach some conclusion. The results of the first experiment showed that mechanosorptive creep acceleration decreases the visco-elastic creep rate and there was some kind of interaction between the two creep mechanisms. Hence the two creep mechanisms should not be considered as a separate phenomenon. On the other hand, the results of the second experiment showed that the visco-elastic and mechanosorptive creep are not a single process but are related processes. Moreover, it was deduced that the mechano-sorptive creep does not depend on the accumulation of the visco-elastic creep rate.

A comparison of the creep/duration of load (DOL) performance of a new structural wood composite material called Parallam® parallel strand lumber (PSL) to two grades of machine-stress-rated (HSR) Douglas-fir lumber is presented in this thesis. Evaluation of the creep/DOL performance was made on nominal 2x4 members under constant bending stress at three stress levels. A total of 306 test specimens were evaluated for a 15-1/2 month time period. The analysis suggests that the duration of load effect for Parallam PSL was consistent with the Madison curve for the time period studied while the MSR Douglas-fir lumber was consistent with recent duration of load models developed for structural lumber. The analysis also indicates that the current duration of load adjustment factors can be applied to develop working stresses for Parallam. The creep behaviour of the Parallam PSL was found to be equivalent or better than the two MSR lumber grades under dry-service conditions. Furthermore, evidence of linear viscoelastic behaviour was found for all test materials within the range of applied stresses evaluated. Two mathematical models of creep were fitted to the creep data and compared. A '4-parameter linear viscoelastic' model fitted the creep data better than an empirical 'power curve' model. The model parameters developed provide a basis for estimating the mean creep behaviour and variability in creep response for these materials under in-service load conditions for dry-service environments.
Forestry, Faculty of
Graduate

Thesis (Ph. D.)--Ohio State University, 2004.
Title from first page of PDF file. Document formatted into pages; contains xvii, 167 p.; also includes graphics (some col.). Includes abstract and vita. Advisor: Robert H. Wagoner, Dept. of Materials Science and Engineering. Includes bibliographical references (p. 158-167).

Pour prévoir et mettre au point la mise en forme d'objets en verre à partir de feuilles plates, les simulations numériques par éléments finis sont développées pour deux grands procédés, le Press-bend et le Blow-bend. Ces deux procédés, utilisés actuellement par les fabricants verriers, comportent deux phases essentielles: le pressage et le fluage pour le premier, le soufflage et le fluage pour le second. Ces modes de production nécessitent la maitrise d'un nombre important de paramètres. Les délais de conception et la minimisation des couts de revient limitent le nombre de mises au point expérimentales. C'est à ce niveau que les simulations numériques en tant qu'outil d'étude de faisabilité et d'optimisation, joue un rôle déterminant. Celles-ci sont associées à une stratégie originale intégrant la conception assistée par ordinateur pour décrire les entités géométriques et surfaciques, et à la métrologie afin de vérifier les critères dimensionnelles. A partir des prescriptions initiales du concepteur, la confrontation des résultats numériques et des relevés expérimentaux pour quatre volumes formes par Press-bend et Blow-bend permettent la validation des paramètres process utilisés. Finalement, une méthode inverse est mise en œuvre afin d'optimiser, dans un premier temps, la carte de températures dans le volume avant formage puis, la topologie des outils de mise en forme.

São descritas as recomendações da Norma Brasileira NBR-6118/1978 e do Código Modelo do Comité Euro-internacional du Béton CEB-1990 para a verificação da estabilidade de pilares esbeltos de concreto armado com seção variável submetidos à flexão normal composta, empregando-se a teoria do método geral. Para a obtenção dos momentos de segunda ordem são descritos os métodos de Engesser-Vianello e da integração numérica das curvaturas das seções transversais ao longo do pilar. Para a determinação do momento fletor absorvido pelas seções em função da curvatura proveniente da flexão do pilar sob a ação de uma força normal, desenvolvem-se as expressões para seções retangulares e circulares (cheias e vazadas) com base nas relações de tensão-deformação dos materiais segundo a norma e o código supracitados. Apresenta-se também um programa para microcomputador, elaborado em linguagem PASCAL, destinado à verificação da estabilidade de pilares com opção de adoção dos critérios da NBR-6118/1978 ou do CEB-1190. Não foram abordados os efeitos decorrentes de vibrações, fazendo-se apenas a descrição dos métodos de consideração dos efeitos decorrentes da deformação lenta.
The recommendations of Brazilian Code NBR-6118/1978 and Comité Euro-Internacional du Béton Model Code CEB-1990 are described, for stability verification of concrete slender columns with variable cross sections, subjected to axial load and bending moment, using the exact method. To obtain the second order bending moments, the Engesser-Vianello method and the numeric integration of the cross section curvatures along the longitudinal axial of column method are described. To obtain of bending moment supported by the cross sections due to curvature resultant of the column flexure under axialload, expressions for rectangular and circular (full and hollow) cross sections are developed, using stress-strain relations for materials proposed by model codes above mentioned. It\'s also presented a software written in PASCAL language for microcomputer and destined to column stability verification, with option to adopt the NBR-6118/1978 or CEB-1990 model code recommendations. Effects due to vibrations were nor included. Methods to take in account creep effects were described.

Cette thèse porte sur le traitement de la farine de bois et son utilisation comme renforts de composites à matrice polymère. En effet, l'incompatibilité entre le bois, hydrophile, et les polymères, généralement hydrophobes, est la principale difficulté de mise en oeuvre des composites bois-polymère. Un traitement des renforts bois est alors nécessaire pour améliorer les propriétés mécaniques et hygroscopiques des composites. Un traitement par fluoration directe est ici proposé pour diminuer le caractère hydrophile du bois et augmenter son adhésion avec la matrice polymère. Après une phase de développement et de réglage du traitement, une caractérisation physico-chimique de la farine de bois non-traitée ou fluorée a été réalisée afin d'observer l'effet de la fluoration. Le greffage de fluor sur les constituants du bois a été validé, et une diminution du nombre des sites de sorption de l'eau dans le bois a été constatée. Cette réduction a conduit à une diminution de la teneur en eau du bois après fluoration. En outre, la structure physique des particules et le comportement thermique du bois n'ont pas été modifiés par le traitement. Des composites bois-polyester ont été fabriqués par moulage par compression à chaud, avec des renforts non-traités et fluorés. Ceux-ci ont été testés en traction, en flexion, et en fluage afin d'observer l'influence du traitement sur les propriétés mécaniques des composites. Par ailleurs, une caractérisation hygroscopique a également été conduite. Les résultats montrent que la fluoration directe des renforts permet d'améliorer les propriétés mécaniques et hygroscopiques des composites bois-polyester.

Tragwerke aus Stahlbeton weisen infolge des Kriechens und Schwindens des Betons ein zeitveränderliches Materialverhalten auf. Die Folge sind Umlagerungen der im Querschnittsinneren wirkende Kräfte und im Zeitverlauf zunehmende Verformungen. Zur Beurteilung dieses Langzeitverhaltens sind geeignete Berechnungsmodelle erforderlich, die im Planungsstadium eine zuverlässige Prognose ermöglichen. Dabei spielen nicht nur reine Stahlbetonkonstruktionen eine Rolle, sondern im Zuge von Ertüchtigungsmaßnahmen werden zur Erhöhung der Tragfähigkeit zunehmend auch textile Bewehrungen aus Carbon- und AR-Glasfasern eingesetzt. Durch die beanspruchungsgerecht aufzubringenden Bewehrungsstrukturen und einen speziellen Feinbeton können sehr geringe Betonschichtdicken realisiert werden. Es entsteht ein Verbundquerschnitt mit unterschiedlichen Betonrezepturen, gleichfalls unterschiedlichem Betonalter und mit mehreren verschiedenen Bewehrungskomponenten. Um Aussagen zum Langzeitverhalten derartiger Konstruktionen treffen zu können, ist eine ganzheitliche Betrachtung über alle diese im Verbund liegenden Komponenten mit ihren jeweiligen Materialeigenschaften erforderlich. Im Rahmen der vorliegenden Arbeit sind in einem ersten Schritt die Stoffgesetze für die beteiligten Materialien Beton, Stahl- und Textilfaserbewehrung zu formulieren. Im Mittelpunkt steht dabei das viskoelastische Verhalten des Betons, für dessen baumechanische Beschreibung ein geeignetes rheologisches Modell in Form einer Feder-Dämpfer-Kombination dargestellt und die zugehörige Spannungs-Dehnungs-Zeit-Beziehung hergeleitet wird. Ferner wird aufgezeigt, wie die erforderlichen Materialparameter mit Hilfe üblicher Berechnungsansätze für Kriechen und Schwinden (z.B. nach EUROCODE 2) kalibriert werden können. Die betrachteten Textilfasern werden zunächst mit linear-elastischem Verhalten in Rechnung gestellt. Auf alternative Ansätze, die auch hier viskoelastische Eigenschaften berücksichtigen, wird hingewiesen, und das Berechnungsmodell ist dahingehend erweiterbar gestaltet. In einem zweiten Schritt werden die Materialmodelle der Einzelkomponenten nach den mechanischen Grundprinzipien von Gleichgewicht und Verträglichkeit und unter der BERNOULLIschen Annahme eines eben bleibenden Querschnittes miteinander in Beziehung gesetzt. Hierfür ist eine inkrementelle Vorgehensweise erforderlich, die mit dem Zeitpunkt der ersten Lastaufbringung beginnt und schrittweise den darauffolgenden Zustand berechnet. Im Ergebnis entsteht ein Algorithmus, der die am Querschnitt stattfindenden Veränderungen im Spannungs- und Dehnungsverhalten unter Einbeziehung der Stahlbewehrung sowie einer ggf. vorhandenen Textilbetonschicht wirklichkeitsnah erfaßt. Für statisch bestimmte Systeme mit bekanntem Schnittkraftverlauf wird gezeigt, wie sich so zu jeder Zeit an jeder Stelle der vorliegende Dehnungszustand und aus diesem über die Krümmung die Durchbiegung berechnen läßt. Der dritte und für viele praktische Anwendungen wichtigste Schritt besteht darin, die am Querschnitt hergeleiteten Beziehungen in ein finites Balkenelement zu überführen und dieses in ein FE-Programm zu implementieren. Auch das gelingt auf inkrementellem Wege, wobei für jedes Zeitinkrement die Spannungs- und Verformungszuwächse aller Elemente mit Hilfe des NEWTON-RAPHSON-Verfahrens über die Iteration des Gleichgewichtszustandes am gesamten System bestimmt werden. Hierzu werden einige Beispiele vorgestellt, und es werden die Auswirkungen des Kriechens und Schwindens mit den sich daraus ergebenden Folgen für das jeweilige Tragwerk erläutert. Ferner wird gezeigt, wie textilbewehrte Verstärkungsmaßnahmen gezielt eingesetzt werden können, um das Trag- und Verformungsverhalten bestehender Bauwerke unter Beachtung des zeitveränderlichen Materialverhaltens kontrolliert und bedarfsgerecht zu beeinflussen
Structures of reinforced concrete show a time-varying material behaviour due to creeping and shrinking of the concrete. This results in the rearrangement of the stresses in the cross-section and time-depending increase of the deformations. Qualified calculation models enabling a reliable prediction during the design process are necessary for the assessment of the long-term behavior. Not only pure reinforced concrete structures play an important role, but within retrofitting actions textile reinforcements of carbon and AR-glass fibres are applied in order to enhance the load-bearing capacity. A small concrete-layer-thickness can be achieved by the load-compatible application of reinforced textile configurations and the usage of a special certain fine-grained concrete. It leads to a composite section of different concrete recipes, different concrete ages and also several components of reinforcement. To give statements for the long-term behaviour of such constructions, a holistic examination considering all this influencing modules with their particular material properties is necessary. Within this dissertation in a first step the material laws of the participated components, as concrete, steel and textile reinforcement, are defined. The focus is layed on the visco-elastic behaviour of the concrete. For its mechanical specification a reliable rheological model in terms of a spring-dashpot-combination is developed and the appropriate stress-strain-time-relation is derived. Furthermore the calibration of the required material parameters considering creep and shrinkage by means of common calculation approaches (e.g. EUROCODE 2) is demonstrated. For the textile fibres a linear-elastic behaviour is assumed within the calculation model. It is also refered to alternative approaches considering a visco-elastic characteristic and the calculation model is configured extendable to that effect. In a second step the material models of the single components are correlated taking into account the mechanical basic principles of equilibrium and compatibility as well as the BERNOULLIan theorem of the plane cross-section. Therefore an incremental calculation procedure is required, which starts at the moment of the first load-application and calculates the subsequent configuration step by step. In the result an algorithm is derived, that realistically captures the occuring changings of stress and strain in the cross-section by considering the steel reinforcement as well as a possibly existing layer of textile concrete. For statically determined systems with known section force status it is demonstrated how to calculate the existing condition of strain and following the deflection via the curvaturve at every time and at each position. The third step - for many practical applications the most important one - is the transformation of the derived relations at the cross-section into a finite beam-element and the implementation of this in a FE-routine. This also takes place in an incremental way, whereat for each time-increment the increase of stress and strain for all elements is identified by using the NEWTON-RAPHSON-method within the iteration process for the equilibrium condition of the whole system. Meaningful numerical examples are presented and the effects of creep and shrinkage are explained by depicting the consequences for the particular bearing structure. Moreover it is shown how the purposeful use of textile reinforcement strengthening methodes can influence and enhance the load-bearing and deflection characteristics of existing building constructions by considering the time-varying material behaviour

Etat de degradation en fonction du temps d'un materiau soumis a des sollicitations thermomecaniques sur des plaques unidirectionnelles et des tubes (esais, traction, fluage, eclatement, flexion). Utilisation d'emission acoustique pour suivre l'evolution de la structure. Cinetique d'endommagement

The diploma‘s thesis deals with the design and assessment of prestressed cocrete tank of biogas station at ultimate limit state and also at serviceability limit state, according to ČSN EN 1992-1-1: Design of concrete structures. Specifically, the design of wall of the tank and foundation slab. The text part contains general information on the desing of the structure, static idealization of the structure, type of load and method for calculation of inner forces. Attachments contain drawing documentation and structural analysis, which completes the text part and solves in detail said structure.

Materials for engineering components operating at high temperatures during their service like rockets, aero-turbines, automobile engines, nuclear power plants, steam power plants and micro-electronics are often designed for creep resistance. Creep studies for such systems are performed either as a material selection routine or for investigating the residual-life, integrity and reliability of the components. Conventionally, uniaxial tests, which are the most reliable creep testing method, are adopted to study the dominant creep mechanism of materials systems at a particular stress and temperature. However, with increased demand for improving system efficiency, high throughput testing and miniaturization, a need for alternate testing methodology is realised. This work analyses the structure-property correlation of bending creep in the power-law regime and aims to establish it as one such alternative testing method with the prospects of obtaining reliable creep data from a small sample volume and at a yield rate that is 10 times faster than the conventional testing routes. In this work, the possibility of an uniaxially equivalent deformation state in bending was investigated. Digital image correlation was adopted to obtain strain and strain-rates at multiple locations in a cantilever. This coupled with the stress information obtained using developed analytical equations and numerical methods enables gathering hundreds of “stress-strain-rate” pairs from a single cantilever. Furthermore, the strain-time information obtained from digital image correlation was used to calculate primary and steady-state creep parameters from a single test performed on a cantilever. The developed method was applied to various material systems, such as commercially pure Pb and Al, and high purity poly- and single-crystal Al. An excellent match was observed between the creep parameters, including those relevant for describing primary and steady-state stages, obtained from bending and uniaxial creep tests. In addition, the effect of sample size on the creep properties obtained from cantilevers was investigated. It was realised that the creep properties obtained from cantilevers of small thickness show a linear hardening in the size range of 3 to 0.5 mm which is inversely proportional to the cantilever thickness. On the other hand, samples tested in uniaxial compression show a softening in creep below a size range below 2 mm diameter with comparable sample thickness and substructure size. A possible interplay between the hardening in creep behaviour due to straingradients and softening due to small sample size is discussed in the context of dislocation substructures. Experimental evidence for the same from EBSD patterns is presented. These are used to discuss the differential creep behaviour along the length and thickness of a cantilever where regimes of hardening, bulk behaviour and softening with respect to uniaxial bulk EE behaviour are observed in a single cantilever. Overall, this study aims at establishing creep of cantilevers as a high throughput creep testing methodology. Testing a cantilever is shown to accurately reproduce all relevant creep parameters as observed in uniaxial tests in the powerlaw regime and in a significantly shorter time. This is possible as long as the sample thickness is such that the strain gradients are small and allows development of equilibrium substructures as per the operable creep mechanism.

碩士
國立成功大學
工程科學系碩博士班
100
In this study, the relative mechanical behavior and buckling failure of sharp-notched SUS 304 stainless steel tubes with notch depths of 0.2, 0.4, 0.6, 0.8 and 1.0 mm subjected to pure bending creep are investigated. The pure bending creep is to bend the tube to a desired moment and keep that moment for a period of time. From the experimental result, the creep curvature and ovalization increase with time until the tube buckles. Higher desired moment leads to the higher creep curvature and ovalization. Finally, referring the Bailey-Norton formulation for uniaxial creep, a formulation is proposed for simulating the first and second stages of the creep curvature – time for SUS 304 stainless steel tubes subjected to pure bending creep. Through comparing with the experimental finding, the theoretical analysis can reasonably describe the experimental result.

碩士
國立成功大學
工程科學系碩博士班
93
Abstract 　Material parameters in the deformation kinetics (DK) and the J2 theory are determined from Darveaux and Banerji shear creep data of 97.5Pb/2.5Sn at 299K, 353K and 407K. The theoretical predictions of two different theories are in very well agreement with the experimental data, but at 299K the theoretical predictions by DK is about 10 times larger than those of J2 theory. However, the predictions of creep strain rate under uniaxial tensile stress of the two theories are not very alike. At various temperatures, the predictions of DK in the stress range higher than the engineering cut-off stress are larger than the predictions of J2, and their tendencies are deviated each other. From Bending/Torsion steady creep analysis of 97.5Pb/2.5Sn thin-walled tubes via DK and J2 theories, the analysis show that (1) under constant temperatures and constant torque with changing bending moment, the coupling effect of bending and twisting of DK on twist rates, are smaller than J2 theory, while the curvature rates of two theories are on the contrary. (2) under constant temperatures and constant bending moment with changing torque, the plot of torsion vs. curvature rate illustrates the coupling effect by bending and twisting of DK on curvature rate , are smaller than J2 theory, while the coupling effect of twist rates are opposite. The methodology of Bending/Torsion steady creep employed in this paper can be used for engineering design.

The present work is aimed at the extraction of material’s yield and creep parameters using cantilevers by conducting constant deflection rate and constant load tests respectively. Under the assumption of Euler-Bernoulli’s beam theory, stress and strain components are considered along beam length only. A stress and strain gradient throughout the cantilever makes bending rich in information as stress-strain rate-strain at every location of the beam corresponds to a single uniaxial test. By extracting these stress-strain rate strain information from multiple locations of the cantilever throughout deformation, high throughput feature of bending is utilized by extracting all the material yield and creep parameters from a single cantilever. The position dependent strain is measured using digital image correlation (DIC) in this work. The estimation of stress in a cantilever during non-linear deformation (i.e., non-linear dependence of stress on strain and/or strain rate), however, needs numerical methods which solve for stress distribution throughout the cantilever using equilibrium equations based on material’s constitutive behavior, beam geometry, loading and boundary conditions. Such numerical methods are developed to gain understanding of the stress redistribution which is found to be transient in nature and finally saturates when permanent (plastic or creep) strain are large (∼3-4%) such that elastic strain are negligible. The influence of material’s yield and creep parameters on stress redistribution profile and associated timelines is studied and limitations of the existing analytical expressions for saturated stress profile are discussed. The numerical method is also utilized to estimate the effect of limitations on strain accuracy measured by DIC on the extracted parameters. Therefore, the present work aims at developing cantilever bending combined with DIC for strain measurement as an alternative testing technique to extract material’s yield and creep parameters from a single cantilever. The procedure for extraction of yield parameters like yield strength and strain hardening exponent is established for a beam of tension-compression symmetric, strain rateinsensitive material using the concept of the ‘invariant point’. These points are identified using numerical methods as the unique locations at every cross-section of the cantilever, one each in tensile and compressive regions, where stress remains almost unchanged during stress redistribution. The hardening exponent and yield strength are measured from a single cantilever with better statistics and thereby improved reliability using strain measured at the invariant points in tensile and compressive regions within an accuracy of 99.5% and 90% respectively for pure copper and aluminium alloys. The procedures for measuring strain rate sensitivity for a tension-compression symmetric, strain-rate sensitive material and yield parameters for a tension-compression asymmetric, strain-rate insensitive material are also proposed and the challenges are highlighted with the understanding that bending has the potential to measure yield parameters for these latter systems as well in future. The timelines associated with stress saturation under creep deformation have been quantified using numerical methods in terms of a parameter stress saturation time (SST). Based on the recommendations obtained from SST, loading profile for T22 boiler steel is redesigned in the form of small loading steps due to which stress gets sufficient time to relax during loading itself and does not exceed yield strength during redistribution. Thus, creep parameters can be extracted at loads as high as yield strength, which is not possible otherwise. This makes testing faster and thereby efficient because creep rates are higher at high load and steady state is achieved faster. In contrast, a high SST at low loads has been identified to explain the misinterpretation of experimental data in terms of mechanism shift at low loads for P91 steel. The numerical method is further developed to include primary creep response at loads above yield which holds relevance to room-temperature creep response of an hcp system, i.e., titanium alloy Ti-6Al. In case of Ti-6Al, it is found from uniaxial creep tests conducted above yield that prior plastic deformation does not affect creep behaviour which implies that plasticity affects only the initial stress distribution. The invariant points which remain invariant to stress redistribution even under the combined effect of creep and plastic deformation are identified based on the numerical methods. The strain at these locations in tension as well as in compression, measured using DIC, are utilized to extract equivalent primary creep response for Ti-6Al using a single cantilever. Therefore, the present work aims at establishing bending as the testing technique to measure yield and creep parameters for a range of materials (FCC, BCC, HCP) and testing conditions (RT-600oC) utilizing minimum sample volume with reduced testing and better statistics.
ARDB 0242, IMPRINT 0009