The influence of manganese (Mn) addition on the tensile properties of recovery-annealed aluminum alloy sheet was studied. After 200 ~ 220¢X C annealing, these alloys exhibit hardening as compared to the as-rolled state. Besides the amount of Mn in solution, the presence of Fe and Si in these alloys was also a vital factor responsible for the hardening. These recovery annealed aluminum alloy sheets show increased tensile elongation with increasing annealing temperature, which is mainly due to the contribution of post-uniform elongation (PUE). The plastic deformation during PUE is associated with development of fine slip bands distributed in the gauge length of the specimen. It was noted that after recovery anneal, the alloy with 0.74 wt% Mn exhibit better combination of strength and ductility as compared to alloys with lower Mn content. In general, these alloys in partially annealed condition show poor tensile ductility at RT, which is mainly attributed to the low work hardening rate associated with the UFG structure. These partially annealed aluminum alloys exhibit highly anisotropic tensile properties, specially a rather poor ductility along the direction of 45o or 90o from the rolling direction at RT. The poor ductility in 45o or 90o direction could be related to flow localization associated with intense shear banding. Discontinuous yielding plays a pivotal role to trigger the flow localization which is affected by the strain path change. However, for alloys tested at 77 K in 45¢X or 90¢X direction, the deformation proceeds by the propagation of Lüders band initially and followed by strain hardening. In general, the tensile elongation can be enhanced greatly irrespective of the stress direction, because a higher work hardening rate can be maintained due to reduced dynamic recovery rate. The yield stress is orientation dependent, which is in the order of 90¢X > 0¢X > 45¢X. The anisotropic tensile behavior has its origin in the rolling texture. The Schmid factor analysis indicates that specimens tested in 45o direction would have lower yield strength as compared to those tested in 0o or 90o direction. Both experimental measurements and simulation indicate that after 30% tensile straining, the copper texture in the partially annealed aluminum alloy is enhanced 0¢X test, and the brass texture is enhanced in 90¢X test, while the texture distribution does not change significantly in 45¢X test. It is suggested that the texture evolution during tensile straining has significant effect on the anisotropy of work hardening rate.

In this thesis, the relationship among friction stir processing (FSP) parameters, microstructure evolution, texture development, and mechanical hehavior of AZ31B Mg alloy was investigated. First of all, in order to reveal the correlation among the deformation conditions, dynamic recrystallization (DRX) mechanisms, and microstructure evolution in the Mg alloy, hot compression tests at a wide range of Zener-Hollomon parameter (Z) values were conducted. Through optical microscopic examination, it was found out that above a critical Z value, twinning influences the DRX process resulting in a more effective grain refinement, which is manifested in a significant change in the slope of the Z-drec relationship, where drec is the recrystallized grain size. Moreover, EBSD examination revealed that the twinning also contributed to a distinct change in the recrystallization texture. Compression tests were performed along both through-thickness and in-rolling-plane directions of the plate to study the orientation dependency of twinning activities and its influence on the DRX process. X-ray line profile analysis (XLPA) provides further insights by highlighting the differences in the dislocation density/types, subgrain sizes, and twin densities during the DRX processes operating with or without the twinning. Secondly, the constitutive behaviour study was applied to the investigation of microstructure evolution during FSP. By varying the key FSP parameters systematically, i.e. rotation and travel rates of the tool, a series of FSP specimens were prepared with a wide range of thermo-mechanical inputs in terms of Z. The resulting tensile behavior in the stir zone (SZ) showed a dramatic change as a function of Z, caused by a systematic change in the texture within SZ measured by neutron diffraction. A three-dimensional transient model was developed to investigate the detailed deformation history including the temperature and strain rate profiles and material flow pattern during FSP of the Mg alloy. Such deformation history can be combined with the constitutive study from the compression tests in order to analyze the developments of micro-texture and DRX grains during FSP, which will, in turn, dominate the mechanical properties. Based on the studies above, new fundamental understandings were gained on the governing mechanisms for the deformation and recrystallization processes during FSP and the influence of thermo-mechanical input during FSP on ductility enhancement in the Mg alloy.

京都大学 / 0048 / 新制・課程博士 / 博士(工学) / 甲第19708号 / 工博第4163号 / 新制||工||1642(附属図書館) / 32744 / 京都大学大学院工学研究科材料工学専攻 / (主査)教授 辻 伸泰, 教授 白井 泰治, 教授 乾 晴行 / 学位規則第4条第1項該当 / Doctor of Philosophy (Engineering) / Kyoto University / DFAM

Η παρουσία της βλάβης διάβρωσης στις αεροπορικές δομές, έχει αποδειχθεί ότι συμβάλλει σημαντικά στην υποβάθμιση της δομικής τους ακεραιότητας. Ταυτόχρονα, οι χρονοβόρες και αυξημένου κόστους διεργασίες και έλεγχοι που πραγματοποιούνται για την αποφυγή ή την επιδιόρθωση της διάβρωσης δεν είναι πάντα αποδοτικές. Στην τρέχουσα βιομηχανική πρακτική, σε περιπτώσεις στατικής φόρτισης, η βλάβη διάβρωσης αντιμετωπίζεται θεωρώντας ότι έχει επέλθει μείωση της φέρουσας διατομής ίση με το βάθος της διαβρωτικής προσβολής και ακολουθεί εκ νέου υπολογισμός των τάσεων. Στις δυναμικές φορτίσεις, τα τρήμματα θεωρούνται ως πιθανά σημεία έναρξης των ρωγμών κόπωσης. Όμως, παρά την αναγνώριση της διάβρωσης ως έναν από τους μηχανισμούς που επιδρούν αρνητικά στη δομική ακεραιότητα μιας αεροπορικής δομής, ούτε και στον σχεδιασμό των αεροπορικών κατασκευών με ανοχή στη βλάβη, ο οποίος είναι ο σύγχρονος τρόπος σχεδιασμού και βασίζεται στις αρχές της θραυστομηχανικής, η βλάβη διάβρωσης λαμβάνεται υπόψη. Ως αποτέλεσμα, η επίδραση της διάβρωσης στη δομική ακεραιότητα των υλικών κάποιες φορές υποεκτιμάται. Σκοπός της παρούσας διδακτορικής διατριβής είναι να συμβάλλει στην ανάπτυξη ικανοτήτων για την εκτίμηση της υποβάθμισης των μηχανικών ιδιοτήτων του διαβρωμένου υλικού με δεδομένα τα μεταλλογραφικά χαρακτηριστικά της βλάβης διάβρωσης. Στο πλαίσιο αυτό, αναπτύσσεται μια μεθοδολογία η οποία επιτρέπει την υπολογιστική προσομοίωση της συμπεριφοράς εφελκυσμού του διαβρωμένου υλικού με δεδομένα τα μεταλλογραφικά χαρακτηριστικά της βλάβης διάβρωσης. Η εργασία περιλαμβάνει ένα πειραματικό και ένα υπολογιστικό σκέλος. Το πειραματικό σκέλος περιλαμβάνει την εκτενή μεταλλογραφική μελέτη της βλάβης διάβρωσης, τη διεξαγωγή μηχανικών δοκιμών εφελκυσμού και τη μελέτη των επιφανειών θραύσης για την αναγνώριση των φυσικών μηχανισμών της βλάβης σε δοκίμια προ-διαβρωμένα για διαφορετικούς χρόνους έκθεσης στο διαβρωτικό περιβάλλον. Τα κύρια αποτελέσματα της πειραματικής διαδικασίας συνοψίζονται ως εξής: Η βλάβη διάβρωσης συσσωρεύεται βαθμιαία και εξελίσσεται, σε συνάρτηση με τον χρόνο έκθεσης, από τρημματική σε διάβρωση αποφλοίωσης. Οι μηχανικές δοκιμές εφελκυσμού στο διαβρωμένο υλικό έδειξαν μια μέτρια υποβάθμιση των ιδιοτήτων αντοχής, αλλά ταυτόχρονα μια σημαντική πτώση των ιδιοτήτων ολκιμότητας. Η μελέτη των επιφανειών θραύσης ανέδειξε την ύπαρξη ψαθυροποιημένων περιοχών κάτω από το στρώμα διάβρωσης. Η ύπαρξη τέτοιων ψαθυροποιημένων ζωνών έχει αποδοθεί από προηγούμενες εργασίες στην προσρόφηση υδρογόνου που παράγεται κατά την διαδικασία της διάβρωσης. Το υπολογιστικό σκέλος περιλαμβάνει την ανάπτυξη μοντέλου πεπερασμένων στοιχείων σε μίκρο και μάκρο-κλίμακα. Για την προσομοίωση της συμπεριφοράς εφελκυσμού του διαβρωμένου υλικού σε μίκρο-κλίμακα αναπτύχθηκε μια Αντιπροσωπευτική Μοναδιαία Κυψελίδα (Representative Unit Cell) που περιλαμβάνει τρήμματα αντιπροσωπευτικά του χρόνου έκθεσης σε διαβρωτικό περιβάλλον. Η τοπικά υποβαθμισμένη συμπεριφορά εφελκυσμού του υλικού, λόγω της ύπαρξης των τρημμάτων, προσδιορίζεται από τις αντιπροσωπευτικές μοναδιαίες κυψελίδες και εισάγεται τοπικά σε ένα μοντέλο μάκρο-κλίμακας. Το μοντέλο λαμβάνει υπόψη την διαφορετική ένταση της βλάβης διάβρωσης στις διάφορες περιοχές του δοκιμίου. Τα αποτελέσματα του μοντέλου έδειξαν ικανοποιητική σύγκλιση με τις μηχανικές δοκιμές σε ότι αφορά στις ιδιότητες αντοχής. Αντίθετα, έδειξαν υποτίμηση της πειραματικά παρατηρούμενης μείωσης των ιδιοτήτων ολκιμότητας. Η υποτίμηση αυτή οφείλεται στο γεγονός ότι το μοντέλο, κα' αρχήν, δεν λαμβάνει υπόψη τους φυσικούς μηχανισμούς της ψαθυροποίησης του υλικού λόγω διάβρωσης. Η μεθοδολογία που αναπτύχθηκε αποτελεί βήμα για τη σύνδεση της βλάβης διάβρωσης με τις απομένουσες μηχανικές ιδιότητες του υλικού και, επομένως, την ασφαλέστερη εκτίμηση της απομένουσας αντοχής διαβρωμένων αεροπορικών δομών. / Corrosion damage accumulation represents a major threat for the structural integrity of metallic aircraft structures and moreover has a strong effect on the load bearing capacity of aging aircraft structures. Corrosion damage is evaluated by means of metallographic features such as pitting density, depth and shape of pits, onset of exfoliation, etc. For the case of static loading, corrosion damage is usually accounted through reducing the metal thickness by the depth of corrosion attack and then calculating the corresponding stress increase. For the case of fatigue, corrosion pits are considered as possible onsets for fatigue cracks. The aim of the present PhD thesis is to contribute to establish a link between the metallographic features of corrosion damage and the degradation of the mechanical properties of a corroded material. Towards this objective, a methodology is developed which allows the numerical simulation of the tensile behavior of the corroded material based on the metallographic features of the corrosion damage. The present work is divided in two parts: a) the experimental investigation and b) the numerical analysis. The experimental part includes an extensive metallographic investigation of the occurring corrosion damage. Moreover, tensile tests were performed on the pre-corroded material which was exposed to the corrosive solution for several exposure periods. Finally, an examination of the fracture surfaces for the identification of the physical mechanisms of the damage has also been conducted. The main conclusion extracted from the metallographic procedure is that corrosion damage evolves from pitting to exfoliation progressively. The tensile tests performed on the pre corroded material revealed a moderate reduction concerning the tensile strength but a significant degradation of the tensile ductility even after short exposure periods. The examination of the fracture surfaces revealed the presence of quasi-cleavage zones beneath the depth of corrosion attack. The formation of these zones has been attributed by previous investigations to hydrogen diffusion and trapping into the corroded material during the corrosion process. The simulation procedure involves the development of a multi scale finite element model. The corrosion damage has been accounted for by introducing 3D Representative Unit Cells (RUCs) developed in the micro scale, with geometrical characteristics obtained by the metallographic analysis data of the corroded material. The degradation of the Representative Unit Cell’s mechanical properties due to the presence of the damage has been recorded. A 3D Finite Element model of a tensile specimen has been developed. This model has been used to simulate the tensile behavior of the corroded material, by including elements with degraded properties extracted from the RUC analysis. For the different exposure times RUCs with different geometrical characteristics were used so as to account for the evolving corrosion damage. The simulation results correlate well with the respective tensile behavior of the alloy obtained by the mechanical tests. As far as tensile ductility is concerned a significant deviation was observed, due to the fact that the finite element model does not account for the embrittlement of the material due to hydrogen absorption. The developed methodology represents a step towards the establishment of a link between the metallographic features of the corrosion damage and the residual mechanical properties of the material, and thus the more reliable estimation of the residual strength of the corroded aircraft structures.

No description available.

This study investigates the tensile behavior of anchor pull-out tests from super early age concrete(less than 12h) by finite element(FE) software ANSYS Workbench. In previous experiment, several series of pull-out tests were finished and analyzed. In each per hour, different speeds(1mm/s, 0.2mm/s, 0.1mm/s and 0.833mm/s) were evaluated, getting the results about correlation of pull-out force and displacement(F-D curve). It is difficult to evaluate the specific development of tensile strength in super young concrete, due to the super plasticity that makes itself soft and unstable. The first step of this study is to collect relevant empirical formula, theoretical varying material properties with time and pull-out force of experimental applied anchors. Comparison of simulation analysis results and empirical formulas determines whether the establishment of the finite element model and adapted constitutive model of known natural hardened concrete(NHC) are valid or not. The second procedure is that the material properties of NHC are replaced by different age values and modified until getting the same simulation results as experiment outcome. The propose of this paper is to investigate a more accurate modified formula to describe the development of tensile behavior in super early age concrete:1. Introduction 2. Background 2.1 Modes of failure 2.2 A new failure mode 2.3 Finite Element Numerical Simulation 3. Research Questions 4. Aims/Objectives of the Research 5. Proposed Research Method 5.1 Previous Empirical theory 5.1.1 Cubic Compressive strength of Early Age Concrete 5.1.2 Tensile Strength of Early Age Concrete 5.1.3 Modulus of Elasticity in Early Age Concrete 5.1.4 Prediction of pull-out maximum force to headed studs from concrete 5.2 Pervious Experiment 5.3 Numerical Simulation 6. Significance/Contribution to the Discipline 7. Experiment Program 7.1 Experiment Setup 7.2 Experiment Result 8. Numerical simulation and analysis 8.1 Material Properties 8.2 Modelling Setup 8.3 The first pull-out test 8.4 Comparison Results at different stages 9. Discussion and Results 10. Summary and Conclusion 11. Recommendation for future studies 12. References 13. Appendix

La tesis se enmarca en la caracterización, a nivel material, de la fractura del hormigón reforzado con fibras de acero (SFRC) bajo solicitaciones de tracción y cortante, y sobre la determinación de parámetros que representan la tenacidad del material sometido a esos dos modos de carga. Asimismo, se han realizado ensayos hasta rotura por cortante de elementos estructurales a escala real, los cuales se han analizado utilizando formulaciones existentes en distintos códigos de diseño.El comportamiento a tracción uniaxial del hormigón reforzado con fibras de acero se caracteriza utilizando cilindros entallados, elaborados con hormigones de resistencia normal y alta, con y sin fibras de acero. La metodología se extiende también para testigos extraídos de elementos de mayor tamaño. Los resultados se utilizan para definir parámetros de tenacidad y resistencia equivalentes de post-pico utilizables para representar el comportamiento del material y para un posible diseño estructural. Además, se desarrolla un estudio paramétrico experimental, que considera diferentes variables del ensayo y forma de probetas, para definir una configuración confiable del ensayo. Se analizan los modos de rotura observados y se evalúa la respuesta tensión-ancho de fisura. Asimismo, se propone una relación tensión-apertura de fisura característica para el diseño y análisis estructural. El comportamiento a tracción uniaxial se compara también con el de flexión y tracción por compresión diametral.La fractura por cortante se estudia a nivel material, en hormigones de resistencia normal y alta, con y sin fibras de acero, utilizando la configuración de cortante directo denominada push-off. Se analizan el modo de rotura y la respuesta tensión-desplazamiento. Además, se definen parámetros basados en la tenacidad y tensiones equivalentes de cortante para una posible utilización en el diseño estructural.Con el fin de obtener resultados que validen la utilización de las fibras de acero como refuerzo de cortante y al mismo tiempo estudiar la fractura por cortante a nivel estructural, se han realizado ensayos a escala real sobre vigas de sección rectangular y en T. Se analizan las respuestas carga-flecha y carga-ancho de fisura de vigas rectangulares de hormigón reforzado con fibras de acero variando su altura y de vigas T variando las dimensiones del ala. Los resultados obtenidos experimentalmente se utilizan para verificar la aplicabilidad de los métodos de diseño existentes en el caso del hormigón reforzado con fibras de acero. Además, se presenta una propuesta para el diseño a cortante basada en la respuesta tensión-desplazamiento relativo obtenida a partir del ensayo push-off de cortante directo. / The thesis deals with the characterization of the failure of steel fiber reinforced concrete (SFRC) in tension and shear, on the material level, and the determination of parameters that represent the toughness in these two modes of failure. Tests have been performed on large-scale beams failing under shear failure, which have been analyzed using existing design code formulas. The toughness parameters determined from the material are used in the design against such failure.The uniaxial tensile behavior of SFRC is characterized using notched cylinders of normal and high strength concretes, with and without steel fibers. The methodology is also extended to cores extracted from large elements. Results are used to define toughness parameters and equivalent post-peak strengths to be used for representing the material behavior and for possible structural design. Furthermore, a parametric study considering different test variables and specimen shape is carried out in order to define a reliable test configuration. The observed modes of failure are analyzed and the stress-crack width response is evaluated. Also, a characteristic stress-crack width response is proposed for structural analysis and design. The uniaxial tension behavior is also compared with that of flexural and splitting-tension.The shear failure is studied using the direct shear push-off test configuration, in normal and high strength concretes with and without steel fibers. The mode of failure and the stress-slip and stress-crack opening responses are analyzed. Toughness parameters and equivalent shear strengths based on the test results are defined for structural design.In order to provide results for validating the use of steel fibers as shear reinforcement and for studying shear failure at the structural level, full-scale tests on rectangular and T-beams were performed. The load-deflection and load-crack width responses are analyzed and compared with results of plain concrete beam tests. The experimentally-obtained results are used to evaluate the applicability of existing design methods for steel fiber reinforced concrete. Furthermore, a proposal for shear design based on the shear stress versus slip relationship from the push-off shear test is presented.

During the last four decades, fiber reinforced concrete has been increasingly used in structural applications. It is generally accepted that addition of steel fibers significantly increases tensile toughness and ductility, also slightly enhances the compressive strength. Although several studies have reported previously the favorable attributes of steel fiber reinforced concrete (SFRC), little general data is related to performance modeling. There are studies on the effect of fibers on compression, tension and shear behavior of concrete. As models proposed so far can, at best, describe only a few aspect of SFRC with a given type and amount of fibers, establishing simple and accurate generalized equations to describe the behavior of SFRC in tension, compression and shear that take into account the fiber type and content is essential. Therefore, a comprehensive experimental research on SFRC is conducted in University of Canterbury to develop generalized equations to represent the characteristics of SFRC. In this research, standard material tests of SFRC are carried out in tension, compression and shear to enable the parametric characterization and modeling of SFRC to be conducted. The tests are conducted using two different propriety fiber types (NovotexTM and DramixTM) with volumetric ratios ranging from 0 to 2 percent of the Novotex fibers and with 1 percent Dramix fibers. Compression tests are conducted on small and large cylinders. For characterization of tensile behavior, several different test methods are used including: direct tension of SFRC alone; SFRC with tension applied to an embedded longitudinal rebar; and flexural bending test. Similarly direct shear tests are conducted to investigate the additional shear resistance contributed by steel fibers. Variations in the results of different specimens are reconciled through normalization of stress and strain parameters. Based on the experimental results, empirical relations are derived for modeling and analysis of SFRC.

Titanium alloys are important engineering alloys that are extensively used in various industries. This is due to their unique combination of mechanical and physical properties such as low density combined with high strength and toughness as well as outstanding corrosion resistance. An additional benefit associated with Ti alloys, in general, is that their properties are relatively temperature-insensitive between cryogenic temperature and ~500 °C. Amongst the Ti alloys, Ti-6Al-4V (referred as Ti64) is a widely used alloy. Conventionally cast Ti64 possesses classical Widmanstätten microstructure of (hcp) α and (bcc) β phases. However this microstructure suffers from large prior β grain size, which tends be in the order of a few mm. Such large grain sizes are associated with poor processability as well as inferior mechanical performance. The necessity to break this coarse as-cast microstructure down, through several successive thermo-mechanical processing steps, adds considerably to the cost of finished Ti alloy products, making them expensive vis-à-vis other competing alloys. The addition of small amount of B (~0.1%) to Ti64 alloys, on the other hand reduces the cast grain size from couple of mm to ~200 µm. Moreover, addition of B to Ti alloys produces the intermetallic TiB needles during solidification by an in situ chemical reaction. The overall objective of this work is to gain insights into the role of microstructural modifications, induced by B addition to Ti64, on the mechanical performance of the alloys, in particular the room temperature damage tolerance (fracture toughness and fatigue crack growth) characteristics. The key questions we seek to answer through this study are the following: (a) What role does the microstructural refinement plays on the quasistatic as well as fracture and fatigue behavior and high temperature deformability of the alloys? (c) A hierarchy of microstructural length scales exist in Ti alloys. These are the lath, colony and grain sizes. Which of these microstructural parameters control the mechanical performance of the alloy? (b) What (possibly detrimental) role, if any, do the TiB needles play in influencing the mechanical performance of Ti64 alloys? This is because TiB being much stiffer, strain incompatibility between the matrix and the TiB phase could lead to easy nucleation of cracks during cyclic loading as well as can pose problems during dynamic deformation. (d) What is the optimum amount of B that can be added to Ti64 such that the most desirable combination of properties can be achieved? Five B-modified Ti64 alloys with B content varying from 0.0 to 0.55 wt.% were utilised to answer the above questions. Marked prior β grain size reduction was noted with up to 0.1 wt.% B addition. Simultaneous refinement of α/β colony size has also been observed. The addition of B to Ti64, on the other hand increases the α lath size. The TiB needles that form in-situ during casting are arranged in a necklace like structure surrounding the grain boundaries for higher B added Ti64 alloys. An anomalous enhancement in elastic modulus, E, of the alloy with only 0.04 wt.% B to Ti64 was found. E has been found to follow the same trend of variation with B content at higher temperatures (up to 600 °C) as well. Nanoindentation experiments were conducted to evaluate the moduli of the various phases present in the microstructure and then rationalize the experimental trends within the framework of approximate models. Marginal but continuous enhancement in strength of the alloys with B addition was observed. It correlates well with the grain size refinement according to Hall-Petch relationship. Ductility on the other hand increases initially with up to 0.1 wt.% B addition followed by a reduction. While the former is due to the microstructural refinement, the latter is due to the presence of significant amount of brittle TiB phase. Room temperature fracture toughness decreases with B addition to Ti64. Such reduction in fracture toughness with the refinement of prior β grain size has been justified with Ritchie-Knott-Rice model. Contradictory roles of microstructural refinement have been observed for notched and un-notched fatigue. While reduction in length scale has a negative role in crack propagation, it enhances the fatigue strength of the alloy owing to better resistance to fatigue crack initiation. TiB needles on the other hand act as sites for crack initiation and hence limit the enhancement in fatigue strength of alloys with 0.30 and 0.55 wt.% B. An investigation of the high temperature deformability of the alloys has been performed over a wide range of temperature (within the two phase α+β regime) and strain rate windows. Results show that microstructural refinement does not alter the high temperature deformation characteristics as well as optimum processing conditions of the alloys. TiB needles, however act as sites for instability owing to differences in compressibility between the matrix and the whisker phase. In summary, this study suggests that the addition of ~0.1 wt.% B to Ti64 can lead to the elimination of certain thermo-mechanical processing steps that are otherwise necessary for breaking the as-cast structure down and hence make finished Ti components more affordable. In addition, it leads to marginal enhancement in the quasi-static properties and significant benefits in terms of high cycle fatigue performance.

No description available.