Rozprawy doktorskie na temat „Strain Rate Sensitivity (SRS)”

Doutoramento em Ciência e Engenharia de Materiais
Nesta tese são apresentados estudos experimentais e microestruturais para a análise e controlo da sensibilidade à velocidade de deformação (SRS) da liga AA5182 e do aço TWIP com o objetivo de melhorar o comportamento mecânico destes materiais. Os aços TWIP são materiais com elevada resistência mecânica e excecional capacidade de encruamento, parâmetros que conduzem à absorção de uma quantidade significativa de energia antes de rotura. As ligas de AlMg são materiais leves, com boa resistência à corrosão e boas propriedades mecânicas. A larga variedade de aplicações, como por exemplo na indústria automóvel, permitirá melhorar a performance dos produtos e economizar energia. O maior problema destes materiais prende-se com a baixa ou negativa sensibilidade à velocidade de deformação que conduz a uma deformação heterogénea e limita a deformação após estricção. Neste trabalho são estudados métodos para melhorar a SRS das ligas de AlMg através de combinação de deformação plástica severa e tratamentos térmicos, e é investigada a origem física da baixa ou até negativa SRS do aço TWIP através de ensaios à escala macro, micro e nano. Estes estudos são complementados e sustentados por um amplo programa de observações microestructurais através de técnicas de microscopia TEM, SEM e EBSD. A deformação plástica severa na liga de AlMg foi aplicada através de laminagem. Foi demonstrado que o tipo de laminagem (simétrica versus assimétrica), o grau de redução de laminagem e o tratamento térmico realizado após a laminagem são os principais fatores que afetam a evolução da SRS. Especificamente, o aumento do grau de laminagem (de 50% para 90%) resulta num aumento da SRS. A técnica de laminagem assimétrica inversa (ASRR) revelou ser a mais eficiente no aumento do SRS, sendo que esta produz a maior deformação equivalente no material. Adicionalmente, para este tipo de laminagem e uma redução da espessura de 90%, verificou-se que a tensão de cedência aumenta para um tratamento térmico mais longo (de 30min a 120min). Conjetura-se que o processo físico associado ao comportamento observado está relacionado com a movimentação de ida e volta de solutos de Mg da solução sólida para precipitados/cachos durante o processo de laminagem e posterior tratamento térmico. A investigação à sensibilidade da velocidade de deformação de aço TWIP com base em testes mecânicos e caracterização microestrutural foi outro objetivo desta tese. Demonstrou-se que as amostras testadas com uma velocidade de deformação reduzida apresentam uma densidade de maclas maior do que as amostras testadas a uma velocidade de deformação maior. À escala macroscópica este traduz-se numa taxa de encruamento maior para velocidades reduzidas, conduzindo a um coeficiente de sensibilidade à velocidade de deformação em termos de taxa de encruamento negativo. Foi observada uma diminuição da SRS com o aumento da deformação, passando de valores positivos a negativos. O presente estudo demonstrou a importância da medida de escala utilizada na investigação do SRS através de uma combinação de testes de micro- e nano-indentações. Nomeadamente, quando o material é testado a uma escala nanométrica, através de nano-indentação, as amostras pré-deformadas em tração com taxas de deformação menores apresentam sistematicamente uma dureza menor do que as amostras pré-deformadas com taxas mais elevadas. À medida que o volume de material testado aumenta, a dureza relativa das duas amostras passa gradualmente da tendência observada à escala nano para aquela observada à escala macroscópica. O efeito está ligado ao mecanismo de interação entre as estruturas de deslocações e maclas.
In this thesis are presented experimental and microstructural studies for strain rate sensitivity (SRS) control and analysis of AA5182 and Twinning Induced Plasticity steel for improved mechanical performance. TWIP steels are materials with very high strength and exceptional strain hardening capability, parameters leading to large energy absorption before failure. Al-Mg alloys are lightweight materials with good corrosion resistance and adequate material properties. The broader use of these materials, for example in the automotive industry, would allow improved product performance and energy savings. The formability of these materials is strongly affected by their negative strain rate sensitivity (SRS) which leads to early failure and limits the post necking deformation. In this work we study ways to improve the strain rate sensitivity of Al-Mg alloys through a combination of severe plastic deformation and annealing, and we investigate the physical origins of the low and potentially negative strain rate sensitivity of TWIP steel through macro, micro and nanoscale testing. These studies are supported by extensive microstructural observations. The severe plastic deformation applied to Al-Mg alloys is applied by rolling. It is shown that the type of rolling (symmetric versus asymmetric), the rolling reduction degree and the applied heat treatment performed after rolling are the main factors affecting the evolution of SRS. Specifically, SRS increases with increasing the degree of rolling for given post-rolling heat treatment. The reversed asymmetric rolling technique appears to be the most efficient in increasing SRS since it produces the largest equivalent plastic strain in the sample. Furthermore, the evolution of tensile flow stresses depends on the chosen thermal treatment; it was observed that the yield stress increases with increasing the annealing time for rolling reduction of 85%. It is conjectured that the physical process responsible for the observed behavior is related to the movement of Mg from solid solution to precipitates/clusters and back during rolling and subsequent annealing. The investigation of the strain rate sensitivity of TWIP steel based on mechanical tests and microstructural characterization is another objective of this thesis. It was demonstrated that slower-deformed samples have a higher twin density, which leads to larger flow stress measured in a macroscopic uniaxial test and results in negative strain hardening rate sensitivity. The SRS is observed to decrease with strain, becoming negative for larger strains. The correlation between SRS and the probing scale was revealed by a combination of micro- and nano-indentation experiments. When probed at the nanoscale by nano-indentation, samples pre-deformed in tension at smaller strain rates exhibit systematically smaller hardness than samples pre-deformed at higher rates. As the volume of material probed increases, the relative hardness of the two types of samples gradually shifts from the trend observed at the nanoscale to that observed macroscopically. The effect is linked to the dislocation-twin interaction mechanism.

Includes bibliographical references (leaves 210-219)
An investigation into high strain rate behaviour of polymer composites was performed by developing a finite element model for a fibre reinforced polymer (FRP) plates impacted at varying strain rates. The work was divided into three facets, firstly to characterize the FRP material at varying strain rates, to develop a constitutive model to elucidate the relationship between strain rate and ultimate stress and lastly to use the experimental data to develop a finite element model. Experimental work performed in support of this model includes material characterization of unidirectional carbon and glass fibre reinforced epoxy at varying impact strain rates. The data is then used to develop a suite of constitutive equations that relate the strain rate, ultimate stress and material loading type. The model is of a linear and non-linear viscoelastic type, depending on the type of loading and is applicable to a FRP plate undergoing out-of-plane stresses. This model incorporates techniques for approximating the quasi-static and dynamic response to general time-varying loads. The model also accounts for the effects of damage, the linear and non-linear viscoelastic constitutive laws reporting failure by instantaneously reducing the relevant elastic modulus to zero. An explicit solver is therefore utilised in order to ensure stability of the numerical procedure. Glass fibre reinforced plastics (GFRP) was found to be more strain rate sensitive in all directions when compared to carbon fibre reinforced plastics (CFRP). The validation process therefore involves plate impact experimental testing on GFRP plates. The data from these experiments compare to within 8% of the finite element model that incorporates both damage and the developed strain rate sensitivity constitutive equations. For the first time a model that includes progressive damage with built-in strain rate sensitivity is developed for these particular FRP systems. Furthermore, the ultimate stress has been related to strain rate using an empirical technique. This technique allows for the prediction of dynamic ultimate stresses given the quasi-static ultimate stresses, again for this particular material systems.

The stress-strain behavior and failure of composite materials are strain rate sensitive, and influenced by the dimensions of the structure. To elucidate the combined effects of scaling and strain rate on the strength of unnotched continuous fiber reinforced composites, an experimental investigation has been conducted on Newport NB321/7781 fiberglass/epoxy and Toray T800/3900-2B unitape/epoxy materials. The experimental results have been characterized in terms of failure strength, failure modes and the Weibull modulus m. A 2D-scaling approach has been followed and composite coupons were fabricated with [0]4 and [±45]s stacking sequences. The experimentation has been conducted at strain rates ranging from quasi-static (0.0002 s^-1) to high strain rate (50 s^-1), to study the mechanical responses and associated failure modes. Subsequently, the Weibull statistical model was utilized to characterize the scaling behavior at different strain rates. The average failure stress of [0]4 carbon, [0]4 fiberglass and [±45]s fiberglass specimens were observed to decrease with increasing specimen size at each strain rate. However, at high strain rate, the percentage of strength reduction was observed to be lower in comparison to the quasi-static strain rate. Owing to the free edge effects, the scaling effect was maximum for [+45/-45]s carbon unitape specimens. But unlike the other stacking sequences, the percentage of strength reduction at higher strain rates was higher compared to quasi-static strain rate, indicating increased scaling effects with strain rate. Weibull modulus m for the specimens tended to increase with increasing strain rate indicating diminishing scaling effects, while [+45/-45]s carbon specimens exhibited opposite trend. Failure at multiple locations was observed in larger coupons at high strain rate, which results in size and strain rate dependent fracture behavior.
Thesis (M.S.)--Wichita State University, College of Engineering, Dept. of Aerospace Engineering.

Plasticity in crystalline solids is controlled by the microscopic line defects known as “dislocations”. Decisive role of dislocations in crystal plasticity in addition to fundamentals of plastic deformation are presented in the current thesis work. Moreover, major features of numerical modeling method “Discrete Dislocation Dynamics (DDD)” technique are described to elucidate a powerful computational method used in simulation of crystal plasticity. First part of the work is focused on the investigation of strain rate effect on the dynamic deformation of crystalline solids. Single crystal copper is chosen as a model crystal and discrete dislocation dynamics method is used to perform numerical uniaxial tensile test on the single crystal at various high strain rates. Twenty four straight dislocations of mixed character are randomly distributed inside a model crystal with an edge length of 1 µm subjected to periodic boundary conditions. Loading of the model crystal with the considered initial dislocation microstructure at constant strain rates ranging from 103 to 105s1 leads to a significant strain rate sensitivity of the plastic flow. In addition to the flow stress, microstructure evolution of the sample crystal demonstrates a considerable strain rate dependency. Furthermore, strain rate affects the strain induce microstructure heterogeneity such that more heterogeneous microstructure emerges as strain rate increases. Anisotropic characteristic of plasticity in single crystals is investigated in the second part of the study. Copper single crystal is selected to perform numerical tensile tests on the model crystal along two different loading directions of [001] and [111] at two high strain rates. Effect of loading orientation on the macroscopic behavior along with microstructure evolution of the model crystal is examined using DDD method. Investigation of dynamic response of single crystal to the mechanical loading demonstrates a substantial effect of loading orientation on the flow stress. Furthermore, plastic anisotropy is observed in dislocation density evolution such that more dislocations are generated as straining direction of single crystal is changed from [001] to [111] axis. Likewise, strain induced microstructure heterogeneity displays the effect of loading direction such that more heterogeneous microstructure evolve as single crystal is loaded along [111] direction. Formation of slip bands and consequently localization of plastic deformation are detected as model crystal is loaded along both directions.

QC 20151015

High-entropy alloys (HEAs) that demonstrate excellent mechanical properties over steel-based alloys are not exempt from the common dilemma of strength–ductility trade-off, which limits their potential applications. One way to improve the property of CrMnFeCoNi HEA is by using the rotationally accelerated shot peening technique to introduce a gradient structure. Two gradient profiles—a thin gradient layer with an undeformed core and a fully deformed structure—are introduced by adjusting the processing parameters. The effects of these gradient profiles on mechanical properties and microstructural evolution at various loading conditions and temperatures are systematically explored. In this thesis, various mechanical tests are performed to investigate the effect of the gradient structure on mechanical properties such as tensile properties at room and cryogenic temperatures, compression at different strain rates and dynamic compression at high strain rates. Material characterisations are performed using various electron microscopic techniques to build a structure–property relationship and investigate microstructural evolution.

En raison de l'augmentation de la demande d’allègement pour les véhicules automobiles, des solutions doivent être créées pour permettre aux constructeurs automobiles de passer d'aciers hautement formables mais lourds à des alliages d'aluminium moins formables mais plus légers pour les carrosseries en blanc. Les alliages d'aluminium de la série 6xxx, basés sur le système Al-Mg-Si-Cu, se sont révélés prometteurs en termes de résistance mécanique et de résistance à la corrosion, mais, l'une de leurs principales limitations concerne leur formabilité. Cette thèse vise à comprendre l'effet des additions de Si, Mg et Cu sur les propriétés mécaniques et de formabilité de la série AA6xxx. La calorimétrie différentielle à balayage et les essais de dureté sont utilisés pour identifier les effets de l'addition de solutés sur la microstructure d’amas de solutés après vieillissement naturel et pré-vieillissement. Les essais de traction donnent accès aux principales propriétés mécaniques : limite d'élasticité, résistance à la traction, taux d’écrouissage et allongement uniforme. Le test de sensibilité à la vitesse de déformation est effectué à l'aide de sauts de vitesse afin d'obtenir non seulement la sensibilité à la vitesse ascendante, mais moins classiquement la sensibilité à la vitesse descendante. Enfin, à l'aide d'équations constitutives, les propriétés mécaniques sont utilisées dans une modélisation par éléments finis pour saisir l'évolution de la déformation et de la vitesse de déformation dans la transition de la striction diffuse à localisée. Dans le cas du vieillissement naturel d'un mois (NA1m), deux types d'amas ont été détectés, une espèce moins stable thermiquement ayant une forte dépendance aux teneurs en Cu et Mg, et une espèce plus stable thermiquement ayant la même sensibilité à toutes les espèces de solutés. Lorsque les échantillons sont pré-vieillis, puis laissés pendant un mois (sNA1m), seule l’espèce d’amas thermiquement plus stables et également sensible à tous les ajouts de solutés existe. La formation de ces différents types d’amas en fonction du traitement thermique s'est traduite par les effets de l'ajout de solutés spécifiques sur les propriétés mécaniques observées. Dans l'état NA1m, les effets des additions de Cu et de Mg à l'alliage ont montré les plus fortes augmentations de la limite d'élasticité et du taux d’écrouissage, par rapport aux additions de Si. Ceci contraste avec la condition sNA1m pour laquelle les additions de Cu, Mg et Si augmentent toutes la limite d'élasticité de façon égale tandis que les additions de Cu se sont avérées avoir le plus fort effet sur l'augmentation du taux de durcissement par déformation, suivies par l'effet d’additions de Si, tandis que les additions de Mg n'ont pas eu d'effet. Les tests de sensibilité à la vitesse de déformation ont révélé une asymétrie entre les tests de variation vers le haut et vers le bas, selon laquelle la sensibilité à la vitesse de variation vers le bas est plus grande que la sensibilité à la vitesse de variation vers le haut. De plus, on a constaté que les ajouts de Si augmentent à la fois la sensibilité à la vitesse de déformation à variation ascendante et à variation descendante dans les conditions NA1m et sNA1m. Enfin, l'application de ces propriétés mécaniques à l'étude de l'évolution des strictions diffuse et locale a démontré que l'augmentation de l'exposant d’écrouissage retarde l'apparition du col diffus, par ailleurs l’augmentation de la sensibilité à la vitesse de déformation permet une distribution plus uniforme des déformations et des vitesses de déformation, permettant ainsi la stabilisation et la propagation du col de striction et retardant l'apparition du col local. L'effet de la sensibilité à la vitesse de déformation ascendante s'est révélé plus important que la variation descendante en raison de l'intensité de l'augmentation de la vitesse de déformation à l'intérieur du col sur une zone beaucoup plus petite
Due to the increased demand for light weighting in automotive vehicles, solutions need to be created to allow automotive manufacturers to switch from highly formable but heavy steels to less formable but lighter aluminium alloys for body-in-white components; doors, roofs, hood. The 6xxx-series of aluminium alloys, based on the system of Al-Mg-Si-Cu, have shown promise for providing adequate strength and corrosion resistance but still, in the current state, one of their main limitations concerns their formability. This thesis aims to understand the effect of Si, Mg, and Cu additions under two different processing routes on the mechanical and formability properties of the AA6xxx-series. Differential scanning calorimetry and hardness testing are used to identify the effects of solute additions on the cluster states after natural ageing and pre-ageing. Tensile testing is used to capture the main mechanical properties: yield strength, tensile strength, strain hardening rate, and uniform elongation. Strain rate sensitivity testing is performed using dynamic strain rate changes to obtain not only the strain rate sensitivity due to rate-change increases (termed up-change), but uniquely, the strain rate sensitivity for rate-change decreases (termed down-change). Finally, using constitutive equations, the mechanical properties are used in combination with finite element modeling to capture the evolution of the strain and strain rate distribution in the evolution and transition of diffuse to local necking. It was found that in the case of natural ageing for one month (NA1m) two cluster types were detected, a less thermally stable species having a high dependency on the Cu and Mg contents, and a more thermally stable species being equally sensitive to all solute species. When samples were first pre-aged, then allowed to naturally age for one month (sNA1m) only the more thermally stable cluster species being equally sensitive to all solute additions existed. The formation of these different cluster types dependent on the heat treatment translated into the effects of specific solute additions on the observed mechanical properties. In the NA1m condition, the effects of Cu and Mg additions to the alloy showed the largest increases on the yield strength and strain hardening rate, as compared to Si additions. This is in contrast to the sNA1m condition whereby Cu, Mg, and Si additions all increased the yield strength equally while Cu additions proved to have the strongest effect on increasing the strain hardening rate, followed by the effect of Si additions, while Mg additions did not have an effect. From the strain rate sensitivity tests, an asymmetry between the up-change and down-change tests was observed whereby the down-change strain rate sensitivity was found to be larger than the up-change strain rate sensitivity. Additionally, Si additions were found to increase both the up-change and down-change strain rate sensitivity in both the NA1m and sNA1m conditions. Finally, the application of these mechanical properties to the onset and evolution of the diffuse and local neck demonstrated that increasing the strain hardening exponent delays the onset of diffuse necking, while increasing both the up-change and down-change strain rate sensitivities provides a more uniform strain and strain rate distribution around the neck, permitting the stabilization and propagation of the neck and delaying the onset of local necking. The effect of the up-change strain rate sensitivity was found to be more important than the down-change due to the intensity of the strain rate increase in the interior of the neck occurring over a much smaller area

Magnesium alloys have been of growing interest to various engineering applications, such as the automobile, aerospace, communication and computer industries due to their low density, high specific strength, good machineability and availability as compared with other structural materials. However, most Mg alloys suffer from poor plasticity due to their Hexagonal Close Packed structure. Grain refinement has been proved to be an effective method to enhance the strength and alter the ductility of the materials. Several methods have been proposed to produce materials with nanocrystalline grain structures. So far, most of the research work on nanocrystalline materials has been carried out on Face-Centered Cubic and Body-Centered Cubic metals. However, there has been little investigation of nanocrystalline Mg alloys. In this study, bulk coarse-grained and nanocrystalline Mg alloys were fabricated by a mechanical alloying method. The mixed powder of Mg chips and Al powder was mechanically milled under argon atmosphere for different durations of 0 hours (MA0), 10 hours (MA10), 20 hours (MA20), 30 hours (MA30) and 40 hours (MA40), followed by compaction and sintering. Then the sintered billets were hot-extruded into metallic rods with a 7 mm diameter. The obtained Mg alloys have a nominal composition of Mg–5wt% Al, with grain sizes ranging from 13 μm down to 50 nm, depending on the milling durations. The microstructure characterization and evolution after deformation were carried out by means of Optical microscopy, X-Ray Diffraction, Scanning Electron Microscopy, Transmission Electron Microscopy, Scanning Probe Microscopy and Neutron Diffraction techniques. Nanoindentaion, compression and micro-compression tests on micro-pillars were used to study the size effects on the mechanical behaviour of the Mg alloys. Two kinds of size effects on the mechanical behaviours and deformation mechanisms were investigated: grain size effect and sample size effect. The nanoindentation tests were composed of constant strain rate, constant loading rate and indentation creep tests. The normally reported indentation size effect in single crystal and coarse-grained crystals was observed in both the coarse-grained and nanocrystalline Mg alloys. Since the indentation size effect is correlated to the Geometrically Necessary Dislocations under the indenter to accommodate the plastic deformation, the good agreement between the experimental results and the Indentation Size Effect model indicated that, in the current nanocrystalline MA20 and MA30, the dislocation plasticity was still the dominant deformation mechanism. Significant hardness enhancement with decreasing grain size, down to 58 nm, was found in the nanocrystalline Mg alloys. Further reduction of grain size would lead to a drop in the hardness values. The failure of grain refinement strengthening with the relatively high strain rate sensitivity of nanocrystalline Mg alloys suggested a change in the deformation mechanism. Indentation creep tests showed that the stress exponent was dependent on the loading rate during the loading section of the indentation, which was related to the dislocation structures before the creep starts. The influence of grain size on the mechanical behaviour and strength of extruded coarse-grained and nanocrystalline Mg alloys were investigated using uniaxial compression tests. The macroscopic response of the Mg alloys transited from strain hardening to strain softening behaviour, with grain size reduced from 13 ìm to 50 nm. The strain hardening was related to the twinning induced hardening and dislocation hardening effect, while the strain softening was attributed to the localized deformation in the nanocrystalline grains. The tension–compression yield asymmetry was noticed in the nanocrystalline region, demonstrating the twinning effect in the ultra-fine-grained and nanocrystalline region. The relationship k tensions < k compression failed in the nanocrystalline Mg alloys; this was attributed to the twofold effect of grain size on twinning. The nanocrystalline Mg alloys were found to exhibit increased strain rate sensitivity with decreasing grain size, with strain rate ranging from 0.0001/s to 0.01/s. Strain rate sensitivity of coarse-grained MA0 was increased by more than 10 times in MA40. The Hall-Petch relationship broke down at a critical grain size in the nanocrystalline region. The breakdown of the Hall-Petch relationship and the increased strain rate sensitivity were due to the localized dislocation activities (generalization and annihilation at grain boundaries) and the more significant contribution from grain boundary mediated mechanisms. In the micro-compression tests, the sample size effects on the mechanical behaviours were studied on MA0, MA20 and MA40 micro-pillars. In contrast to the bulk samples under compression, the stress-strain curves of MA0 and MA20 micro-pillars were characterized with a number of discrete strain burst events separated by nearly elastic strain segments. Unlike MA0 and MA20, the stress-strain curves of MA40 micro-pillars were smooth, without obvious strain bursts. The deformation mechanisms of the MA0 and MA20 micro-pillars under micro-compression tests were considered to be initially dominated by deformation twinning, followed by dislocation mechanisms. For MA40 pillars, the deformation mechanisms were believed to be localized dislocation activities and grain boundary related mechanisms. The strain hardening behaviours of the micro-pillars suggested that the grain boundaries in the nanocrystalline micro-pillars would reduce the source (nucleation sources for twins/dislocations) starvation hardening effect. The power law relationship of the yield strength on pillar dimensions in MA0, MA20 supported the fact that the twinning mechanism was correlated to the pre-existing defects, which can promote the nucleation of the twins. Then, we provided a latitudinal comparison of the results and conclusions derived from the different techniques used for testing the coarse-grained and nanocrystalline Mg alloy; this helps to better understand the deformation mechanisms of the Mg alloys as a whole. At the end, we summarized the thesis and highlighted the conclusions, contributions, innovations and outcomes of the research. Finally, it outlined recommendations for future work.

Les procédés de mise en forme de tôles minces sont très largement répandus dans l'industrie. Néanmoins, l'utilisation de ces procédés est limitée par le niveau de formabilité du matériau formé, notamment dans le cas des alliages d'aluminium. Afin d'améliorer ces limites de formabilité, des procédés de mise en forme à chaud peuvent être envisagés. L'objectif de cette thèse est d'étudier à l'aide d'approches expérimentale et numérique l'effet de la température et de la vitesse de déformation sur la formabilité des tôles en alliage d'aluminium AA5086 et de proposer une modélisation capable de prédire ces effets. Une campagne d'essais a été réalisée sur ce matériau à partir d'un essai d'emboutissage de type Marciniak. Des courbes limites de formage (CLF) ont été établies sur une plage de température allant de l'ambiant jusqu'à 200°C et pour des vitesses de déformation allant du quasi-statique à 2s-1. Des effets, positif de la température et négatif de la vitesse de déformation sur la formabilité ont été mis en évidence. La prise en compte des effets de la température et de la vitesse de déformation dans les modèles prédictifs des CLF, qu'ils soient analytiques ou numériques, est à ce jour très limitée. Dans ce travail, un modèle numérique prédictif basé sur la simulation par éléments finis du modèle géométrique de Marciniak et Kuczynski (M-K) est proposé. Les déformations limites obtenues avec de ce modèle sont très sensibles à la description du comportement thermo-viscoplastique du matériau et à la calibration du défaut géométrique pilotant l'apparition de la striction dans le modèle M-K. Des essais de traction uniaxiale réalisés dans les mêmes conditions opératoires que les essais de mise en forme de Marciniak ont permis d'identifier des lois d'écrouissage de nature très différentes (rigidifiante, saturante ou mixte). Ces lois conduisent à des prédictions très différentes de la formabilité du matériau pour une valeur donnée du défaut géométrique du modèle EF M-K. Différentes stratégies de calibration de la taille de ce défaut initial ont été envisagées. L'utilisation du point expérimental de la CLF correspondant à des conditions de déformation plane permet de calibrer de manière satisfaisante la valeur de ce défaut. Cette procédure de calibration a été appliquée pour l'ensemble des lois identifiées. Les lois de nature rigidifiante de type Ludwick se sont montrées les plus effficaces alors que les lois saturante de type Voce se sont avérées incapables de prédire la formabilité du matériau pour certaines conditions opératoires. Finalement, il est démontré qu'une valeur constante du défaut géométrique ne peut être retenue pour l'ensemble des conditions opératoires étudiées même si le modèle M-K s'est avéré assez efficace pour représenter l'effet de la température plutôt que celui de la vitesse de déformation.

The objective of this study was to evaluate and develop an understanding of durability of an adhesive bonded system, for application in a future high speed civil transport (HSCT) aircraft structure. The system under study was comprised of Ti-6Al-4V metal adherends and a thermosetting polyimide adhesive, designated as FM-5, supplied by Cytec Engineered Materials, Inc. An approach based on fracture mechanics was employed to assess Ti-6Al-4V/FM-5 bond durability. Initially, wedge tests were utilized to find a durable surface pretreatment for the titanium adherends. Based on an extensive screening study, chromic acid anodization (CAA) was chosen as the standard pretreament for this research project. Double cantilever beam specimens (DCB) were then made and aged at 150° C, 177° C, and 204° C in three different environments; ambient atmospheric air (14.7 psia), and reduced air pressures of 2 psi air (13.8 KPa) and 0.2 psi air (1.38 KPa). Joints were aged for up to 18 months (including several intermediate aging times) in the above environments. The strain energy release rate (G) of the adhesive joints was monitored as a function of exposure time in the different environments. A 40% drop in fracture toughness was noted over the 18 month period, with the greatest degradation observed in samples aged at 204° C in ambient atmospheric air pressure. The loss in adhesive bond performance with time was attibutable to a combination of physical and chemical aging phenomena in the FM-5 resin, and possible degradation of the metal-adhesive interface(s). Several mechanical and material tests, performed on the bonded joints and neat FM-5 resin specimens, confirmed the above statement. It was also noted that physical aging could be "erased" by thermal rejuvenation, partially restoring the toughness of the FM-5 adhesive material. The FM-5 adhesive material displayed good chemical resistance towards organic solvents and other aircraft fluids such as jet fuel and hydraulic fluid. The results from the FM-5 adhesive and its bonded joints were compared and contrasted with VT Ultem and REGULUS polyimide adhesives. The FM-5 adhesive showed the best performance among the three adhesive systems. The effect of mode-mixity on the fracture toughness of the Ti-6Al-4V/FM-5 adhesive bonded system was also evaluated. DCB tests in conjunction with end-notched flexure (ENF) and mixed-mode flexure (MMF) tests, were used to fracture the bonded joints under pure mode I, pure mode II, and a combination of mode I and II loadings. The results showed that the mode I fracture toughness was twice as large as the mode II toughness. This was a rather surprising find, in sharp contrast to what several researchers have observed in the past. Our current understanding is that the crack path selection during the failure process plays a significant role in explaining this anomalous behavior. Finally, failure envelopes were generated for the titanium/FM-5 bonded system, both prior to and following thermal aging. These envelopes could serve as useful tools for engineers designing with Ti-6Al-4V/FM-5 bonds.
Ph. D.

The application of polymers in robust engineering designs is on the rise due to their excellent mechanical properties such as high fracture toughness, specific strength, durability, as well as, thermal and chemical resistances. Implementation of some advanced polymeric solids is limited due to the lack of available mechanical properties. In order for these materials to endure strenuous engineering designs it is vital to investigate their response in multiple loading rates and conditions. In this thesis, the mechanical response of polyethermide (PEI) is characterized under quasi-static, high strain rate, and multiple impact conditions. Standard tension, torsion, and compression experiments are performed in order to distinguish the multi-regime response of PEI. The effects of physical ageing and rejuvenation on the quasi-static mechanical response are investigated. The strain softening regime resulting from strain localization is eliminated by thermal and mechanical rejuvenation, and the advantages of these processes are discussed. The dynamic fracture toughness of the material in response to notched impact via Charpy impact test is evaluated. The high strain-rate response of PEI to uniaxial compression is evaluated at rates exceeding 104/s via miniaturized Split Hopkinson Pressure Bar (MSHPB), and compared to the quasi-static case to determine strain-rate sensitivity. The elastic response of the aged material to multiple loading conditions are correlated using the Ramberg-Osgood equation, while the elastoplastic response of rejuvenated PEI is correlated using a both the Ramberg-Osgood equation and a novel model. The strain-rate sensitivity of the strength is found to be nominally bilinear and transition strains are modeled using the Ree-Erying formulation. Finally, multiple impact experiments are performed on PEI using the MSHPB and a model is proposed to quantify damage as a result of collision.
B.S.M.E.
Bachelors
Engineering and Computer Science
Mechanical and Aerospace Engineering

Thesis (Ph. D.)--Materials Science and Engineering, Georgia Institute of Technology, 2008.
Committee Chair: Thadhani, Naresh; Committee Member: Doyoyo, Mulalo; Committee Member: Kecskes, Laszlo; Committee Member: Li, Mo; Committee Member: Sanders, Thomas; Committee Member: Zhou, Min.

Le procédé de soudage laser est largement utilisé dans les travaux d'assemblage, en particulier, dans ledomaine de l'industrie automobile. L'acier dual phase DP600 est un acier à haute résistance qui permet deréduire le poids de l'automobile dans le cadre de l'allègement des structures. Notre travail s' estessentiellement basé sur l'évaluation des contraintes résiduelles générées dans l'acier DP600 lors du soudagepar laser. Deux approches ont été réalisées. L'approche expérimentale a été réalisée à l'aide de méthodes derayon X et par neutrons pour calculer les contraintes résiduelles. L'approche de simulation a été réalisée parcouplage de différentes formulations numériques.Numériquement, le formalisme de la mécanique continue a été utilisé par des simulations par éléments finis(FEM) pour analyser et évaluer les contraintes résiduelles. Sur la base de tests de traction expérimentaux, lemodèle constitutif élasto-thermo-plastique de l'acier DP600 a été identifié. L'écrouissage du matériau a étéétudié par la loi de Ludwik et de Voce. A partir de résultats experimentaux, un modèle a été proposé et lesrésultats analysés en utilisant une loi de mélange martensite (écrouissage Ludwik) et ferrite (adoucissementde Voce). De même, nous avons étudié la sensibilité à la température en utilisant plusieurs modèles :Johnson-Cook, Khan, Chen. A partir de cette étude, nous avons proposé un modèle de sensibilité à tatempérature. Enfin, un modèle de sensibilité à la déformation plastique, à la vitesse de déformation issu destravaux d'A.Gavrus et un modèle d'anisotropie planaire définit par la théorie de Hill ont été ajoutés.Une méthode d'automate cellulaire (CA) 2D a été programmée pour simuler l'évolution de la microstructurelors de la solification liée au processus de soudage laser. Dans ce modèle, les phénomènes de nucléationavec prise en compte de l'orientation de la croissance, de la concentration et de la vitesse de croissance àl'interface solide/liquide, l'anisotropie de la tension de surface, de la diffusion, ainsi que la fraction desphases en présence ont été pris en compte. De plus, les équations de conservation ont été étudiées en détail etanalysés. Les résultats ainsi que le champ de température issu du modèle FEM ont été importés dans lemodèle CA. En comparant la simulation et les résultats expérimentaux, de bonnes concordances ont ététrouvées.Par la suite, nous avons réalisés un couplage des deux modèles CA et FEM. Concernant le procédé laser, lesrésultats du modèle par éléments finis ont été analysés. La géométrie de l'échantillon, la source de chaleur,les conditions aux limites, le comportement thermo-mécanique de l'acier dual phase DP600 telles que laconductivité, la densité, la chaleur spécifique, l'expansion, l'élasticité et la plasticité sont introduites. Lesmodèles d'analyse du terme d'écrouissage, de la sensibilité à la vitesse de déformation, de la sensibilité à latempérature, de l'anisotropie plastique et de l'anisotropie élastique ont été simulés. Les fractions volumiquesconcernant ta nature des deux phases en présence ont été également étudiées.Les résultats numériques finaux tes contraintes résiduelles ont été étudiées. Les comparaisons avec desmesures experimentales ont montré à la fois quels phénomènes étudiés sont prépondérants et tes effets moinsinfluents sur l'évaluation des contraintes résiduelles. Les résultats tes plus probants ont montré des bonnesconvergences entre l'approche numérique et expérimentale. Ces résultats confortent la robustesse du modèlenumérique developpé
Laser welding process is widely used in assembly work of automobi le industry. DP600 dual phase steeis a high strength steel to reduce automobile weight. Residual stresses are produced during laser weldingDP600. Continuum mechanics is used for analyzing res idual stresses by finite element simulation.Based on experimental tensile tests, the DP600 steel constitutive model are identified. The hardening termaccording to Ludwik law, Voce law and a proposed synthesis model are studied. The temperature sensitivityof Johnson-Cook, Khan, Chen and a proposed temperature sensitivity model are investigated. The strain ratesensitivity model proposed by A. Gavrus and planar anisotropy defined by Hi ll theory are also used.Cellul ar Automaton (CA) 20 method are programed for the simulation of solidification microstructureevolution during laser welding process. The temperature field of CA are imported from finite element analysimodel. The analysis function of nucleation, solid fraction, interface concentration, surface tension an isotropy,diffusion, interface growth ve locity and conservation equations are presented in detail. By comparing thesimulation and experimental results, good accordances are found.Modelling by a finite element method of laser welding process are presented. Geometry of specimen, heatsource, boundary conditions, DP600 dual phase steel material properties such as conductivity, density, specifiheat, expansion, elasticity and plasticity are introduced. Models analyzing hardening term, strain ratesensitivity, temperature sensitivity, plastic an isotropy and elastic an isotropy are simulated.The numerical results of laser welding DP600 steel process are presented. The influence of hardening term,strain rate sensitivity, temperature sensitivity and anisotropy on residual stresses are analyzed. Comparisonwith experimental data show good numerical accuracy.Keywords: Laser Welding, DP600, Residual Stress, Cellular Automaton, Hardening, Temperature sensitivity,Strain Rate Sensitivity, Anisotropy, Mixture dual phase law

Les durées de vie classiques des pièces en aéronautique sont de plusieurs dizaines d’années. Cependant, certaines applications en marge impliquent des durées de vie bien plus courtes, sans réparation ou récupération des pièces. Les modèles de conception classiques doivent être adaptés et la démarche du choix matériau se faire « au juste besoin », autorisant l’utilisation des matériaux aux conditions limites de leur intégrité. Afin d’estimer ces limites, la caractérisation à plus hautes températures d’alliages existants est entreprise. C’est dans cette optique que se placent les travaux de thèse présentés dans ce manuscrit. L’alliage étudié est le Ti-6Al-4V (TA6V) forgé qui possède à l’issu du traitement thermomécanique une microstructure duplex. Il est actuellement l’alliage de titane le plus couramment utilisé en aéronautique et son utilisation est généralement limitée aux environs de 350°C pour des durées de vie classiques. Dans le but d’utiliser cet alliage pendant une dizaine d’heure, l’étude menée consiste à caractériser le TA6V de 20°C à 600°C. La caractérisation se centre, dans un premier temps, sur l’état métallurgique de la matière initiale issue du galet forgé et sur sa stabilité en température. Ensuite, le comportement mécanique du TA6V est étudié de 20°C à 600°C en traction, mettant en évidence une sensibilité de la contrainte d’écoulement à la vitesse de déformation dépendant de la température. Ce comportement est mis en lien avec le phénomène de vieillissement dynamique. La caractérisation du comportement mécanique est poursuivie par une campagne étendue de fluage de 20°C à 600°C pour différents niveaux de contraintes (de 0,3 à 1 fois la limite d’élasticité en traction). Ces essais montrent différents comportements en fonction de la température. La matière déformée en traction et en fluage est analysée en microscopie électronique en transmission afin d’apporter des informations sur les mécanismes de déformation gouvernant les différents comportements de l’alliage. Les campagnes de caractérisation en traction et en fluage ont permis d’établir un modèle de comportement viscoplastique du TA6V de 20°C à 600°C validé par l’ajustement des résultats obtenus à l’issue d’essais thermomécaniques complexes avec la simulation de ces essais par éléments finis. La corrélation des résultats en traction et en fluage et la détermination des mécanismes de déformation conduit à une discussion sur le comportement viscoplastique du TA6V, pour finalement aboutir à une proposition de modélisation du fluage du TA6V de 20°C à 600°C. Le modèle permet de reproduire qualitativement des courbes de fluage à partir de la sensibilité à la vitesse de déformation mesurée au cours d’essais de traction
Classical life time of aeronautic parts lasts several decades. However, for some special applications with short life time and without repairs or recovery of parts, material design is tailored “close to real needs”. This justifies characterization at higher temperatures of well-known alloys and not developing new alloys. The study presented in this manuscript is included within this frame of short life applications. Forged Ti-6Al-4V (Ti-64) alloy with a bimodal microstructure is the most common titanium alloy in aeronautic and is usually limited below 350°C applications during classical life time. In order to use this alloy during a ten hour application, this thesis consists in characterizing Ti-64 from 20°C to 600°C. In a first time, characterization is focused on initial metallurgical state coming from a forged billet and on its thermal stability. Then, mechanical behavior of Ti-64 is studied by tensile testing from 20°C to 600°C, highlighting strain rate sensitivity (SRS) of flow stress. SRS is depending on temperature. This dependency is usually due to dynamic strain ageing phenomenon. Mechanical behavior characterization continues with creep testing from 20°C to 600°C for several stress levels (from 0.3 to 1 time yield stress values). Different behaviors versus temperature are revealed. Deformed samples by tensile testing and creep testing are analyzed by transmission electronic microscopy to bring information about deformation mechanisms controlling the different behaviors of the alloy. Thanks to tensile and creep testing, a viscoplastic modeling of Ti-64 from 20°C to 600°C has been performed and validated by fitting results from complex thermo mechanical tests with finite elements simulations. Comparison of mechanical behavior with deformation mechanisms leads to a discussion about viscoplasticity of Ti-64, and finally results in a proposal modeling creep behavior of Ti-64 from 20°C to 600°C. The model is able to estimate qualitatively creep curves using strain rate sensitivity measured during tensile tests

Etude experimentale sur les alliages al-li soumis a des essais de traction et de compression pour des vitesses de deformation comprises entre 10**(-4) et 310**(3) s**(-1) et de 10**(-3) a 210**(3) s**(-1) respectivement. Auparavant ces alliages ont ete traites thermiquement. Analyse de la sensibilite a la vitesse de deformation. Influence du mecanisme d'activation thermique sur le processus de deformation. Determination du mode de rupture. Analyse des facies de rupture pour les echantillons deformes en traction et de la formation des cellules de dislocation dans ceux deformes en compression. Simulation numerique du processus de penetration a grande vitesse d'une cible mince par un projectile circulaire en utilisant un modele de fluide elasto-plastique. Comparaison avec des resultats experimentaux

Ever since the failure of the fan blades of the Rolls Royce gas turbine engine that powered the Lockheed Martin tristar aircraft in 1972, ambient temperature dwell fatigue (DF) in titanium alloys has received considerable research attention. Now, it is well known that titanium alloys (alpha, near-alpha and few alpha-beta) are sensitive to a dwell period at the peak load of an otherwise normal fatigue loading cycle. Many industrially relevant α and near α Ti alloys are susceptible to this rather unusual phenomenon. The phenomenon is very interesting in the fact that it is most pronounced at ambient temperature (~25 °C) and vanishes above 200 °C. This study is aimed to develop a comprehensive understanding of the effect of microstructure and texture on the dwell fatigue of α and near α titanium alloys. Firstly, the temperature sensitivity of dwell fatigue was investigated with the help of phenomenological modelling using Kocks Mecking approach. The key role played by the strain hardening and strain rate sensitivity of the material was understood. The conditions that result in the highest dwell fatigue sensitivity was also identified. Further, the in-plane anisotropy in the dwell fatigue behavior of commercially pure titanium (cp-Ti) was investigated using Electron backscattered diffraction (EBSD) and crystal plasticity FFT simulations. The role of texture in regulating the strain hardening and strain rate sensitivity and the subsequent control over the anisotropy in dwell fatigue response was vividly elucidated. Also, the microstructural heterogeneity in deformation of cp-Ti and its implications on dwell fatigue was studied. The conditions that result in incompatibilities at grain boundaries resulting in the formation of grain boundary affected zones and grain boundary sliding which in turn affected the dwell fatigue life was identified. A detailed investigation of the crack nucleation mechanisms in a near alpha titanium alloy using 2D and 3D EBSD techniques was carried out. Site-specific TEM using FIB-TEM lamella preparation and TEM-OIM was also done to understand finer microstructural features resulting in crack nucleation. Finally, the effect of thermomechanical processing on the dwell fatigue behavior of this near alpha alloy was studied. Volume fraction of primary alpha was identified as the most important parameter that affects the dwell fatigue response of this alloy.

碩士
國立高雄應用科技大學
土木工程與防災科技研究所
96
This research is to investigate the strain rate effect on the stress-strain curve of RPC composites by using the inclusion theory and the secant moduli. The composite material is examined at the age of 7 days with RPC mortar as the matrix, and three volume concentrations, 1％, 2％ and 3％, of the steel fiber as the inclusion respectively. Strain rates with 5×10^-6/s、5.5×10^-5/s、1×10^-4/s、1×10^-3 /s、1×10^-2 /s and 1×10^-1/s by MTS, and with 2×10^2/s ~1×10^3 /sby SHPB are applied to the materials, respectively. A four-parameter mechanics model in term of the strain rate is proposed to simulate the stress-strain curves of the RPC matrix. From the simulated results of the RPC matrix, the mean-field approach and the secant modulus method are used to simulate the stress-strain relationship of the RPC composites. Results show that dynamic four-parameter mechanics model can be derived from the static mechanics model, and four parameters with K1、K2、η1 and η2 are determined from the experimental stress-stain curves of the matrix simulated the RPC matrix. Meanwhile, the simulated stress-strain curves and the experimental ones are pretty close to the each other in RPC composites with different strain rates at the peak strain 2×10^-3 ~ 3×10^-3. Thus, the proposed approach by combining the inclusion theory and the secant modulus is suitable for predicting the stress-strain relationship of the RPC composites with dynamic different strain rates.

D.Ing.
In mechanical design and analysis the mechanical properties of the material used are crucial to achieve effective design or analysis. In designing structures that are susceptible to dynamic loading different mechanical properties of the material may be needed than those used for quasi-static situations. Usually when one refers to the dynamic properties of a metal one refers to the notch toughness of the material. That is the resistance of the material to crack propagation under dynamic loading. Another less well known dynamic property of a metal is strain rate sensitivity. This implies that mechanical properties like yield strength, tensile strength and rupture strain varies according to strain rate. Typical applications where these properties are of use are in impact situations such as vehicle collisions and cold and hot working of metals in the manufacturing industry. The mechanical properties of certain metallic components or structures may change when the component or structure are subjected to dynamic loading that causes permanent deformation. The purpose of this investigation is to investigate the strain rate sensitive behaviour of certain stainless steels. The steels investigated are AISI Types 304, 316 and 430 stainless steels, 3CR12 corrosion resisting steel (a proprietary alloy also known as Type 1.4003) and mild steel which acts as a reference. The strain rate sensitivity of the above mentioned steels are investigated experimentally at room temperature for strain rates between 10' to approximately 100 s -1 . The steels are all tested in as delivered sheet form and testing is conducted in both rolling directions. The testing at the medium strain rates necessitated the design and construction of a dynamic tensile tester, the design of which, is also presented. The implementation of strain rate sensitive material properties into structural design and analysis are investigated and a constitutive model is proposed. The implementation of the proposed constitutive model into numerical methods analysis tools such as the finite element method is discussed and presented. The practical implementation of the proposed constitutive model is illustrated by numerically analysing the problem of a clamped beam struck transversely by a mass and comparing this with available experimental data. The validity of a typical constant velocity tensile test that is used to determine strain rate sensitive material properties is also investigated numerically to place the experimental results obtained into perspective. All the steels tested are found to be strain rate sensitive. Their behaviour is satisfactorily described by the constitutive model presented. No general trend regarding strain rate sensitivity is found when the results of the two rolling directions are compared. The importance of including strain rate sensitivity into structural design and analysis is illustrated by the analysis of the clamped beam struck transversely by a mass. The numerical results compare well with the available experimental data. It transpires from the numerical analysis of a typical constant velocity tensile test that it is difficult to obtain a constant strain rate throughout the gauge length of a typical test specimen. It also shows that there exists an optimum specimen geometry where the strain rate variation in the gauge length is at a minimum.

With the focus of the automotive industry on decreasing vehicle weight and improving fuel efficiency, aluminum is being used for structural components in automobiles. Given the high strain rates associated with vehicle impact, it is necessary to understand the rate sensitivity of any potential alloy (eg. AA5754) in order to accurately predict deformation behaviour. Furthermore, the magnitude and strain path associated with the residual strains remaining after forming of the component also play a major role in how the material will behave. It has been found that AA5754 sheet exhibits negative rate sensitivity up to a strain rate of 0.1/s, and positive strain rate sensitivity at strain rates between 0.1/s and 1500/s. Increasing the strain rate also has the effect of increasing the yield stress as well as the ductility. When a strain path change is involved between the prestrain stage and subsequent uniaxial loading, it has the effect of reducing the rate sensitivity of the material as well as reducing the overall flow stress. A rate-sensitive adaptation of the Voce material model was successfully implemented in LS-DYNA and used to predict the response of AA5754 sheet in bending for applied strain rates of 0.001/s and 0.1/s.
Thesis (Ph.D, Mechanical and Materials Engineering) -- Queen's University, 2008-09-23 20:11:30.829

M.Ing.
In the design and analysis of components and structures, detailed information on the material behaviour and its properties is required. When a material is loaded dynamically, such as in metal punching, the material properties may not be the same as when loaded statically. This is known as the strain rate sensitivity of a material, which implies that properties such as the yield strength, tensile strength and ductility may vary with the rate at which the material is loaded. South Africa is one of the large stainless steel producing countries. Seventy percent of the known chromium ore reserves are found in the Bushveld Igneous Complex in the Northern Province and Mpumalanga. To compete on the global stainless steel market it is essential that the South African producers have all the relevant product information directly available. Considerable research has been performed on mild steel at different strain rates and temperatures[1]. Work has also been done on some austenitic stainless steels. Very little, or no work has been done in this regard on ferritic and martensitic stainless steels and on the proprietary alloy 3CR12[2]. The aim of this thesis is to investigate the strain rate sensitivity of Types 304, 430 and 316 stainless steel, 3CR12 corrosion resistant steel and mild steel at different temperatures. To achieve this, tensile tests are performed. at strain rates between 10's -1 to approximately 100s -1 and at temperatures ranging from -40°C to 140°C. Shear tests are also performed at various strain rates, to investigate the effect that material behaviour has on a typical metal working process. The results obtained show that all the materials tested are strain rate sensitive. The strain rate sensitivity varies as a function of the material tested and the testing temperature. Constitutive models which take into account the strain rate sensitivity at room temperature for all the materials are also presented. These models describe the behaviour of the material fairly accurately. Three dimensional plots are also presented which depict how the yield strength, tensile strength and elongation vary as a function of both strain rate and temperature. These plots clearly show material trends for the strain rates and temperatures tested.

碩士
國立海洋大學
材料工程研究所
84
In this study, slow strain rate tension will be performed in order to investigate the effect of hydrogen on the plates and laser welds of Ti-strengthened maraging steel. The influence of tensile strain rate, environment and microstructure aged at various conditions will be evaluated. Further understanding of the hydrogen effects on the Ti-strengthened maraging steel will be obtained. The result indicated that the T-250 laser weld and plate subjected to a 482℃(900℉) aging treatment have the highest strength and the lowest elongation. The specimens subjected to a 426℃(800℉) aging have the highest sensitivity of strain rate, the strength and elongation are largely decreased as the tensile strain rate is decreased or the amount of hydrogen is increased. Testing in the hydrogen environment, the feature of fracture surface is mostly intergranular fracture. As the aging temperature is increased, less sensitive to the strain rate or hydrogen embrittlement is obtained.For the fatigue crack growth test: The crack growth rate and the effect of environments reduced with increasing aging temperature. For the specimens subjected to a 593℃(1100℉) aging, the precipitation of reverted austenite rearrenged themself in various orientation. The result indicated that reverted austenite could impinge the crack propagation or blunt the crack tip, so better fatigue properties would be expected.

This study describes the strength, toughness and strain-rate sensitivity of fibre-reinforced cement-based foams subjected to variable loading rates. Drop-weight impact tests were conducted on beams with cast density between 475 - 1200 kg/cu.m. The study shows that under quasi-static loading, the compressive strength, elastic modulus and the modulus of rupture of plain mixes scale with the square of the relative density. On the other hand, the flexural toughness factor scaled linearly with it. Fibres were seen to increase the flexural strength at all rates of loading, regardless of cast density. Further, cement based foams were seen to be strain-rate sensitive. The resistance of cement-based foams to sulphate exposure was also investigated. Heavier cement-based foams are more susceptible to sulphate attack and perform poorly with an increase in the duration of exposure when compared to the lightest mix which showed improved responses up to 30 days of exposure due to self-healing.
Structural Engineering

Research Doctorate - Doctor of Philosophy (PhD)
In the last two decades a great deal of research has been focused on the development and characterisation of metallic foams for special purpose applications. Due to their high strength to weight ratios and highly porous structures, metallic foams have unique energy absorption, damping, and thermal properties. However, these materials have not yet been widely used in industry, simply because of their higher costs when compared to their polymeric competitors in the market. In recent years, researchers have shown considerable interest in metallic syntactic foams, which are produced by embedding hollow or porous low density heat resistant particles in a metallic matrix. Owing to their relatively simple manufacturing processes, metallic syntactic foams have lower costs when compared to other foams. However, the typical aluminium syntactic foams have significantly higher densities (reportedly more than 1.4 g/cm³). This is mainly due to the high densities of the filler particles (typically more than 0.6 g/cm³) and the failure of particles during the manufacturing process. In this thesis, the major limitations of the metallic syntactic foams, i.e., their high densities and relatively high costs, are addressed by introducing a novel light porous filler material, Expanded Perlite (EP). A large volume fraction of internal porosity (≥95%) reduces the density of this natural volcanic glass down to only 0.18 g/cm³. Being mined in large quantities, to the author’s knowledge EP has the lowest price when compared to its competitors. The large particle size range, from 300 μm to 6 mm, allows for the simple, cost efficient manufacture of foams with the desired properties. EP/A356 aluminium syntactic foams were successfully fabricated using a melt infiltration technique. Depending on the manufacturing parameters, the densities of the foams may vary between 0.7 and 1.05 g/cm³, which are the lowest among the typical syntactic foams. The produced foams were then subjected to a wide range of microstructural, structural, and mechanical testing for a comprehensive characterisation of the material. With a special focus on the energy absorption capabilities of the foams, attempts were made to improve the mechanical responses of the foams by adjusting their structures and microstructures. Heat treatment, a smaller EP particle size, and a higher sphericity of the particles were shown to be effective parameters which increase the mechanical strength and energy absorption capacities of the foams. The positive strain rate sensitivity of the compressive properties makes this foam attractive for crash cushioning applications. The foams also showed outstanding performances under cyclic compressive loading conditions. Following the major characterisations, an application of EP/aluminium syntactic foam, as the core of hollow steel tubes, was investigated. The compressive and bending properties of the foam filled tubes improved considerably when compared to empty tubes. A second novel filler material, with a higher density and crushing strength than those of expanded perlite, was employed for the manufacture of high strength syntactic foams, while maintaining a low price. Syntactic foams with a density of 1.5 g/cm³ were made by the infiltration of packed beds of pumice, a natural porous volcanic glass with a particle density of 0.75 g/cm³, with molten aluminium. The pumice/aluminium syntactic foams showed a 35% increase in their energy absorption capacities when compared to the EP/aluminium syntactic foams.