Thesis (Ph.D.)--McMaster University, 2005. / Supervisor: J. D. Embury. Includes bibliographical references (leaves 238-245).

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<p><strong>In this study, detailed numerical analysis is carried out to investigate the effects of strain hardening on necking improvement by using finite element package ABAQUS. In addition, the response of laminated composite in necking, pure bending and hydroforming is also examined. It is concluded that architectured structure, especially corrugated reinforcement is an efficient method to significantly improve necking strain.</strong></p> <p><strong>The necking strain is proportional to the strain hardening rate and volume fraction of the cladding material for laminated composite. In pure bending process, the residual stress distribution varies according to different material composition. The extent of springback is linearly related to the bending moment.</strong></p> <p><strong>After the unloading in hydroforming process, the volume change of the specimen is linearly related to the fluid pressure while the slope of the linear function is independent to the material composition.</strong></p> <p><strong>Under 2D plane strain tension, corrugated reinforcement is able to provide high strain hardening rate at large strain, and hence significantly improve necking strain of the composite. Small scale corrugation is superior to large scale ones in both necking strain and strength improvement. An optimal scale exists for highest necking strain and strength while further decrease of scale deteriorates the tensile response.</strong></p> <p><strong>The anisotropic improvement of necking strain by 2D corrugation can be extended to other directions by 3D cone reinforcement. Under plane strain condition, the cone reinforcement is superior to the flat reinforcement in necking strain while remaining comparable strength.</strong> <h1></h1></p> / Master of Applied Science (MASc)

Thesis (MScEng (Civil Engineering))--University of Stellenbosch, 2010. / ENGLISH ABSTRACT: Reinforced concrete is susceptible to cracking. This makes it less durable than it would be had it been crack-free. Ingress of harmful substances into reinforced concrete through cracks – which causes corrosion of steel – is not desirable. This can be mitigated by the use of fibre reinforced-concretes or mortars showing strain hardening properties accompanied by improved ductility and multiple cracking under tensile loading. Such materials are called Strainhardening cementitious composites (SHCC’s). At Stellenbosch University, work has been done in previous years on SHCC to determine its behaviour under various loading conditions. Some of the aspects of the material that have been studied are the behaviour under uni-axial tensile loading, uni-axial compression and shear. The behaviour of SHCC under biaxial stresses was investigated to enable the prediction of the material behaviour under complex stress conditions. Square plate specimens of nominal dimensions 100 x 100 x 20 mm were cast and subjected to biaxial compressive loading at stress path angles of 00, 150, 300 and 450; which were equivalent to vertical/horizontal stress ratios of 0/1, 0.27/1, 0.58/1 and 1/1 respectively, at ages 23 to 33 days. Comparisons were made between specimens tested using steel platens and those tested using Vesconite sheets. Those tested using Vesconite yielded lower failure stresses. Vesconite was used to reduce the retraining effect of the frictional force between the specimens and the steel platens. Poisson’s ratios were calculated for specimens tested using steel and those tested using Vesconite. The values for Vesconite were found to be higher than for steel. Additionally, the values for the uni-axial case were different from those obtained for other stress ratios. This could have been due to the assumption made that plane stress was realised and that Elastic moduli in tension and in compression was the same. The possibility of the existence of a triaxial stress state could render the calculated Poisson’s ratios incorrect. Shear slip type failure accompanied by wedging was observed. Vertical to near diagonal cracks were evident on the front faces of the specimens for the stress path angles of 00 to 450 respectively. The observed crack patterns showed closely spaced multiple micro-cracking on the narrow edges of specimens indicating Strain-hardening behaviour. The stress-strain curves also showed a slight indication of strain-hardening where tensile strains were measured. / AFRIKAANSE OPSOMMING: Gewapende beton is vatbaar vir krake. Dit maak dit minder duursaam as wanneer dit kraak-vry is. Instroming van skadelike stowwe in gewapende beton deur middel van krake - wat korrosie van staal veroorsaak - is nie wenslik nie. Dit kan verbeter word deur die gebruik van veselversterkte beton of mortel wat vervormingsverharding eienskappe toon, vergesel deur verbeterde rekbaarheid en veelvuldige krake onder trekspanning. Sulke materiaal word Strainhardening cementitious composites (SHCC's) genoem. Die Universiteit Stellenbosch, het in vorige jare werk gedoen om SHCC se gedrag te bepaal onder verskillende belastingstoestande. Sommige van die aspekte van die materiaal wat bestudeer is, is gedrag onder uni-aksiale trek, uni-aksiale druk en skuif. Die gedrag van SHCC onder biaksiale spannings is ondersoek om voorspelling van materiaalgedrag onder komplekse spanningstoestande moontlik te maak. Vierkantige plaat monsters van nominale dimensies 100 x 100 x 20 mm is gemaak en aan biaksiale drukkragte onderwerp, met spannningspad hoeke van 00, 150, 300 en 450; wat soortgelyk is aan die horisontale spanning verhoudings van 0/1, 0.27/1, 0.58/1 en 1/1 onderskeidelik, op ouderdomme 23-33 dae. Vergelykings is getref tussen monsters getoets met staal plate en diegene getoets word met Vesconite plate. Die proefstukke getoets met Vesconite het laer falingsspannings opgelewer. Vesconite is gebruik om die uitwerking van die wrywingskrag tussen die monsters en die staal plate te verminder. Poisson se verhouding is bereken vir die staal en Vesconite monsters afsonderlik. Daar is gevind dat die Vesconite waarde hoër was as die vir staal. Daarbenewens het die waardes vir die uni-aksiale geval, verskil van dié vir ander spanningsverhoudings. Dit kan wees as gevolg van die aanname van vlakspanning en dat die Elastiese moduli in druk en in trek dieselfde is. Die moontlikheid van die bestaan van 'n drie-dimensionele spanningstoestand, kan beteken dat die berekende Poisson’s verhoudings onakkuraat is. Skuif-glip tipe faling, vergesel deur vaswigting is waargeneem. Vertikale tot feitlik diagonale krake is duidelik sigbaar op die voorkant van die monsters vir spanningspadhoeke van 0-450 onderskeidelik. Die waargeneemde kraak patrone het nou gespasieerde, veelvuldige mikro-krake op die smal randte van die monsters, wat dui op vervormingsverharding. Die spanningsvervormings kurwes het ook effense aanduidings getoon van die vervormingsverharding waar trekvervorming gemeet is.

This thesis investigates the quantity Crack Tip Opening Displacement (CTOD) as a means to assess fracture toughness when measured in the Single Edge Notched Bend (SENB) specimen setup. A particular objective is to assess the effectiveness of the test when used for high strain-hardening materials (e.g. stainless steels). This has been an increasing concern as the current available methods were generally designed for lower strain hardening structural steel. Experimental work on CTOD tests included silicone casting of the crack, and constant displacement tests were also performed. The silicone castings enable physical measurement of the crack under an optical microscope. Results from a series of Finite Element (FE) models were validated from the experiments. δ5 surface measurements were obtained using Digital Image Correlation (DIC) as a courtesy of TWI, which were compared to surface CTOD measurements from the silicone castings. In addition to the experiments and Finite Element modelling, archived test data from TWI was processed, showing analytical differences between current Standard CTOD equations. CTOD calculations from BS 7448, ISO 12135, ASTM E1820 and WES 1108 were compared to the experimental and FE modelling results. For high strain hardening material, CTOD predicted by Standard equations (apart from those in BS 7448 and single point CTOD from ISO 12135) were lower than the values determined from silicone measurements and modelling. This potentially leads to over conservative values to be used in Engineering Critical Assessments (ECA) or material approval. Based on a series of different strain hardening property models, a relationship between strain hardening and the specimen rotational factor, rp was established. An improved equation for the calculation of CTOD is proposed, which gave good estimation of the experimental and Finite Element modelling results. The improved equation will be proposed for future amendments of the ISO 12135 standard. The results of this research enable the accurate fracture characterisation of a range of engineering alloys, with both low and high strain hardening behaviour in both the brittle and ductile fracture regime.

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.

Roll-forming is an efficient sheet forming process that is used in manufacturing long parts with constant cross-section. The theoretical, experimental and numerical analyses of the process are limited since the sheet takes a complex 3D shape during the process. In this study proper finite element method models to simulate the roll-forming process are examined both numerically and experimentally. In addition, the applicability of 2D plane strain models to the simulation of the process is investigated. To reveal the deformation of the sheet, important geometrical parameters of the sheet and the rollers are introduced. The effect of these parameters on the strain hardening and deformation of the sheet is analyzed at distinct parts of the sheet that undergoes different types of deformations. Having revealed the deformation mechanisms, the assumptions behind the theoretical knowledge is criticized. The mentioned studies are verified with a case study in which a roll-formed product is analyzed under service loads. The manufacturing of the product and service load application are simulated and the results are compared with the experiments. In addition, effects of cold forming on the behaviour of the product under service loads are examined. It is concluded that under some conditions, 2D plane strain simulations can be used to predict the strain hardening in the material that occurs during roll-forming and this hardening has a considerable effect on the response of the material under loading.

Environmental stress cracking (ESC) is one of the main, and probably the most common, failure mechanisms involved in polymer fractures. This type of failure is critically important as it occurs suddenly, without any visible pre-fracture deformation. Such failure can be catastrophic and costly in cases where structural integrity is required. In polyethylene (PE), ESC occurs through a slow crack growth mechanism. Cracks initiate from stress-concentrated imperfections, propagate through the bulk of PE, and ultimately result in a brittle fracture. In order to predict the environmental stress cracking resistance (ESCR) of PE, it is necessary to fully understand the molecular structure of the resin. In this thesis, attempts were made to find relationships between molecular structure characteristics and material responses, mainly inter-lamellar entanglements and strain hardening behaviour of PE resins, through mechanical and rheological experiments. Inter-lamellar entanglements are believed to be the main factor controlling slow crack growth of PE. Extent of entanglements and entanglement efficiency were investigated by monitoring the strain hardening behaviour of PE resins in the solid state through a uniaxial tensile test, and in the melt state, through extensional rheometry. ESCR is usually assessed by unreliable and time consuming testing methods such as the notch constant load test (NCLT) on notched PE specimens in the presence of an aggressive fluid and elevated temperatures. In this thesis, a practical, yet reliable, tensile test was developed for the evaluation and prediction of ESCR. The developed test offers a more reliable and consistent ESCR picture without the drawbacks of the subjective notching process and presence of aggressive fluids. Through this test, a factor called ???corrected hardening stiffness (cHS)??? was developed, which can easily be used for a relative ranking of ESCR of different PE resins. Studies were next extended to the melt state via shear and extensional rheometry. Through studies in the shear mode, a molecular weight-normalized average characteristic relaxation time (??N) was found to be efficient in predicting the extent of chain entanglements in resins. This provided a potential melt indicator for a relative measure of ESCR, for linear low density polyethylene (LLDPE), with different short chain branching levels. Extensional studies were conducted to evaluate the strain hardening behaviour in the melt state. An inverse correlation was obtained between ESCR and the melt strain hardening coefficient (MSHC), found from Sentmanat Extensional Rheometry (SER). This indicated an inverse relationship between ESCR and chain extensibility in the melt. In addition, a new factor called ???melt hardening stiffness (mHS)??? was developed from the slope of a stress-strain line, obtained from SER. This factor, analogous to cHS, can be used for a practical and reliable ranking of ESCR of PEs. ESCR is usually associated with classical crystalline phase property indicators, such as crystallinity and lamella thickness. In this thesis, the effect of processing and post processing temperature on the extent of inter-lamellar entanglements were investigated, evaluated, and correlated to ESCR. Also, analysis of the lamella surface area (LSA) was pursued since LSA reflects changes in phase interconnectivity more precisely. The focus of this part of the study was on the effect of temperature on LSA to identify the optimum processing and post-processing conditions which yield a higher LSA. It was reasonable to presume that PE with larger lamella lateral surface areas will have more inter-lamellar entanglements, hence higher ESCR. Finally, a well-controlled ultraviolet (UV) photoinitiated reactive extrusion (REX) process was developed for selective formation of long chain branches in the PE structure. This was conducted to impose restrictions against stretching of the polymer chain, which consequently enhanced ESCR.

Thesis (Ph. D.)--Ohio State University, 2006. / Title from first page of PDF file. Includes bibliographical references (p. 144-155).

abstract: As the demand of sustainable construction materials increases, use of fibers and textiles as partial or full reinforcement in concrete members present a tremendous opportunity. Proper characterization techniques and design guides for hybrid materials are therefore needed. This dissertation presents a comprehensive study on serviceability-based design of strain softening and strain hardening materials. Multiple experimental procedures are developed to document the nature of single crack localization and multiple cracking mechanisms in various fiber and fabric reinforced cement-based composites. In addition, strain rate effects on the mechanical properties are examined using a high speed servo-hydraulic tension test equipment. Significant hardening and degradation parameters such as stiffness, crack spacing, crack width, localized zone size are obtained from tensile tests using digital image correlation (DIC) technique. A tension stiffening model is used to simulate the tensile response that addresses the cracking and localization mechanisms. The model is also modified to simulate the sequential cracking in joint-free slabs on grade reinforced by steel fibers, where the lateral stiffness of slab and grade interface and stress-crack width response are the most important model parameters. Parametric tensile and compressive material models are used to formulate generalized analytical solutions for flexural behaviors of hybrid reinforced concrete (HRC) that contains both rebars and fibers. Design recommendations on moment capacity, minimum reinforcement ratio etc. are obtained using analytical equations. The role of fiber in reducing the amount of conventional reinforcement is revealed. The approach is extended to T-sections and used to model Ultra High Performance Concrete (UHPC) beams and girders. The analytical models are extended to structural members subjected to combined axial and bending actions. Analytical equations to address the P-M diagrams are derived. Closed-form equations that generate the interaction diagram of HRC section are presented which may be used in the design of multiple types of applications. The theoretical models are verified by independent experimental results from literature. Reliability analysis using Monte Carlo simulation (MCS) is conducted for few design problems on ultimate state design. The proposed methodologies enable one to simulate the experiments to obtain material parameters and design structural members using generalized formulations. / Dissertation/Thesis / Doctoral Dissertation Civil and Environmental Engineering 2016