The understanding of the strengthening mechanisms is crucialboth in the development of new materials with improvedmechanical properties and in the development of better materialmodels in the simulation of industrial processes. The aim ofthis work has been to study different strengthening mechanismsfrom a fundamental point of view that enables the developmentof a general model for the flow stress. Two differentmechanisms namely, solid solution strengthening and grain sizestrengthening have been examined in detail. Analytical modelsproposed in the literature have been critically evaluated withrespect to experimental data from the literature. Two differentexperimental surface techniques, atomic force microscopy (AFM)and electron backscattered diffraction (EBSD) were used tocharacterize the evolving deformation structure at grainboundaries, in an ultra low-carbon (ULC) steel. A numericalmodel was also developed to describe experimental featuresobserved locally at grain boundaries. For the case of solid solution strengthening, it is shownthat existing models for solid solution strengthening cannotexplain the observed experimental features in a satisfactoryway. In the case of grain size strengthening it is shown that asimple model seems to give a relatively good description of theexperimental data. Further, the strain hardening in materialsshowing a homogenous yielding, is controlled by grainboundaries at relatively small strains. The experimentalresults from AFM and EBSD, indicate more inhomogenousdeformation behaviour, when the grain size is larger. Bothtechniques, AFM and EBSD, correlate well with each other andcan be used to describe the deformation behaviour both on alocal and global scale. The results from the numerical modelshowed a good qualitative agreement with experimentalresults. Another part of this project was directed towards thedevelopment of continuum models that include relevantmicrostructural features. One of the results was the inclusionof the pearlite lamellae spacing in a micromechanically basedFEM-model for the flow stress of ferriticperlitic steels.Moreover a good agreement was achieved between experimentalresults from AFM and FEM calculations using a non-local crystalplasticity theory that incorporates strain gradients in thehardening moduli. The main philosophy behind this research has been to combinean evaluation of existing strengthening models, with newexperiments focused on studying the fundamental behaviour ofthe evolving dislocation structure. This combination can thenbe used to draw general conclusions on modelling thestrengthening mechanisms in metals. <b>Keywords:</b>strengthening mechanisms, flow stress, solidsolution strengthening, grain size strengthening,micromechanical modelling, AFM, EBSD

<p>The understanding of the strengthening mechanisms is crucialboth in the development of new materials with improvedmechanical properties and in the development of better materialmodels in the simulation of industrial processes. The aim ofthis work has been to study different strengthening mechanismsfrom a fundamental point of view that enables the developmentof a general model for the flow stress. Two differentmechanisms namely, solid solution strengthening and grain sizestrengthening have been examined in detail. Analytical modelsproposed in the literature have been critically evaluated withrespect to experimental data from the literature. Two differentexperimental surface techniques, atomic force microscopy (AFM)and electron backscattered diffraction (EBSD) were used tocharacterize the evolving deformation structure at grainboundaries, in an ultra low-carbon (ULC) steel. A numericalmodel was also developed to describe experimental featuresobserved locally at grain boundaries.</p><p>For the case of solid solution strengthening, it is shownthat existing models for solid solution strengthening cannotexplain the observed experimental features in a satisfactoryway. In the case of grain size strengthening it is shown that asimple model seems to give a relatively good description of theexperimental data. Further, the strain hardening in materialsshowing a homogenous yielding, is controlled by grainboundaries at relatively small strains. The experimentalresults from AFM and EBSD, indicate more inhomogenousdeformation behaviour, when the grain size is larger. Bothtechniques, AFM and EBSD, correlate well with each other andcan be used to describe the deformation behaviour both on alocal and global scale. The results from the numerical modelshowed a good qualitative agreement with experimentalresults.</p><p>Another part of this project was directed towards thedevelopment of continuum models that include relevantmicrostructural features. One of the results was the inclusionof the pearlite lamellae spacing in a micromechanically basedFEM-model for the flow stress of ferriticperlitic steels.Moreover a good agreement was achieved between experimentalresults from AFM and FEM calculations using a non-local crystalplasticity theory that incorporates strain gradients in thehardening moduli.</p><p>The main philosophy behind this research has been to combinean evaluation of existing strengthening models, with newexperiments focused on studying the fundamental behaviour ofthe evolving dislocation structure. This combination can thenbe used to draw general conclusions on modelling thestrengthening mechanisms in metals.</p><p><b>Keywords:</b>strengthening mechanisms, flow stress, solidsolution strengthening, grain size strengthening,micromechanical modelling, AFM, EBSD</p>

No description available.

Fifteen tests were conducted to evaluate the shear performance of beams with carbon fiber reinforced polymer (CFRP) laminates and CFRP anchors. The specimens consisted of 24-in. deep T-beams. The specimens were strengthened in shear with CFRP laminates that were anchored using several different CFRP end anchorage details. Load was applied to the reinforced concrete members at three different shear span-to-depth ratios. Observations of the behavior and data from the tests were used to evaluate the performance of the CFRP laminates and CFRP anchors. Overall, a 30-40% increase in shear strength was observed when anchored CFRP laminates were installed on members loaded at a shear span-to-depth ratio greater than two. The CFRP strengthening system performed well when properly detailed CFRP anchors were installed. Design recommendations regarding the installation of the CFRP anchors were developed. The CFRP anchorage detail developed in this study provided additional CFRP material in critical locations to reinforce the anchor and prevent premature failures from occurring due to anchor rupture. Theoretical calculations predicting the shear strength of the retrofitted concrete members were carried out and compared to the measured strengths of the members. Based on this analysis, a design equation was developed that produced conservative results for all of the specimens tested. / text

<p>Material failure is a ubiquitous problem, and it is known that materials fail at much lower stresses than the theoretical maximum calculated from the number and strength of the individual bonds along the material cross-section. The decreased strength is attributed to inhomogeneous stress distributions under load, thus causing the stress to accumulate at localized regions, initiating microcrack formation and subsequent propagation. In many cases, these initiation and propagation steps involve covalent bond scission. </p><p>Over the past decade there has been increased interest in channeling the mechanical forces that typically trigger destructive processes (e.g., chain scission) during use into constructive chemical transformations. In an ideal system, these stress-induced chemical transformations would redistribute load prior to material failure, thus extending material lifetime. In this Dissertation, the work of developing constructive transformations through the response of a small molecule "mechanophore" is discussed. </p><p>The gem-dihalocyclopropane mechanophore is capable of undergoing a non-scissile electrocyclic ring opening reaction under molecular scale tensile load. The mechanochemistry is demonstrated both in solution via pulsed ultrasound (Chapter 2) and in the bulk via extrusion and uniaxial tension (Chapter 3). In solution, dramatic remodeling at the molecular level occurs under the elongational flow experienced during pulsed ultrasound. Because elongational flow results in regiospecific stress distributions along a polymer main chain, this remodeling converts a gem-dichlorocyclopropane-laden homopolymer into phase separating diblock-copolymers. In the bulk, it is shown that the increased reactivity of an activated gem-dibromocyclopropane mechanophore towards nucleophilic displacement reactions leads to more non-destructive intermolecular bond-forming reactions than chain scissions, indicating the potential of the gem-dibromocyclopropane mechanophore as a self-strengthening platform. </p><p>Coupling the idea of mechanophore activation under high forces and covalent bond formation, an autonomous remodeling platform is developed, utilizing the gem-dibromocyclopropane mechanophore and a carboxylate nucleophile (Chapter 4). The system can be either two components, with a mechanophore-based polymer and a small molecule cross-linker, or a one-component system in which the mechanophore and nucleophile are embedded within the same polymer backbone. Both in the bulk and in solution, the autonomous remodeling polymer undergoes mechanophore activation followed by covalent bond formation, creating a cross-linked network in response to high shear forces. This form of remodeling leads to orders of magnitude increases in elastic modulus in response to forces that otherwise degrade polymer molecular weight and material properties. In all cases, the covalent bond formation through nucleophilic displacement of the allylic bromine by a carboxylate is confirmed as the source of polymer remodeling by FTIR as well as numerous control studies. </p><p>Together, these studies show that covalent polymer mechanochemistry can be used as a constructive tool for polymer chemistry (the direct conversion of homopolymers into well-ordered diblock copolymers) and materials science (polymers that self-strengthen in response to an applied force). This work paves the way for the future development of new mechanophores that will optimize the proof-of-principle behaviors demonstrated here.</p> / Dissertation

The actual shear capacity of existing concrete structures is often unable to meet current standard requirements. This may be attributable to increased load requirements, inadequate shear provisions in the original design or increased demand in shear capacity owing to flexural strengthening. However, available methods of assessment are often conservative, and the actual strength may be sufficient to sustain the specified assessment load. Therefore, it is important that realistic assessment methods are employed. This research comprises an investigation into the shear capacity of prestressed concrete bridges and into the feasibility of a novel strengthening approach, both through comprehensive laboratory experimentation and theoretical analyses. The laboratory testing indicates that the shear capacity of prestressed concrete bridges, post-tensioned transversely to form a deck, can be significantly greater than suggested by the relevant standards. The strengthening method proposed, namely deep embedment of steel or fibre-reinforced polymer (FRP) bars, is found to be feasible and very effective for reinforced concrete (RC) and prestressed concrete (PSC) beams of any size. Analytical models based on the upper-bound theorem of plasticity theory are successfully developed and applied, resulting in much more realistic predictions than those from current standards and codes when assessing shear capacity. For the strengthened beams, in addition to an upper-bound model, a strengthening design method based on a truss analogy is developed, which can be directly implemented into codes of practice. The analytical methods permit the assessment of existing longitudinally and laterally prestressed concrete bridges for shear capacity in a rational manner, and then to determine the capacity of a practical shear strengthening system if the bridge turns out to actually be understrength. Use of the proposed methodology will allow significant savings, as the costs associated with replacing or strengthening the structure can be avoided or minimised, encouraging a sustainable approach.

Indivíduos com Síndrome do Impacto apresentam alterações nos movimentos da escápula em relação ao tórax como uma diminuição da rotação superior, inclinação posterior e aumento da rotação medial, que associada a diminuição do controle e do recrutamento da musculatura estabilizadora podem causar pinçamento dos tecidos moles. O objetivo deste estudo foi determinar o efeito de um protocolo de fortalecimento com carga e complexidade progressivas e aumento da percepção cinestésica na cinemática escapular de indivíduos com síndrome do impacto, enfatizando os músculos serrátil anterior, romboides e porções superior, média e inferior do trapézio. Vinte e cinco indivíduos com diagnóstico da síndrome foram submetidos ao programa de fortalecimento muscular e aumento do controle motor que teve duração de oito semanas, realizados três vezes por semana. O sistema eletromagnético de aquisição de dados foi utilizado para avaliar a cinemática em três planos antes e após o protocolo. A dor e a função do ombro foram avaliadas pelo Shoulder Pain and Disability Index (SPADI-Brasil). O modelo linear de efeitos mistos foi utilizado para as comparações. A escápula apresentou alterações pós-intervenção com redução da rotação interna no repouso, plano sagital e frontal, diminuição da inclinação anterior nos três planos e redução da rotação superior no plano frontal e escapular . Houve diminuição da dor e melhora da função avaliada pelo SPADI-Br. O protocolo de controle motor e fortalecimento muscular altera a cinemática da articulação escapulotorácica e gera diminuição da dor e melhora da qualidade de vida. / Patients with subacromial impingement syndrome have changes in the scapula relative to the thorax movements with decreased upward rotation, posterior tilt and increased medial rotation that if associated with control and recruitment of stabilizer muscles decrease, could result on soft tissues impingement. The aim of this study is to determine the effect of a strengthening and kinesthetic awareness protocol with progressive and complexity load in scapular kinematics of individuals with impingement syndrome, emphasizing the serratus anterior, rhomboids and upper, middle and lower trapezius. Twenty-five diagnosed patients with the syndrome did undergo to the muscle strengthening and increased motor control program during eight weeks, three times a week. The electromagnetic data acquisition system was used to evaluate the kinematic at three planes before and after the protocol. The shoulder pain and function were evaluated by Shoulder Pain and Disability Index (SPADI-Brazil). The linear mixed-effects model was used for comparisons pre- and post-intervention. The scapula showed post-intervention changes with reduced internal rotation at rest, sagittal and frontal plane, decreased anterior tilt in three planes and reducing the upward rotation on the frontal and the scapular planes, decreased pain and improvement in function assessed by SPADI-Br. The motor control and strengthening protocol changes the kinematics scapulothoracic joint and results in decreased pain and improved quality of life.

In this thesis, I planned to fabricate Cu/Ni metallic multilayers with equal layer thicknesses on different substrates by using magnetic sputtering technique. My objective was to characterize the texture, structure and hardness, in order to study strengthening mechanisms and nanotwins in the Cu/Ni multilayers. Sputtered, highly textured (111) and (100) Cu/Ni multilayers with individual layer thickness, h, vary from 1 to 200 nm. At greater h, X-ray diffraction (XRD) patterns of Cu and Ni (100 or 111) peaks are clearly separated indicating that the interface between Cu and Ni is semi-coherent. When h decreases to 5 nm or less, XRD spectra show significant peak distortions due to coherency stress. High resolution microscopy studies confirm the coexistence of nanotwins and coherent layer interfaces in highly (111) textured Cu/Ni mutilayers. Nanoscale twins can be formed in Cu at all h and in Ni at smaller h. Multilayer hardnesses increase with decreasing h, approach maxima at h of 2.5-5 nm, and show softening thereafter. A detail comparison between (111) and (100) textured Cu/Ni is made in both microstructure and strengthening. In this thesis, the possible mechanisms to form high density growth twins in Ni are discussed. Furthermore, the influences of both coherent layer interfaces and twin interfaces on strengthening mechanisms are discussed.

Many bridges are handling heavier loads than those expected at design, making it increasingly necessary to strengthen existing members or conduct repairs on damaged structural members. Carbon Fiber Reinforced Polymer (CFRP) materials have been broadly used to repair and strengthen reinforced concrete structures. Using CFRP materials as the strengthening material is an excellent solution because of their mechanical properties. CFRP has properties of high strength, corrosion resistance, and light weight. CFRP materials are being widely used for shear and flexural strengthening. Most studies have focused on uni-directional layout of CFRP strips in high shear regions of beams. Recent shear tests on full-scale I-girders have shown that the use of bi-directional CFRP layouts with CFRP anchors led to much higher shear strength increases than when using uni-directional layouts. The objective of the study is to determine the mechanism that governs shear strengthening of bridge girders using bi-directional CFRP and, in doing so, demonstrate the feasibility of using bi-directional CFRP for shear strengthening of large bridge I- and U-beams. Small-scale panel tests have been conducted to investigate parameters that influence the shear strength provided by bi-directional CFRP layouts. Panels were tested under compressive forces to simulate the compression struts that develop in the webs of I-beams. The applied loads generated bottle-shaped compressive struts. CFRP anchors were used to prevent early failure due to CFRP strip delamination from the panel surface. The panels, while not fully reproducing the boundary condition of girder webs, were tested ahead of full-scale girders to investigate a wide range of parameters in a cost-effective manner. The variables considered include the amount of CFRP and steel reinforcement, the inclination of CFRP fibers, and the layout and spacing of CFRP strips. The panel tests provide qualitative comparisons between the influence of the various parameters. The relative strength contributions of CFRP strips, steel stirrups, and concrete were evaluated. / text

The objective of this research is the evaluation of shear behavior of full-scale reinforced concrete T-beams strengthened with carbon fiber reinforced polymer (CFRP) sheets and CFRP anchors. Although the CRFP material has high tensile strength, premature failure due to debonding CFRP sheets prevents utilizing that strength. The use of CFRP anchors prevents this failure, so the CFRP sheets are able to reach ultimate strain. The current shear design is based on plasticity, which assumes that all steel (ductile material) stirrups, across the critical section yield at ultimate. However the strain in the CFRP (brittle material), is essential to estimate the shear contribution of CFRP. To evaluate the validity of CFRP strengthening for shear, 24 tests were conducted with several parameters including shear-span-to-depth ratio, depth of beams, different transverse reinforcement ratios, and the layout of CFRP strips. In addition, a simple shear behavior model was developed to explain the differences between ductile and brittle material. From test observation, the use of CFRP anchors resulted in U-wrap application to perform like continuous wrapping which implies that a CFRP strip reached rupture strain because the anchors prevented debonding failure. However, all FRP strips did not rupture simultaneously because the strain distribution across a critical crack was not uniform. The average strain across the critical crack was about 0.005. Therefore a conservative value of effective strain (0.004) was selected for design purposes. In addition, when a beam is strengthened with CFRP, interactions between the contributions of the CFRP, steel or concrete must be taken into account. Factors ka, ks, and kf were introduced in the proposed shear design equations. Factor ka reflects the change in the material contributions as the shear span to depth ratio (a/d ratio) changes in deep beams. Factors ks and kf account for the change in steel or CFRP shear contribution due to the change in the critical crack angle as well as the interactions between the steel and FRP transverse reinforcement. As the amount of either steel or FRP material increase, the efficiency of the other material decreases. / text