The failure behavior of collagenous soft tissues is important for clinical problems of plaque rupture and trauma. Cyclic tests require high frequencies that may affect the strength properties of the soft tissues. Experimental results of mechanical response of blood vessels to physiologic loads can be used to model and predict plaque rupture and direct medical therapy or surgical intervention. The goal of the study is to measure the mechanical failure properties of arteries to determine if they are strain rate and cycle dependant and to measure the progressive damage of arteries with time dependent loading. Ring specimens of porcine carotid arteries were preconditioned and then pulled to failure. In all cases, the intima broke first. Ultimate stress increased as a weak function of increasing strain rates. The ultimate stress at 100 mm/s was 4.54 MPa, greater than the 3.26 MPa at 0.1 mm/s. Strain rates between 1 and 100 mm/s correspond to a cyclic frequency of 0.5 Hz to 5 Hz for fatigue testing. In contrast, ultimate strain in arteries was independent of strain rate over the range tested. The creep tests showed a logarithmic relationship between stress magnitude and stress duration for this soft tissue. The creep testing indicates that damage is accumulating above certain threshold stress levels. The values of ultimate strength showed a 35% increase after 10,000 cycling loading. In contrast, the ultimate strain had a 13% decrease after cycling and the difference was statistically significant with p=0.018. The testing results showed that there were no significant differences on strength among fresh arteries and arteries stored at 5¡ã C for up to two weeks. The test results may be useful for developing a mathematical model to predict the behavior of arterial soft tissues and may be extended to estimate fracture and fatigue in the atherosclerotic plaque cap.

This study presents characterization of thermo-mechanical viscoelastic and long-term behaviors of thick-section multi-layered fiber reinforced polymer composite materials. The studied multi-layered systems belong to a class of thermo-rheologically complex materials, in which both stress and temperature affect the time-dependent material response. The multi-layered composites consist of alternating layers of unidirectional fiber (roving) and randomly oriented continuous filament mat. Isothermal creep-recovery tests at various stresses and temperatures are performed on E-glass/vinylester and Eglass/ polyester off-axis specimens. Analytical representation of a nonlinear single integral equation is applied to model the thermo-mechanical viscoelastic responses for each off-axis specimen. Long-term material behaviors are then obtained through vertical and horizontal time shifting using analytical and graphical shifting procedures. Linear extrapolation of transient creep compliance is used to extend the material responses for longer times. The extended long-term creep strains of the uniaxial E-glass/vinylester specimens are verified with the long-term experimental data of Scott and Zureick (1998). A sensitivity analyses is then conducted to examine the impact of error in material parameter characterizations to the overall long-term material behaviors. Finally, the calibrated long-term material parameters are used to study the long-term behavior of multi-layered composite structures. For this purpose, an integrated micromechanical material and finite element structural analyses is employed. Previously developed viscoelastic micromodels of multi-layered composites are used to generate the effective nonlinear viscoelastic responses of the studied composite systems and then implemented as a material subroutine in Abaqus finite element code. Several long-term composite structures are analyzed, that is; I-shaped columns and flat panels under axial compression, and a sandwich beam under the point bending and transmission tower under lateral forces. It is shown that the integrated micromechanical-finite element model is capable of predicting the long-term behavior of the multilayered composite structures.

Disposed waste materials remained from demolished buildings have been an environmental problem especially for developing countries. Recycled Cement Concrete (RCC) is one of the abundant components of waste materials that include quality aggregates. Use of RCC in asphalt concrete pavements is economically a feasible option as it not only helps in recycling waste materials but also preserves natural resources by fulfilling the demand for quality aggregate in pavement constructions. However, due to variability in RCC characteristics, a detailed evaluation of its effect on asphalt concrete performance is required. In this study, effect of RCC content on rutting potential of asphalt concrete is investigated using laboratory prepared specimens. Rutting susceptibility of the specimens is determined using repeated creep tests performed in the uniaxial stress mode. Because of the aspect ratio requirements for the repeated creep test, the standard Marshall mix design procedures were modified based on the energy concept by changing the compactor device and the applied design number of blows. The modified specimens were tested to determine a number of parameters that can describe the rutting behavior of the tested mixes. The findings indicate that slope constant and flow number give relatively stronger relationships with rutting behavior as compared to the other rutting parameters. While increasing the RCC content yields improved rutting performance for coarse graded specimens, it dramatically reduces the performance for fine graded specimens.

The mechanical property of polymeric materials changes over time, especially when they are subjected to long-term loading scenarios. To predict the time-dependent viscoelastic behaviors of epoxy-based adhesive materials, it is imperative that reliable accelerated tests be developed to determine their long-term performances under different exposed environments. A neat epoxy resin system and a commercial structural adhesive system for bonding aluminum substrates are investigated. A series of moisture diffusion tests have been performed for more than three months in order to understand the influence of the absorbed moisture on creep behavior. The material properties, such as elastic modulus and glass transition temperature, are also studied under different environmental conditions. The time-temperature superposition method produces a master curve allowing the long-term creep compliance to be estimated. The physics-based Coupling model is found to fit well the long-term creep master curve. The equivalence of the temperature and moisture effect on the creep compliance of the epoxy adhesives is also addressed. Finally, a methodology for predicting the long-term creep behavior of epoxy adhesives is proposed.

In this work, a constitutive theory is developed, within the context of continuum mechanics, to describe the creep deformation of single crystal superalloys. The con- stitutive model that is developed here is based on the fact that as bodies deform the stress free state that corresponds to the current configuration (referred to as the "natural configuration", i.e., the configuration that the body would attain on the removal of the external stimuli) evolves. It is assumed that the material possesses an infinity of natural (or stress-free) configurations, the underlying natural configuration of the body changing during the deformation process, with the response of the body being elastic from these evolving natural configurations. It is also assumed that the evolution of the natural configurations is determined by the tendency of the body to undergo a process that maximizes the rate of dissipation. Central to the theory is the prescription of the forms for the stored energy and rate of dissipation functions. The stored energy reflects the fact that the elastic response exhibits cubic symmetry. Consistent with experiments, the elastic response from the natural configuration is assumed to be linearly elastic and the model also takes into account the fact that the symmetry of single crystals does not change with inelastic deformation. An ap- propriate form for the inelastic stored energy (the energy that is `trapped' within dislocation networks) is also utilized based on simple ideas of dislocation motion. In lieu of the absence of any experimental data to corroborate with, the form for the inelastic stored energy is assumed to be isotropic. The rate of dissipation function is chosen to be anisotropic, in that it reflects invariance to transformations that belong to the cubic symmetry group. The rate of dissipation is assumed to be proportional to the density of mobile dislocations and another term that takes into account the damage accumulation due to creep. The model developed herein is used to simulate uniaxial creep of <001>, <111> and <011> oriented single crystal nickel based su- peralloys for a range of temperatures. The predictions of the theory match well with the available experimental data for CMSX-4. The constitutive model is also imple- mented as a User Material (UMAT) in commercial finite element software ABAQUS to enable the analysis of more general problems. The UMAT is validated for simple problems and the numerical scheme based on an implicit backward difference formula works well in that the results match closely with those obtained using a semi-inverse approach.

<p>Today there is an even higher demand on IT-projects success rate compared to a few years ago. IT-projects must be handled more efficiently and result in a more profitable investment. In order to make the handling of IT-projects more efficient and create profitability the developers must have a broad understanding of the IT-projects characteristics and which factors that affect the result of the projects. Changes within IT-projects are often carried out through the development of a system. Since this change is more common when it comes to IT-projects it is important to handle risk and change within these kinds of projects in order to prevent and avoid uncontrolled changes. Our purpose with this thesis is to investigate if change is a problem in IT-projects. Thereafter we identify and examine procedures to avoid uncontrolled changes within IT-projects and to prevent an unplanned expansion of the project scope. The thesis also results in a method of how this change can be controlled. The thesis has been conducted by studying theory about IT-projects, system development and change management. Further on a qualitative study has been conducted where three persons from different system development companies has been interviewed. After an analysis of the theoretical framework and the empirical research a method has been developed. This method describes the change management process and should function as a support for a more effective handling of changes in order to prevent that uncontrolled changes arises. The method illuminates the importance of a common understanding, planning and identification & priority as three factors in order to create an effective change management. Further on we emphasize the necessity of having a structure of control which covers all steps in the method. Finally we describe how the step of handling change is conducted by presenting the necessary steps to prevent that uncontrolled changes arises.</p> / <p>Det ställs allt högre krav på IT-projekt idag jämfört med för några år sedan. IT-projekt måste hanteras effektivare och resultera i lönsammare investeringar för att projekten skall bli genomförda. För att kunna effektivisera hanteringen av IT-projekt och skapa lönsamhet måste dagens systemutvecklare ha en bred förståelse över IT-projektens karakteristika och vad som kan påverka dess resultat. Ofta sker förändringar inom IT-projekt under utvecklingen av ett system. Eftersom denna förändring är mer påtaglig vad det gäller IT-projekt är det viktigt att hantera risk och förändring vid utförandet av IT-projekt. Detta för att kunna förebygga och undvika att okontrollerade förändringar uppstår. Vårt syfte med uppsatsen är att undersöka om förändring ses som ett problem inom IT-projekt. Därefter identifierar och undersöker vi vilka arbetssätt som finns för att undvika okontrollerade förändringar inom IT-projekt och på så sätt förhindrar en oplanerad expansion av projektomfattningen. Uppsatsen resulterar även i ett tillvägagångssätt för hur denna förändringshantering kan se ut. Uppsatsarbetet har genomförts genom en litteraturstudie där teori om IT-projekt, systemutveckling, riskhantering och förändringshantering har behandlats. Uppsatsarbetet har vidare resulterat i en kvalitativ empirisk undersökning där tre personer som arbetar i systemutvecklingsföretag har intervjuats ingående. Litteraturstudien och den empiriska undersökningen har efter analysering resulterat i en egenutvecklad modell. Denna modell beskriver förändringshanteringsprocessen och skall fungera som ett underlag till en effektiv hantering av förändringar så att okontrollerade förändringar inte uppstår. I modellen belyser vi vikten av förståelse, planering och identifiering & prioritering som tre grundfaktorer för en effektiv hantering av förändring. Vi betonar även vikten av att ha en kontrollstruktur som sträcker sig över samtliga steg i modellen. Slutligen poängterar vi hur själva hanteringssteget skall utföras genom att presentera de steg som måste tas för att undvika att okontrollerade förändringar uppstår.</p>

Considering the unlikely core melt down scenario for a light water reactor (LWR) a possible failure mode of the reactor pressure vessel (RPV) and its failure time has to be investigated for a determination of the loadings on the containment. Worldwide several experiments have been performed accompanied with material properties evaluation, theoretical, and numerical work. At the Institute of Safety Research of the FZR a finite element model has been de-veloped simulating the thermal processes and the viscoplastic behaviour of the ves-sel wall. An advanced model for creep and material damage has been established and has been validated using experimental data. The thermal and the mechanical calculations are sequentially and recursively coupled. The model is capable of evalu-ating fracture time and fracture position of a vessel with an internally heated melt pool. The model was applied to pre- and post test calculations for the FOREVER test se-ries representing the lower head RPV of a PWR in the geometrical scale of 1:10. These experiments were performed at the Royal Institute of Technology in Stock-holm. The results of the calculations can be summarised as follows: # The creeping process is caused by the simultaneous presence of high tem-perature (>600 °C) and pressure (>1 MPa) # The hot focus region is the most endangered zone exhibiting the highest creep strain rates. # The exact level of temperature and pressure has an influence on the vessel failure time but not on the failure position # The failure time can be predicted with an uncertainty of 20 to 25%. This uncer-tainty is caused by the large scatter and the high temperature sensitivity of the viscoplastic properties of the RPV steel. # Contrary to the hot focus region, the lower centre of the vessel head exhibits a higher strength because of the lower temperatures in this zone. The lower part moves down without significant deformation. Therefore it can be assumed, that the vessel failure can be retarded or prevented by supporting this range. # The development of a gap between melt crust and vessel wall could not be proofed. First calculations for a PWR geometry were performed to work out differences and commonalities between prototypic scenarios and scaled experiments. The results of the FOREVER-experiments cannot be transferred directly to PWR geometry. The geometrical, mechanical and thermal relations cannot be scaled in the same way. Because of the significantly higher temperature level, a partial ablation of the vessel wall has to be to expected in the PWR scenario, which is not the case in the FOREVER tests. But nevertheless the FOREVER tests are the only integral in-vessel retention experiments up to now and they led to a number of important insights about the behaviour of a vessel under the loading of a melt pool and pressure.

To get an improved understanding and knowledge of the processes and phenomena during the late phase of a core melt down accident the FOREVER-experiments (Failure of Reactor Vessel Retention) are currently underway. These experiments are simulating the lower head of a reactor pressure vessel under the load of a melt pool with internal heat sources. The geometrical scale of the experiments is 1:10 compared to a common Light Water Reactor. During the first series of experiments the Creep behaviour of the vessel is investigated. Due to the multi-axial creep deformation of the three-dimensional vessel with a non-uniform temperature field these experiments are on the one hand an excellent possibility to validate numerical creep models which are developed on the basis of uniaxial creep tests. On the other hand the results of pre-test calculations can be used for an optimized experimental procedure. Therefore a Finite Element model is developed on the basis of the multi-purpose commercial code ANSYS/Multiphysics®. Using the Computational Fluid Dynamic module the temperature field within the vessel wall is evaluated. The transient structural mechanical calculations are performed applying a creep model which is able to take into account great temperature, stress and strain variations within the model domain. The new numerical approach avoids the use of a single creep law with constants evaluated for a limited stress and temperature range. Instead of this a three-dimensional array is developed where the creep strain rate is evaluated according to the actual total strain, temperature and equivalent stress for each element. Performing post-test calculations for the FOREVER-C2 experiment it was found that the assessment of the experimental data and of the numerical results has to be done very carefully. A slight temperature increase during the creep deformation stage of the experiment for example could explain the creep behaviour which appears to be tertiary because of the accelerating creep strain rate. Taking into account both - experimental and numerical results - gives a good opportunity to improve the simulation and understanding of real accident scenarios.

<p>Materials to be used in thermal power plants have to resist creep deformation for time periods up to 30 years. The role of alloying elements for creep strength of 9-12% Cr steels is analysed. The creep strength in these steels relies on minor additions of alloying elements. Precipitates give rise to the main strengthening and remaining elements produce solid solution hardening. Nucleation, growth and coarsening of particles are predicted by thermodynamic modelling. Phase fractions and size distributions of M₂₃C₆ carbides, MX carbonitrides and Laves phase are presented. The size distributions are needed in order to determine the particle hardening during creep. At elevated temperatures the climb mobility is so high that the dislocations can climb across particles instead of passing by making Orowan loops.</p><p>By solving Fick's second law the concentration profile around a moving dislocation can be determined. The results show an accumulation of solutes around the dislocation that slows down dislocation movement. When Laves phase grows a decrease in creep strength is observed due to a larger loss in solid solution hardening than strength increase by particle hardening. Solid solution hardening also gives an explanation of the low dislocation climb mobility in 9-12% Cr steels.</p><p>Three different dislocation types are distinguished, free dislocations, immobile dislocation and immobile boundary dislocations. This distinction between types of dislocations is essential in understanding the decreasing creep with strain during primary creep. The empirical relation with subgrain size inversely proportional to stress has been possible to predict. The total creep strength can be predicted by adding the contribution from individual mechanisms.</p>

Due to their structural efficiency and architectural elegance, concrete arches have long been used in bridge applications. However, the construction of concrete arches requires significant temporary supporting structures, which prevent their widespread use in modern bridges. A relatively new form of arch bridges is the network arch, in which a dense arrangement of inclined hangers is used. Network arches are subjected to considerably smaller bending moments and deflections than traditional arches and are therefore suitable for modern, accelerated construction methods in which the arches are fabricated off-site and then transported to the bridge location. However, service-level stresses, which play a critical role in the performance of the structure, are relatively unknown for concrete network arches and have not been sufficiently investigated in the previous research on concrete arches. The primary objective of this dissertation is to improve the understanding of short-term and time-dependent stresses in concrete arches, and more specifically, concrete network arches. The research presented herein includes extensive field monitoring of the West 7th Street Bridge in Fort Worth, Texas, which is the first precast network arch bridge and probably the first concrete network arch bridge in the world. The bridge consists of twelve identically designed concrete network arches that were precast and post-tensioned before they were transported to the bridge site and erected. A series of vibrating wire gages were embedded in the arches and were monitored throughout the construction and for a few months after the bridge was opened to traffic. The obtained data were processed, and structural response parameters were evaluated to support the safe construction of the innovative arches, identify their short-term and time-dependent structural behavior, and verify the modeling assumptions. The variability of stresses among the arches was also used to assess the reliability of stress calculations. The results of this study provide valuable insight into the elastic, thermal, and time-dependent behavior of concrete arches in general and concrete network arches in particular. The knowledge gained in this investigation also has broader applications towards understanding the behavior of indeterminate prestressed concrete structures that are subjected to variable boundary conditions and thermal and time-dependent effects.