| 1. |
- Dahlin, Peter, 1971-
(författare)
-
Growth of fatigue cracks subjected to non-proportional Mode I and II
- 2005
-
Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- This thesis deals with some aspects of crack growth in the presence of cyclic loading, i.e. fatigue. The cyclic load cases studied here are primary of non-proportional mixed mode type. Under non-proportional loading the principal stress directions rotate and, generally, the ratio between the principal stresses vary. A new criterion has been presented for prediction of incipient crack path direction after changes in load from steady Mode I to non-proportional loading.The criterion is based on FE-simulations which are used to compute the actual elasto-plastic stress state in the vicinity of the crack tip. The predictions of the criterion capture several phenomena observed in the literature, which indicates that plasticity effects have to be included in a criterion for crack path predictions under non-proportional loading. The effects of Mode II overloads on subsequent Mode I crack growth have been studied relatively little in the literature. Also, the results deviates substantially. In the present thesis, this load case has been investigated in detail, both experimentally and analytically. The results show that the Mode I crack growth rate decreases after a single Mode II load, if the R-ratio is not as high as to keep the entire Mode I load cycle above the closure level. This is based on the fact, shown in this thesis, that the reduction is caused by crack closure due to tangential displacement of crack-surface irregularities.A new loading device is presented. With this device, it is possible to apply sequential loading in Mode I and Mode II in an automated way, without having to dismount the specimens. This loading device is used to study the influence of periodic Mode II loading on Mode I crack growth. The main parameters concerning the influence of periodic Mode II loading on Mode I crack growth are; (i) the Mode I R-ratio, (ii) the Mode II magnitude and (iii) the Mode II periodicity, M (number of Mode I loads for every Mode II load). The mechanisms involved are mainly RICC (Roughness-Induced Crack Closure) and a Mode II mechanism that increases the growth rate temporary at every Mode II load. Hence, the latter becomes more significant for low M-values. The higher the Mode I R-ratio the smaller is the reduction.
|
|
| 2. |
|
|
| 3. |
|
|
| 4. |
|
|
| 5. |
- Barsoum, Imad
(författare)
-
Ductile failure and rupture mechanisms in combined tension and shear
- 2006
-
Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- This licentiate thesis is generally concerned with the ductile failure and rupture mechanisms encountered under combined tension and torsion loading. In the first part entitled Paper A, an experimental investigation of the rupture mechanisms in a mid-strength and a high strength steel was conducted employing a novel test configuration. The specimen used was a double notched tube specimen loaded in combined tension and torsion at a fixed ratio. The effective plastic strain, the stress triaxiality and the Lode parameter was determined in the centre of the notch at failure. Scanning electron microscopy of the fractured surfaces revealed two distinctively different ductile rupture mechanisms depending on the stress state. At high stress triaxiality the fractured surfaces were covered with large and deep dimples, suggesting that growth and internal necking of voids being the governing rupture mechanism. At low triaxiality it was found that the fractured surfaces were covered with elongated small shear dimples, suggesting internal void shearing being the governing rupture mechanism. In the fractured surfaces of the high-strength steel, regions with quasi-cleavage were also observed. The transition from the internal necking mechanism to the internal shearing mechanism was accompanied by a significant drop in ductility.In the second part entitled Paper B, a micromechanics model based on the theoretical framework of plastic localization into a band introduced by Rice is developed. The model employed consists of a planar band with a square array of equally sized cells, with a spherical void located in the centre of each cell. The periodic arrangement of the cells allows the study of a single unit cell for which fully periodic boundary conditions are applied. The micromechanics model is applied to analyze failure by ductile rupture in experiments on double notched tube specimens subjected to combined tension and torsion carried out by the present authors. The stress state is characterized in terms of the stress triaxiality and the Lode parameter. Two rupture mechanisms can be identified, void coalescence by internal necking at high triaxiality and void coalescence by internal shearing at low triaxiality. For the internal necking mechanism, failure is assumed to occur when the deformation localizes into a planar band and is closely associated with extensive void growth. For the internal shearing mechanism, a simple criterion based on the attainment of a critical value of shear deformation is utilized. The two failure criteria capture the transition between the two rupture mechanisms successfully and are in good agreement with the experimental result.
|
|
| 6. |
- Barsoum, Imad, 1978-
(författare)
-
The effect of stress state in ductile failure
- 2008
-
Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The industrial application of high strength steels in structural components has increased the demand on understanding the ductile failure behavior of this type of materials. In practical situations the loading experienced on components made out of these materials can be very complex, which may affect the failure behavior.The objective of this work is to study the effect of stress state on ductile failure and the mechanisms leading to rupture in high strength steels. The stress state is characterized by the stress triaxiality T and the Lode parameter L, which is a deviatoric stress state parameter that discriminates between axisymmetric or shear dominated stress states. For this purpose experiments on two different specimen configurations are performed; a double notched tube (DNT) specimen tested in combined tension and shear and a round notched bar (RNB) specimen tested in uniaxial tension. The two specimens give rise to different stress states at failure in terms of T and L. The failure loci for the DNT specimen show an abrupt change in ductility, indicating a transition between the rupture mechanisms necking of intervoid ligaments and shearing of intervoid ligaments. A clear difference in ductility between the two specimen configurations is also observed, which is closely associated with the difference in stress state at failure.A micromechanical model is developed, which assumes that ductile material failure occurs when the deformation becomes highly non-linear and localizes into a band. The model, which is applied to analyze the experiments, consists of a band with a square array of equally sized cells, with a spherical void located in the center of each cell. The model, extended with a shear criterion, captures the experimental trend rather well. The model also shows that the effect of the deviatoric stress state (L) on void growth, void shape evolution and coalescence is significant, especially at low levels of T and shear dominated stress state.
|
|
| 7. |
|
|
| 8. |
- Biasetti, Jacopo
(författare)
-
Physics of blood flow in arteries and its relation to intra-luminal thrombus and atherosclerosis
- 2013
-
Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Vascular pathologies such as Abdominal Aortic Aneurysm (AAA) and atherosclerosis are complex vascular diseases involving biological, mechanical, and fluid-dynamical factors. This thesis follows a multidisciplinary approach and presents an integrated fluid-chemical theory of ILT growth and analyzes the shear-induced migration of red blood cells (RBCs) in large arteries with respect to hypoxia and its possible role in atherosclerosis. The concept of Vortical Structures (VSs) is employed, with which a theory of uid-chemically-driven ILT growth is formulated. The theory proposes that VSs play an important role in convecting and activating platelets in the aneurysmatic bulge. In particular, platelets are convected toward the distal aneurysm region inside vortex cores and are activated via a combination of high residence times and relatively high shear stress at the vortex boundary. After vortex breakup, platelets are free to adhere to the thrombogenic wall surface. VSs also convect thrombin, a potent procoagulant enzyme, captured in their core, through the aneurysmatic lumen and force its accumulation in the distal portion of the AAA. This framework is in line with the clinical observation that the thickest ILT is usually seen in the distal AAA region. The investigation of the fluid-dynamics in arteries led to the study of the shear-induced migration of RBCs in large vessels such as the abdominal aorta and the carotid artery. Marked RBCs migration is observed in the region of the carotid sinus and in the iliac arteries, regions prone to atherogenesis. This leads to the hypothesis that oxyhemoglobin availability can decrease in the near-wall region thus contributing to wall hypoxia, a factor implicated in atherosclerosis. The thesis proposes a new potential mechanism of ILT growth, driven by fluid and chemical stimuli, which can be used to study ILT progression over physiologically relevant timeframes and be used as a framework to test new hypotheses; the thesis also provides new insights on the oxyhemoglobin availability in the near-wall region with direct inuence on atherosclerosis.
|
|
| 9. |
|
|
| 10. |
|
|
| 11. |
- Borodulina, Svetlana
(författare)
-
Micromechanical Behavior of Fiber Networks
- 2013
-
Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Paper is used in a wide range of applications, each of which has specific requirements on mechanical and surface properties. The role of paper strength on paper performance is still not well understood. This work addresses the mechanical properties of paper by utilizing fiber network simulation and consists of two parts.In the first part, we use a three-dimensional model of a network of fibers to describe the fracture process of paper accounting for nonlinearities at the fiber level (material model and geometry) and bond failures. A stress-strain curve of paper in tensile loading is described with the help of the network of dry fibers; the parameters that dominate the shape of this curve are discussed. The evolution of network damage is simulated, the results of which are compared with digital speckle photography experiments on laboratory sheets. It is concluded that the original strain inhomogeneities due to the structure are transferred to the local bond failure dynamics. The effects of different conventional and unconventional bond parameters are analyzed. It has been shown that the number of bonds in paper is important and that the changes in bond strength influence paper mechanical properties significantly.In the second part, we proposed a constitutive model for a fiber suitable for cyclic loading applications. We based the development of the available literature data and on the detailed finite-element model of pulp fibers. The model provided insights into the effects of various parameters on the mechanical response of the pulp fibers. The study showed that the change in the microfibril orientation upon axial straining is mainly a geometrical effect and is independent of material properties of the fiber as long as the deformations are elastic. Plastic strains accelerate the change in microfibril orientation. The results also showed that the elastic modulus of the fiber has a non-linear dependency on a microfibril angle,with elastic modulus being more sensitive to the change of microfibril angle around small initial values of microfibril angles. These effects were incorporated into a non-linear isotropic hardening plasticity model for beams and tested in a fiber network in cycling loading application model, using the model we estimated the level of strains that fiber segments accumulate at the failure point in a fiber network.The main goal of this work is to create a tool that would act as a bridge between microscopic characterization of fiber and fiber bonds and the mechanical properties that are important in the papermaking industry. The results of this work provide a fundamental insight on mechanics of paper constituents in tensile as well as cyclic loading. This would eventually lead to a rational choice of raw materials in paper manufacturing and thus utilizing the environment in a balanced way.
|
|
| 12. |
- Borodulina, Svetlana
(författare)
-
Micromechanics of Fiber Networks
- 2016
-
Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The current trends in papermaking involve, but are not limited to, maintaining the dry strength of paper material at a reduced cost. Since any small changes in the process affect several factors at once, it is difficult to relate the exact impact of these changes promptly. Hence, the detailed models of the network level of a dry sheet have to be studied extensively in order to attain the infinitesimal changes in the final product.In Paper A, we have investigated a relation between micromechanical processes and the stress–strain curve of a dry fiber network during tensile loading. The impact of “non-traditional” bonding parameters, such as compliance of bonding regions, work of separation and the actual number of effective bonds, is discussed. In Paper B, we studied the impact of the chemical composition of the fiber cell wall, as well as its geometrical properties, on the fiber mechanical properties using the three-dimensional model of a fiber with helical orientation of microfibrils at a range of different microfibril angles (MFA). In order to accurately characterize the fiber and bond properties inside the network, via statistical distributions, microtomography studies on the handsheets have been carried out. This work is divided into two parts: Paper C, which describes the methods of data acquisition and Paper D, where we discuss the extracted data. Here, all measurements were performed at a fiber level, providing data on the fiber width distribution, width-to-height ratio of isotropically oriented fibers and contact density. In the last paper, we utilize data thus obtained in conjunction with fiber morphology data from Papers C and D to update the network generation algorithm in order to produce more realistic fiber networks. We also successfully verified the models with the help of experimental results from dry sheets tested under uniaxial tensile tests. We carry out numerical simulations on these networks to ascertain the influence of fiber and bond parameters on the network strength properties.
|
|
| 13. |
|
|
| 14. |
- Cadario, Alessandro, 1975-
(författare)
-
Life prediction and mechanisms for the initiation and growth of short cracks under fretting fatigue loading
- 2006
-
Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Fretting fatigue is a damage process that may arise in engineering applications where small cyclic relative displacements develop inside contacts leading to detrimental effects on the material fatigue properties. Fretting is located in regions not easily accessible, which makes it a dangerous phenomenon. It is therefore important to be able to make reliable predictions of the fretting fatigue lives.The work presented in this thesis has its focus on different aspects related to fretting fatigue in the titanium alloy Ti-17. A fretting experiment was developed which allowed for separate control of the three main fretting loads. Initially, the evolution of the coefficient of friction inside the slip region was investigated experimentally and analytically. Subsequently, 28 fretting tests were performed in which large fatigue cracks developed.The fretting tests were firstly evaluated with respect to fatigue crack initiation through five multiaxial fatigue criteria. The criteria predicted a too high fretting fatigue limit. A possible clue to the discrepancy was found in the fretting induced surface roughness with the asperity-pit interactions.The fatigue growth of the large fretting cracks was numerically modelled through a parametric crack growth procedure. The predicted lives were compared to the experimental outcome. The numerical simulations showed that linear elastic fracture mechanics was an appropriate tool for the prediction of fretting fatigue propagation lives in the long crack regime.Fatigue cracks spend most of their propagation life in the small crack regime. The possibility of modelling the small crack behaviour is therefore very important from the engineering point of view. The fatigue growth of through thickness short cracks was studied experimentally and numerically in the four-point bend configuration. It was found that linear elastic fracture mechanics and closure-free material growth data furnished conservative estimates for cracks longer than 50 μm.One method to improve fretting fatigue life is to shot peen the contact surfaces. Experimental results on fretting life with or without shot peening were simulated. The fatigue life enhancement in shot peened specimens could be explained by slower crack growth in the surface material layer with residual compressive stresses.
|
|
| 15. |
|
|
| 16. |
- Dahlberg, Carl F. O.
(författare)
-
On the Role of Interfaces in Small Scale Plasticity
- 2011
-
Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The strong evidence for a size dependence of plastic deformation in polycrystalline metalsis the basis for the research presented in this thesis. The most important parameter for this, and arguably also the most well known, is the grain size. As the size of the grains in a microstructure is decreased the yield stress increases. This is known as the Hall–Petch relation and have been confirmed for a large number of materials and grain sizes. Other structural dimensions may also give rise to a similar strengthening effect, such as the thickness of films and surface coatings, the widths of ligaments and localization zones and the diameter of thin wires, to mention a few. The work presented in this thesis is shown to be able to model these effects.Size dependent plastic deformation have here been modeled in a continuum mechanical setting by an extension of the standard theory of solid mechanics. Specifically, the work in this thesis is formulated in terms of the higher order strain gradient plasticity (SGP) theory presented by Gudmundson [Gudmundson, P., 2004. A unified treatment of strain gradient plasticity. Journal of the Mechanics and Physics of Solids, 52]. This allows size dependent plasticity phenomena to be modeled and a yield stress that is proportional to the inverse of the geometric dimension of the problem is predicted.The ability to model interfaces have been of specific importance to the work presented here. The state at internal interface is shown, via a physically motivated constitutive description, to be of great importance to capture size effects. The surface energy at grain boundaries is shown to influence both the local and the macroscopic behavior. At the smallest scales an additional deformation mechanism have been introduced at the internal boundaries. This allowed the strengthening trend associated with decreasing grains size to be halted, in qualitative agreement with reported experiments on the behavior of ultrafine and nanocrystalline polycrystals. In the later part of the thesis the focus is aimed at modeling grains structures to bring some insight into the different regions of deformation mechanisms in relation to grainsize and interface strength. A deformation mechanism map for polycrystals is suggested based on the results from structures with both hexagons and log-normal size distributed Voronoi tessellations, and the implication of a statistical variation in grain size have been explored.A finite element implementation of the theory have been developed that is a fully implicit backward-Euler algorithm with tangent operators consistent with the stress update scheme, which give excellent convergence properties and is numerically very stable. Higher order finite elements have been implemented for modeling of both bulk material and internal interfaces. A plane strain version have been used to model metal-matrix composites and explore the implication of some of the more exotic features of the theory.
|
|
| 17. |
|
|
| 18. |
- Dahlberg, Johan, 1977-
(författare)
-
On the asperity point load mechanism for rolling contact fatigue
- 2007
-
Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Rolling contact fatigue is a damage process that may arise in mechanical applications with repeated rolling contacts. Some examples are: gears; cams; bearings; rail/wheel contacts. The resulting damage is often visible with the naked eye as millimeter sized surface craters. The surface craters are here denoted spalls and the gear contact served as a case study.The work focused on the asperity point load mechanism for initiation of spalls. It was found that the stresses at asperity level may be large enough to initiate surface cracking, especially if the complete stress cycle was accounted for.The gear contact is often treated as a cylindrical contact. The thesis contains experimental and numerical results connected to rolling contact fatigue of cylindrical contacts. At the outset a stationary cylindrical contact was studied experimentally. The stationary test procedure was used instead of a rolling contact. In this way the number of contact parameters was minimized. The cylindrical contact resulted in four different contact fatigue cracks. The two cracks that appeared first initiated below the contact. The other two cracks developed at the contact surface when the number of load cycles and the contact load increased.The influence of a surface irregularity (asperity) was studied numerically with the Finite Element Method (FEM). Firstly, the stationary contact was modelled and investigated numerically. At the cylindrical contact boundary a single axisymmetric was included. The partially loaded asperity introduced a tensile surface stress, which seen from the asperity centre was radially directed. Secondly, FE simulations were performed where a single axisymmetric asperity was over-rolled by a cylindrical contact. The simulations were performed for pure rolling and rolling with slip. For both situations, tensile forward directed stresses in front of the asperity were found. The presence of slip and a surface traction greatly increased the stresses in front of the asperity. Finally, when rolling started from rest with applied slip, the distance to steady-state rolling was determined for elastic similar cylindrical rollers.
|
|
| 19. |
- Dersjö, Tomas, 1981-
(författare)
-
Methods for reliability based design optimization of structural components
- 2012
-
Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Cost and quality are key properties of a product, possibly even the two most important. Onedefinition of quality is fitness for purpose. Load-bearing products, i.e. structural components,loose their fitness for purpose if they fail. Thus, the ability to withstand failure is a fundamentalmeasure of quality for structural components. Reliability based design optimization(RBDO) is an approach for development of structural components which aims to minimizethe cost while constraining the probability of failure. However, the computational effort ofan RBDO applied to large-scale engineering problems has prohibited it from employment inindustrial applications. This thesis presents methods for computationally efficient RBDO.A review of the work presented on RBDO algorithms reveals that three constituentsof an RBDO algorithm has rendered significant attention; i ) the solution strategy for andnumerical treatment of the probabilistic constraints, ii ) the surrogate model, and iii) theexperiment design. A surrogate model is ”a model of a model”, i.e. a computationally cheapapproximation of a physics-based but computationally expensive computer model. It is fittedto responses from the physics-motivated model obtained via a thought-through combinationof experiments called an experiment design.In Paper A, the general algorithm for RBDO employed in this work, including the sequentialapproximation procedure used to treat the probabilistic constraints, is laid out. A singleconstraint approximation point (CAP) is used to save computational effort with acceptablelosses in accuracy. The approach is used to optimize a truck component and incorporatesthe effect that production related design variables like machining and shot peening have onfatigue life.The focus in Paper B is on experiment design. An algorithm employed to construct anovel experiment design for problems with multiple constraints is presented. It is based onan initial screening and uses the specific problem structure to combine one-factor-at-a-timeexperiments to a several-factors-at-a-time experiment design which reduces computationaleffort.In Paper C, a surrogate model tailored for RBDO is introduced. It is motivated by appliedsolid mechanics considerations and the use of the first order reliability method to evaluate theprobabilistic constraint. An optimal CAP is furthermore deduced from the surrogate model.In Paper D, the paradigm to use sets of experiments rather than one experiment at atime is challenged. A new procedure called experiments on demand (EoD) is presented. TheEoD procedure utilizes the core of RBDO to quantify the demand for new experiments andaugments it by a D-optimality criterion for added robustness and numerical stability.
|
|
| 20. |
|
|
| 21. |
- Elmukashfi, Elsiddig, et al.
(författare)
-
A multiscale continuum modeling of strain-induced cavitation damageand crystallization in rubber-like materials
- 2015
-
Rapport (övrigt vetenskapligt/konstnärligt)abstract
- A multiscale continuum model for strain-induced cavitation damage and crystallization for rubber-like materials is proposed. The constitutive behavior is determined by homogenization over different length scales, namely, the nano-scale, micro-scale and macro-scale. The microstructure of a filled rubber-like material is seen as interaction between clusters of the filler particles and long-chain molecules that form two networks, between cross-links and between the filler aggregates. The network between cross-links in the nano-scale is modeled using the full network approach of semi-crystalline chains. A phenomenological law is proposed to describe the crystallite nucleation law. The network between the filler particles is described by statistical mechanics in the nano- and/or micro-scale where the polymer chains sliding on and/or debonding from filler aggregate surface is incorporated. The debonding process is regarded as the main mechanism of the nucleation of nano-cavities which introduces non-affine deformation to the network between cross-links. Hence, The nanoscopic initially non-cavitated network between cross-links is homogenized over the micro-scale assuming a spherical representative volume element using the kinematics proposed by Hang-Sheng and Abeyaratne (Hang-Sheng and Abeyaratne in Journal of the Mechanics and Physics of Solids 40 (3), 571592,1992). The constitutive model is presented in form of an averaged strain energy function. The predictive capabilities of the model are then tested via comparisons with experimental data from literature.
|
|
| 22. |
- Elmukashfi, Elsiddig
(författare)
-
An experimental method for estimating the tearing energy inrubber-like materials using the true stored energy
- 2015
-
Rapport (övrigt vetenskapligt/konstnärligt)abstract
- A method for determining the critical tearing energy in rubber-like materials is proposed. In this method, the energy required for crack propagation in a rubber-like material is determined by the change of the recovered elastic energy. Hence, the dissipated energy due to different inelastic processes is deducted from the total strain energy applied to a system. Therefore, the classical method proposed by Rivlin and Thomas (Rivlin and Thomas in Journal of Polymer Science 10(3):291-318,1953) using the pure shear tear test is modified using the actual stored elastic energy. The elastically stored energy in a pure shear is determined experimentally using cyclic loading. The experimental results show that the classical method overestimates the critical tearing energy by approximately $15\%$. Moreover, the effect of the unloading rate on the determination of the elastically stored energy is investigated and found to be minimal suggesting that the crack propagation velocity has a minor effect in the change of the elastically stored energy.
|
|
| 23. |
- Elmukashfi, Elsiddig
(författare)
-
Experimental investigations of crack propagation in rubber under dif-ferent loading rates, temperatures and fracture modes
- 2015
-
Rapport (övrigt vetenskapligt/konstnärligt)abstract
- In the present paper, the fracture behavior of carbon-black natural rubber material is experimentally studied. The cracked pure shear and the single edge notch specimens were used for investigating both pure mode I and mixed mode I and II fracture behavior, respectively. Further, different testing conditions were employed in the case of the cracked pure shear specimens. The specimens were subjected to three different loading rates and they were tested in two different temperatures. For studying the crack growth, a high speed camera at up to 7000 frames/s was used to follow the progress of the crack and later a post-processor was used to obtain the crack trajectory and velocity at different stages. The method introduced previously by the present author (Elmukashfi in report 580, Department of Solid Mechanics, Royal Institute of Technology (KTH), 2015) was used to obtain the critical tearing energy using the cracked pure shear specimens. Hence, the uncracked pure shear specimens were subjected to cyclic loading history in order to obtain the true elastic energy in pure shear. The single edge notch specimens were tested in room temperature under quasi-static loading. The pure mode I results suggest that the critical tearing depends strongly on the loading rate as well as the temperature. The tearing behavior shows stick-slip pattern at low tearing rates and smooth propagation at high velocities. The size of the stick-slip region is reduced significantly by increasing the loading rate as well as the temperature. In the mixed mode I and II, the transition from the stick-slip to smooth propagation and the transition from mixed mode I and II take place approximately simultaneously.
|
|
| 24. |
- Elmukashfi, Elsiddig
(författare)
-
Modeling of fracture and damage in rubber under dynamic and quasi-static conditions
- 2015
-
Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Elastomers are important engineering materials that have contributed to the different technical developments and applications since the 19th century. The study of crack growth mechanics for elastomers is of great importance to produce reliable products and therefore costly failures can be prevented. On the other hand, it is fundamental in some applications such as adhesion technology, elastomers wear, etc. In this thesis work, crack propagation in rubber under quasi-static and dynamic conditions is investigated.In Paper A, theoretical and computational frameworks for dynamic crack propagation in rubber have been developed. The fracture separation process is presumed to be described by a cohesive zone model and the bulk behavior is assumed to be determined by viscoelasticity theory. The numerical model is able to predict the dynamic crack growth. Further, the viscous dissipation in the continuum is found to be negligible and the strength and the surface energy vary with the crack speed. Hence, the viscous contribution in the innermost of the crack tip has been investigated in Paper B. This contribution is incorporated using a rate-dependent cohesive model. The results suggest that the viscosity varies with the crack speed. Moreover, the estimation of the total work of fracture shows that the fracture-related processes contribute to the total work of fracture in a contradictory manner.A multiscale continuum model of strain-induced cavitation damage and crystallization in rubber-like materials is proposed in Paper C. The model adopts the network decomposition concept and assumes the interaction between the filler particles and long-chain molecules results in two networks between cross-links and between the filler aggregates. The network between the crosslinks is assumed to be semi-crystalline, and the network between the filler aggregates is assumed to be amorphous with the possibility of debonding. Moreover, the material is assumed to be initially non-cavitated and the cavitation may take place as a result from the debonding process. The cavities are assumed to exhibit growth phase that may lead to complete damage. The comparison with the experimental data from the literature shows that the model is capable to predict accurately the experimental data.Papers D and E are dedicated to experimental studies of the crack propagation in rubber. A new method for determining the critical tearing energy in rubber-like materials is proposed in Paper D. The method attempts to provide an accurate prediction of the tearing energy by accounting for the dissipated energy due to different inelastic processes. The experimental results show that classical method overestimates the critical tearing energy by approximately 15%. In Paper E, the fracture behavior of carbon-black natural rubber material is experimentally studied over a range of loading rates varying from quasi-static to dynamic, different temperatures, and fracture modes. The tearing behavior shows a stick-slip pattern in low velocities with a size dependent on the loading rate, temperature and the fracture mode. Smooth propagation results at high velocities. The critical tearing depends strongly on the loading rate as well as the temperature.
|
|
| 25. |
|
|
| 26. |
|
|
| 27. |
- Fallqvist, Björn, 1985-
(författare)
-
Mechanical response of cross-linked actin networks
- 2013
-
Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The ability to predict the mechanical properties of cells should be seen in the light of the close connection between abnormal cell states and a change in the cell response to stimuli. For example, it has been found that the stiffness of cancer cells is much lower than their healthy counterparts, influencing metastasis and cell migration. On the contrary, malaria cells have been found to exhibit a significant increase in stiffness.The major structural entity of the cell is called the cytoskeleton, an interior network consisting of three types of protein filaments - actin filaments, intermediate filaments and microtubules. The remodelling ability of the cytoskeleton through polymerisation provides the cell with the ability to adapt its response to external forces accordingly. The properties of interfilament cross-links in terms of stiffness and ability to detach can be expected to influence the mechanical response. The work presented herein focuses on the mechanical response of cross-linked actin networks. The results indicate a strong dependence of the mechanical properties on cross-link dynamics and characteristics.In Paper A, a constitutive model for the response of transiently cross-linked networks is developed using a continuum framework. The deformation is split into viscous (representing sliding of filaments) and elastic deformation. A strain energy function is proposed in the form of a neo-Hookean model, modified in terms of chemically activated cross-links. The disassociation rate constant is modified in terms of an exponential function taking into account the amount of strain energy available to break bonds. The constitutive model was compared with experimental relaxation tests and it was found that the initial region of fast stress relaxation can be attributed to breaking of bonds, and the subsequent slow relaxation to sliding of filaments.In Paper B, a finite element framework was used to assess the influence of numerous geometrical and material parameters on the response of cross-linked actin networks. It was shown that considering the presence of a statistical dispersion in filament lengths has a significant effect on the mechanical properties of the network. Further, the compliance of the crosslinks was shown to influence the stress-strain curve and shift the region of strain hardening. The influence of boundary conditions and the effect of network parameters on experiments in terms of local and global effects were also addressed. Finally, a micromechanically motivated constitutive model in a continuum framework was presented, capturing some essential characteristic features of cross-linked actin networks.
|
|
| 28. |
|
|
| 29. |
|
|
| 30. |
- Fredriksson, Per, 1976-
(författare)
-
Modelling and simulation of plastic deformation on small scales : interface conditions and size effects of thin films
- 2008
-
Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Contrary to elastic deformation, plastic deformation of crystalline materials, such as metals, is size-dependent. Most commonly, this phenomenon is present but unnoticed, such as the effect of microstructural length scales. The grain size in metallic materials is a length scale that affects material parameters such as yield stress and hardening moduli. In addition, several experiments performed in recent years on specimens with geometrical dimensions on the micron scale have shown that these dimensions also influence the mechanical behaviour. The work presented in this thesis involves continuum modelling and simulation of size-dependent plastic deformation, with emphasis on thin films and the formulation of interface conditions.A recently published strain gradient plasticity framework for isotropic materials [Gudmundson, P., 2004. A unified treatment of strain gradient plasticity. Journal of the Mechanics and Physics of Solids 52, 1379-1406] is used as a basis for the work. The theory is higher-order in the sense that additional boundary conditions are required and, as a consequence, higher-order stresses appear in the theory. For dimensional consistency, length scale parameters enter the theory, which is not the case for conventional plasticity theory. In Paper A and B, interface conditions are formulated in terms of a surface energy. The surface energy is assumed to depend on the plastic strain state at the interface and different functional forms are investigated. Numerical results are generated with the finite element method and it is found that this type of interface condition can capture the boundary layers that develop at the substrate interface in thin films. Size-effects are captured in the hardening behaviour as well as the yield strength. In addition, it is shown that there is an equivalence between a surface energy varying linearly in plastic strain and a viscoplastic interface law for monotonous loading.In paper C, a framework of finite element equations is formulated, of which a plane strain version is implemented in a commercial finite element program. Results are presented for an idealized problem of a metal matrix composite and several element types are examined numerically. In paper D, the implementation is used in a numerical study of wedge indentation of a thin film on an elastic substrate. Several trends that have been observed experimentally are captured in the theoretical predictions. Increased hardness at shallow depths due to gradient effects as well as increased hardness at more significant depths due to the presence of the substrate are found. It is shown that the hardening behaviour of the film has a large impact on the substrate effect and that either pile-up or sink-in deformation modes may be obtained depending on the material length scale parameter. Finally, it is qualitatively demonstrated that the substrate compliance has a significant effect on the calculated hardness of the film.
|
|
| 31. |
- Grytsan, Andrii, 1986-
(författare)
-
Abdominal aortic aneurysm inception and evolution - A computational model
- 2016
-
Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Abdominal aortic aneurysm (AAA) is characterized by a bulge in the abdominal aorta. AAA development is mostly asymptomatic, but such a bulge may suddenly rupture, which is associated with a high mortality rate. Unfortunately, there is no medication that can prevent AAA from expanding or rupturing. Therefore, patients with detected AAA are monitored until treatment indication, such as maximum AAA diameter of 55 mm or expansion rate of 1 cm/year. Models of AAA development may help to understand the disease progression and to inform decision-making on a patient-specific basis. AAA growth and remodeling (G&R) models are rather complex, and before the challenge is undertaken, sound clinical validation is required.In Paper A, an existing thick-walled model of growth and remodeling of one layer of an AAA slice has been extended to a two-layered model, which better reflects the layered structure of the vessel wall. A parameter study was performed to investigate the influence of mechanical properties and G&R parameters of such a model on the aneurysm growth.In Paper B, the model from Paper A was extended to an organ level model of AAA growth. Furthermore, the model was incorporated into a Fluid-Solid-Growth (FSG) framework. A patient-specific geometry of the abdominal aorta is used to illustrate the model capabilities.In Paper C, the evolution of the patient-specific biomechanical characteristics of the AAA was investigated. Four patients with five to eight Computed Tomography-Angiography (CT-A) scans at different time points were analyzed. Several non-trivial statistical correlations were found between the analyzed parameters.In Paper D, the effect of different growth kinematics on AAA growth was investigated. The transverse isotropic in-thickness growth was the most suitable AAA growth assumption, while fully isotropic growth and transverse isotropic in-plane growth produced unrealistic results. In addition, modeling of the tissue volume change improved the wall thickness prediction, but still overestimated thinning of the wall during aneurysm expansion.
|
|
| 32. |
- Grytsan, Andrii, 1986-
(författare)
-
Computational model of abdominal aortic aneurysm inception and evolution
- 2014
-
Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Incidence of abdominal aortic aneurysm (AAA) is increasing in the aging society of the western world. Development of AAA is mostly asymptomatic and is characterized by a bulge in the abdominal aorta. However, AAA may suddenly rupture, which results in an internal bleeding associated with a high mortality rate. Patients with AAA undergo regular screening until treatment indication. To date, statistical criteria are used to decide whether the risk of rupture exceeds the risk of intervention. Models of AAA development help to understand the disease progression and to yield patient-specific criterion for AAA rupture.Up to date, sophisticated models of AAA development exist. These models assume the abdominal aorta as a thin-walled structure, which saves the computational effort. This thesis aims at investigating the importance of employing a thick-walled model of the aorta. The effects on AAA development that cannot be captured with a thin-walled model are of interest. In Paper A, the thick-walled model of growth and remodeling of one layer of a AAA slice has been extended to a two-layered model. The parameter study has been performed to investigate the influence of mechanical properties and growth and remodeling (G&R) parameters of two individual layers on the gross mechanical response and G&R of the artery. It was concluded that the adventitia acts to protect the arterial wall against rupture even in pathological state.In Paper B, the model was extended to an organ level model of AAA development. Furthermore, the model was incorporated into a so-called Fluid-Solid-Growth (FSG) framework, where the AAA development is loosely coupled to the blood flow conditions such as wall shear stress. One patient-specific geometry of the abdominal aorta is used to illustrate the model capabilities. A transmurally non-uniform distribution of the strains of individual arterial constituents was observed. In addition, an increased aneurysm tortuosity was observed in comparison to a thin-walled approach. These findings signify the importance of a thick-walled approach to model the aneurysm development. Finally, the proposed methodology provides a realistic basis to further explore the growth and remodeling of AAA on a patient-specific basis.
|
|
| 33. |
- Hagman, Anton, 1984-
(författare)
-
Influence of inhomogeneities on the tensile and compressive mechanical properties of paperboard
- 2016
-
Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The in-plane properties of paperboard have always been of interest to paper scientists. Tensile properties are crucial when the board is fed through converting machines at high speeds. Compressive properties are essential in the later use. Inhomogeneities affect both the compressive and tensile properties. For the tensile properties, it is the inherent heterogeneity of the paperboard that might cause problems for the board-maker. Varying material properties, through the thickness of the paperboard, are on the other hand used to achieve high bending stiffness with low fiber usage. It is of interest to know how this practice affects the local compressive properties. Papers A and B aims to address this, while C, D and E focus on in-plane heterogeneities. Paper A investigates the mechanism that causes failure in the short span compression test (SCT). It was concluded that the main mechanism for failure in SCT is delamination due to shear damage. In paper B the effect of the through-thickness profiles on the local compression strength was examined. It was concluded that the local compression is governed by in-plane stiffness and through thickness delamination. The latter was in turn dependent on the local shear strength and in-plane stiffness gradients. In paper C the tensile test is investigated with focus on sample size and strain distributions. The strain behavior was dependent on the length to width ratio of the sample and was caused by activation of local zones with high strainability. Paper D focuses on the strain zones seen in C. The thermal response in paper was studied. It was observed that an inhomogeneous deformation pattern arose in the paper samples during tensile testing. It was concluded that the heat patterns observed coincided with the deformation patterns. It could be shown that the formation was the cause of the inhomogeneous deformation. In final paper, E, the virtual field method was applied on data from C.
|
|
| 34. |
|
|
| 35. |
- Hagman, Anton, 1984-
(författare)
-
Investigations of In-Plane Properties of Paperboard
- 2013
-
Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- In-plane properties of paperboard have always been of interest to paper mechanical researchers. The reason for this is that they play a large role for the usability of the paperboard throughout its lifespan. Tensile properties are crucial when the board is fed through printing and converting machines at high speeds in the beginning of its life. While compressive properties are essential in the later use of e.g. packages. In this thesis some methods for evaluating in-plane properties are reinvestigated.In Paper A the tensile test was investigated with focus on sample size and strain distributions. Three different multiply paperboards were examined with varying sample sizes using speckle photography. Different strain behaviour was found for different sample sizes. This difference was dependent on the length to width ratio of the sample and was caused by the activation of strain zones in the sample. These zones were of a constant size and therefore occupied different amounts of the total sample area.Paper B investigates the mechanism that causes failure in the short span compression test (SCT). Three different multiply paperboards were examined, this time chosen to have distinctly different through-thickness profiles. The boards were characterized and the data was used to simulate a SCT test with the three different boards. The simulation was conducted with a finite element model consisting of layers of continuum elements with cohesive interfaces in-between. From the model it was concluded that the main mechanism for failure in SCT is delamination that was caused by shear damage.
|
|
| 36. |
- Hagman, Anton, 1984-, et al.
(författare)
-
Thermographical analysis of paper during tensile testing and comparison to digital image correlation
- 2016
-
Rapport (övrigt vetenskapligt/konstnärligt)abstract
- The thermal response in paper has been studied using thermography. It was observed that an inhomogenous deformation pattern arose in the paper samples during tensile testing. In the plastic regime a pattern of warmer streaks could be observed in the samples. On the same samples digital image correlation (DIC) was used to study local strain fields. It was concluded that the heat patterns observed by thermography coincided with the deformation patterns observed by DIC. Due to the fibrous network structure paper has an inhomogenous microstructure, called formation. It could be shown that the formation was the cause of the inhomogenous deformations in paper. Finite element simulations were used to show how papers with different amount of homogeneity would deform. Creped papers, where the strain at break has been increased, were analysed. For these paper it was seen that an overlaid permanent damage was created during the creping process. During tensile testing this was recovered as the paper network structure was strained.
|
|
| 37. |
- Hannes, Dave, 1983-
(författare)
-
On fatigue crack growth modelling of surface initiated rolling contact fatigue using the asperity point load mechanism
- 2014
-
Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Load transfer in applications or between machine components is generally achieved through contact. In case of recurrent high contact loads in combination with a rolling motion, i.e. with a relatively small amount of slip, the contact surface may eventually suffer from rolling contact fatigue (RCF). The damage consists then of cracks and craters or spalls, which can cause dysfunctionality of the application leading to inefficiency or increased maintenance costs. Ultimately the damage may cause total failure of the machine component. The damage process is still not fully understood due to the complexity of the problem. Different mechanisms have been suggested to explain initiation and propagation of RCF damage. The current work focused on crack growth modelling of surface initiated RCF in case hardened gear steel. The study was based on the asperity point load mechanism, which emphasizes the importance of the surface roughness in the damage process. Asperities on the contact surface act as stress raisers inducing locally high tensile surface stress when entering the contact. Improved understanding of the damage process and further validation of the asperity point load mechanism was achieved.In Paper A, the crack path of surface initiated RCF was simulated in the symmetry plane of the damage with the trajectory of the largest principal stress in the uncracked material. The mode I fracture mechanism was found applicable as well as linear elastic fracture mechanics (LEFM). The evolvement of the asperity contact parameters during the load cycle was determined through a finite element (FE) contact model based on an equivalent contact geometry. The predicted RCF crack path agreed with experimental spall profiles both in entry details as in overall shape. An experimental series was performed in Paper B to investigate the crack closure behaviour in presence of large negative minimum loads. The experimental results suggested a crack closure limit close to zero. The choice of the equivalent mixed-mode stress intensity factor range and especially the crack closure limit had a significant effect on the predicted RCF or spalling life. The two-dimensional crack growth model was further developed in Paper C and used to investigate the influence of asperity size, friction and residual surface stress on the simulated RCF damage. The simulations agreed qualitatively with experimental observations where reduced surface roughness, improved lubrication and compressive residual surface stress increased RCF resistance. In Paper D, a three-dimensional stationary crack was studied using an FE model and a simplified RCF load. A new crack geometry was proposed allowing the investigation of the spall opening angle of the typical vshaped damage. Crack arrest through crack closure was suggested as explaining mechanism. A qualitative study indicated increased spread of the surface damage with increased friction. The results also depended on the crack inclination angle. The different studies supported the asperity point load mechanism to explain not only fatigue initiation but also fatigue crack propagation.
|
|
| 38. |
- Huang, Hui
(författare)
-
Numerical and experimental investigation of paperboard creasing and folding
- 2011
-
Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- This licentiate thesis aims to increase the understanding of deformation and damage mechanisms of paperboard during converting, especially creasing and folding will be analyzed. A simple two dimensional creasing simulation was performed. In this model, paperboard was modeled as a combination of an anisotropic elastic-plastic continuum model with isotropic hardening and a softening cohesive interface model. The paperboard was composed of four plies with uniform material parameters. Creasing simulations were done on both machine direction (MD) and cross machine direction (CD) samples to two crease depths 0.0 mm and 0.2 mm, respectively. The simulation results showed good agreement with experimental results. The out-of-plane shear properties are dominating factors for creasing and folding. Therefore, a test method to determine shear properties was proposed. This part of the work is based on the most recently proposed test method, the laminated double notch shear test. To improve the technique, double notches with declined slopes, called tilted double notch shear test, were used instead of uniform depth double notches. The influence of shear zone length was also investigated. The results reveal the short shear zone lengths gave higher shear strength and more pronounced shear strength profile. The results from the rst two analyses were utilized to study folding of paperboard. The simulation model was the same as in the creasing simulations. However, to improve the model and better account the actual micro structure of paperboard a new material mapping method was proposed. The continuum properties of the plies were assumed to vary in the thickness direction. The shear strengths of the interfaces were determined by using the tilted double notch shear test using a short shear zone length, L= 5 mm. The agreement between simulation results and experiment results was good, and most of the folding properties were captured.
|
|
| 39. |
- Huang, Hui
(författare)
-
Numerical and Experimental Investigation on Paperboard Converting Processes
- 2013
-
Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- An investigation of the mechanical properties of paperboard and its influence on converting processessuch as creasing, folding and forming, from both an experimental and numerical perspective wasperformed. Fundamental research to establish a material model for paperboard, and an experimentalout-of-plane shear test method was suggested. Research where the models were used for verificationwas also done. The numerical model is a combination of continuum and interface models. Thecontinuum model represents the paperboard plies, which is an orthotropic elastic-plastic model withHill criteria and isotropic hardening. The interface model is used for connecting the paperboard pliesand also contributing to the delamination properties during converting processes. The interface modelhas linear elastic behavior followed by the initiation and evolution of damage. Both of these twomodels are available in ABAQUS. An experimental characterization scheme consisting of threeexperiments: in-plane tensile test, double notch shear test and density measurements, was shown to besufficient to predict the creasing and folding behavior.The creasing and folding performance can be well predicted by the model. The impact of ply andinterface properties on different paperboards were investigated by numerical simulations, in order tomimic different production strategies. It was shown that the interface strengths mainly influenced thefolding behavior, whereas different ply properties affected the required creasing force.The forming investigation was conducted in a three dimensions deep pear-shape mould. The numericalinvestigation included the effect of pressure, boundary conditions, material properties, differentdeformation and damage mechanisms, i.e. delamination and plasticity. The results showed thesimulation can capture the failure pattern of experiments and the mechanisms during forming. Toachieve better forming performance with anisotropic commercial paperboard in an axis-symmetricmould, a combination of fixed and free boundary conditions can be used to minimize in-plane straincomponents while enabling delamination. Modification of material properties would enable an evenbetter optimization. Additionally, reduction of anisotropy can improve the forming performance.
|
|
| 40. |
- Huang, Hui, et al.
(författare)
-
Quasi static analysis of creasing and folding for three paperboards
- 2013
-
Rapport (övrigt vetenskapligt/konstnärligt)abstract
- The creasing and folding behavior of three paperboards have been studied both experimentally and numerically. Creasing and folding studies were performed on strips in both the machine direction and the cross machine direction. A finite element model that mimicked the experimental creasing and folding setup was developed, and the creasing and folding behavior could be well predicted for all three paperboards.An experimental characterization scheme consisting of three experiments was proposed, and was shown to be sufficient to predict the creasing and folding behavior. For the whole paperboard the shear strength profiles in the through thickness direction was determined with the notched shear test. Each ply was laid free by grinding, and density measurements and in-plane tension tests were performed on the bottom, middle and top plies of each paperboard. Instead of assuming uniform properties in each ply, the shear strength profiles were used to map the measured properties in the through thickness direction.Numerical simulations were performed when the ply and interface properties of the paperboards were altered to follow different shear strength profiles. This was done in order to mimic different production strategies. It was shown that the interface strengths mainly influenced the folding behavior. Whereas altered the ply properties affected the creasing force needed.
|
|
| 41. |
- Huang, Hui, et al.
(författare)
-
The dependency of shear zone lengh on the shear strength profiles in paperboard
- 2011
-
Rapport (övrigt vetenskapligt/konstnärligt)abstract
- In this work, the notched shear strength test (NST) has been further improved. In order to simplify and accelerate the testing procedure, the notches with declined slopes were used. With the proposed procedure, the shear strength profile in the thickness direction of a paperboard can be measured using one sheet only. By using the test setup, the dependency of shear zone length on shear strength was investigated. Experimental results show that both the measured shear strength values as well as the shear strength profile varied significantly with different shear zone length. Longer shear zone gave lower shear strength values and flatter profiles, while a shorter shear zone gave higher strength values and more pronounced shear strength profiles that better followed the paperboard ply structure.
|
|
| 42. |
- Jelagin, Denis, 1979-
(författare)
-
Frictional Effects on Hertzian Contact and Fracture
- 2007
-
Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- This thesis addresses normal axisymmetric contact of dissimilar elastic solids at finite interfacial friction. It is shown that in the case of smooth and convex but otherwise arbitrary contact profiles and monotonically increasing loading a single stick-slip contour evolves being independent of loading and profile geometry. This allows developing an incremental procedure based on a reduced problem corresponding to frictional rigid flat punch indentation of an elastic half-space. The reduced problem, being independent of loading and contact region, was solved by a finite element method based on a stationary contact contour and characterized by high accuracy. Subsequently, a tailored cumulative superposition procedure was developed to resolve the original problem to determine global and local field values for two practically important geometries: flat and conical profiles with rounded edges and apices. Results are given for relations between force, depth and contact contours together with surface stress distributions and maximum von Mises effective stress, in particular to predict initiation of fracture and plastic flow. It is also observed that the presence of friction radically reduces the magnitude of the maximum surface tensile stress, thus retarding brittle fracture initiation.Hertzian fracture through indentation of flat float glass specimens by steel balls has been examined experimentally for a full load cycle. It has been observed that if the specimen survived during loading to a maximum level it frequently failed at decreasing load. It has been proposed by Johnson et al. (1973) that the underlying physical cause of Hertzian fracture initiation during load removal is that at unloading frictional tractions reverse their sign over part of the contact region. Guided by these considerations a robust computational procedure has been developed to determine global and local field values in particular at unloading at finite friction. In contrast to the situation at monotonically increasing loading, at unloading invariance properties are lost and stick-slip regions proved to be severely history dependent and in particular with an opposed frictional shear stress at the contact boundary region. This causes an increase of the maximum tensile stress at the contour under progressive unloading. It is shown that the experimental observations concerning Hertzian fracture initiation at unloading are at least in qualitative correlation with the effect friction has on the maximum surface tensile stress.A contact cycle between two dissimilar elastic bodies at finite Coulomb friction has been further investigated analytically and numerically for a wider range of material parameters and contact geometries. With the issue of Hertzian fracture initiation in mind, results concerning the influence of the friction coefficient and compliance parameters on the absolute maximum surface tensile stress during a frictional contact cycle are reported along with the magnitudes of the relative increase of maximum tensile stresses at unloading. Based on a critical stress fracture criterion it is discussed how the predicted increases will influence the critical loads required for crack initiation.Fracture loads are measured with steel and tungsten carbide spherical indenters in contact with float glass specimens at monotonically increasing loading and during a load cycle. Computational predictions concerning the fracture loads are given based on Hertz and frictional contact theories combined with a critical stress fracture criterion. The computational results obtained for frictional contact are shown to be in better agreement with experimental findings as compared to the predictions based on the Hertz theory. The remaining quantitative discrepancy was attributed to the well-known fact that a Hertzian macro-crack initiates from pre-existing defects on the specimen’s surface. In order to account for the influence of the random distribution of these defects on the fracture loads at monotonic loading, Weibull statistics was introduced. The predicted critical loads corresponding to 50% failure probability were found to be in close agreement with experimentally observed ones.
|
|
| 43. |
|
|
| 44. |
- Karlén, Kristoffer, 1982-
(författare)
-
Probabilistic modeling of fatigue failures
- 2012
-
Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Fatigue is a well known failure mode in engineering that can have catastrophic consequences, such as loss of human life. Thus design against fatigue failure is very important. There are many sources of scatter present in fatigue, for instance; the difference in load in-between users of a product, material scatter and scatter in the production. The material scatter will be studied in this thesis.In order to quantify the material scatter, experiments have to be performed. Both finite life tests, i.e. experiments at a constant stress levels where all specimen fail and the number of cycles to failure is stored, and fatigue limit tests, experiments where some experiments are run-outs and some experiments fail, have to be performed. The SN-space contains both the finite life part and the fatigue limit part. In order to model the material scatter, the Weakest Link (WL)-integral can be used. This integral, which was derived by Waloddi Weibull at KTH, takes the entire volumetric stress distribution into account. The outcome from this integral is a fatigue failure probability for a specimen or a structure. Thus if this integral is used a structure is designed with respect to a fatigue failure probability instead of a peak stress. Such a peak stress, or hot-spot stress, is related to the fatigue limit and is typically reduced by a safety factor.In paper A fatigue limit tests performed on a custom-made specimen with two notches of different size are presented. The predictive capabilities of the weakest link integral were studied here, where the WL-model was fitted to the experimental outcome in both notches separately and then to both notches simultaneously. It was observed that the WL-integral is in good agreement with the experimental outcome when fitted to the experimental outcome in one notch, but poor when fitted to both notches,The weakest link integral was evaluated at the specimen surface area and as a volumetric phenomenon in paper B. The conclusions in this paper was that the area and volume formulation of the WL-integral show similar results.A new model for the entire SN-space, the PES-model was analyzed in paper C. Here, an equivalent stress measure (a scalar stress value) was introduced in order to have the same stress measure for finite life and the fatigue limit regime. The investigated equivalent stresses were; the point stress (largest occurring stress value), the gradient adjusted point stress (largest occurring stress value reduced with the stress gradient), the area stress (an effective measure of the surface stresses using the weakest link) and the volume stress (a similar measure that summarize the volumetric stresses). It was observed that the choice of equivalent stress had a small effect for finite life both a large effect at the fatigue limit regime.In paper D a model that combines two failure mechanisms is presented, the DS-model. This model combines a defect based model, D, that is taken to be the weakest link integral (both area and volume versions) with a stress based model, S, taken to be the normal distribution where the stress measure used is either the point stress or the gradient adjusted point stress. It is assumed that the two failure mechanisms are independent. It was observed that, the D-model was dominating for low failure probabilities and the S-model for high failure probabilities.In order to study the experimental scatter in another way, the estimated fatigue failure locations were studied in paper E. The stress was then evaluated and the estimated fatigue failure sites and the local failure probability could be estimated. In order to better understand the spatial scatter in the estimated fatigue failure sites a modified stress gradient was used. Further, experiments where fatigue failure could occur in both notches were performed. It was seen that the spatial scatter was large, in terms of location and in stress. None of the fatigue limit models could describe the experimental trend from the competing fatigue failure site experiments.The effect of random defect is studied in paper F. In this paper defects of different size, which are treated as circular cracks, are placed at random positions in the specimen. A fatigue crack growth analysis is performed for each crack and thus the fatigue life is obtained. The main conclusion in this paper was that the computed fatigue crack growth life does not agree with the experimentally found fatigue life.
|
|
| 45. |
|
|
| 46. |
- Kiousis, Dimitrios, 1979-
(författare)
-
Computer Aided Angioplasty : Patient-specific arterial modeling and smooth 3D contact analysis of the stent-balloon-artery interaction
- 2006
-
Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Paper A: In this paper, the development and implementation of a contact algorithm based on C2-continuous surface representations is discussed. In 3D contact simulations involving models with arbitrarily curved surfaces (as in the case of vessel walls), the discretization of the contact surfaces by means of facet-based techniques could lead to numerical instabilities and finally loss of quadratic convergence. These instabilities arise mainly due to the sliding of contractor (slave) nodes over the boundaries of target (master) contact facets, where jumps of the normal vector are experienced. The paper addresses successfully this problem, by discretization of the target surfaces by means of C2-continuous parameterization schemes. Initially, the uniform cubic B-spline surfaces are introduced. Next, in an attempt for more accurate representations of the geometric models of the contact surfaces, a new parameterization based on the expression of cubic B-splines is developed. The two approaches are implemented into a finite element framework and more specifically, into the multipurpose finite element analysis program FEAP. The special merits of the developed algorithms and the advantages of the smooth surfaces over facet-based approaches are exhibited through a classical contact mechanics problem, considering incompressibility, finite deformations and large slidings. Next, a simulation of balloon angioplasty with stenting is presented, where the contact between both medical devices (balloon and stent) with the arterial wall is modeled. The arterial wall is modeled in this first approach, as hyperelastic, homogeneous, isotropic, while a cylindrically orthotropic model is developed to capture the nonlinear, anisotropic behavior of the balloon catheter under pressure. Two stents with the same geometry but different strut thickness, are studied. Both are considered elasto-plastic. The performed simulations point out the outcome of the balloon angioplasty and stenting in terms of luminal gain and mechanical strains. Finally, a comparison between the two stent configurations is presented.Paper B: The second paper makes use of the contact tool developed in Paper A and focuses on the changes of the mechanical environment of the arterial wall due to stenting, as a function of a set of stent design parameters. In particular, Paper B presents a detailed geometric and material model of a postmortem human iliac artery, composed by distinct tissue components, each associated with specific mechanical properties. The constitutive formulation for the artery considers anisotropic, highly nonlinear mechanical characteristics under supraphysiological loadings. The material and structural parameters of the arterial model are obtained through uniaxial tensile tests on stripes extracted from the several arterial tissues that form the stenosis, axially and circumferentially oriented. Through cooperation with a well-established stent manufacturing company, an iliac stent was acquired. The dimensions of the stent are measured under a reflected-light microscope, while it is parameterized in such a way as to enable new designs to be simply generated through variations of its geometric parameters. The 3D balloon-stent-artery interaction is simulated by making use of the smooth contact surfaces with C2-continuity, as previously mentioned. Next, scalar quantities attempt to characterize the arterial wall changes after stenting, in form of contact forces induced by the stent struts, stresses within the individual components and luminal change. These numerically derived quantities allow the determination of the most appropriate stent configuration for an individual stenosis. Therefore, the proposed methodology has the potential to provide a scientific basis for optimizing treatment procedures, stent material and geometries on a patient-specific level.
|
|
| 47. |
- Kulachenko, Artem
(författare)
-
Mechanics of paper webs in printing press applications
- 2006
-
Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The mechanics of paper is a difficult subject because paper is a unique material. It is very thin, flexible at bending, unstable in compression and stiff at tension. Dealing with paper we have to account for orthotropy and heterogeneities created during the manufacturing process.This thesis addresses two topics in mechanics of paper webs in printing press applications. First is the dynamic behaviour of the travelling webs. Second is so-called “fluting” after heat-set web-fed offset printing.There are a number of challenges in simulating the dynamics of the paper web. It is necessary to include the influence of the paper web transport velocity. Due to initial sag or vibrations, gyroscopic forces affect the dynamics of the webs. Furthermore, the transport velocity reduces the stress stiffening of the web. A good theoretical model should account for large displacements and should be capable of simulating wrinkles, which is essentially a post-buckling phenomenon. Finally, the paper web is surrounded with air which reduces the natural frequencies substantially by “adding" mass to the paper. A non-linear finite element formulation has been developed in this study for simulation of travelling webs. The results of the studies shows that for the tension magnitudes used in the printing industry the critical web speed lies far above those used today. Speed limitations are rather caused by ink setting and tension control problems. If the web tension profile is skew, however, edge vibrations are inevitable even at small external excitations.Fluting is a permanent wavy distortion of the paper web after heat-set web offset printing. It is often seen in high quality printing products, especially in areas covered with ink. It is generally accepted that tension and heat are required to create fluting. However, there have been certain disputes as to the mechanism of fluting formation, retention and key factors affecting this phenomenon. Most of the existing studies related to fluting are based on linear buckling theories. A finite element model, capable of simulating a post-buckling behaviour has been developed and experimentally verified. Studies show that none of the existing theories can consistently explain fluting. A new basic mechanism of fluting formation has been proposed and numerically demonstrated. Fluting was explained as a post-buckling phenomenon due to small scale moisture variations developing during through-air drying. It was concluded that air permeability variation is the key factor affecting fluting tendency. Fluting is retained due to inelastic deformations promoted by high drying temperatures.
|
|
| 48. |
- Lagerblad, Ulrika, 1987-
(författare)
-
A fixed-lag smoother for solving joint input and state estimation problems in structural dynamics
- 2016
-
Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- In this thesis we have investigated different numerical filters for joint input and state estimation, with the aim of designing a robust algorithm capable of monitoring the continuous motion and loading in a truck chassis. The algorithm has to be able to use sparse measurements of the motion on different parts of the truck as it is excited by road induced vibrations, and transform this data into knowledge of the state in the entire system. To do this, the algorithm has to be supplied with information about the dynamic properties of the current system.In Paper A we have developed and implemented a fixed-lag smoother for joint input and state estimation in linear time-invariant dynamic structures. A fixed-lag smoother maximizes the use of information available in the measurements by allowing a small time lag in the estimation. As input, external forces as well as support motions can be computed. Furthermore, both measurement noise and model errors are accounted for and simulated as stochastic processes. The filter is firstly verified with straightforward numerical simulations of a simply supported beam, followed by a more involved simulation of a truck fuel tank. It is shown that the fixed-lag smoother performs very well, it estimates both input and states with a high accuracy even though the signals are contaminated with noise and the model contains errors.In Paper B the fixed-lag smoother is applied on real measurements. We investigate the capabilities of the proposed filter by analysing acceleration measurements from a truck side skirt excited by road induced vibrations. In this study, we focus on estimating the state in the side skirt body from a minimum number of measurement sensors. The dynamic properties of the side skirt are obtained experimentally from an operational modal analysis. It is shown that the fixed-lag smoother estimates the state very well. The results also shows that the smoothing effect is larger when fewer measurement sensors are used.
|
|
| 49. |
|
|
| 50. |
|
|
