Fretting fatigue initiation was studied for a shrink-fit pin at rotating bending. Eight assemblies with four different grips were manufactured from soft normalized steel and tested at loads well below bending endurance. All pins displayed rust-red fretting oxides deep into the contact and black oxidised fretting scars with fretting fatigue cracks at the rim. The slip evolution was simulated in a three-dimensional FE model including assembly, bending and sufficiently many rotations to reach a steady-state. The extension of cyclic slip agreed with the black oxidised scar. Deeper into the contact a monotonic slip developed to the positions with rust-red oxides. Asymmetric slip and traction on the interface sides together with a slight twist of the pin in the hub and the slip development process, illustrated that a three-dimensional analysis was required for the interface. Both the stress amplitude and the Findley multi-axial criterion predicted fretting fatigue of the pin although the rotating bend stress was well below the endurance limit.

The behaviour of inclusion initiated fatigue was studied for a high strength bearing steel with a bainite micro-structure. The analysis included experiments and numerical simulations. It was realized that the stress-state was multi-axial in the matrix material that met the inclusion also for a uniaxial far field stress. Fatigue initiation risk at the interface between the inclusion and matrix material was therefore predicted with the Findley multi-axial critical plane criterion. The fatigue parameters were determined from independent experiments on smooth specimens with tensile surface stress gradients. Crack growth from the inclusion to final rupture was modelled as a penny shaped crack with closure compensated effective material parameters. The growth simulations suggested that the majority of the fatigue life was consumed as fatigue crack initiation at the non-metallic inclusion.

High-porosity, three-dimensional wood fiber networks made by foam forming present experimentally accessible instances of hierarchically structured, athermal fiber networks. We investigate the large deformation compression behavior of these networks using fiber-resolved finite element analyses to elucidate the role of microstructures in the mechanical response to compression. Three-dimensional network structures are acquired using micro-computed tomography and subsequent skeletonization into a Euclidean graph representation. By using a fitting procedure to the geometrical graph data, we are able to identify nine independent statistical parameters needed for the regeneration of artificial networks with the observed statistics. The compression response of these artificially generated networks and the physical network is then investigated using implicit finite element analysis. A direct comparison of the simulation results from the reconstructed and artificial network reveals remarkable differences already in the elastic region. These can neither be fully explained by density scaling, the size effect nor the boundary conditions. The only factor which provides the consistent explanation of the observed difference is the density and fiber orientation nonuniformities; these contribute to strain-localization so that the network becomes more compliant than expected for statistically uniform microstructures. We also demonstrate that the experimentally manifested strain-stiffening of such networks is due to development of new inter-fiber contacts during compression.

To meet the need of lightweight chassis in the near future, a technological step of introducing anisotropic materials like Carbon Fibre Reinforced Plastics (CFRP) in structural parts of cars is a possible way ahead. Though there are commercially available tools to find suitability of Fibre Reinforced Plastics (FRPs) and their orientations, they depend on numerical optimization and complexity increases with the size of the model. Nevertheless, the user has a very limited control of intermediate steps. To understand the type of material system that can be used in different regions for a lightweight chassis, especially during the initial concept phase, a more simplified, yet reliable tool is desirable.The thesis aims to provide a framework for determining fibre orientations according to the most-ideal loading path to achieve maximum advantage from FRP-materials. This has been achieved by developing algorithms to find best-fit material orientations analytically, which uses principal stresses and their orientations in a finite element originating from multiple load cases. This thesis takes inspiration from the Durst criteria (2008) which upon implementation provides information on how individual elements must be modelled in a component subjected to multiple load cases. This analysis pre-evaluates the potential of FRP-suitable parts. Few modifications have been made to the existing formulations by the authors which have been explained in relevant sections.The study has been extended to develop additional MATLAB subroutines which finds the type of laminate design (uni-directional, bi-axial or quasi-isotropic) that is suitable for individual elements.Several test cases have been run to check the validity of the developed algorithm. Finally, the algorithm has been implemented on a Body-In-White subjected to two load cases. The thesis gives an idea of how to divide the structure into sub-components along with the local fibre directions based on the fibre orientations and an appropriate laminate design based on classical laminate theory.

A numerical analysis, using the finite element method, of the mechanical behavior at powder compaction at higher densities was performed. In this investigation the material behavior is modeled using an advanced macroscopic constitutive description initially presented by Brandt and Nilsson [1]. This material model, like many other models describing powder compaction at higher densities, includes a large number of constitutive parameters and as a result, a complete material characterization is a difficult task to perform or at least requires a large number of different experiments. A remedy to this problem is to apply inverse modeling, i.e. optimization, for determination of relevant material properties from comparatively simple experiments. It is then of course important, in order to achieve high accuracy results from the optimization procedure, that the stress fields produced during the experiments involves high gradients of stress. Adhering to simple uniaxial die compaction experiments the main parameter that can be used in order to achieve such a feature is the geometry of the punch used for load application. In the present investigation a number of punch profiles are studied and it is found that a skewed punch geometry is the most appropriate one to be used for experimental die compaction aiming at a constitutive description of the powder material based on inverse modeling. The main efforts are devoted towards an analysis based on the previously mentioned material model by Brandt and Nilsson [1] but also relevant results for another type of constitutive model will be presented. (C) 2010 Elsevier B.V. All rights reserved.

Modeling of hard metal powder inserts is analyzed based on a continuum mechanics approach. In particular, one commonly used cutting insert geometry is studied. For a given advanced constitutive description of the powder material, the material parameter space required to accurately model the mechanical behavior is determined. These findings are then compared with the corresponding parameter space that can possibly be determined from a combined numerical/experimental analysis of uniaxial die powder compaction utilizing inverse modeling. The analysis is pertinent to a particular WC/Co powder and the finite element method is used in the numerical investigations of the mechanical behavior of the cutting insert.

In order to decrease the extensive experimental work in product development of WC/Co powder tooling equipment, an investigation of the applicability of inverse procedures to industrially relevant material models is performed. The powder die compaction process of WC/Co powder is modeled using the finite element method and an advanced plasticity-based constitutive model. An angled indenter is used to increase the sensitivity of constitutive parameters with respect to the measurable data from experiment, which is done using an instrumented die. Here, the inverse modeling (optimization) is performed (using iterative reduction of variable space and a combination of genetic (GA) and gradient search algorithms) in order to determine the yield surface of the investigated material. The results indicate that inverse modeling can be a useful tool in order to reduce the experimental efforts at material characterization of powder materials described by advanced material models.

In order to decrease the amount of experimental work at material characterization of powder mixtures, optimization procedures are often used. In the present study, characterization of WC/Co powders described by an advanced plasticity model is at issue. In particular, uniaxial die compaction is analyzed numerically in order to determine the correlation between material parameters, scalar and functional, and measurable information from experiments. Such information include radial (contact) pressure between powder and die walls, press force as function of indentation depth and frictional effects between powder and die walls. The commercial finite element solver LS-Dyna and the optimization module LS-OPT are used in the present investigation.

Dry pressing powder compaction is analyzed using the finite element method (FEM) and in particular then the explicit commercial FEM program LS-DYNA. The aim is then to determine the sensitivity of mechanical local and global parameters with respect to different constitutive as well as geometrical variables. A very advanced constitutive description (not available as standard in commercial FEM packages but implemented into LS-DYNA for the present investigation) is used in the numerical analysis, Brandt and Nilsson [1], and the results are particularly helpful for a forthcoming inverse analysis where it is crucial, for reasons of efficiency, to reduce the number of variables in the optimization procedure aiming at a accurate description of the mechanical behavior of the powder material. It should be clearly stated that the geometrical parameters to be investigated include different possible press geometries to be used for experimental verification of the results.

The purpose of this thesis project is to develop a methodology that can be used to minimize the number of spot-welds in a mechanical structure, this is done in a reliable manner via optimization methods. The optimization considers fatigue life in spot-welds and also stiffness and eigenfrequency values. The first chapter of this thesis presents a spot-weld fatigue model proposed by Rupp (1995), common FEmodels of spot-welds and also important aspects about structural optimization in general. The second chapter further describes how topology optimization and size (parameter) optimization are applied on a simple multi-weld model with respect to the aforementioned structural constraints. The topology optimization is later used on a full-size car model, while the size optimization is used to optimize the multiweld model by adding an non-linear structural constraint - a crash indentation constraint. The spot-weld fatigue model proposed by Rupp (1995), is also verified by comparing FE results using different FE-models of spot-welds compared to fatigue data by Long and Khanna (2007). Both optimization methods successfully minimize the total amount of spot-welds on the multi-weld model. The topology optimization,accompanied with thegradient based MFD algorithm,minimizes th etotal spot-welds with around 15% and 3% on the multi-weld model and car body respectively. The size optimization, using design of experiments and response surfaces, manage storeduce the number of welds in the multi-weldmodel by 25%. However, with the size optimization the computational time is several orders of magnitude longer-even without the formulation of the crash constraint. The fatiguespot-weld model fares reasonably well compared to the experimental fatigue data, regardless of the FE model of the spot-weld. It is concluded that the ACM model would be recommended based on its compatibility with fatigue and optimization methods, mesh-independence and also other studies have shown its ability to represent stiffness and eigenfrequency correctly.

The introduction of particles and precipitates into a matrix material results in strengthening effects. The two main mechanisms involved in this matter are referred to as Orowan and shearing. To numerically study this phenomenon is the motivation to the research done, which is presented here in this thesis. The heterogeneous microscale state of deformation in such materials brings in size scale effects into the picture. A strain gradient plasticity (SGP) theory is used to include effects of small scale plasticity. In addition, a new interface formulation is proposed which accounts for the particle-matrix interactions. By changing a key parameter, this interface model can mimic the level of coherency of particles, and hence is useful in studying different material systems.

The governing equations and formulations are then implemented into an in-house SGP FEM program. The program is equipped with axi-symmetric and three-dimensional modelling capabilities. Different distributions of particles are considered, from which proper representative volume elements (RVEs) are constructed. These RVEs are then analyzed under different loadings, and homogenization methods are utilized to evaluate macroscopic response of the material. A quantity of interest is the increase in yield stress of material due to presence of particles and precipitates. Comprehensive parametric studies are carried out to study the effects of different parameters on the strengthening. A closed formsolution is obtained, which suggests the strengthening increases by increasing the surface area of particles per unit volume of material.

The work done is presented in four appended papers. Paper A uses an axi-symmetric model to set the theoretical basis for the rest of the papers. Effects of different key parameters on the strengthening are studied and presented in this paper. Since the axisymmetric model is numerically cheap, an extensive amount of analyses are carried out. Paper B is about the expansion of the theory introduced in the first paper into 3D space. The micromechanical model is composed of a cuboid RVE with eight different particles, one at each corner. The inclusion of more than one particle is a key parameter in studying the effects of size distribution.

The idea of having the most general micromechanical model is the theme of Paper C. Here, a completely random distribution of particles in 3D space is taken into account. In addition, the results of all carried out analyses are tested against experimental results from different material systems. Last paper, Paper D, summarizes a successful effort to include Shearing mechanism in the micromechanical model. The RVE is equipped withan embedded slip plane, and yet has the features introduced in previous papers. Hence, it has the ability to cover both strengthening mechanisms observed in precipitated materials.

In manufacturing of paper and paperboard, optimized fiber usage has crucial importance for process efficiency and profitability. Dry strength of paper is one of the important quality criteria, which can be improved by adding dry strength additive that affect fiber to fiber bonding. This study is using the micromechanical simulations which assist interpretation of the experimental results concerning the effect of strength additives.

A finite element model for 3D dry fiber network was constructed to study the effect of bond strength, bond area and the number of bonds numerically on the strength of paper products. In the network, fibers’ geometrical properties such as wall thickness, diameter, length and curl were assigned according to fiber characterization of the pulp and SEM analyses of dry paper cross-section. The numerical network was created by depositing the fibers onto a flat surface which should mimic the handsheet-making procedure. In the FE model, each fiber was represented with a number of quadratic Timoshenko beam elements where fiber to fiber bonds were modelled by beam-to-beam contact. The contact model is represented by cohesive zone model, which needs bond strength and bond stiffness in normal and shear directions. To get a reasonable estimate of the bond stiffness, a detailed finite element model of a fiber bond was used. Additionally, the effect of different fiber and bond geometries on bond stiffness were examined by this model since the previous work [13] indicated that the bond stiffness can have a considerable effect on dry strength of paper.

The network simulation results show that the effect of the strength additive comes through improving the bond strength primarily. Furthermore, with the considered sheet structure, both the fiber bond compliance and the number of bonds affect the stiffness of paper. Finally, the results of the analyses indicated that the AFM measurements of the fiber adhesion could not be used directly to relate the corresponding changes in the bond strength.

The fiber bond simulation concluded that fiber wall thickness has the most significant effect on the fiber bond compliance. It was also affected by micro-fibril orientation angle, bond orientation and the degree of pressing.

Abstract

Upon trying to reach higher strengths, when designing steels, inevitably susceptibility to one

type of cracking known as hydrogen cracking increases. In present work, this complexity with

regard to high strength structural steel of Weldox 1300 and wear plate of Hardox 500 was

studied.

Effect of low temperature tempering (200 ) and hard leveling on hydrogen embrittlement was

qualitatively explored by fracture toughness testing. Tests were performed on SENB specimens of two types of Weldox 1300 in air and 3.5% NaCl solution. In-situ testing of as-quenched Weldox caused K value for crack growth initiation to drop to almost 20% of that for reference specimen tested in air. However, Weldox 1300 in tempered and leveled condition exhibited considerably improved resistance against hydrogen cracking by almost 50% compared to asquenched condition.

It is believed that formation of transit carbides acting as strong traps due to tempering, and alteration in dislocations’ structure and level of tensile residual stresses thanks to combined effects of tempering and leveling have considerable impact on crack growth kinetics which results in improved resistance. The influence of tempering and leveling was not investigated separately.

Additionally, by using four-point-bending test it was attempted to screen a method suitable for study of hydrogen embrittlement. Test variables were tried to be adjusted to meet the failure criteria. Precharged samples were subjected to bending stresses and left in outdoor atmosphere. Hydrogen measurement after passing 41 days on one of the samples containing a stress concentrator showed that hydrogen had been trapped and still present into the sample. Although hydrogen measurement showed the effect of stress fields on hydrogen trapping, test results along with FEM simulation indicated that such a test method might not be practicable for this special combination of materials and expectations.

Folding of coated paper is examined numerically using the finite element method. Particular emphasis is put on the behaviour of field variables relevant for cracking of the coating layers. In the numerical analysis, the basepaper is modelled as an anisotropic elastic-plastic material (both elastic and plastic anisotropy is accounted for) while the constitutive behaviour of the coating layers are approximated by classical (Mises) elastoplasticity. The numerical results suggest, among other things, that particular forms of plastic anisotropy can substantially reduce the maximum strain levels in the coating. It is also shown that delamination buckling, in the present circumstances, will have a very small influence on the strain levels in the coating layer subjected to high tensile loading.

Folding of coated papers is examined numerically using the finite element method. The analysis is focused on the influence from dynamic effects on the folding process. In particular, the behaviour of field variables relevant for cracking of the coating layers are studied in some detail. The results presented indicate that dynamic effects are of little importance as regards maximum strain levels in the coating but will influence the stress and strain distributions. Accordingly, a quasi-static analysis of the problem will be sufficient in order to describe many of the important features related to cracking.

A finite element procedure, developed in order to account for the effect of high anisotropy at folding of coated papers, is presented. The anisotropic behaviour (with very low stiffness in the thickness direction) is modelled using stiff structural elements (trusses and beams). The numerical results indicate that high elastic anisotropy leads to lower strain levels at folding than reported in previous analyses where this effect was not accounted for. High plastic anisotropy, on the other hand, will contradict this result.

Microindentation as a method for determining important material properties of paper coating materials is studied experimentally and numerically The bulk of the investigation is concentrated upon the short-lived elastic part of a spherical indentation test, but determination of the failure stress of the coating is also discussed. The results indicate that microindentation can be a powerful tool for material characterization of these materials, but only if careful efforts arc, made to account for the influence from plasticity as well as from boundary effects.

For a company like Scania CV AB, a vast number of laws and regulations has to be considered when developing a truck. In the constant struggle to keep the generated noise below the allowed levels, the gears are made more slender and flexible. The slenderness in combination with case hardening has brought a new type of gear fracture into the light.The Tooth Interior Fatigue Fracture, TIFF. A 2D-method, and a tool for engineers, was developed in the early 2000’s. However, this tool did not provide sufficient accuracy andcompatibility with the current design process to be adopted by the engineers at Scania.This thesis expands on the current 2D-model and attempts to improve the accuracy by bringing the analysis to 3D. Furthermore, the computational tool is developed in Pythonto allow for a more streamlined interface with the current workflow.The proposed method approximates the tooth as a cantilever-beam, and is only evaluated for this case. However, the stresses are computed with good accuracy. The onlydiscrepancy is one of the stress components, where the error is about 50%. This error isderived from the decision to, in torsion, model the cross-section of the gear tooth as an ellipse. The method has potential to be incorporated into the current design process, but the accuracy of the stresses due to torsion has to be improved, and some of the equations has to be adapted before real gear geometries can be considered.

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.

A micromechanical model consisting of a band with a square array of equally sized cells, with a spherical void located in each cell, is developed. The band is allowed a certain inclination and the periodic arrangement of the cells allow the study of a single unit cell for which fully periodic boundary conditions are applied. The model is based on the theoretical framework of plastic localization and is in essence the micromechanical model by Barsoum and Faleskog (Barsoum, I., Faleskog, J., 2007. Rupture mechanisms in combined tension and shear-micromechanics. International Journal of Solids and Structures 44(17), 5481-5498) with the extension accounting for the band orientation. The effect of band inclination is significant on the strain to localization and cannot be disregarded. The macroscopic stress state is characterized by the stress triaxiality and the Lode parameter. The model is used to investigate the influence of the stress state on void growth and coalescence. It is found that the Lode parameter exerts a strong influence on the void shape evolution and void growth rate as well as the localized deformation behavior. At high stress triaxiality level the influence of the Lode parameter is not as marked and the overall ductility is set by the stress triaxiality. For a dominating shear stress state localization into a band cannot be regarded as a void coalescence criterion predicting material failure. A coalescence criterion operative at dominating shear stress state is needed.

Objective: To date no diagnostic tool is yet available to objectively assess the in vivo biomechanical properties of the uterine cervix during gestation. Methods: We show the first clinical application of an aspiration device to assess the in vivo biomechanical properties of the cervix in pregnancy with the aim to describe the physiological biomechanical changes throughout gestation in order to eventually detect pregnant women at risk for cervical insufficiency (CI). Results: Out of 15 aspiration measurements, 12 produced valid results. The stiffness values were in the range between 0.013 and 0.068 bar/mm. The results showed a good reproducibility of the aspiration test. In our previous test series on non-pregnant cervices our repetitive measurements showed a standard deviation of > 20% compared to <+/- 10% to our data on pregnant cervices. Stiffness values are decreasing with gestational age which indicates a progressive softening of cervical tissue towards the end of pregnancy. Three pregnant women had two subsequent measurements within a time interval of four weeks. Decreasing stiffness values in the range of 20% were recorded. Discussion: This preliminary study on the clinical practicability of aspiration tests showed promising results in terms of reproducibility (reliability) and clinical use (feasibility). Ongoing studies will provide further insights on its usefulness in clinical practice and in the detection of substantial changes of the cervix in pregnancy indicative for threatened preterm birth or cervical insufficiency.

Fatigue failures are critical in armored vehicle design but the lack of knowledge on fatigue phenomena forces the use of large safety factors which induces weight and costs increase. This master's thesis has been driven by the will of improving the simulation of low cycle fatigue phenomenon of metal parts and welded assemblies in order to reduce safety margins. Two distinct but complementary paths are investigated: improvement of the fatigue simulation model and awareness of fatigue phenomenon dispersion. Firstly, the implementation of a new lifetime prediction model is proposed based on the ONERA fatigue criterion that has been developed for both low cycle and high cycle fatigue phenomena. The method introduced in this report is composed of the identification of material behavior laws, the simulation of experimental tests by finite element analysis and the calibration of fatigue criterion parameters. Low cycle fatigue experimental tests (uniaxial traction-compression tests) data for base metal and welded samples are used for the identifications. The material behavior follows Chaboche's theory and integrates two nonlinear kinematic hardenings and one isotropic hardening. However, the use of isotropic hardening is nearly impossible in practice due to computation cost and it has been decided to work with stabilized cycle laws. The finite element analyses are controlled by tests stresses. The ONERA fatigue criterion is only partially identified because the full identification requires additional experimental tests that have not been performed yet. Despite those simplifications, the lifetime prediction value is accurate and better than the ones given by previously used model. The process still has to be validated on other geometries. During the implementation of the whole process, elastic limit and fatigue limit appeared to be the most influential variables and the refinement of their values will increase the model's accuracy. Secondly, it is well known that fatigue phenomenon shows high dispersion. One way to take into account this aspect is to assign a stochastic aspect to lifetime prediction model variables and propagate it through the model. However, the combination with finite element analysis is impossible because of the computation time cost of each simulation forcing the use of metamodelling methods. PhimecaSoft offers to gather in a user-friendly interface a broad range of methods to execute uncertainties propagations in structural design environment. A coupling with in-house fatigue model has been set up to study the influence of model variables on the lifetime. The definition of the probability laws of the input variables is crucial. In the framework of this thesis it has been necessary to truncate the laws to keep plausible dispersions of the input variables and realistic values of the lifetime. Different functionalities of PhimecaSoft have been investigated such as: the use of Sobol's coefficients for global sensitivity analysis or the generation of metamodels. Reliability analysis is also part of PhimecaSoft and can be done whether by simulations methods or approximation methods. The use of reliability index is encouraged in the future to move from safety factor's based design toward reliability based design optimization. This second part of the thesis confirms that the elastic limit and fatigue limit are the most influential variables of the problem.

The finite element method and casting simulations have reached higher popularity and accuracy partly due to increase in computer power over the last couple of years. The increase in computer power has led to better simulations and hence a better reflection of reality. The development in simulations has made it possible to connect different kinds of physics and simulation tools, a type of multiphysics. In this work the connection between casting simulations and strength analysis (with finite element method) has been evaluated with focus on improving the computation accuracy at Scania.This work indicates that by implementing data from casting simulation into a strength analysis the result changes. These changes are local and often located in areas where the stress levels are large. This emphasise the importance of using casting simulation data in strength analyses. Furthermore there are a large room for improvement and some calibration should be executed before usage.A method has been developed on how to implement casting simulations into a strength analysis. This method requires a interdisciplinary connection between different groups at Scania. The result of this connection is not only a better simulation but also an exchange of knowledge regarding the product that is of interest for all involving groups.

The correlatioin between residual stresses and the global properties from an indentation test, i.e. hardness and size of the contact area, has been studied frequently in recent years. The investigations presented have been based on experimental, theoretical and numerical methods and as a result, the basic features of the problem are fairly well understood in the case of residual equi-biaxial surface stresses. The more general case, when the principal surface stresses are not necessarily equi-biaxial, has received nuch less attention and it is therefore the aim of the present study to remedy this shortcoming. In doing so, qualitative results are of immediate interest in this initial study but possible ways of quantitative descriptions are also discussed for future purposes. The present analysis is based on numerical methods and in particular the finite element method (FEM) is relied upon. Classical Mises elastoplastic material behavior is assumed throughout the investigation.

Wood-fiber reinforced polylactide is a biodegradable compositewhere both fibers and matrix are from renewableresources. When designing new materials of this kind, itis useful to measure the influence of fiber–matrixinterface properties on macroscopic mechanicalproperties. In particular, a quantitative measure of thedynamic stress transfer between the fibers andthe matrix when the material is subjected tocyclic loading would simplify the development of wood-fibercomposites. This is obtained by comparing themechanical dissipation of the composite with avalue predicted by a viscoelastic micromechanical model basedon perfect interfacial stress transfer. Theloss factors predicted by the model are 0.12 and 0.16 at dryand humid conditions, respectively, which amountto 63 and 66% of the experimentally determinedvalues. For Young's moduli the predicted values are 1.01 and0.88 GPa, which correspond to 92% of the experimentallydetermined values. The mismatch between thepredicted and experimental values may be attributed toimperfect interfaces with restrained stress transfer.Loss factors are also determined for specificmolecular bonds using dynamic Fourier transform infrared(FT-IR) spectroscopy. These values show the sametrends with regard to moisture content as themacroscopically determined loss factors.

Reliable drop test simulations of electronic packages require reliable material characterization of solder joints. Mechanical properties of lead-free solder were here experimentally investigated for both monotonous and cyclic loading at different strain rates. With regards to the observed complex material behavior, the non-linear mixed hardening Armstrong and Fredrick model combined with the Perzyna viscoplastic law was chosen to fit the experimental data. This model was subsequently implemented into a commercial finite element code and used to simulate drop tests. Actual drop test experiments were conducted in parallel and experimental results were compared to simulations. Prediction discrepancies were analyzed and explanations suggested.

Following the growing use of amorphous polymers in an expanding range of applications, interest for polymer mechanical modeling has greatly increased. Together with reliable constitutive models, stable, accurate and rapid integration algorithms valid for large deformations need to be developed. Here, in a framework of hyperelasto-viscoplasticity and multiplicative split formulation, three integration algorithms (explicit, fully implicit and forward gradient) are applied to the polymer model developed by Anand and Gurtin (2003) and respective stability is investigated. The explicit and fully implicit algorithms were furthermore implemented in a commercial Finite Element code and simulations of a simple tensile test are shown to capture the actual deformation behavior of polymers.

Mechanical behaviour of lead-free solder has been investigated experimentally and thereafter modelled analytically. Material test specimens subjected to uniaxial cyclic loadings at different strain rates exhibit noticeable properties: viscoplasticity, non-linear mixed hardening within each cycle and hardening followed by softening between consecutive cycles. To accurately describe these behaviours, a modified Armstrong and Fredrick model was combined to the Perzyna viscoplastic evolution equation. The set of parameters was determined by both curve-fitting and use of analytical relations. © 2006 IEEE.

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.

We have investigated a relation between micromechanical processes and the stress-strain curve of a dry fiber network during tensile loading. By using a detailed particle-level simulation tool we investigate, among other things, the impact of "non-traditional" bonding parameters, such as compliance of bonding regions, work of separation and the actual number of effective bonds. This is probably the first three-dimensional model which is capable of simulating the fracture process of paper accounting for nonlinearities at the fiber level and bond failures. The failure behavior of the network considered in the study could be changed significantly by relatively small changes in bond strength, as compared to the scatter in bonding data found in the literature. We have identified that compliance of the bonding regions has a significant impact on network strength. By comparing networks with weak and strong bonds, we concluded that large local strains are the precursors of bond failures and not the other way around.

As fiber and bond characterization tools become more sophisticated, the information from the fiber scale becomes richer. This information is used for benchmarking of different types of fibers by the paper and packaging industries. In this work, we have addressed a question about the effect of variability in the fiber and fiber bond properties on the average stiffness and strength of fiber networks. We used a fiber scale numerical model and reconstruction algorithm to address this question. The approach was verified using the experimental sheets having fiber data acquired by a fiber morphology analyzer and corrected by microtomographic analysis of fibers in these sheets. We concluded, among other things, that it is sufficient to account for the average bond strength value with an acceptable number of samples to describe dry network strength, as long as the bond strength distribution remains symmetric. We also found that using the length-weighted average for fiber shape factor and fiber length data neglects the important contribution from the distribution in these properties on the mechanical properties of the sheets.

As fiber and bond characterization tools become more sophisticated, the information from the fiber scale becomes richer. This information is used for benchmarking of different types of fibers by the paper and packaging industries. In this work, we have addressed a question about the effect of variability in the fiber and fiber bond properties on the average stiffness and strength of fiber networks. We used a fiber-scale numerical model and reconstruction algorithm to address this question. The approach was verified using the experimental sheets having fiber data acquired by a fiber morphology analyzer and corrected by microtomographic analysis of fibers in these sheets. We concluded, among other things, that it is sufficient to account for the average bond strength value with an acceptable number of samples to describe dry network strength, as long as the bond strength distribution remains symmetric. We also found that using the length-weighted average for fiber shape factor and fiber length data neglects the important contribution from the distribution in these properties on the mechanical properties of the sheets.

This article presents an evaluation of two different crack prediction approaches based on a comparison of the stress intensity factor distribution for three example problems. A single edge notch specimen and a quarter circular corner crack specimen subjected to shear displacements and a three point bend specimen with a crack inclined to the mid-plane are examined. The stress intensity factors are determined from the singular stress field close to the crack front. Two different fracture criteria are adopted for the calculation of an equivalent stress intensity factor and crack deflection angle. The stress intensity factor distributions for both numerical methods agree well to available reference solutions. Deviations are recorded at crack front locations near the free surface probably due to global contraction effects and the twisting behaviour of the crack front. Crack propagation calculations for the three point bending specimen give results that satisfy intuitive expectations. The outcome of the study encourages further pursuit of a crack propagation tool based on a combination of elements.

A crack-insertion tool for automatic non-planar crack propagation calculations is under development. The final volume mesh of the cracked structure is achieved by a new approach. The cracked structure boundary is modelled as a discretized skin. A tube-like domain enclosing the crack-front is traced and filled with hexahedral elements while the remaining cracked structure is meshed with tetrahedral elements. The two separate meshes are finally connected by linear MPC equations. Results show that the method works very well for curved crack shapes in complex structures.

A fretting experiment with separate control of bulk stress, normal and tangential contact loads is presented. For the initial test series, constant normal and bulk loads were combined with a cyclic tangential load. Both the spherical indenter and the plane specimen were manufactured from alpha + beta titanium alloys. Strain gauges and acoustic emission measurements were employed for the determination of the time to crack initiation and the propagation life. The fretting cracks always initiated inside the slip zone at positions that were spread over the whole slip zone. Crack nucleation was investigated with five multiaxial fatigue criteria. It was concluded that macroscale stresses from the contact and bulk load alone could not explain all aspects of crack initiation. A possible explanation was found in the fretting-induced roughness. The growth of the fretting crack was simulated numerically by a parametrical description. It was noted that a cyclic bulk load was required to drive the crack growth through the specimen to final failure.