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The development of additive manufacturing (AM) is opening new possibilities and new fields of application. The use of AM structural parts is already a reality in some cases [1]. However, when aiming at high end and demanding sectors, such as the aeronautic and aerospace industries, in the instances where the structural aspect plays a critical role, the use of metal AM components is still limited. AM parts often have defects (porosity and roughness) that influence fatigue resistance in a negative way. There is a need to get rid of these defects typical of the AM technology. Efforts to achieve this can consist on AM manufacturing parameters optimisation. These parameters can be tuned to reduce roughness [2], but subsurface porosity is created at the same time, reducing the potential influence of the former type of defect but increasing that of the latter. In this research the post-treatment route was explored to investigate and tackle the fatigue resistance issue. A representative material, laser powder bed fusion AlSi10Mg, was used. As built samples were subjected to the following post-treatments: Stress relief heat treatment (SRHT), hot isostatic pressing (HIP) and friction stir processing (FSP). The first is a thermal treatment and the other two are thermomechanical. Through tensile-tensile fatigue testing to obtain SN or Wöhler curves and fatigue crack propagation testing a thorough study of the fatigue behaviour of the material in the different states was performed. Quantitatively, in contrast with the SRHT and HIP, that showed no significant improvement, an increase of up to 100 times in fatigue life was achieved with FSP. The reason behind these differences seems to be linked mainly to the porosity reduction effect of FSP, which makes it a promising post-processing technique to improve fatigue resistance [3,4].

Ductile fracture results from the nucleation, growth and coalescence of small internal cavities. In aluminium alloys, the void population generally nucleates by the fracture of iron rich intermetallic particles. The objective of this study is to understand and model the effect of microstructure heterogeneities on damage accumulation in three 6xxx series aluminium alloys. The three alloys, i.e. Al 6005A, Al 6061 and Al 6056, exhibit a volume fraction of iron rich particles close to 1%. However, samples of similar yield strengths, owing to appropriate heat treatments, show very different fracture strain for these three alloys. A sort of cellular automaton type model has been developed to describe the growth and coalescence of voids nucleated from 3D particle fields accounting for the spatial, shape and size particle distributions. The model treats local interaction between neighbouring cavities in a simplified way and captures cluster effects on coalescence. The model parameters are extracted from a detailed microstructure analysis. High resolution 3D X-ray synchrotron tomography is used to characterize the size and position distribution of the iron-rich intermetallics and initial cavities in the three alloys. In addition, a statistical study performed on polished fractured tensile samples allows extracting nucleation stresses and the probability of fracture as a function of the size of the intermetallic particle. The damage model is validated on tensile tests of various alloys tempers and on in-situ tension 3D X-ray synchrotron tomography.

Analysis of the serviceability performances of the bridge by genetic algorithm approaches

Evolution of the bridge structural properties during the construction stages

Seismic Action Modeling and Long Suspension Bridge Response Computation

A modern conceptual design of a bridge structure should be open to wider criteria, which assure that the structure is endowed with static, dynamic, and ductile characteristics sufficient to tackle the seismic events. Correct conception of the structural morphology, other than the achievement of better guarantees with regard to the seismic behavior, may also lead to economic advantages either in the realization of the whole structural system or in the adoption of seismic devices, with a consequent possible reduction of the management cost of the structure as well. For the satisfaction of these requirements and for the exploration and the verification of innovative structural schemes, refined and subtle analysis formulations and tools must be considered. Such a kind of structural scheme is based on the two-blade slender bridge piers. These are elements subdivided into two parts with different geometric and mechanical properties. The first part has a box section, which is highly stiff. Two flexible blades connected to the top compose the second part. Because of the uncertainties involved in the problem, the geometrical and mechanical properties that define the structural problem cannot be considered as deterministic quantities. This chapter presents a paper that models such uncertainties by using a fuzzy criterion. © 2003 Elsevier Science Ltd. All rights reserved.

A novel in-plane bending test was used to study edge ductility in DP800 as a common advanced high-strength steel in the car industry. The test utilized the digital image correlation technique to measure the local and average fracture strain values along the edge of the specimen. In contrast to the widely used hole expansion capacity test, the impact of punch friction, contact stress, and out-of-plane strain on edge ductility is eliminated by removing the punch. Also, the strain gradient inherent to the beam bending provides a controlled crack propagation path, making crack tracking easier than the sheared edge tensile test. The proposed bending test was utilized to investigate the influence of material orientation, cutting parameters, and global strain gradient on edge fracture strain. A correlation was observed between edge ductility, material orientation, and cutting tool sharpness, while the average fracture strain was independent of the strain gradient. The outcome shows that the in-plane bending test is reliable for determining edge ductility in any desired material orientation.

Expedient simulations and numerical modelling of forming processes depend on a good knowledge of the friction at the tool–sheet metal contact. The absolute value of the friction force or relative changes of this force due to changing conditions are vital input for a reliable numerical model. For that reason, a good selection of a laboratory experiment that can measure the friction under forming conditions is an essential step. This work is part of a large European Union project named ASPECT, which evaluates the friction force evolution in sheet metal forming as a result of changes in contact conditions. Simplified tribological lab-scale techniques showed an temperature dependence of friction opposite to what is measured in state-of-the-art macroscopic flat strip drawing tests. To understand and overcome this contradiction, a new method was developed to integrate the deformation aspect into a friction test; this approach assures closer resemblance to the actual forming conditions. The aim is to develop a method that measures the effect of temperature and speed on the resistance to sliding and deformation, and to provide an efficient prescreening and ranking of forming oils.

Despite its promising advantages, the application of directed energy deposition (DED) to produce large metal parts is hindered by challenges inherent to the process. Undesired residual stresses, distortions and heterogeneous material properties mainly originate from a part’s thermal history. Fast part-scale thermal models therefore facilitate improved applicability of DED by enabling the prediction and mitigation of these unwanted effects. In this work, the efficiency of a discontinuous Galerkin-based thermal model with heat input by hot element addition, is evaluated and improved to allow such fast simulations. It is found that the model permits the use of a coarse discretization around the heat source, which significantly reduces simulation time while maintaining accurate solutions. It is also shown that the model naturally facilitates the use of local time stepping, which can considerably improve the efficiency of thermal additive manufacturing simulations.

In this paper, macroscopic behavior obtained from crystal plasticity finite element simulations of irregularly shaped 3D and 2D volume elements (VEs) are compared. These morphologically periodic VEs are generated using the open-source software library Voro++. Periodic boundaryconditions are utilized to homogenize the material response employing a prescribed macroscopic deformation gradient tensor. To accelerate the assignment of periodic boundary conditions, a conformalmesh is employed by which periodic couples of faces on the hull of the volume element have identicalmesh patterns. In the simulations, plane strain conditions are assumed, which means that the averagethickness strain in 3D VEs is set to zero. However, grains are allowed to strain in the thickness direction. In the case of 2D VEs, plane strain elements are used. The principal goal of this comparison isto evaluate the accuracy of 2D VEs simulations. In the current study, two kinds of 2D VEs are generated: 1) Slicing 3D VEs normal to the thickness direction, 2) Separately generating 2D VEs. The firstmethod corresponds to sectioning 3D microstructures using EBSD. This approach is generally usedas an assumed more accurate alternative to 2D VEs. Based on the results, there is a large gap betweenthe flow curves of 2D and 3D VEs. Additionally, 2D sectioning of 3D VEs does not necessarily endup in higher precision in material behavior predictions.

With directed energy deposition (DED), metal parts of up to multiple meters in size can be additively manufactured. The widespread industrial application of DED is, however, limited by undesired effects such as the development of excessive distortions or residual stresses. The origin of these effects is often studied using numerical models, but conventional DED process simulations are often slower than the actual process. This thesis aims to address this shortcoming by proposing methods to accelerate thermal and mechanical DED simulations. Thermal DED simulations are accelerated by means of a simplified heat input model and an efficient discretisation scheme. A hot element addition heat input model is proposed that efficiently combines material deposition and heat input. Spatial discretisation with the discontinuous Galerkin finite element method allows to correctly capture the temperature discontinuity between newly deposited hot elements and already present cooler elements. Accurate temperature predictions are obtained even on coarse meshes, so that thermal DED simulations can be accelerated by limiting the size of the system of equations that has to be solved. With the fully explicit solution scheme, the thermal history is obtained through numerically cheap matrix-vector multiplications at the element level. This scheme naturally enables local time-stepping, allowing solution components of large elements to be updated less often than those of small elements, which further accelerates thermal DED simulations of realistic geometries. Mechanical DED simulations are accelerated using local model-order reduction based on proper orthogonal decomposition (POD). Displacements in regions of the model that behave linearly are approximated with a limited set of deformation modes, whereas those in nonlinear regions are resolved in full detail. A dual-primal domain decomposition method efficiently couples the reduced-order and full-order regions. A novel type of POD-based deformation modes, termed covariant modes, is proposed to obtain accurate predictions of both overall deformations and local stresses. The proposed local MOR-method significantly accelerates the mechanical simulation of DED-processes, while accurately predicting deformations, stresses and plastic strains during the deposition process.

Steel sheets are an essential raw material for a wide range of applications, such as household appliances, packaging, construction, shipbuilding, industrial machinery, automotive, and energy industries. The worldwide market for steel sheets faces intense competition and increasing demand for light-weight metal components to reduce CO2 emissions. As a result, newly developed grades of advanced high-strength steel (AHSS) have gained attention, especially from the automotive industry. AHSS allows for down-gauging due to its higher strength compared to other conventional steel grades. However, due to the low thickness of the sheets, rolling of AHSS is a critical process that may suffer from instabilities, such as pinching. Pinching represents a complex type of phenomena related to inhomogeneous stress distributions in the strip, which may arise from disruptions during the rolling process. Similarly to other shape defects, pinches can be related to uneven strip deformations in the roll bite, which result in inhomogeneous stress distributions across the strip’s width. Pinching defects in steel sheets appear as surface marks, wrinkling, repetitive rippled areas, and local ruptures. In the most severe cases, the strip breaks completely, causing damage to the rolls and considerable manufacturing downtime. Controlling the stability and enhancing the performance of the rolling process are top priorities for steel manufacturers. These tasks aim to minimize the occurrence of defects, ensure consistent product quality, and enhance the efficiency of the manufacturing process. Therefore, better understanding of instability phenomena like pinching is required for determining suitable solutions to prevent them and to obtain a stable rolling process. However, despite being a commonly reported issue among steel manufacturers, pinching has been poorly understood in terms of its underlying mechanism. Currently, there is a lack of research examining the mechanisms behind pinches, both in terms of experimental and numerical investigations. Without a comprehensive understanding of these phenomena, it is unfeasible to develop effective measures to prevent pinches and ensure stable operations of rolling mills. Therefore, the aims of this study are: firstly, to identify the mechanism and possible causes of pinching, and secondly, to develop a simulation tool that can be used to analyze pinching phenomena and design guidelines for the selection of robust production settings in cold rolling mills. To this end, both experimental study and numerical modelling are performed, as presented in this work. The experimental investigation of pinching phenomena presented in this work provide an in-depth understanding of the circumstances that lead to pinching through a series of cold rolling tests and the analysis and characterization of pinching defects. To study pinching phenomena, an appropriate tool is needed to replicate and investigate actual pinching events. Simulation models are essential for predicting the occurrence of pinching during the rolling process. However, existing numerical models of rolling do not succeed to reproduce the occurrence of pinching. This is because pinching is a complex phenomenon that depends on the strong interplay between local deformations within the roll bite and the stress state outside the roll bite. To capture this complexity, a numerical tool must be capable of modeling the process both at a millimeter (or sub-millimeter) scale within the roll bite and at a meter scale outside the roll bite. Moreover, to effectively study pinching events, a three-dimensional rolling model is necessary, as the distribution of stresses and strains across the strip's width is a crucial factor. The finite element method (FEM) is a well-established numerical tool for simulating metal forming processes, and is therefore a suitable technique for analyzing and predicting defects during rolling. However, accounting for all the relevant physics of the rolling process in a conventional 3D FEM model would result in an unfeasible computational time. This work proposes a numerical strategy to decrease the computational expense of 3D sheet rolling FEM simulations. The method involves coupling a global model, which represents the behavior and stress state of the strip outside the roll bite, with a local model that reproduces the deformation mechanics inside the roll bite. The global model is a shell finite element model of the sheet, while the local model is a high resolution 2D plane strain model of the roll bite. The developed approach has been validated by comparing its results to those of a conventional full 3D rolling model under stable rolling conditions. Additionally, this model has been employed to carry out a qualitative analysis of instability phenomena that arise during thin strip rolling. Such phenomena include flatness defects that result from disruptions in the frictional conditions. The simulation results demonstrate that locally varying friction induce local variations in the thickness strain, which cause stress re-distributions in the rolled sheet, resulting in flatness defects. Therefore, the proposed model offers a cost-effective alternative to more expensive 3D FEM models in the analysis of complex instability phenomena that can lead to defects during sheet metal rolling processes.

AlSi coating fracture during press hardening of boron steel can significantly increase tool wear and reduce the product quality. During heating of AlSi-coated press hardening steel, the coating layer evolves due to diffusion. This results in various Fe Al intermetallic compounds (such as FeAl, Fe2Al5), voids throughout the coating and rise in surface roughness. The goal of this study is to investigate the effect of heating parameters on AlSi coating micro-structure and thereby on its fracture behavior during deformation at elevated temperatures. Heating and quenching experiments are performed and later the coating layer is inspected under the microscope to check the distribution of intermetallics, voids and surface roughness. Subsequently, the coating fracture for the micro-structures resulting from different heating parameters is investigated during uniaxial tensile deformation at 700 °C. After the test, the severity of coating cracks is correlated in terms of the percentage of Fe-rich compounds and void distributions in the AlSi coating. The results show that by increasing the amount of Fe-rich compounds in the coating, its crack density is significantly reduced. Finally, it is shown that by adjusting the heating stage parameters, it is possible to further improve the ductility of coating micro-structure and minimize coating fracture upon tensile deformation.

This work proposes a novel framework able to optimise both topology and fibre angle concomitantly to minimise the compliance of a structure. Two different materials are considered, one with isotropic properties (nylon) and another one with orthotropic properties (onyx, which is nylon reinforced with chopped carbon fibres). The framework optimises, in the same particular sub–step, first the topology, and second, the fibre angle at every element throughout the domain. For the isotropic material, only topology optimisation takes place, whereas, for the orthotropic solid, both topology and fibre orientation are considered. The objective function is to minimise compliance, and this is done for three volume fractions of material inside the design domain: 30%, 40%, and 50%. Two classical benchmark cases are considered: 3-point and 4-point bending loading cases. The optimum topologies are further treated and manufactured using the fused filament fabrication (FFF) 3D printing method. Key results reveal that the absolute stiffness, density–normalised and volume–normalised stiffness values within each admissible volume are higher for onyx than for nylon, which proves the efficiency of the proposed concurrent optimisation framework. Moreover, although the objective function was to minimise compliance, it was also effective to improve the strength of all parts. The excellent quality and geometric tolerance of the 3D–printed parts are also worth mentioning.

In this paper, a Finite Element Model Updating (FEMU) procedure is developed to find the best creep parameters for filament-wound cylinders under radial compression in harsh environmental conditions. Three winding angles are considered, each under three different hygrothermal conditions. The two-stage creep model captures i) primary creep through a time-hardening approach whilst ii) secondary creep is captured by Norton's law. Given the high number of parameters in this two-stage creep model and the complexity of determining them experimentally, the FEMU routine utilises an optimisation scheme that sequentially couples a Genetic Algorithm (GA) with a gradient-based (GB) Levenberq-Marquardt Algorithm (LMA) to find all required creep input parameters to feed the model that best simulates experimental results. This framework finds the global optimum through an initial screening of the optimum area within the design space with GA, clearing the path to allow the GB algorithm to find the global optimum, substantially reducing the chance or even avoiding falling in local minima. The global search is driven by experimental data of cylinders loaded in radial compression under aggressive environments. The numerical results show excellent agreement with experimental results with reasonably low computational efforts.(c) 2022 The Author(s). Published by Elsevier Ltd. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/).

Three approaches for construction of a surrogate model of a result field consisting of multiple physical quantities are presented. The first approach uses direct interpolation of the result space on the input space. In the second and third approaches a Singular Value Decomposition is used to reduce the model size. In the reduced order surrogate models, the amplitudes corresponding to the different basis vectors are interpolated. A quality measure that takes into account different physical parts of the result field is defined. As the quality measure is very cheap to evaluate, it can be used to efficiently optimize hyperparameters of all surrogate models. Based on the quality measure, a criterion is proposed to choose the number of basis vectors for the reduced order models. The performance of the surrogate models resulting from the three different approaches is compared using the quality measure based on a validation set. It is found that the novel criterion can effectively be used to select the number of basis vectors. The choice of construction method significantly influences the quality of the surrogate model.

We present a computational framework for two-scale asymptotic homogenization to determine the intrinsic magnetic permeability of composites. To this end, considering linear magnetostatics, both vector and scalar potential formulations are used. Our homogenization algorithm for solving the cell problem is based on the displacement method presented in Lukkassen et al. 1995, Composites Engineering, 5(5), 519-531. We propose the use of the meridional eccentricity of the permeability tensor ellipsoid as an anisotropy index quantifying the degree of directionality in the linear magnetic response. As application problems, 2D regular and random microstructures with overlapping and nonoverlapping monodisperse disks, all of which are periodic, are considered. We show that, for the vanishing corrector function, the derived effective magnetic permeability tensor gives the (lower) Reuss and (upper) Voigt bounds with the vector and scalar potential formulations, respectively. Our results with periodic boundary conditions show an excellent agreement with analytical solutions for regular composites, whereas, for random heterogeneous materials, their convergence with volume element size is fast. Predictions for material systems with monodisperse overlapping disks for a given inclusion volume fraction provide the highest magnetic permeability with the most increased inclusion interaction. In contrast, the disk arrangements in regular square lattices result in the lowest magnetic permeability and inadequate inclusion interaction. Such differences are beyond the reach of the isotropic effective medium theories, which use only the phase volume fraction and shape as mere statistical microstructural descriptors.

While society benefits from Arctic shipping, it is necessary to recognize that ship operations in Arctic waters pose significant risks to people, the environment, and property. To support the management of those risks, this article presents a comprehensive approach addressing both short-term operational risks, as well as risks related to long-term extreme ice loads. For the management of short-term operational risks, an extended version of the Polar Operational Limit Assessment Risk Indexing System (POLARIS) considering the magnitude of the consequences of potential adverse events is proposed. For the management of risks related to long-term extreme ice loads, guidelines are provided for using existing analytical, numerical, and semi-empirical methods. In addition, to support the design of ice class ship structures, the article proposes a novel approach that can be used in the conceptual design phase for the determination of preliminary scantlings for primary hull structural members.

Microstructurally small fatigue crack growth (FCG) rate in body-centred cubic (BCC) ferritic stainless steel is investigated by using a novel domain misorientation approach for EBSD microstructural deformation analyses, in conjunction with in situ digital imaging correlation (DIC). The DIC analyses revealed that shear strain local- isations occur ahead of the crack tip during propagation and correlate well with the FCG rate retardations. Grain boundaries can be found at both peaks and valleys of the FCG rate curve and alter the interaction between crack growth and shear strain localisations. At the microstructural level, the deformation is associated with the dislocation-mediated plastic deformation process, showing increased formation of grain sub-structures in the regions of the strain localisation. Consequently, material experiences local hardening causing the FCG retarda- tion events. If the crack avoids the hardened material region through a macroscopic cross-slip mechanism, retardation is minor. On the contrary, if the crack penetrates the hardened region, retardation is significant.

In this work, metamodel-based robust optimization is performed using measured scatter of noise variables. Principal component analysis is used to describe the input noise using linearly uncorrelated principal components. Some of these principal components follow a normal probability distribution, others however deviate from a normal probability distribution. In that case, for more accurate description of material scatter, a multimodal distribution is used. An analytical method is implemented to propagate the noise distribution via metamodel and to calculate the statistics of the response accurately and efficiently. The robust optimization criterion as well as the constraints evaluation are adjusted to properly deal with multimodal response. Two problems are presented to show the effectiveness of the proposed approach and to validate the method. A basketball free throw in windy weather condition and forming of B-pillar component are presented. The significance of accounting for non-normal distribution of input variables using multimodal distributions is investigated. Moreover, analytical calculation of response statistics, and adjustment of the robust optimization problem are presented and discussed.

In a world where materials are becoming more and more scarce, it is of key importance to produce with the least amount of waste as possible. This is also the case for metal forming processes. Unfortunately, metal forming processes are subjected to uncertainty, such as variations in the raw material or the production environment. These disturbances must be taken into account when designing the process in order to manufacture products with high accuracy and low scrap rates. The procedure of finding the design parameters that lead to the least variation in the final product despite the influence of the uncertain parameters, is referred to as robust optimization. To reduce the time and costs associated with process development using physical experiments, computational models are often used. Metal forming processes are commonly modelled with transient Finite Element Analysis (FEA), which is computationally expensive, with model evaluation times usually in the order of minutes or hours. In general, optimization requires many evaluations of the objective function. Optimizing the production process based on the FE-model is therefore not computationally feasible. Surrogate models are commonly used to overcome this computational burden. A surrogate model is a cheap-to-evaluate model that mimics the output of the expensive model and is constructed using a data set of results from a limited number of evaluations of the expensive model. In this work, methods for surrogate modelling and robust optimization of multi-stage metal forming processes using computational models are studied. The dissertation focuses on the construction of surrogate models that describe the entire output field of FE-models. Special attention is paid on how the available data should be preprocessed to get as much information as possible from them when different phyiscal quantities need to be modelled. Furthermore it is investigated how Radial Basis Function interpolation can be applied to get the best interpolation of preprocessed data. The obtained knowledge is applied in order to construct a surrogate model of a production stage of a two-stage metal forming process. The output from the surrogate model of the first stage is propagated to the FE-model of the subsequent stage and compared with the output of a surrogate model that describes both stages at once. Lastly, an adaptive sampling strategy for scalar metamodel-based robust optimization with multi-stage models is presented. For this purpose an algorithm is proposed that takes account of the simulation time of different stages.

Funding Information: The authors are grateful for the financial support from the Academy of Finland through the project (309830) Ice Block Breakage: Experiments and Simulations (ICEBES). Publisher Copyright: © 2022. The Authors. Laboratory experiments on saline ice are often performed on cold, isothermal and dry specimens out of convenience versus working with warm specimens or specimens floating in water. The laboratory conditions, thus, usually involve non-natural conditions. This study compares cyclic loading experiments covering the main range of ocean wave periods performed on both dry, isothermal and warm, floating ice specimens. Results indicate that −2.5°C isothermal dry specimens have higher moduli than floating specimens with an average temperature of −2.5°C with a naturally occurring temperature gradient. Moreover, the dislocation density estimated using a physics-based model and the strain energy density dissipated in 10−3–10−2 Hz loading-unloading cycles are much lower for the −2.5°C dry specimens than for the −2.5°C floating specimens. Although the precise reason for the dislocation density difference requires further study, the results nonetheless contribute to the understanding and implementation of ice rheology and related geophysical modeling.

Herein, an experimental investigation into the occurrence of “pinching” defects during cold-rolling of thin metal sheets is presented. Pinched strips are typically characterized by repetitive ripples and local ruptures, which can cause strip breaks. Even though pinches are a widely experienced phenomenon in both hot- and cold-rolling of steel strips, no previous studies are known that have investigated the underlying mechanism for pinching during continuous rolling processes. Therefore, a set of rolling experiments is performed in a single-stand pilot mill to create pinches by applying sudden perturbations in the process conditions. For the rolling settings chosen in the performed experiments, it is found that disruptions in the lubrication state are a powerful approach to induce shape defects, which develop as pinches. Herein, a first extensive description of the pinching mechanism is provided, as observed during the trials, by monitoring the strip's behavior and analyzing the damaged sheets. It is shown that ripples in these pinched strips are the result of folds, which form in the roll bite. The folds originate from the waviness of the strip upstream of the bite, being created by nonuniform conditions over the width of the strip.

We investigate the predictive performance of specific analytical and numerical methods to determine the effective magnetic properties of two-phase steels at the macroscale. We utilize various mixture rules reported in the literature for the former, some of which correspond to rigorous bounds, e.g., Voigt (arithmetic) and Reuss (harmonic) averages. For the latter, we employ asymptotic homogenization together with the finite element method (FEM) and periodic boundary conditions (PBC). The voxel-based discretization of the representative volume element is conducted with digital image processing on the existing micrographs of DP600-grade steel. We show that unlike the considered isotropic mixture rules, which use only the phase volume fraction as the statistical microstructural descriptor, finite element method-based first-order asymptotic homogenization allows prediction of both phase content and directional dependence in the magnetic permeability by permitting an accurate consideration of the underlying phase geometry.

Sheet metal surfaces are generally textured to improve tribological performance in deep drawing applications. Variations in coefficient of friction in forming processes is one of the major causes of defective products. The major reasons for an unstable friction condition are the tool wear and inhomogeneity in lubricant amount. Textured surfaces can offer enhanced and stable friction condition. However, there is no clear design guidelines available for texturing sheet metal surfaces for a robust friction condition. Various types of texturing methods are available. In this study, the friction sensitivity of surface texture made by laser-texturing method to variations in tool wear and inhomogeneity in lubricant distribution is investigated. The laser-textured surface parameters such as crater diameter and texture density are chosen within the physically attainable range such that a robust friction behavior during forming process is achieved. A multi-scale friction model is used to determine coefficient of friction for textured surfaces in boundary and mixed lubrication conditions. The friction model in combination with surface generating algorithm is used to optimize individual crater geometry and their spacing. The objective is to determine the surface texture which is least sensitive to the potential variations in the tool roughness and lubricant amount in sheet metal forming applications.

Gradient structured materials have been proven to have excellent mechanical properties, such as strength–ductility synergy and excellent strain hardening. In this study, the deformation mechanism of heterostructured bulk aluminum with submicron deformation mechanisms was investigated using a mechanism-based strain-gradient plasticity model, whose gradient information was obtained using a discrete gradient computation method. The model was then used to simulate bending of the material and investigate extra strain hardening. The microstructure of the material was characterized using electron backscattered diffraction analysis. The complicated dislocation reactions occurring during the deformation of multilayer deposition material were determined from the simulation results. The distribution and evolution of geometrically necessary dislocations (GNDs) were numerically determined. The simulation results demonstrate that the GNDs and the number of material gradient cycles have a direct influence on plastic hardening. Inclusion of more layer periods in the material resulted in additional large-scale strain gradient across its thickness. The results of this study advances the understanding of the underlying deformation mechanisms that control ductility and strengthening over periods and gradients and provides the possibility of obtaining multilayer materials with exceptional mechanical properties.

With the recent surge in development of offshore wind in the Baltic Sea, Bohai Sea and other ice-prone regions, a need has arisen for new basin tests to qualify the interaction between offshore wind turbines and sea ice. To this end, a series of model tests was performed at the Aalto ice basin as part of the SHIVER project. The tests were aimed at modeling the dynamic interaction between flexible, vertically-sided structures and ice failing in crushing. A real-time hybrid test setup was used which combines numerical and physical components to model the structure. This novel test setup enabled the testing of a wide range of structure types, including existing full-scale structures for which ice-induced vibrations have been documented, and a series of single-degree-of-freedom oscillators to obtain a better understanding of the fundamental processes during dynamic ice-structure interaction. The tests were primarily focused on the dynamic behavior of support structures for offshore wind turbines under ice crushing loads. First results of the campaign show that the combination of the use of cold model ice and not scaling time and deflection of the structure can yield representative ice-structure interaction in the basin. This is demonstrated with experiments during which a scaled model of the Norströmsgrund lighthouse and Molikpaq caisson were used. The offshore wind turbine tests resulted in multi-modal interaction which can be shown to be relevant for the design of the support structure. The dataset has been made publicly available for further analysis.

Transformation induced plasticity (TRIP) phenomenon and mechanical behavior of TRIP-aided steels are influenced by the ambient as well as local temperature which raises due to plastic deformation and latent heat of transformation. In order to study this effect, tensile tests with samples made of DH800 steel were carried out at various temperatures, while the temperature rise was monitored during the test. In parallel, a physically-based model was applied to predict the change in material behavior as a consequence of the TRIP effect. In this model, martensitic transformation is mainly stress-driven, and the self-consistent scheme was adopted to achieve a homogenized material behavior. In addition, temperature rise as a result of plastic energy dissipation and latent heat of mechanically-induced retained austenite-martensite transformation was taken into account. Empirical flow curves at various temperatures indicated that at sub-zero values, the hardening of material due to the TRIP effect is much more pronounced. It was observed that a temperature change between +80 °C and −40 °C increases the flow stress by 200 MPa. Measured temperature rise, especially at sub-zero levels, showed an initial sharp increase at low strain stages due to phase transformation that flattens afterward. The numerical results are in agreement with experimental measurements of material flow curves and retained austenite fraction.

In hot stamping severe tool wear occurs. The main tool wear mechanism is the so-called compaction galling, in which fractured particles from the coating of the part (blank) being build-up and become compacted upon the tools. Since fractured particles play an important role in the wear mechanism, fracture of the AlSi coating is investigated. For this purpose, simplified B-pillar parts are pressed at several temperatures to investigate the effect of temperature, strain and tool contact on the fracture behavior of the AlSi coating. Three positions which exhibit different strain modes are analysed by means of optical topography and optical microscopy measurements. The results indicate a strong correlation between the AlSi coating fracture, strain and temperature. For instance, at relatively low forming temperatures (i.e. 650 ºC), mode I fracture as well as spallation of the coating and at higher temperatures (750 and 800 ºC) mainly mode I coating fractures are observed. Interestingly, this type of mode I coating fractures are found to widen, eventually giving rise to mode II coating fractures along the coating-substrate interface. The onset strain of coating fracture is low and approximately at strains of 0.017, coating cracks are observed at all start temperatures. At contact-dominated regions, additional coating cracks are observed mainly due to a drop in blank temperature and added shear stresses due to the relative sliding between the blank and tools.

Background Accurate and up-to-date assessment of demographic metrics is crucial for understanding a wide range of social, economic, and public health issues that affect populations worldwide. The Global Burden of Diseases, Injuries, and Risk Factors Study (GBD) 2019 produced updated and comprehensive demographic assessments of the key indicators of fertility, mortality, migration, and population for 204 countries and territories and selected subnational locations from 1950 to 2019. Methods 8078 country-years of vital registration and sample registration data, 938 surveys, 349 censuses, and 238 other sources were identified and used to estimate age-specific fertility. Spatiotemporal Gaussian process regression (ST-GPR) was used to generate age-specific fertility rates for 5-year age groups between ages 15 and 49 years. With extensions to age groups 10-14 and 50-54 years, the total fertility rate (TFR) was then aggregated using the estimated age-specific fertility between ages 10 and 54 years. 7417 sources were used for under-5 mortality estimation and 7355 for adult mortality. ST-GPR was used to synthesise data sources after correction for known biases. Adult mortality was measured as the probability of death between ages 15 and 60 years based on vital registration, sample registration, and sibling histories, and was also estimated using ST-GPR. HIV-free life tables were then estimated using estimates of under-5 and adult mortality rates using a relational model life table system created for GBD, which closely tracks observed age-specific mortality rates from complete vital registration when available. Independent estimates of HIV-specific mortality generated by an epidemiological analysis of HIV prevalence surveys and antenatal clinic serosurveillance and other sources were incorporated into the estimates in countries with large epidemics. Annual and single-year age estimates of net migration and population for each country and territory were generated using a Bayesian hierarchical cohort component model that analysed estimated age-specific fertility and mortality rates along with 1250 censuses and 747 population registry years. We classified location-years into seven categories on the basis of the natural rate of increase in population (calculated by subtracting the crude death rate from the crude birth rate) and the net migration rate. We computed healthy life expectancy (HALE) using years lived with disability (YLDs) per capita, life tables, and standard demographic methods. Uncertainty was propagated throughout the demographic estimation process, including fertility, mortality, and population, with 1000 draw-level estimates produced for each metric. Findings The global TFR decreased from 2.72 (95% uncertainty interval [UI] 2.66-2.79) in 2000 to 2.31 (2.17-2.46) in 2019. Global annual livebirths increased from 134.5 million (131.5-137.8) in 2000 to a peak of 139.6 million (133.0-146.9) in 2016. Global livebirths then declined to 135.3 million (127.2-144.1) in 2019. Of the 204 countries and territories included in this study, in 2019, 102 had a TFR lower than 2.1, which is considered a good approximation of replacement-level fertility. All countries in sub-Saharan Africa had TFRs above replacement level in 2019 and accounted for 27.1% (95% UI 26.4-27.8) of global livebirths. Global life expectancy at birth increased from 67.2 years (95% UI 66.8-67.6) in 2000 to 73.5 years (72.8-74.3) in 2019. The total number of deaths increased from 50.7 million (49.5-51.9) in 2000 to 56.5 million (53.7-59.2) in 2019. Under-5 deaths declined from 9.6 million (9.1-10.3) in 2000 to 5.0 million (4.3-6.0) in 2019. Global population increased by 25.7%, from 6.2 billion (6.0-6.3) in 2000 to 7.7 billion (7.5-8.0) in 2019. In 2019, 34 countries had negative natural rates of increase; in 17 of these, the population declined because immigration was not sufficient to counteract the negative rate of decline. Globally, HALE increased from 58.6 years (56.1-60.8) in 2000 to 63.5 years (60.8-66.1) in 2019. HALE increased in 202 of 204 countries and territories between 2000 and 2019. Interpretation Over the past 20 years, fertility rates have been dropping steadily and life expectancy has been increasing, with few exceptions. Much of this change follows historical patterns linking social and economic determinants, such as those captured by the GBD Socio-demographic Index, with demographic outcomes. More recently, several countries have experienced a combination of low fertility and stagnating improvement in mortality rates, pushing more populations into the late stages of the demographic transition. Tracking demographic change and the emergence of new patterns will be essential for global health monitoring. Copyright (C) 2020 The Author(s). Published by Elsevier Ltd.

The accurate description of friction is critical in the finite element (FE) simulation of the sheet metal forming process. Usually, friction is oversimplified through the use of a constant Coulomb friction coefficient. In this study, the application of an existing multiscale friction model is extended to the hot stamping process. The model accounts for the effects of tool and sheet metal surface topography as well as the evolution of contact pressure, temperature, and bulk strain during hot stamping. Normal load flattening and strip drawing experiments are performed to calibrate the model. The results show that the model can relatively well predict friction in strip draw experiments when the tool surface evolution due to wear is incorporated. Finally, the application of the formulated multiscale friction model was demonstrated in the FE simulation of a hot-stamped part.

A panoptic view of architectured planar lattices based on star-polygon tilings was developed. Four star-polygon-based lattice sub-families, formed of systematically arranged triangles, squares, or hexagons, were investigated numerically and experimentally. Finite-element-based homogenization allowed computation of Poisson's ratio, elastic modulus, shear modulus, and planar bulk modulus. A comprehensive understanding of the range of properties and micromechanical deformation mechanisms was developed. Adjusting the star-polygon angle achieved an over 250-fold range in elastic modulus, over a 10-fold range in density, and a range of $-0.919$ to $+0.988$ for Poisson's ratio. Additively manufactured lattices, achieved by novel printing strategies, showed good agreement in properties. Parametric additive manufacturing procedures for all lattices are available on \url{www.fullcontrol.xyz/#/models/1d3528}. Three of the four sub-families exhibited in-plane elastic isotropy. One showed high stiffness with auxeticity at low density and a primarily axial deformation mode as opposed to bending deformation for the other three lattices. The range of achievable properties, demonstrated with property maps, proves the extension of the conventional material-property space. Lattice metamaterials with Triangle-Triangle, Kagome, Hexagonal, Square, Truncated Archimedean, Triangular, and Truncated Hexagonal topologies have been studied in the literature individually. Here, it is shown that these structures belong to the presented overarching lattice family.

Funding Information: IL wishes to acknowledge the financial support from Alfred Kordelin Foundation, Finland, and from Aker Arctic Technology Inc. Finland and Finnish Maritime Foundation through the Industry-Academia Graduate School in Aalto University Department of Mechanical Engineering, Finland. The authors are also grateful for the financial support from Business Finland, Aker Arctic Technology Inc. Finland, ABB Marine, Arctia Shipping, Finland, Technip Offshore Finland Oy, Suomen Hy?tytuuli Oy, Finland, Finnish Transport Agency and Ponvia Oy through the ARAJ?? research-project. The authors express their appreciation to Aalto Ice Tank staff Otto Puolakka, Teemu P?iv?rinta and Lasse Turja for their work on the experiments. Funding Information: IL wishes to acknowledge the financial support from Alfred Kordelin Foundation, Finland , and from Aker Arctic Technology Inc., Finland and Finnish Maritime Foundation through the Industry-Academia Graduate School in Aalto University Department of Mechanical Engineering, Finland . The authors are also grateful for the financial support from Business Finland , Aker Arctic Technology Inc., Finland , ABB Marine, Arctia Shipping, Finland , Technip Offshore Finland Oy , Suomen Hyötytuuli Oy, Finland , Finnish Transport Agency and Ponvia Oy through the ARAJÄÄ research-project. The authors express their appreciation to Aalto Ice Tank staff Otto Puolakka, Teemu Päivärinta and Lasse Turja for their work on the experiments. Publisher Copyright: © 2021 The Author(s) Laboratory-scale experiments on ice-structure interaction process in shallow water were performed by pushing a ten-meter-wide ice sheet against an inclined structure of the same width. Seven experiments were performed in three series: In one of the series, the compressive and flexural strengths were both about 50kPa, in the two other test series the ice strength was two and four times higher. The ice thickness was about 50 mm in all experiments. The loading process showed two phases: the ice load on the structure (1) first increased linearly with a rate that was constant for all experiments, after which (2) the loading process reached a steady-state phase with approximately constant load. The magnitude of ice loads was not proportional to ice strength, as the weakest ice yielded higher loads than the ice having twice its strength. The ice rubble grounded in all experiments, but the bottom carried only a small portion of the load. The load records could be normalized by a factor combining the weight and the characteristic length of the intact ice. Based on the normalization, a model explaining the loading process was derived; the weight of the incoming ice has a dominant role during phase (1), while buckling explains the change in the process to phase (2) when the ice is strong enough. The loading process for the weakest ice was different from that for the other two ice types used. For example, instead of forming a rubble pile consisting of distinct ice blocks, weakest ice formed a dense pile of slush. The normalized ice load data highlighted the differences in the loading process.

In this paper we alleviate limitations of the conventional embedded reinforcement formulation for applications with dense fiber contents. We demonstrate that by condensing the fiber degrees of freedom during the assembly stage, the condition number of the resultant stiffness matrix is effectively reduced and the use of iterative solvers is facilitated even without preconditioning. Numerical benchmarks consisting of very large numbers of high aspect ratio discrete fibers are performed, and major advantages are reported in terms of the computational efficiency of the solution method. We apply the solution method to a set of examples in the modeling of the fiber reinforced composites, specifically the estimation of the homogenized mechanical properties of the discontinuous fiber composites and modeling of the compression molding process. In the latter case, a specimen containing more than 2 million discrete fibers is analyzed.

Background Flow curves can easily be obtained by uniaxial tensile tests, but strains are then limited by diffuse necking. For many applications, the flow stress must be known above this limit. Objective The main objective of this paper is to obtain flow curves for material with low uniform elongation to relatively high strains compared to a uniaxial tensile test. Method A novel in-plane sheet bending experiment and stress evaluation procedure is presented. The developed bending device can be mounted in a tensile test machine and can produce very high bending curvatures compared to previously proposed pure bending setups. The bending angle and curvature are obtained by image processing and the bending moment is calculated directly from the force measured from the tensile test machine and the bending angle. The moment–curvature relation is used to determine the uniaxial stress–strain relation using an analytical approach, without presuming any hardening model. The bending process and the analytical procedure are validated by a numerical simulation as well as by experiments. Results The numerical validation shows good agreement between the stress–strain curve obtained from the bending process and that of the uniaxial input flow curve up to 12% strain. Experimentally the model is validated by comparing the stress–strain curve obtained from the bending test with the results directly obtained from a tensile test for mild steel. Good agreement is observed up to 12% strain. As an application example, bending tests were performed on a martensitic steel (MS) with low uniform strain (less than 3%). For this material, flow curves could be obtained up to relatively high strains (~12%), compared to a tensile test. Conclusion This bending test setup allows to study materials with low uniform elongation up to significantly higher strains than are readily obtained in a tensile test.

Production efficiency in metal forming processes can be improved by implementing robust optimization. In a robust optimization method, the material and process scatter are taken into account to predict and to minimize the product variability around the target mean. For this purpose, the scatter of input parameters are propagated to predict the product variability. Consequently, a design setting is selected at which product variation due to input scatter is minimized. If the minimum product variation is still higher than the specific tolerance, then the input noise must be adjusted accordingly. For example this means that materials with a tighter specification must be ordered, which often results in additional costs. In this article, an inverse robust optimization approach is presented to tailor the variation of material and process noise parameters based on the specified product tolerance. Both robust optimization and tailoring of material and process scatter are performed on the metamodel of an automotive part. Although the robust optimization method facilitates finding a design setting at which the product to product variation is minimized, the tighter product tolerance is only achievable by requiring less scatter of noise parameters. It is shown that the presented inverse approach is able to predict the required adjustment for each noise parameter to obtain the specified product tolerance. Additionally, the developed method can equally be used to relax material specifications and thus obtain the same product tolerance, ultimately resulting in a cheaper process. A strategy for updating the metamodel on a wider (noise) base is presented and implemented to obtain a larger noise scatter while maintaining the same product tolerance.

Despite all the efforts to optimize the meniscus prosthesis system (geometry, material, and fixation type), the success of the prosthesis in clinical practice will depend on surgical factors such as intra-operative positioning of the prosthesis. In this study, the aim was therefore to assess the implications of positional changes of the medial meniscus prosthesis for knee biomechanics. A detailed validated finite element (FE) model of human intact and meniscal implanted knees was developed based on a series of in vitro experiments. Different non-anatomical prosthesis positions were applied in the FE model, and the biomechanical response during the gait stance phase compared with an anatomically positioned prosthesis, as well as meniscectomized and also the intact knee model. The results showed that an anatomical positioning of the medial meniscus prosthesis could better recover the intact knee biomechanics, while a non-anatomical positioning of the prosthesis to a limited extent alters the knee kinematics and articular contact pressure and increases the implantation failure risk. The outcomes indicate that a medial or anterior positioning of the meniscus prosthesis may be more forgiving than a posteriorly or laterally positioned prosthesis. The outcome of this study may provide a better insight into the possible consequences of meniscus prosthesis positioning errors for the patient and the prosthesis functionality. Graphical abstract Electronic supplementary material The online version of this article (10.1007/s11517-020-02158-0) contains supplementary material, which is available to authorized users.

A decomposition strategy for the structural optimization of a fiber-reinforced aircraft wing box is proposed. Theproposed method decomposes the wing-box optimization into two levels: a system-level and a subsystem-leveloptimization. The ribs are the subsystems of the problem. Each rib has a local set of design variables andconstraints. The loads on the ribs are the crushing loads caused by the bending of the wing. At the system level,the wing-box skins are optimized while accounting for the effect of the skin design on the loads applied to the ribs. Thesensitivity of the rib mass to the applied loads is evaluated using the Lagrange multipliers of the optimized rib design.To enhance the numerical efficiency of the two-level optimization, the changes of the loads on the ribs are subjected toa reduction by principal component analysis (PCA). In both the wing-level and rib-level optimization problems, thelevel-set strategyfor the optimization of compositestructures, previouslyintroduced by the authors, is employed.Thismethod permits an advantageous use of coarse and fine finite element models employing a standard commercial finiteelement code. The proposed method is applied to the design of a composite horizontal tail plane. The accuracy of andthe computational time savings by the proposed PCA-based reduction scheme are quantified.

Funding Information: This work was funded though the Finland Distinguished Professor programme ”Scaling of Ice Strength: Measurements and Modeling”, and through the ARAJ research project, both funded by Business Finland and the industrial partners Aker Arctic Technology, Arctech Helsinki Shipyard, Arctia Shipping, ABB Marine, Finnish Transport Agency, Suomen Hyȵtytuuli Oy, and Ponvia Oy. This financial support is gratefully acknowledged. The second author (J.P.D.) thanks Business Finland for support by the FiDiPro Professorship from Aalto University, and the sabbatical support from Aalto University, which collectively supported an annual visit 2015–2016, and summer visits 2017–2019. Publisher Copyright: © 2021 The Author(s) This work investigates the influence of loading type, loading rate, and test size on the fracture energy of columnar freshwater S2 ice. The ice sheet in the Ice tank at Aalto University was very warm (above -0.5 ∘C) and thick (0.34