A new approach to determine the accuracy of morphology–elasticity relationships in continuum FE analyses of human proximal femur

AbstractContinuum finite element (FE) models of bones are commonly generated based on CT scans. Element material properties in such models are usually derived from bone density values using some empirical relationships. However, many different empirical relationships have been proposed. Most of these will provide isotropic material properties but relationships that can provide a full orthotropic elastic stiffness tensor have been proposed as well. Presently it is not clear which of these relationships best describes the material behavior of bone in continuum models, nor is it clear to what extent anisotropic models can improve upon isotropic models. The best way to determine the accuracy of such relationships for continuum analyses would be by quantifying the accuracy of the calculated stress/strain distribution, but this requires an accurate reference distribution that does not depend on such empirical relationships. In the present study, we propose a novel approach to generate such a reference stress distribution. With this approach, stress results obtained from a micro-FE model of a whole bone, that can represent the bone trabecular architecture in detail, are homogenized and the homogenized stresses are then used as a reference for stress results obtained from continuum models. The goal of the present study was to demonstrate this new approach and to provide examples of comparing continuum models with anisotropic versus isotropic material properties.Continuum models that implemented isotropic and orthotropic material definitions were generated for two proximal femurs for which micro-FE results were available as well, one representing a healthy and the other an osteoporotic femur. It was found that the continuum FE stress distributions calculated for the healthy femur compared well to the homogenized results of the micro-FE although slightly better for the orthotropic model (r=0.83) than for the isotropic model (r=0.79). For the osteoporotic bone also, the orthotropic model did better (r=0.83) than the isotropic model (r=0.77). We propose that this approach will enable a more relevant and accurate validation of different material models than experimental methods used so far

The Content of Native American Cultural Stereotypes in Comparison to Other Racial Groups

abstract: Despite a large body of research on stereotypes, there have been relatively few empirical investigations of the content of stereotypes about Native Americans. The primary goal of this research was to systematically explore the content of cultural stereotypes about Native Americans and how stereotypes about Native Americans differ in comparison to stereotypes about Asian Americans and African Americans. Building on a classic paradigm (Katz and Braly, 1933), participants were asked to identify from a list of 145 adjectives those words associated with cultural stereotypes of Native Americans and words associated with stereotypes of Asian Americans (Study 1) or African Americans (Study 2). The adjectives associated with stereotypes about Native Americans were significantly less favorable than the adjectives associated with stereotypes about Asian Americans, but were significantly more favorable than the adjectives associated with stereotypes about African Americans. Stereotypes about Native Americans, Asian Americans and African Americans were also compared along the dimensions of the stereotype content model (SCM; Fiske, et al., 2002), which proposes that stereotypes about social groups are based on the core dimensions of perceived competence, warmth, status, and competitiveness. Native Americans were rated as less competent, less of a source of competition, and lower in social status than Asian Americans, and less competent and lower in social status than African Americans. No significant differences were found in perceived warmth across the studies. Combined, these findings contribute to a better understanding of stereotypes about Native Americans and how they may differ from stereotypes about other racial groups.Dissertation/ThesisM.S. Psychology 201

Temperature Dependent Friction Modelling: The Influence of Temperature on Product Quality

In the stamping of industrial parts, friction and lubrication play a key role in achieving high quality products and reducing scrap. Especially in the start-up phase of new production runs, transient effects can have a significant impact on the product quality. Before a steady-state production run is established, heating up of tools influence tribological conditions. This influences the performance of the forming operation and, consequently, the quality of the formed product. In the development process of new industrial parts, it is therefore crucial to accurately account for these transient effects in sheet metal forming simulations. This paper presents the modeling of the frictional behavior of two tribological systems as developed within the ASPECT project. The first tribology systems consist of a stainless steel with corresponding drawing oil and tool material. The sheet material of the second tribology system is a hot dip galvanized bake hardened steel. Subsequently, it is shown how temperature affects the frictional behavior of these tribology systems. Finally, generated friction models have been applied to a spare wheel well of Opel. The spare wheel well is modelled using a generally applicable approach to account for transient effects under industrial sheet metal forming process conditions. For varying temperature and tribological conditions, the spare wheel well can show cracks and differences in thinning and draw-in. This emphasizes the strong influence of transient effects on both part quality and the overall production stability

An insight in friction and wear mechanisms during hot stamping

Hot stamping is often used in the automotive industry to combine formability and strength. However, during forming process at high temperatures, friction and tool wear are determining factors that can affect the efficiency of the whole process. The goal of this paper is to investigate the effects of temperature on the local coefficient of friction and tool wear and to provide an insight in the phenomena which take place at the tool-sheet metal interface during hot stamping processes. For this purpose, hot friction draw tests between uncoated tool steel and Al-Si coated press hardening steel were carried out at several temperatures between 500-700°C. Consecutive tests were performed to mimic industrial hot stamping process and to investigate the effect of tool wear on the friction phenomenon. Finally, tool-sheet metal tribological behavior and the interaction between the friction and tool wear mechanisms were analyzed using different imaging and chemical characterization techniques. The results show that several stages can be distinguished at the interface between tool and sheet metal coating during hot stamping: flattening due to initial normal contact, ploughing of tool asperities through coating, secondary ploughing in the coating by adhered material on the tooling, and abrasive wear in the tool by embedded particles in the sheet metal coating. Furthermore, tool wear shows some major differences in the temperature range of 500-700°C. At high temperature a larger abrasive area and more severe compaction galling occurs that can be explained by material properties of Al-Si coating at elevated temperatures. The results of this study can be used for more efficient process design and a more realistic modelling of the hot stamping process.</jats:p

A novel approach to estimate trabecular bone anisotropy from stress tensors

Continuum finite element (FE) models of bones and bone-implant configurations are usually based on clinical CT scans. In virtually all of these models, material properties assigned to the bone elements are chosen as isotropic. It has been shown, however, that cancellous bone can be highly anisotropic and that its elastic behavior is best described as orthotropic. Material models have been proposed to derive the orthotropic elastic constants from measurements of density and a fabric tensor. The use of such relationships in FE models derived from CT scans, however, is hampered by the fact that the measurement of such a fabric tensor is not possible from clinical CT images since the resolution of such images is not good enough to resolve the trabecular micro-architecture. In this study, we explore an alternative approach that is based on the paradigm that bone adapts its micro-architecture to the loading conditions, hence that fabric and stress tensors should be aligned and correlated. With this approach, the eigenvectors and eigenvalues of the element continuum-level stress tensor are used as an estimate of the element fabric tensor, from which the orthotropic material properties then are derived. Using an iterative procedure, element orthotropic material properties and fabric tensors are updated until a converged situation is reached. The goals of this study were to investigate the feasibility and accuracy of such an iterative approach to derive orthotropic material properties for a human proximal femur and to investigate whether models derived in this way can provide more accurate results than isotropic models. Results were compared to those obtained from models of the same femurs for which the fabric was measured from micro-CT scans. It was found that the iterative approach could well estimate the orientation of the fabric principal directions. When comparing the stress/damage values in the models with material properties based on estimated and measured fabric tensors, the differences were not significant, suggesting that the material properties based on the estimated fabric tensor well reflected those based on the measured fabric tensor. Errors were less than those obtained when using isotropic models. It is concluded that this novel approach can provide a reasonable estimate of anisotropic material properties of cancellous bone. We expect that this approach can lead to more accurate results in particular for models used to study implants, which are usually anchored in highly anisotropic cancellous bone regions

An industrial-scale cold forming process highly sensitive to temperature induced frictional start-up effects to validate a physical based friction model

It is widely realized in the metal forming industry that the processes suffer from start-up effects, which can cause products that fall out of their specification limits after a certain period of running. Attempts are made to solve this by modelling the underlying tribological phenomena in order to design robust processes. Another possibility is to adapt the processes to changes of the tribological system through control systems. For both approaches the ability to account for tribological effects in numerical simulations is a crucial requirement. To showcase this, a real-life demonstrator process is built, that is specifically designed to be sensitive to temperature induced frictional effects. This enables the validation of the physical based tribological models on an industrial scale, without the need to add complexity from other research domains in the field of metal forming. After designing the tooling, the process' sensitivity to temperature induced frictional effects is verified by implementing and applying all simulations tools created [1]. The industrial scale demonstrator process is ran, while measuring tool temperature and geometrical product parameters (quality features) in real-time, at different stroke rates until a steady-state temperature is reached. The recorded data sets, combined with the products themselves, show during the full validation that the predictions from the simulations are generally in good agreement with the results from production

Analytical, numerical and experimental studies on ploughing behaviour in soft metallic coatings

A coating layer is often present on engineering surfaces. An example is a zinc coating on steel sheet, this being a soft metallic coating on a hard substrate. To characterize the tribological behaviour of these engineered surfaces, it is necessary to understand the mechanical behaviour of the coating during ploughing. The material point method (MPM)-based ploughing model has been used to compute friction and the ploughed profile when an asperity is ploughing through a coated surface. An analytical ploughing model has also been used to study the effect of the thickness and hardness of the coating relative to the substrate on coefficient of friction using rigid-plastic material behaviour and its results have been compared with the MPM-model results. The MPM-based ploughing model has been experimentally validated and is shown to agree well with the ploughing experiments using rigid spherical indenters sliding through lubricated-zinc coated steel, uncoated steel and bulk zinc over a range of applied loads

Multi-scale contact modeling of coated steels for sheet metal forming applications

Friction in sheet metal forming is a local phenomenon which depends on continuously evolving contact conditions during the forming process. This is mainly influenced by local contact pressure, surface textures of the sheet metal as well as the forming tool surface profile and material behavior. The first step for an accurate prediction of friction is to reliably estimate real area of contact at various normal loads. In this study, a multi-scale contact model for the normal load is presented to predict asperity deformation in coated steels and thus to estimate the real area of contact. Surface profiles of the zinc layer and steel substrate are modelled explicitly obtained from confocal measurements. Different mechanical properties are assigned to the zinc coating and the steel substrate. The model was calibrated and validated relative to lab-scale normal load tests using different samples of zinc coated steel with distinct surface textures. The results show that the model is able to predict the real area of contact in zinc-coated steels for various contact pressures and different surface textures. Current multi-scale model can be used to determine the local friction coefficient in sheet metal forming processes more accurately.</jats:p