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

Cracking Behavior of Coating during Hot Tensile Tests of AlSi-Coated Press Hardening Steel

For industrial hot stamping applications, press hardening steel is usually coated with Al-10wt.%Si, in order to prevent substrate decarburization and oxidation at elevated temperatures. However, during hot stamping, the AlSi coating layer fractures, causing severe tool wear, substrate oxidation and increased friction coefficient between the tool and stamped part. The initiation of coating fracture can largely be attributed to the formation of several intermetallic compounds (i.e., FeAl & Fe2Al5) via Fe-diffusion, which also results in void formation throughout the coating layer. These intermetallics are formed mainly during the heating stage, with decreasing Fe-content from the coating-substrate interface. Due to distinctive thermo-mechanical properties of intermetallics compared to the steel substrate, the interaction between different intermetallics, including voids, causes high strain localization around the voids, leading to coating fracture. The goal of this study is to detect the initiation of cracks in a ~45 mu-m coating layer during uniaxial tensile deformation of a 1.5 mm AlSi-coated press hardening steel. For this purpose, isothermal tensile tests were performed at elevated temperatures. The coating cracks were detected by means of acoustic emission (AE) sensors during deformation. The distribution of coating cracks at hot stamping condition was examined via optical measurements. The tensile strain was measured from a strain grid on the sample gauge. The experiment involves heating the coated steel in a furnace to 920 ̊C for 6 minutes, followed by uniaxial tensile deformation (at 600 ̊C and 800 ̊C), and finally quenching at ambient air. The first AE signal from the sample was observed during the tensile deformation at 600 ̊C, indicating that tensile strain initiates fracture in the coating layer. At cooling stage, the temperature change with time triggered more AE signals, which may correspond to substrate phase transformation and additional fractures in the coated steel; the latter is owing to thermal expansion mismatch between the intermetallics in the coating layer, and steel substrate. Interestingly, no AE signals were observed during the heating stage; i.e., no coating cracks occur prior to deformation and quenching

An efficient strategy to describe the propagation of variation through multi-stage metal forming processes

In this work an efficient strategy to describe the propagation of uncertainty throughout a production process is proposed and validated. A two-stage metal forming process is used to demonstrate the proposed strategy. A metamodel of the first stage is built using a singular value decomposition combined with multiquadric radial basis functions. The strategy is validated by comparing the propagated results from the first stage, of both the metamodel and a finite element model, to a second stage finite element model for a new set of validation points. Using the proposed strategy, it will be possible to efficiently optimize multi-stage processes under uncertainty.</p

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

Mesh management methods in finite element simulations of orthodontic tooth movement

In finite element simulations of orthodontic tooth movement, one of the challenges is to represent long term tooth movement. Large deformation of the periodontal ligament and large tooth displacement due to bone remodelling lead to large distortions of the finite element mesh when a Lagrangian formalism is used. We propose in this work to use an Arbitrary Lagrangian Eulerian (ALE) formalism to delay remeshing operations. A large tooth displacement is obtained including effect of remodelling without the need of remeshing steps but keeping a good-quality mesh. Very large deformations in soft tissues such as the periodontal ligament is obtained using a combination of the ALE formalism used continuously and a remeshing algorithm used when needed. This work demonstrates that the ALE formalism is a very efficient way to delay remeshing operations