Showing papers by "Georg Schitter published in 2011"
TL;DR: Digital image correlation is used to link stress whitening in bone, reported to be correlated to microdamage, to quantitative local strain values, suggesting that damage initiation in trabecular bone is asymmetric in tension and compression, with failure originating and propagation over a large range of tensile strains.
Abstract: The use of bone mineral density as a surrogate to diagnose bone fracture risk in individuals is of limited value. However, there is growing evidence that information on trabecular microarchitecture can improve the assessment of fracture risk. One current strategy is to exploit finite element analysis (FEA) applied to 3D image data of several mm-sized trabecular bone structures obtained from non-invasive imaging modalities for the prediction of apparent mechanical properties. However, there is a lack of FE damage models, based on solid experimental facts, which are needed to validate such approaches and to provide criteria marking elastic-plastic deformation transitions as well as microdamage initiation and accumulation. In this communication, we present a strategy that could elegantly lead to future damage models for FEA: direct measurements of local strains involved in microdamage initiation and plastic deformation in single trabeculae. We use digital image correlation to link stress whitening in bone, reported to be correlated to microdamage, to quantitative local strain values. Our results show that the whitening zones, i.e. damage formation, in the presented loading case of a three-point bending test correlate best with areas of elevated tensile strains oriented parallel to the long axis of the samples. The average local strains along this axis were determined to be (1.6±0.9)% at whitening onset and (12±4)% just prior to failure. Overall, our data suggest that damage initiation in trabecular bone is asymmetric in tension and compression, with failure originating and propagating over a large range of tensile strains.
53 citations
TL;DR: Improved performance of a closed-loop AO system with Piezo-driven Deformable Mirror and high-sampling-rate Wavefront Sensor is improved by means of model-based control and the variance of the residual error has been reduced.
21 citations
TL;DR: In this article, an integrated approach to design a robust feedback controller and topography estimator, taking into account the dynamical uncertainties of the imaging system, is presented, where it is shown that for a given AFM system there exist a direct trade-off between the achievable closed-loop bandwidth and the guaranteed bounds on the topography estimation error due to the dynamic uncertainties in the system.
11 citations
01 Jan 2011
TL;DR: In this paper, the authors discuss some recent developments to compensate for the adverse effects of the positioning stage dynamics in high-speed AFM by utilizing modern control methods and demonstrate that the improvements on both the lateral scanning motion and in controlling the tip-sample interaction force allow significantly faster, and more accurate AFM imaging.
Abstract: In Atomic Force Microscopy (AFM), the dynamics and non-linearities of the positioning stage are major sources of image artifacts and distortion, especially when imaging at high-speed. This contribution discusses some recent developments to compensate for these adverse effects of the positioning stage dynamics in high-speed AFM by utilizing modern control methods. The improvements on both the lateral scanning motion and in controlling the tip-sample interaction force are demonstrated to allow significantly faster, and more accurate AFM imaging.
7 citations