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- ItemDevelopment, design, and realization of a proficiency test for the forensic determination of shooting distances - FDSD 2015(Amsterdam : Elsevier, 2016) Niewöhner, Ludwig; Barth, Martin; Neimke, Dieter; Latzel, Silke; Stamouli, Amalia; Nys, Bart; Gunaratnam, Lawrence; Fries, Kira; Uhlig, Steffen; Baldauf, HenningWithin the framework of the ENFSI Expert Working Group Firearms/GSR a novel proficiency test on the Forensic Determination of Shooting Distances – FDSD 2015 – was implemented. This proficiency test was developed out of collaborative studies which were previously carried out by a number of pre-selected ENFSI laboratories. The aim of this test was to assess the laboratories’ performance in visualizing the lead patterns on a shot object, and compare the questioned patterns with provided test shot patterns. The participating laboratories were requested to estimate the presumed shooting distance following their individual laboratory specific methods (SOPs) for shooting distance/muzzle-to-target determination. The submitted results were compiled by means of z scores according to the IUPAC and EURACHEM guidelines, and an extended statistical evaluation was performed. This is one of the first proficiency tests in the field of qualitative forensic methods where z scores were successfully utilized. This paper summarizes the results of the study and presents the overall performance of the participating laboratories.
- ItemNumerical simulation of the edge stress singularity and the adhesion strength for compliant mushroom fibrils adhered to rigid substrates(Amsterdam : Elsevier, 2016) Balijepalli, R.G.; Begley, M.R.; Fleck, N.A.; McMeeking, R.M.; Arzt, E.Bio-inspired adhesion of micropatterned surfaces due to intermolecular interactions has attracted much research interest over the last decade. Experiments show that the best adhesion is achieved with compliant “mushroom”-shaped fibrils. This paper analyses numerically the effects of different mushroom shapes on adhesion to a rigid substrate. When a remote stress is applied on the free end of a fibril perfectly bonded to a rigid substrate, the resultant stress distribution along the fibril is found to change dramatically between the straight punch and mushroom fibrils. A singular stress field is present at the edge of the fibril where it contacts the substrate and, in this work, the amplitude of the singularity is evaluated for fibrils perfectly bonded to a flat substrate so that sliding cannot occur there. This exercise is carried out for fibril geometries involving combinations of different diameters and thicknesses of the mushroom cap. By assuming a pre-existing detachment length at the corner where the stress singularity lies, we predict the adhesive strength for various mushroom cap shapes. Our study shows that a smaller stalk diameter and a thinner mushroom cap lead to higher adhesive strengths. A limited number of results are also given for other shapes, including those having a fillet radius connecting the stalk to the cap. The results support the rational optimization of synthetic micropatterned adhesives.
- ItemStructural evolution and strain induced mixing in Cu-Co composites studied by transmission electron microscopy and atom probe tomography(Amsterdam : Elsevier, 2015) Bachmaier, Andrea; Aboulfadl, H.; Pfaff, Marina; Mücklich, Frank; Motz, ChristianA Cu–Co composite material is chosen as a model system to study structural evolution and phase formations during severe plastic deformation. The evolving microstructures as a function of the applied strain were characterized at the micro-, nano-, and atomic scale-levels by combining scanning electron microscopy and transmission electron microscopy including energy-filtered transmission electron microscopy and electron energy-loss spectroscopy. The amount of intermixing between the two phases at different strains was examined at the atomic scale using atom probe tomography as complimentary method. It is shown that Co particles are dissolved in the Cu matrix during severe plastic deformation to a remarkable extent and their size, number, and volume fraction were quantitatively determined during the deformation process. From the results, it can be concluded that supersaturated solid solutions up to 26 at.% Co in a fcc Cu–26 at.% Co alloy are obtained during deformation. However, the distribution of Co was found to be inhomogeneous even at the highest degree of investigated strain.
- ItemDetachment of an adhered micropillar from a dissimilar substrate(Amsterdam : Elsevier, 2015) Khaderi, S.N.; Fleck, N.A.; Arzt, E.; McMeeking, R.M.Abstract The mechanics of detachment is analysed for 2D flat-bottomed planar pillars and 3D cylindrical pillars from a dissimilar elastic substrate. Application of an axial stress to the free end of the pillar results in a singularity in stress at the corner with the substrate. An eigenvalue analysis reveals that the stress field near the corner is dominated by two singular eigenfields having eigenvalues ( λ 1 , λ 2 ) with corresponding intensities ( H 1 , H 2 ) . The asymptotic stress field σij is of the form σ ij = H 1 r λ 1 − 1 f ij ( λ 1 , θ ) + H 2 r λ 2 − 1 f ij ( λ 2 , θ ) , where fij describe the angular dependence θ of σij, and r is the radial distance from the corner. The stress intensities ( H 1 , H 2 ) are calculated numerically, using a domain integral approach, as a function of the elastic mismatch between the pillar and substrate. The singular zone extends across approximately 10 of the pillar diameter (in 3D) or pillar width (in 2D). Interfacial failure is predicted for an assumed crack emanating from the corner of pillar and substrate. For the case of an interfacial crack that resides within the domain of corner singularity, a boundary layer analysis is performed to calculate the dependence of the interfacial stress intensity factor K upon ( H 1 , H 2 ) . When the crack extends beyond the domain of corner singularity, it is necessary to consider the full geometry in order to obtain K. A case study explores the sensitivity of the pull-off stress to the flaw size and to the degree of material mismatch. The study has implications for the optimum design of adhesive surface micropatterns, for bonding to either stiffer or more compliant substrates.
- ItemBioinspired polydimethylsiloxane-based composites with high shear resistance against wet tissue(Amsterdam : Elsevier, 2016) Fischer, Sarah C.L.; Levy, Oren; Kroner, Elmar; Hensel, René; Karp, Jeffrey M.; Arzt, EduardPatterned microstructures represent a potential approach for improving current wound closure strategies. Microstructures can be fabricated by multiple techniques including replica molding of soft polymer-based materials. However, polymeric microstructures often lack the required shear resistance with tissue needed for wound closure. In this work, scalable microstructures made from composites based on polydimethylsiloxane (PDMS) were explored to enhance the shear resistance with wet tissue. To achieve suitable mechanical properties, PDMS was reinforced by incorporation of polyethylene (PE) particles into the pre-polymer and by coating PE particle reinforced substrates with parylene. The reinforced microstructures showed a 6-fold enhancement, the coated structures even a 13-fold enhancement in Young׳s modulus over pure PDMS. Shear tests of mushroom-shaped microstructures (diameter 450 µm, length 1 mm) against chicken muscle tissue demonstrate first correlations that will be useful for future design of wound closure or stabilization implants.