Many skeletons (tests) of the fossil echinoid Micraster from the marl limestone strata of the Upper Cretaceous formation of western Navarra (Spain) show small triangular holes on their surface, caused by the loss of a pyramidal fragment. The base of the pyramidal borings is often close to an equilateral triangle. The test of echinoids is a mosaic structure constituted of interlocking monocrystalline plates of bio-calcite which possess a preferred crystalline orientation. In the case of the genus Micraster, the surface of the plates is near-parallel to the basal calcite plane (0001). The geometry of the observed holes, at first sight surprising, is simply explained by the intersection of cracks along the three easy { 1014} cleavage planes of the calcite single crystalline plates constitutive of the skeletons. The fracture process leading to the pyramidal fractures is most likely due to stresses of thermo-elastic origin induced by the heterogeneous dilatational response of the anisotropic tests adhered to the isotropic marly limestone in which they are embedded and which is constitutive of their inner molds.

Volume changes accompanying the plastic deformation at 300 K of nanocrystalline samples of alpha-Fe with a columnar grain structure possessing a < 1 (1) over bar0 > random fiber texture has been obtained from molecular dynamics (MD) simulations. The samples were strained in tension along the common axial direction of the columnar grains. After removal of the elastic volume change, the evolution of plastic volume strain was obtained. Small but non-negligible volume dilations or contractions are observed depending on size (samples of very small grain size show volume contraction). The rate of volume change is high during the first 10% plastic deformation and continues at a low pace thereafter; the first 10% deformation represents a transient in the stress-strain behavior too. The complex behavior observed is reasonably explained by the superposition of contributions from different plastically-induced structural changes to the mass density change: Mainly from changes of grain size, grain boundary structure, dislocation density and density of point-defects. The results are of interest for the development of crystal plasticity theories not restricted by the volume conserving assumption.

An analysis of the loading-unloading behaviour of tensile tests was performed in a transformation-induced plasticity-assisted (TRIP-assisted) steel, duplex high Mn steel (22% Mn-3% Al-3% Si) for a wide range of grain sizes, D. The Bauschinger effect observed during unloading was so strong that plastic deformation was detected before complete unloading was reached. In addition, the yield stress, , or the flow stress , , are related to the grain size according to the Hall-Petch relationship. This behaviour was compared with a twinning-induced plasticity (TWIP), steel (22% Mn-0.6% C). The high levels of internal stresses obtained in both steels can be rationalised on account of the reinforcement of the austenitic matrix by the increasing volume fraction of epsilon-martensite or deformation twins of nanometric thickness.

A series of torsion tests were performed to characterise the plastic response at high strain levels in a twinning-induced plasticity steel (22%Mn-0.6%C in mass) in the interval of temperatures 20 degrees C <= T <= 450 degrees C at three different shear strain rates, , 2.4 center dot 10(-3), 0.2, 6.5 s(-1). The work hardening at room temperature was near-linear typical of this steel. It was found that the work hardening rate and the torsional ductility are strongly influenced by temperatures T >= 150 degrees C. Sub-surface deformed specimens at T = 450 degrees C and 2.4 center dot 10(-3) s(-1) were analysed via high-resolution electron backscatter diffraction system, showed a sub-structure of very fine grains indicating that continuous dynamic recrystallization had taken place.

Six suppliers from several countries were asked to furnish parallelepiped 10 × 30 × 80 mm3 samples of commercial purity tungsten machined from rolled plates in view of selecting a provider of tungsten bricks for the target of the European neutron spallation source, ESS, under construction in Lund (Sweden). Sample surfaces were to be ground and free from visible defects or oxidation. The material should be rolled after sintering. A minimum room temperature tensile strength of 600 MPa was specified. The samples were submitted to different blind mechanical tests, measurement of physical properties and structural observations in order to assess their suitability for the application. We present here a summary of their main measured properties. The dispersion of results is noteworthy; the exercise allowed to sort-out technically eligible candidates for the application.

Hot rolled, laboratory-cast, TWIP steel specimens with composition 22% Mn-0.6% C (in mass %) was cold rolled to reductions of 40%, 50%, 60% and 70% and afterwards isothermally annealed for various times in the interval of temperatures 450 degrees C <= T <= 1100 degrees C. The purpose was to study the precipitation behavior and its plausible effect in the static recrystallization and grain growth kinetics. Two types of precipitates were found in 600 degrees C <= T <= 700 degrees C for long times: (Fe, Mn)(3) C - Cementite and Vanadium Carbonitrides. Recrystallized grain size was very fine, D-0 <= 2 mu m. Also, a weaken retained rolling texture in the recrystallisation process was found. Calculated value of activation energy for recrystallization, Q(soft) = 281 +/- 70 kJ.mol(-1) was obtained which corresponds practically with the activation energy for bulk self-diffusion in austenite (270 kJ.mol(-1) ) and for Mn diffusion in the austenite lattice (265 kJ.mol(-1)). Nevertheless, higher calculated activation energy for grain growth, Q(GG) = 384 +/- 60 kJ.mol(-1) was found with a grain growth exponent of n(GG) similar to 4. Consequently, the most plausible explanation is that the quantity of precipitates is enough to have relevant pinning effect of migrating grain boundaries during grain growth due to the mean length between precipitates, L-prec, is smaller than some threshold value of grain size, L-prec < D-threshold, being, D-0 << D-threshold.

A clear transition in the tensile ductility behavior has been observed for grain sizes D in the range of 15 mu m - 20 mu m (1.50 mu m <= D < 50 mu m) in a 22% Mn, 0.6% C (in mass %) TWIP steel. This behavior is a combination of the intrinsic effect of grain size D on strength and work hardening rate of the material, with an extrinsic effect, superficial decarburization and Mn depletion processes occurred during annealing treatments at T >= 1000 degrees C. In the present work, this extrinsic effect happened in TWIP steel has been studied in depth. GDOES (Glow Discharge Optical Emission Spectrometry) analyses have been carried out in order to study quantitatively the C and Mn concentration profiles. The depth of surface decarburization has been modeled by using Birks-Jackson theory. Two micro-constituents have been observed via Ferritoscope into decarburized volume: alpha'-martensite and gamma-austenite. The ductility of coarse-grained TWIP steel, subjected for high annealing temperatures and long annealing times, declines as a consequence of the formation of alpha'-martensite and less stable gamma-austenite with lower stacking fault energy, SFE, due to the Mn depletion in the decarburized volume.

The tensile elongation of an < 011 > oriented columnar nanocrystalline pure iron structure at a temperature of 300 K has been simulated by molecular dynamics (MD). The simulated sample contains 4.3 x 10(6) atoms and has been subject to free elongation along the < 011 > axis common to the grains. Periodic boundary conditions have been assumed. The grains are randomly oriented around their common < 011 > and the size of their cross section is about 10 nm. The stress-strain curve has been calculated up to 0.5 true strain. After elastic deformation and heterogeneous dislocation nucleation from the grain boundaries, it shows a peak stress of 8 GPa followed by a remarkably stable steady state with a flow stress of 5.15 GPa, where neither the crystallographic texture nor the grain structure show any important change despite the large plastic deformation imparted. Upon a strain reversal, a pronounced Bauschinger effect is then observed (-3.3 GPa compressive yield stress), followed by a hardening transient until the absolute level of the flow stress in compression reaches near the same value it had in tension when the unloading took place. The results of the MD simulation are discussed by comparison with experimental values of the strength and structural evolution of heavily drawn iron wires available in the bibliography.

This work presents a finite element analysis of the indentation size effect (ISE) experimentally observed in tests performed at submicron scale. A 3D model of a conical rigid surface indenting on a Nb single crystal at different depths has been developed. The bcc Nb material has been characterized within a finite-strain framework through a crystal plasticity model incorporating strain-gradient hardening. The hardness evolution for different material orientations and for different initial dislocation densities has been studied. The numerical results are compared with predictions of existing analytical models and with experimental results. (C) 2013 Elsevier B.V. All rights reserved.

The structure and crystallographic texture of zinc strips (Zn-Cu-Ti alloy) produced by the continuous horizontal twin roll strip casting method has been characterized. In longitudinal sections normal to the transverse direction, the strips display an approximately symmetrical chevron patterned structure of columnar grains inclined about 30 degrees from the rolling direction. In association with such structure, the macroscopic texture is mainly < 1 (1) over bar 00 > 'normal' (not cyclic) fibre texture tilted approximately +/-30 degrees around the transverse direction plus a similarly tilted weak < 0001 > fibre texture. A thin layer of small equiaxed grains with a strong (0001) basal texture is present at the free surfaces. The observed structure/texture combination agrees quite well with the expected macrostructure of solidification of the alloy in the twin roll casting process.

This work presents the evaluation of a new control system of Functional Electrical Stimulation (FES) for applying strategies for artificial activation of the ankle joint muscles. In particular, we present an investigation of the direct effects of event-driven open loop FES assistance of the ankle muscles during walking on the lower limb kinematics in able-bodied individuals (n = 5) and one incomplete spinal cord injured (iSCI) patient. We analyzed gait kinematics between walking with and without FES assistance. Effects consistent with normal physiological gait were obtained, but some unexpected alterations also occurred. Therefore, further studies are needed to help adjust the FES strategy.