Showing papers by "Marc A. Meyers published in 2011"

Biological materials: a materials science approach.

Abstract: The approach used by Materials Science and Engineering is revealing new aspects in the structure and properties of biological materials. The integration of advanced characterization, mechanical testing, and modeling methods can rationalize heretofore unexplained aspects of these structures. As an illustration of the power of this methodology, we apply it to biomineralized shells, avian beaks and feathers, and fish scales. We also present a few selected bioinspired applications: Velcro, an Al2O3-PMMA composite inspired by the abalone shell, and synthetic attachment devices inspired by gecko.

Mechanical properties and the laminate structure of Arapaima gigas scales

Abstract: The Arapaima gigas scales play an important role in protecting this large Amazon basin fish against predators such as the piranha. They have a laminate composite structure composed of an external mineralized layer and internal lamellae with thickness of 50-60 μm each and composed of collagen fibers with ~1 μm diameter. The alignment of collagen fibers is consistent in each individual layer but varies from layer to layer, forming a non-orthogonal plywood structure, known as Bouligand stacking. X-ray diffraction revealed that the external surface of the scale contains calcium-deficient hydroxyapatite. EDS results confirm that the percentage of calcium is higher in the external layer. The micro-indentation hardness of the external layer (550 MPa) is considerably higher than that of the internal layer (200 MPa), consistent with its higher degree of mineralization. Tensile testing of the scales carried out in the dry and wet conditions shows that the strength and stiffness are hydration dependent. As is the case of most biological materials, the elastic modulus of the scale is strain-rate dependent. The strain-rate dependence of the elastic modulus, as expressed by the Ramberg-Osgood equation, is equal to 0.26, approximately ten times higher than that of bone. This is attributed to the higher fraction of collagen in the scales and to the high degree of hydration (30% H(2)O). Deproteinization of the scale reveals the structure of the mineral component consisting of an interconnected network of platelets with a thickness of ~50 nm and diameter of ~500 nm.

Armadillo armor: mechanical testing and micro-structural evaluation.

Abstract: The armadillo has a unique protective bony armor, called the osteoderm, which confers to its shell-like skin distinctive mechanical properties. The top layer of the shell is made out of a dark-brown keratin layer with bimodal size scales. Beneath the keratin layer, the osteoderm consists of hexagonal or triangular tiles having a composition that is the same as bone. The tiles are connected by non-mineralized collagen fibers, called Sharpey's fibers. The tough and highly mineralized tiles have a tensile strength of approximately 20 MPa and toughness of around 1.1 MJ/m3. In comparison, the hydrated osteoderm has a lower tensile strength of ∼16 MPa and a toughness of 0.5 MJ/m3. The tensile failure occurs by the stretching and rupture of the Sharpey's fibers. In a specially designed punch test in which an individual tile is pushed out, the shear strength is ∼18 MPa, close to the tensile strength of the osteoderm. This surprising result is interpreted in terms of deformation in the Sharpey's fibers in the hydrated condition. The armadillo shell and a turtle shell are compared, with their corresponding similarities and differences.

Structure and mechanical properties of Saxidomus purpuratus biological shells.

Abstract: The strength and fracture behavior of Saxidomus purpuratus shells were investigated and correlated with the structure. The shells show a crossed lamellar structure in the inner and middle layers and a fibrous/blocky and porous structure composed of nanoscaled particulates (~100 nm diameter) in the outer layer. It was found that the flexure strength and fracture mode are a function of lamellar organization and orientation. The crossed lamellar structure of this shell is composed of domains of parallel lamellae with approximate thickness of 200-600 nm. These domains have approximate lateral dimensions of 10-70 μm with a minimum of two orientations of lamellae in the inner and middle layers. Neighboring domains are oriented at specific angles and thus the structure forms a crossed lamellar pattern. The microhardness across the thickness was lower in the outer layer because of the porosity and the absence of lamellae. The tensile (from flexure tests) and compressive strengths were analyzed by means of Weibull statistics. The mean tensile (flexure) strength at probability of 50%, 80-105 MPa, is on the same order as the compressive strength (~50-150 MPa) and the Weibull moduli vary from 3.0 to 7.6. These values are significantly lower than abalone nacre, in spite of having the same aragonite structure. The lower strength can be attributed to a smaller fraction of the organic interlayer. The fracture path in the specimens is dominated by the orientation of the domains and proceeds preferentially along lamella boundaries. It also correlates with the color changes in the cross section of the shell. The cracks tend to undergo a considerable change in orientation when the color changes abruptly. The distributions of strengths, cracking paths, and fracture surfaces indicate that the mechanical properties of the shell are anisotropic with a hierarchical nature.

Correlation of the mechanical and structural properties of cortical rachis keratin of rectrices of the Toco Toucan (Ramphastos toco).

Abstract: A B S T R A C T Mechanical characterization of the cortex of rectrices (tail feathers) of the Toco Toucan (Ramphastos toco) has been carried out by tensile testing of the rachis cortex in order to systematically determine Young’s modulus and maximum tensile strength gradients on the surfaces and along the length of the feather. Of over seventy-fi ve samples tested, the average Young’s modulus was found to be 2.56 ± 0.09 GPa, and maximum tensile strength was found to be 78 ± 6 MPa. The Weibull modulus for all sets is greater than one and less than four, indicating that measured strength is highly variable. The highest Weibull moduli were reported for dorsal samplings. Dorsal and ventral surfaces of the cortex are both signifi cantly stiffer and stronger than lateral rachis cortex. On the dorsal surface, cortex sampled from the distal end of the feather was found to be least stiff and weakest compared to that sampled from proximal and middle regions along the length of the feather. Distinctive fracture patterns correspond to failure in the superfi cial cuticle layer and the bulk of the rachis cortex. In the cuticle, where supramolecular keratinous fi bers are oriented tangentially, evidence of ductile tearing was observed. In the bulk cortex, where the fi bers are bundled and oriented longitudinally, patterns suggestive of near-periodic aggregation and brittle failure were observed. c

Growth and Collapse of Nanovoids in Tantalum Monocrystals Loaded at High Strain Rate

Abstract: Shock-induced spall in ductile metals is known to occur by the sequence of nucleation, growth and coalescence of voids, even in high purity monocrystals. However, the atomistic mechanisms involved are still not completely understood. The growth and collapse of nanoscale voids in tantalum are investigated under different stress states and strain rates by molecular dynamics (MD) simulations. Three principal mechanisms of deformation are identified and quantitatively evaluated: shear loop emission, prismatic loop formation, and twinning. Dislocation shear loops expand as expected from a crystallographic analysis, and their extremities remain attached to the void surface in tension (if there is no dislocation reaction or cross slip), but can detach in compression and form prismatic loops due to cross slip and reactions. Prismatic loops that detach from the void are also formed by reaction of multiple shear loops sharing the same <111< slip direction during hydrostatic loading. Nanotwins form preferably upon both uniaxial and hydrostatic tensile stress. The void-size effect on plasticity is studied via MD simulations and is modeled based on the shear loop emission mechanism. The stresses required for generation of a free surface step, dislocation and bow are calculated by continuum dislocation theory. The predictions agree well with MD simulation results.

Laser Compression of Nanocrystalline Tantalum

Abstract: Abstract Nanocrystalline tantalum (grain size ∼70 nm) prepared by severe plastic deformation (high-pressure torsion) from monocrystalline [1 0 0] stock was subjected to shock compression generated by high-energy laser (∼350–850 J), creating pressure pulses with initial duration of ∼3 ns and amplitudes of up to ∼145 GPa. The laser beam, with a spot radius of ∼1 mm, created a crater of significant depth (∼135 μm). Transmission electron microscopy revealed few dislocations within the grains and an absence of twins at the highest shock pressure, in contrast with monocrystalline tantalum. Hardness measurements were conducted and show a rise as the energy deposition surface is approached, evidence of shock-induced defects. The grain size was found to increase at a distance of 100 μm from the energy deposition surface as a result of thermally induced grain growth. The experimentally measured dislocation densities are compared with predictions using analyses based on physically based constitutive models, and the similarities and differences are discussed in terms of the mechanisms of defect generation. A constitutive model for the onset of twinning, based on a critical shear stress level, is applied to the shock compression configuration. The predicted threshold pressure at which the deviatoric component of stress for slip exceeds the one for twinning is calculated and it is shown that it is increased from ∼24 GPa for the monocrystalline to ∼150 GPa for the nanocrystalline tantalum (above the range of the present experiments). Calculations using the Hu–Rath analysis show that grain growth induced by the post shock-induced temperature rise is consistent with the experimental results: grains grow from 70 to 800 nm within the post-shock cooling regime when subjected to a laser pulse with energy of 684 J.