Deep penetration of ultra-high molecular weight polyethylene composites by a sharp-tipped punch

The file attached to this record is the author's final peer reviewed version. The Publisher's final version can be found by following the DOI link.The penetration of unidirectional (UD) and [0o/90o] cross-ply ultra-high molecular weight polyethylene fibre composites by sharp-tipped cylindrical punches has been investigated. While the measured penetration pressure for both composite types increased with decreasing punch diameter, the pressure was significantly higher for the cross-ply composites and increased with decreasing ply thickness. A combination of optical microscopy and X-ray tomography revealed that in both composites, the sharp-tipped punch penetrated without fibre fracture by the formation of mode-I cracks along the fibre directions, followed by the wedging open of the crack by the advancing punch. In the cross-ply composites, delamination between adjacent 0o and 90o plies also occurred to accommodate the incompatible deformation between plies containing orthogonal mode-I cracks. Micromechanical models for the steady-state penetration pressure were developed for both composites. To account for material anisotropy as well as the large shear strains and fibre rotations, the deformation of the composites was modelled via a pressure-dependent crystal plasticity framework. Intra and inter-ply fracture were accounted for via mode-I and delamination toughnesses respectively. These models account for the competition between deformation and fracture of the plies and accurately predict the measured steady-state penetration pressures over the wide range of punch diameters and ply thicknesses investigated here. Design maps for the penetration resistance of cross-ply composites were constructed using these models and subsequently used to infer composite designs that maximise the penetration resistance for a user prescribed value of fibre strength

Thermal stress effects in intermetallic matrix composites

Intermetallic matrix composites develop residual stresses from the large thermal expansion mismatch (delta-alpha) between the fibers and matrix. This work was undertaken to: establish improved techniques to measure these thermal stresses in IMC's; determine residual stresses in a variety of IMC systems by experiments and modeling; and, determine the effect of residual stresses on selected mechanical properties of an IMC. X ray diffraction (XRD), neutron diffraction (ND), synchrotron XRD (SXRD), and ultrasonics (US) techniques for measuring thermal stresses in IMC were examined and ND was selected as the most promising technique. ND was demonstrated on a variety of IMC systems encompassing Ti- and Ni-base matrices, SiC, W, and Al2O3 fibers, and different fiber fractions (Vf). Experimental results on these systems agreed with predictions of a concentric cylinder model. In SiC/Ti-base systems, little yielding was found and stresses were controlled primarily by delta-alpha and Vf. In Ni-base matrix systems, yield strength of the matrix and Vf controlled stress levels. The longitudinal residual stresses in SCS-6/Ti-24Al-llNb composite were modified by thermomechanical processing. Increasing residual stress decreased ultimate tensile strength in agreement with model predictions. Fiber pushout strength showed an unexpected inverse correlation with residual stress. In-plane shear yield strength showed no dependence on residual stress. Higher levels of residual tension led to higher fatigue crack growth rates, as suggested by matrix mean stress effects

Ultrasonic Methods for Characterizing the Interface in Composites

Micromechanical modeling studies of composite materials have highlighted the need for better characterization of the interface zone in composite materials. Bulk behavior in composites has been predicted to be strongly influenced by the local elastic properties, residual stresses, and adhesion of the interface. Techniques to nondestructively measure these newly perceived quantities of importance do not exist. Thus it is not possible experimentally to (i) confirm the micromechanical model predictions, (ii) explore the relationships between interface properties and processing variables, and (iii) ensure acceptable interface properties in commercially fabricated composites. We report here the current status of a SDIO/ONR funded research program directed at developing experimental techniques for characterizing the interface zone in composites through the use of ultrasonic interface waves [1]

Surface instabilities in shock loaded granular media

The file attached to this record is the author's final peer reviewed version. The Publisher's final version can be found by following the DOI link.The initiation and growth of instabilities in granular materials loaded by air shock waves are investigated via shock-tube experiments and numerical calculations. Three types of granular media, dry sand, water-saturated sand and a granular solid comprising PTFE spheres were experimentally investigated by air shock loading slugs of these materials in a transparent shock tube. Under all shock pressures considered here, the free-standing dry sand slugs remained stable while the shock loaded surface of the water-saturated sand slug became unstable resulting in mixing of the shocked air and the granular material. By contrast, the PTFE slugs were stable at low pressures but displayed instabilities similar to the water-saturated sand slugs at higher shock pressures. The distal surfaces of the slugs remained stable under all conditions considered here. Eulerian fluid/solid interaction calculations, with the granular material modelled as a Drucker–Prager solid, reproduced the onset of the instabilities as seen in the experiments to a high level of accuracy. These calculations showed that the shock pressures to initiate instabilities increased with increasing material friction and decreasing yield strain. Moreover, the high Atwood number for this problem implied that fluid/solid interaction effects were small, and the initiation of the instability is adequately captured by directly applying a pressure on the slug surface. Lagrangian calculations with the directly applied pressures demonstrated that the instability was caused by spatial pressure gradients created by initial surface perturbations. Surface instabilities are also shown to exist in shock loaded rear-supported granular slugs: these experiments and calculations are used to infer the velocity that free-standing slugs need to acquire to initiate instabilities on their front surfaces. The results presented here, while in an idealised one-dimensional setting, provide physical understanding of the conditions required to initiate instabilities in a range of situations involving the explosive dispersion of particles

Indentation response of a 3D non-woven carbon-fibre composite

The file attached to this record is the author's final peer reviewed version. The Publisher's final version can be found by following the DOI link.The indentation response of a 3D noninterlaced composite comprising three sets of orthogonal carbon-fibre tows in an epoxy matrix is investigated. The 3D composites have a near isotropic and ductile indentation response. The deformation mode includes the formation of multiple kinks in the tows aligned with the indentation direction and shearing of the orthogonally oriented tows. Finite element (FE) calculations are also reported wherein tows in one direction are explicitly modeled with the other two sets of orthogonal tows and the matrix pockets treated as an effective homogenous medium. The calculations capture the indentation response in the direction of the explicitly modeled tows with excellent fidelity but under-predict the indentation strength in the other directions. In contrast to anisotropic and brittle laminated composites, 3D noninterlaced composites have a near isotropic and ductile indentation response making them strong candidates for application as materials to resist impact loading

Lithic technological responses to Late Pleistocene glacial cycling at Pinnacle Point Site 5-6, South Africa

There are multiple hypotheses for human responses to glacial cycling in the Late Pleistocene, including changes in population size, interconnectedness, and mobility. Lithic technological analysis informs us of human responses to environmental change because lithic assemblage characteristics are a reflection of raw material transport, reduction, and discard behaviors that depend on hunter-gatherer social and economic decisions. Pinnacle Point Site 5-6 (PP5-6), Western Cape, South Africa is an ideal locality for examining the influence of glacial cycling on early modern human behaviors because it preserves a long sequence spanning marine isotope stages (MIS) 5, 4, and 3 and is associated with robust records of paleoenvironmental change. The analysis presented here addresses the question, what, if any, lithic assemblage traits at PP5-6 represent changing behavioral responses to the MIS 5-4-3 interglacial-glacial cycle? It statistically evaluates changes in 93 traits with no a priori assumptions about which traits may significantly associate with MIS. In contrast to other studies that claim that there is little relationship between broad-scale patterns of climate change and lithic technology, we identified the following characteristics that are associated with MIS 4: increased use of quartz, increased evidence for outcrop sources of quartzite and silcrete, increased evidence for earlier stages of reduction in silcrete, evidence for increased flaking efficiency in all raw material types, and changes in tool types and function for silcrete. Based on these results, we suggest that foragers responded to MIS 4 glacial environmental conditions at PP5-6 with increased population or group sizes, 'place provisioning', longer and/or more intense site occupations, and decreased residential mobility. Several other traits, including silcrete frequency, do not exhibit an association with MIS. Backed pieces, once they appear in the PP5-6 record during MIS 4, persist through MIS 3. Changing paleoenvironments explain some, but not all temporal technological variability at PP5-6.Social Science and Humanities Research Council of Canada; NORAM; American-Scandinavian Foundation; Fundacao para a Ciencia e Tecnologia [SFRH/BPD/73598/2010]; IGERT [DGE 0801634]; Hyde Family Foundations; Institute of Human Origins; National Science Foundation [BCS-9912465, BCS-0130713, BCS-0524087, BCS-1138073]; John Templeton Foundation to the Institute of Human Origins at Arizona State Universit

Mechanical metamaterials at the theoretical limit of isotropic elastic stiffness

Acknowledgements H.N.G.W. is grateful for support for this work by the ONR (grant number N00014-15-1-2933), managed by D. Shifler, and the DARPA MCMA programme (grant number W91CRB-10-1-005), managed by J. Goldwasser.Peer reviewedPostprintPostprintPostprintPostprin

Sources of acoustic emission during fatigue of Ti-6Al-4V: effect of microstructure

The fundamentals of acoustic emission (AE) analysis of fatigue cracking were applied to Ti-6Al-4V. The effect of microstructure on the characteristics of the AE events generated and the failure mechanisms which produced AE in Ti-6Al-4V were established. Lamellar microstructures generated one to two orders of magnitude more emission than equiaxed microstructures. The combination of larger grain size, more continuous α/β interfaces, more tortuous crack-front geometry, cleavage and intergranular fracture in lamellar microstructures accounts for the greater amount of emission. For lamellar microstructures, most AE events were generated in the upper 20% of the stress range, whereas in equiaxed microstructures, most events were generated at lower stresses. Most AE events were generated during crack opening and also at low stresses. AE events having high level intensities were also generated at stresses other than the peak stress. This is because in titanium alloys, which have both high strength and toughness, AE events are generated from both plastic zone extension and crack extension.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/44721/1/10853_2004_Article_BF00542927.pd

Symmetry and topology in antiferromagnetic spintronics

Antiferromagnetic spintronics focuses on investigating and using antiferromagnets as active elements in spintronics structures. Last decade advances in relativistic spintronics led to the discovery of the staggered, current-induced field in antiferromagnets. The corresponding N\'{e}el spin-orbit torque allowed for efficient electrical switching of antiferromagnetic moments and, in combination with electrical readout, for the demonstration of experimental antiferromagnetic memory devices. In parallel, the anomalous Hall effect was predicted and subsequently observed in antiferromagnets. A new field of spintronics based on antiferromagnets has emerged. We will focus here on the introduction into the most significant discoveries which shaped the field together with a more recent spin-off focusing on combining antiferromagnetic spintronics with topological effects, such as antiferromagnetic topological semimetals and insulators, and the interplay of antiferromagnetism, topology, and superconductivity in heterostructures.Comment: Book chapte