Search

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 compressive response of a three-dimensional (3D) non-interlaced composite comprising three orthogonal sets of carbon fibre tows within an epoxy matrix is analysed. First, the compressive response is measured in three orthogonal directions and the deformation/failure modes analysed by a combination of X-ray tomography and optical microscopy. In contrast to traditional unidirectional and two-dimensional (2D) composites, stable and multiple kinks (some of which zig-zag) form in the tows that are aligned with the compression direction. This results in an overall composite compressive ductility of about 10% for compression in the low fibre volume fraction direction. While the stress for the formation of the first kink is well predicted by a usual micro-buckling analysis, the composite displays a subsequent hardening response associated with formation of multiple kinks. Finite element (FE) calculations are also reported to analyse the compressive response with the individual tows modelled as anisotropic continua via a Hill plasticity model. The FE calculations are in good agreement with the measurements including prediction of multiple kinks that reflect from the surfaces of the tows. The FE calculations demonstrate that the three-dimensionality of the microstructure constrains the kinks and this results in the stable compressive response. In fact, the hardening and peak strength of these composites is not set by the tows in direction of compression, but rather set by the out-of-plane compressive response of the tows perpendicular to the compression direction.

We have investigated the penetration response of confined ceramic targets impacted by Tungsten long-rod projectiles in the so-called Lundberg test setup with the ceramic modelled using a mechanism-based constitutive model. The calculations accurately predict the three observed penetration regimes, viz. interface defeat, dwell followed by penetration and penetration with no/short dwell. More importantly, the calculations suggest that these regimes occur in both a ceramic target and a reference target where microcracking of the ceramic is artificially switched off. This is because penetration occurs by a spherical cavity expansion mechanism with the onset of continued penetration set by the attainment of either a critical size of a plastic or damage zone. The dwell time then correlates with the time required to establish this inelastic zone. The main influence of damage is to reduce the pressure required to setup this critical inelastic zone and thereby reduce the interface defeat velocity as well as increase the penetration rate for a given velocity above the interface defeat velocity. We further show that an increase in the downward force due to the backflow of the deforming impactor is not essential for a dwell regime to exist. The mechanism-based ceramic model includes three material length scales and we demonstrate that the penetration size effect reported in experiments is primarily associated with the length scale associated with rate dependent lattice plasticity and the critical size of the comminuted zone that is required to trigger granular flow. The material model is also used to probe the dependence of material properties such as strength and toughness on the penetration resistance. Two regimes of penetration, viz. a plasticity-dominated and a microcracking-dominated regime emerge from the calculations. These predictions suggest that increasing the strength rather than toughness of the Corbit-98 Alumina will have a greater beneficial influence on its penetration performance.

The response of dry and water saturated sand slugs impacting normally oriented and inclined rigid-stationary targets with four different surface coatings is measured with an emphasis on the quantification of the momentum transmitted from the slugs into the targets. The targets were coated with Alumina, PTFE, Aluminium or sand-paper layers in order to investigate the effect of varying surface hardness and surface roughness. In all the cases, the fraction of the slug momentum transferred into the target was equal for dry and water saturated sand slugs and also independent of the slug velocity over the range 73 m s − 1 − 137 m s − 1 that is investigated here. For normal impacts, the surface coatings had no measurable influence on the momentum transfer into the targets and this was attributed to the symmetry of the impact event. However, the break of symmetry in the inclined impact cases resulted in two non-zero components of the net transmitted momentum into the targets and a strong influence of the surface coatings. This is attributed to friction between the sand particles and the target surface with the resultant transmitted momentum increasing in the order Alumina to PTFE to Aluminium to sand-paper surface coatings. In all cases, the transmitted momentum was less than the corresponding value under normal impact. Coupled discrete particle/Lagrangian simulations of these experiments with the sand particles modelled as spheres captured the normal impact measurements with a high degree of fidelity. However, the simulations underestimated the transmitted momentum for the inclined impacts especially for the rough surface coatings such as the sand-paper: increasing the friction coefficient between the particles and the target in the simulations did not improve the predictions. We demonstrate that this discrepancy is due to the spherical particle assumption: in the experiments the sand particles are sub-spherical and this reduces the tendency of particles to roll on the target surface and thereby increases frictional interactions. Increasing the radius of gyration of particles decreased the discrepancy between the measurements and the predictions but yet could not accurately predict all components of the transmitted momentum. Most numerical calculations tend to use spherical particles to represent the impacting granular media. However, this study demonstrates the need to appropriately parameterise particle shape in such discrete particle calculations to accurately capture the granular media/structure interactions.

The only engineering materials with both high strength and toughness, and with densities less than 1000 kg m−3, are natural materials (woods) and some plastics. Cellular structures such as the octet lattice, when made from periodic arrangements of strong, low-density metallic trusses, are known to have high specific strengths and elastic moduli. However, much less is known of their resistance to fracture. Here we investigate the fracture toughness of a Ti-6Al-4V alloy octet-lattice truss structure manufactured using a ‘snap-fit’ method. The samples had densities between 360 and 855 kg m−3 (relative densities of 8–19%) and free truss lengths between 4 and 15 mm. Their fracture resistance was determined using the J-integral compliance method applied to single-edge notched bend specimens. The toughness is shown to increase linearly with the relative density and with the square root of the cell size, while the strength was confirmed to scale only with relative density and the strength of the solid. A moderate increase in resistance with crack length (an R-curve effect) was seen for the higher relative density and larger cell size samples. With a fracture toughness between 2 and 14 MPa m1/2 and a compressive strength between 20 and 70 MPa, these structures offer a new lightweight engineering material solution for use at temperatures up to 450 °C.

We explore the feasibility of cellular materials concepts for passive and active mitigation of blast overpressures. The passive approach requires a cellular medium that compresses at nominally constant stress and dissipates the kinetic energy acquired by an attached buffer plate. Provided the cellular material is not compressed beyond its densification strain, the transmitted pressure is approximately the dynamic crush strength of the medium. This can be set just below a damage threshold by appropriate selection of the cellular material, its topology and relative density. However, for many realistic blast scenarios, the thicknesses required to avoid excess densification are excessive. The alternative is a deployable, pre-compressed, cellular medium released just prior to the arrival of the blast-created impulse. This accelerates an attached buffer toward the blast and creates momentum opposing that acquired from the blast. Numerical simulations of the fully coupled fluid structure interaction in air show that momentum cancellation is feasible, enabling a protective structure having much smaller volume.

The article focuses on the structural performance of sandwich beams with hollow truss lattice cores made from a ductile stainless steel. The trusses are arranged in an orthogonal (cross-ply) configuration, in either ±45° (diamond) or 0°/90° (square) orientations with respect to the face sheets. The responses in shear, tension and compression, as well as simply supported and fully clamped bending, are measured for specimens with both core orientations. While the two cores perform equally well in compression, the diamond orientation exhibits higher shear strength but lower stretch resistance. For bend-dominated loadings of the sandwich beams, the core in the diamond orientation is preferred because of its higher shear strength. For stretch-dominated loadings encountered in large-displacement, fully clamped bending, the square orientation is superior. Models of core and beam yielding are used to rationalize these observations. Optimizations are then performed to identify strong lightweight designs and to enable performance comparisons with other sandwich structures.

Low-pressure sputtering and ionized vapor deposition processes create atomic fluxes with kinetic energies in the 1.0-20 eV (and above) range. The impact energy of these hyperthermal atoms significantly effects the surface morphology and structure of vapor deposited films. Recent molecular dynamics simulations of metal atom interactions with a metal surface have established the energy and angular dependence of many of the impact energy induced mechanisms of atomic assembly including biased diffusion, atomic reflection, resputtering, and thermal transient induced "athermal" diffusion. These four effects have been incorporated into an earlier two-dimensional kinetic Monte Carlo model that analyzes the thermally driven multipath diffusional processes active during vapor deposition (Y. G. Yang et al., Acta Mater., 45 (1997) 1445). The contributions of the energy-dependent mechanisms to surface morphology were found to grow in importance as the substrate temperature was reduced and/or as the rate of deposition increased. The simulation method- ology was used to establish functional dependence of surface roughness upon the atom's kinetic energy and its direction of incidence during the hyperthermal deposition of nickel vapor. The simulations reveal the existence of a minimum surface roughness at an incident angle which increased with impact atom kinetic energy. Modification of the impact energy is shown to be a viable means for controlling surface morphology during physical vapor deposition under high deposition rate, low deposition temperature growth conditions.  2001 Published by Elsevier Science Ltd on behalf of Acta Materialia Inc.

Anisotropic damage evolution in a 0°/90° laminated Nicalon ™ SiC fiber-reinforced calcium aluminosilicate (CAS) glass–ceramic composite during uniaxial tensile deformation has been investigated using a variety of non-invasive characterization techniques. The elastic constant reduction in the three principal directions was measured from in situ laser-generated ultrasonic velocity measurements in various sound propagation directions. They indicate that, in addition to a large drop in elastic stiffness in the loading direction, the constants characterizing the nominal elastic stiffness transverse to the loading direction were also degraded. Surface replicas taken intermittently during loading revealed that transverse softening of the elastic stiffness was associated with fiber/matrix interface damage mainly in the 0° plies, while the large softening of the elastic stiffness in the loading direction was the result of multiple matrix cracking in both the 0° and 90° plies. While the ultrasonic data allowed a detailed characterization of the anisotropic damage evolution in this laminate, acoustic emission measurements and surface replica data identified the crack initiation stress in the 90° plies and correlated it to macroscopically observable deviations of the stress–strain curve from linear elastic behavior. These matrix cracks were found to have initiated preferentially in the weak 90° plies near the 90°/0° ply boundaries.

Damage transverse to the loading direction of a unidirectionally reinforced ceramic matrix composite has been observed and characterized by ultrasonic methods and acetate replicas. A laser-ultrasonic technique approach was developed and used to measure the ultrasonic velocity anisotropy, thus the complete elastic stiffness tensor, during uniaxial tensile loading/unloading of calcium aluminosilicate (CAS) composites reinforced by Nicalon™ SiC fibers. Acoustic emission and surface replica observations link degradation of the axial stiffness to the accumulation of matrix cracks normal to the loading (and fiber) direction. The substantial loss of transverse stiffness is proposed to originate from the fiber/matrix interface debonding near matrix cracks. These observations may significantly impact the constitutive models used to design multiaxially loaded ceramic matrix composites.

A team had been assembled to address challenges presented by future generations of hydrocarbon-fueled, long-range expendable missiles that operate up to Mach 7. The core issues involve robust materials and designs capable of sustaining pressure loads and extreme heat flux as well as shape-morphing components that allow efficient flight scenarios. The concepts invoke active cooling of the combustion system as well as passive cooling, through the use of heat pipes, of leading edges. The emphasis is on refractory alloys, coated to provide oxidation protection. Structural concepts leading to near-zero thermal expansion are also pursued. The document highlights the principal challenges and the key accomplishments of the program. Additional details are, found in the published papers, listed at the end of the document.