Deformation mechanisms leading to auxetic behaviour in the α-cristobalite and α-quartz structures of both silica and germania

Analytical expressions have been developed in which the elastic behaviour of the α-quartz and α-cristobalite molecular tetrahedral frameworks of both silica and germania are modelled by rotation, or dilation or concurrent rotation and dilation of the tetrahedra. Rotation and dilation of the tetrahedra both produce negative Poisson’s ratios (auxetic behaviour), whereas both positive and negative values are possible when these mechanisms act concurrently. Concurrent rotation and dilation of the tetrahedra reproduces with remarkable accuracy both the positive and negative nu31 Poisson’s ratios observed in silica α-quartz and α-cristobalite, respectively, when loaded in the x3 direction. A parametric fit of the concurrent model to the germania α-quartz experimental nu31 Poisson’s ratio is used to predict nu31 for germania α-cristobalite, for which no experimental value exists. This is predicted to be +0.007. Strain-dependent nu31 trends, due to concurrent rotation and dilation in the silica structures, are in broad agreement with those predicted from pair-potential calculations, although significant differences do occur in the absolute values. Concurrent dilation and rotation of the tetrahedra predicts that an alternative uniaxial stress (sigma3)-induced phase exists for both silica α-quartz and α-cristobalite and germania α-cristobalite, having geometries in reasonable agreement with beta-quartz and idealised beta-cristobalite, respectively

Piezomorphic materials

The development of stress-induced morphing materials which are described as piezomorphic materials is reported. The development of a piezomorphic material is achieved by introducing spatial dependency into the compliance matrix describing the elastic response of a material capable of undergoing large strain deformation. In other words, it is necessary to produce an elastically gradient material. This is achieved through modification of the microstructure of the compliant material to display gradient topology. Examples of polymeric (polyurethane) foam and microporous polymer (expanded polytetrafluoroethylene) piezomorphic materials are presented here. These materials open up new morphing applications where dramatic shape changes can be triggered by mechanical stress

Elastic constants of 3-, 4- and 6-connected chiral and anti-chiral honeycombs subject to uniaxial in-plane loading

Finite Element models are developed for the in-plane linear elastic constants of a family of honeycombs comprising arrays of cylinders connected by ligaments. Honeycombs having cylinders with 3, 4 and 6 ligaments attached to them are considered, with two possible configurations explored for each of the 3- (trichiral and anti-trichiral) and 4- (tetrachiral and anti-tetrachiral) connected systems. Honeycombs for each configuration have been manufactured using rapid prototyping and subsequently characterised for mechanical properties through in-plane uniaxial loading to verify the models. An interesting consequence of the family of 'chiral' honeycombs presented here is the ability to produce negative Poisson's ratio (auxetic) response. The deformation mechanisms responsible for auxetic functionality in such honeycombs are discussed

The in-plane linear elastic constants and out-of-plane bending of 3-coordinated ligament and cylinder-ligament honeycombs

Four novel cylinder-ligament honeycombs are described, where each cylinder has 3 tangentially-attached ligaments to form either a hexagonal or re-entrant hexagonal cellular network. The re-entrant cylinder-ligament honeycombs are reported for the first time. The in-plane linear elastic constants and out-of-plane bending response of these honeycombs are predicted using Finite Element (FE) modelling and comparison made with hexagonal and re-entrant hexagonal honeycombs without cylinders. A laser-crafted re-entrant cylinder-ligament honeycomb is manufactured and characterized to verify the FE model. The re-entrant honeycombs display negative Poisson's ratios and synclastic curvature upon out-of-plane bending. The hexagonal and 'trichiral' honeycombs possess positive Poisson's ratios and anticlastic curvature. The 'anti-trichiral' honeycomb (short ligament limit) displays negative Poisson's ratios when loaded in the plane of the honeycomb, but positive Poisson's ratio behaviour (anticlastic curvature) under out-of-plane bending. These responses are understood qualitatively through considering deformation occurs via direct ligament flexure and cylinder rotation-induced ligament flexure

DIFFERENTIATION OF THE ENERGETIC PARAMETERS BETWEEN MALE AND FEMALE ELITE POLE VAULTERS

This study aimed to examine whether differences were evident in the average energetic waveforms and energy summary results between four male and four female elite pole vaulters. Full body DXA scans and 3D body surface scans were conducted for each of the athletes to allow subject specific body segment inertial parameters to be calculated. A total of 67 vaults were recorded across the 8 vaulters, with a 3D kinematic analysis being performed on each vault. Translational and rotational kinetic energy; and gravitational potential energy were derived from this analysis and used to calculate total vaulter energy. Average summary energy values and energetic waveforms were compared between the male and female vaulters. Significant differences were found in some of the initial energy values as well as variables related to the pole extension phase

AN ANALYSIS OF DIFFERENT POLE VAULTING POLE LOAD-DEFORMATION TESTING REGIMES

This study aimed to determine whether differences in the pole elastic energy characterisation were evident between quasi-static and dynamic load-deformation testing on pole vault poles. A total of 12 pole vault poles were tested from three different manufacturers, who each utilise different materials within their pole construction processes. A quasi-static and a dynamic pole bending test was performed on each pole utilising a custom built rig. A 3D load cell was positioned in series with the rig to obtain the forces required to develop a load-deformation curve for each trial. The results of the bending trials demonstrate a noticeable difference in the load-deformation curves obtained via the two methods, most notably regarding the degree of hysteresis between the loading and unloading phases and the resultant elastic potential energy calculations

Follow-up services for improving long-term outcomes in intensive care unit (ICU) survivors

This is a protocol for a Cochrane Review (Intervention). The objectives are as follows: Our main objective is to assess the effectiveness of follow-up services for ICU survivors that aim to identify and address unmet health needs related to the ICU period. We aim to assess the effectiveness in relation to health-related quality of life, mortality, depression and anxiety, post-traumatic stress disorder, physical function, cognitive function, ability to return to work or education and adverse events. Our secondary objectives are, in general, to examine both the various ways that follow-up services are provided and any major influencing factors. Specifically, we aim to explore: the effectiveness of service organisation (physician versus nurse led, face to face versus remote, timing of follow-up service); possible differences in services related to country (developed versus developing country); and whether participants had delirium within the ICU setting

Fabrication, characterisation and modelling of uniform and gradient auxetic foam sheets

Large sheets of polyurethane open-cell foam were compressed (or stretched) using pins and a conversion mould whilst undergoing thermal softening and controlled cooling. Sheets (final dimensions 355 x 344 x 20 mm) were fabricated with uniform triaxial compression, with and without through-thickness pins, and also with different compression regimes (uniform triaxial compression or through-thickness compression and biaxial planar tension) in opposing quadrants. The samples fabricated under uniform triaxial compression with and without pins exhibited similar cell structure and mechanical properties. The sheets fabricated with graded compression levels displayed clearly defined quadrants of differing cell structure and mechanical properties. The graded foam quadrants subject to triaxial compression displayed similar cell structure, tangent moduli and negative Poisson’s ratio responses to the uniform foams converted with a similar level of triaxial compression. The graded foam quadrants subject to through-thickness compression and biaxial planar tension displayed a slightly re-entrant through-thickness cell structure contrasting with an in-plane structure resembling the fully reticulated cell structure of the unconverted parent foam. This quadrant of graded foam displayed positive and negative Poisson’s ratios in tension and compression, respectively, accompanied by high and low in-plane tangent modulus, respectively. The strain-dependent mechanical properties are shown to be fully consistent with expectations from honeycomb theory. The triaxially compressed quadrants of the graded sheet exhibited ~4 times lower peak acceleration than quadrants with through-thickness compression and biaxial planar tension in 6 J impact tests using a steel hemispherical drop mass

A comparison of novel and conventional fabrication methods for auxetic foams for sports safety applications

This study compares fabrication methods for auxetic foam intended for use in sports safety equipment. Thermo-mechanical conversion methods were applied using: i) cubic moulds (150x150x150 mm), ii) cuboidal moulds (150x150x30 mm) & iii) cuboidal moulds (150x150x30 mm) with through-thickness pins. The cuboidal moulds having one reduced dimension relative to the cubic moulds enable faster heat transfer and more consistent through-thickness compression to the foam during conversion. The through-thickness pins allow greater control of in-plane compression throughout the bulk of the converted foam. Samples were compared using: i) density measurements and measurements of total surface folding (length multiplied by depth), ii) quasistatic compressive load/unload tests to obtain specific strain energy, stress/strain relationship and Poisson's ratio, iii) impact testing on a bespoke drop rig based on a standard for cricket pads (BS 6183-3: 2000, EN 2001) at 5, 10 and 15 J & iv) microscopic images of dissected samples to confirm their auxetic (re-entrant) structure. Samples fabricated in cuboidal moulds show less variation in final density, axial compressive stiffness and specific strain energy between samples than those cut from monoliths fabricated in cubic moulds. Samples created with through-thickness pins exhibited reduced surface folding. Greater control over final properties paves the way for further work designing auxetic foams for sport safety equipment