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

Development of a test method for assessing laceration injury risk of individual cleats during game-relevant loading conditions

Professional rugby union players experience an injury once every 10 matches, and up to 23% of these injuries are skin lacerations. Current regulations to assess laceration injury risk of cleated footwear involve two optional mechanical tests for manufacturers: a drop test and a pendulum test. However, there is limited rationale for these tests and associated impact parameters. A questionnaire among 191 rugby players showed that the ruck is the most prevalent game scenario in which skin laceration injuries occur. During the ruck, laceration injuries result from stamping movements by players wearing cleated footwear. A biomechanical study was conducted to obtain game-relevant impact parameters of stamping in the ruck. Eight participants were asked to perform 10 stamps on an anthropomorphic test device. Kinetic and kinematic data were clustered – identifying two distinct phases of the stamp motion – providing test parameters for mechanical assessment of skin laceration risk. A two-phase mechanical test was designed to quantify laceration injury risk of individual cleats. Phase one represents initial impact and phase two represents the subsequent raking motion as observed in the biomechanical study. Each phase is based on the impact parameters of observed stamping impacts. The developed test method has the potential to be adapted as an international standard for assessing laceration injury risk of cleated footwear. Future research is required to assess the repeatability of this method and its sensitivity to laceration injury

Development of a method for measuring quasi-static stiffness of snowboard wrist protectors

In snowboarding, the wrist is the most common injury site, as snowboarders often put their arms out to cushion a fall. This can result in a compressive load through the carpals coupled with wrist hyperextension, leading to sprains or fractures. Wrist protectors are worn by snowboarders in an effort to reduce injury risk, by decreasing impact forces and limiting wrist hyperextension during falls. However, there is no international standard or universally-accepted performance specification that these products should conform to, resulting in an inability to judge which design elements offer the most protection. EN 14120:2003 prescribes requirements that roller sports wrist protectors should meet, and has been identified as a starting point for developing a snowboarding-specific standard. This paper critiques the EN 14120:2003 test protocol and goes on to present a mechanical test for assessing the ability of snowboard wrist protectors to resist extension of the hand under an applied load. A bespoke rig incorporating the hand/arm surrogate from EN 14120:2003 was mounted to a uniaxial test machine, and wrist protectors were strapped to the surrogate at a set tightness (tight, moderate, loose). Linear displacement of the uniaxial test machine was transferred to angular displacement of the hand via a galvanised steel cable passing through a low friction pulley. Linear displacement was set to 200 mm/min and force was measured at the load cell until 80 N was reached. The test, presented here, found that the ability of the protectors to limit hand extension was dependent on how tightly they were fitted to the surrogate; therefore, strap tightness must be accounted for during further wrist protector safety assessments. This test provides a repeatable way to characterise the ability of snowboarding wrist protectors to limit wrist extension

Effect of surrogate design on the measured stiffness of snowboarding wrist protectors

In snowboarding, the wrist is the most common injury site, as snowboarders often put their arms out to cushion a fall. This can result in a compressive load through the carpals coupled with wrist hyperextension, leading to ligament sprains or carpal and forearm bone fractures. Wrist protectors are worn by snowboarders in an effort to reduce injury risk, by decreasing peak impact forces and limiting wrist extension to prevent hyperextension during falls. There is no international standard or universally accepted performance specification that snowboarding wrist protectors should conform to, resulting in an inability to judge which designs offer the best protection. This study investigated how surrogate arm design affected the stiffness of wrist protectors during quasi-static mechanical testing. Three surrogate arms with increasing design complexity were used to test three wrist protectors. The results show that surrogate design does influence the stiffness of snowboarding wrist protectors. Given that the surrogate does influence protector performance, it is recommended that a standard surrogate design is established for research and product testing

Application of auxetic foam in sports helmets

This investigation explored the viability of using open cell polyurethane auxetic foams to augment the conformable layer in a sports helmet and improve its linear impact acceleration attenuation. Foam types were compared by examining the impact severity on an instrumented anthropomorphic headform within a helmet consisting of three layers: a rigid shell, a stiff closed cell foam, and an open cell foam as a conformable layer. Auxetic and conventional foams were interchanged to act as the helmet’s conformable component. Attenuation of linear acceleration was examined by dropping the combined helmet and headform on the front and the side. The helmet with auxetic foam reduced peak linear accelerations (p < 0.05) relative to its conventional counterpart at the highest impact energy in both orientations. Gadd Severity Index reduced by 11% for frontal impacts (38.9 J) and 44% for side impacts (24.3 J). The conformable layer within a helmet can influence the overall impact attenuating properties. The helmet fitted with auxetic foam can attenuate impact severity more than when fitted with conventional foam, and warrants further investigation for its potential to reduce the risk of traumatic brain injuries in sport specific impacts

Methods for estimating moment of inertia of cricket bats

Mass moment of inertia is a key inertial property of cricket bats and should be used in selection to optimise performance. Players currently rely on a subjective assessment of how the bat feels when swung supported only by a value for bat mass from the manufacturer. This reliance on a subjective assessment is because the moment of inertia of a bat typically requires a pendulum method to measure with sufficient accuracy. In this study, two methods for estimating moment of inertia were tested. The hypotheses were that (1) an acceptable estimate of moment of inertia could be calculated using a beam model approach, and (2) the inertial property first moment could act as a proxy measure for moment of inertia. Experimental values for moment of inertia were obtained using a pendulum method. The two-section beam model showed an error of 0.43–0.53% between model and experimental values based on a Root Mean Square Error of 0.0017 kg m2. First moment data were generated on 5005 bats spread across eight bat shapes. A correlation was shown between the measured value of first moment and the beam model value of moment of inertia, with an R2 value &gt; 0.992 for all bat shapes. This study showed that a two-section beam model and first moment method for estimating cricket bat moment of inertia could be used to improve bat selection

Comparison of depth cameras for three-dimensional Reconstruction in Medicine

KinectFusion is a typical three-dimensional reconstruction technique which enables generation of individual three-dimensional human models from consumer depth cameras for understanding body shapes. The aim of this study was to compare three-dimensional reconstruction results obtained using KinectFusion from data collected with two different types of depth camera (time-of-flight and stereoscopic cameras) and compare these results with those of a commercial three-dimensional scanning system to determine which type of depth camera gives improved reconstruction. Torso mannequins and machined aluminium cylinders were used as the test objects for this study. Two depth cameras, Microsoft Kinect V2 and Intel Realsense D435, were selected as the representatives of time-of-flight and stereoscopic cameras, respectively, to capture scan data for the reconstruction of three-dimensional point clouds by KinectFusion techniques. The results showed that both time-of-flight and stereoscopic cameras, using the developed rotating camera rig, provided repeatable body scanning data with minimal operator-induced error. However, the time-of-flight camera generated more accurate three-dimensional point clouds than the stereoscopic sensor. Thus, this suggests that applications requiring the generation of accurate three-dimensional human models by KinectFusion techniques should consider using a time-of-flight camera, such as the Microsoft Kinect V2, as the image capturing sensor

1956 Ruby Yearbook

A digitized copy of the 1956 Ruby, the Ursinus College yearbook.https://digitalcommons.ursinus.edu/ruby/1059/thumbnail.jp

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