Matt Carré

Review of the frictional properties of finger-object contact when gripping

Abstract Gripping is an everyday task which is taken for granted by many. The current paper examines extant knowledge of howobjects are gripped for manipulation, and the relationship, the coefficient of friction, between finger and object, has on various grip parameters. It is found that friction is an essential part of the feedback and feedforward control system for grip. The friction of the finger-object contact can be measured in several different ways, using methods of either a probe moving on a finger or a finger moving on a flat surface. These friction measurements can then be used to gain information about the effect of normal force, sliding speed, contact area, and level of moisture present. They also can provide information about the changes between test subjects, for example, the effects of age, gender, and race. Knowing the effect of these parameters can help to improve the manoeuvrability of everyday items through inclusive design; designing products to be used by the whole population regardless of age or ability. The current paper also suggests areas of further investigation so that knowledge of skin friction can be expanded and applied to a larger range of materials and applications.

Trajectory analysis of a soccer ball

We performed experiments in which a soccer ball was launched from a machine while two cameras recorded portions of its trajectory. Drag coefficients were obtained from range measurements for no-spin trajectories, for which the drag coefficient does not vary appreciably during the ball’s flight. Lift coefficients were obtained from the trajectories immediately following the ball’s launch, in which Reynolds number and spin parameter do not vary much. We obtain two values of the lift coefficient for spin parameters that had not been obtained previously. Our codes for analyzing the trajectories are freely available to educators and students.

Impact of a non-homogeneous sphere on a rigid surface

Abstract This paper examines the impact behaviour of a non-homogenous sphere (in this case a cricket ball with a rolled core construction) with a rigid surface. Experiments were carried out to measure the force-deflection behaviour of a cricket ball during a normal impact in two orientations (impacting on the seam and perpendicular to the seam). For the two orientations of impact, a disparity was found in terms of the force-deflection behaviour. Greater deformation was found for impacts landing on the seam, compared to those landing perpendicular to the seam. Comparisons with quasi-static test data suggested that only the bottom third of the ball may have been compressed during impact. The dynamic force-deflection behaviour was modelled using a mass attached to a Hertzian spring in parallel with a damper whose damping coefficient varied with the contact area. The coefficients in the model could be described using the velocity before impact alone. The model was found to be in good agreement with the experimental data. The model was then extended to predict oblique impacts by incorporating a measured coefficient of friction. This performed well in predicting the rebound velocity, angle and spin of a cricket ball after oblique impact with a cricket pitch. Inconsistencies in the results were attributed to deformation in the pitch surface.

Understanding the Effect of Seams on the Aerodynamics of an Association Football

Abstract The aerodynamic properties of an association football were measured using a wind tunnel arrangement. A third scale model of a generic football (with seams) was used in addition to a ‘mini-football’. As the wind speed was increased, the drag coefficient decreased from 0.5 to 0.2, suggesting a transition from laminar to turbulent behaviour in the boundary layer. For spinning footballs, the Magnus effect was observed and it was found that reverse Magnus effects were possible at low Reynolds numbers. Measurements on spinning smooth spheres found that laminar behaviour led to a high drag coefficient for a large range of Reynolds numbers, and Magnus effects were inconsistent, but generally showed reverse Magnus behaviour at high Reynolds number and spin parameter. Trajectory simulations of free kicks demonstrated that a football that is struck in the centre will follow a near straight trajectory, dipping slightly before reaching the goal, whereas a football that is struck off centre will bend before reaching the goal, but will have a significantly longer flight time. The curving kick simulation was repeated for a smooth ball, which resulted in a longer flight time, due to increased drag, and the ball curving in the opposite direction, due to reverse Magnus effects. The presence of seams was found to encourage turbulent behaviour, resulting in reduced drag and more predictable Magnus behaviour for a conventional football, compared with a smooth ball.

The dynamic impact characteristics of tennis balls with tennis rackets

Abstract The dynamic properties of six types of tennis balls were measured using a force platform and high-speed digital video images of ball impacts on rigidly clamped tennis rackets. It was found that the coefficient of restitution reduced with velocity for impacts on a rigid surface or with a rigidly clamped tennis racket. Pressurized balls had the highest coefficient of restitution, which decreased by 20% when punctured. Pressureless balls had a coefficient of restitution approaching that of a punctured ball at high speeds. The dynamic stiffness of the ball or the ball-racket system increased with velocity and pressurized balls had the highest stiffness, which decreased by 35% when punctured. The characteristics of pressureless balls were shown to be similar to those of punctured balls at high velocity and it was found that lowering the string tension produced a smaller range of stiffness or coefficient of restitution. It was hypothesized that players might consider high ball stiffness to imply a high coefficient of restitution. Plots of coefficient of restitution versus stiffness confirmed the relationship and it was found that, generally, pressurized balls had a higher coefficient of restitution and stiffness than pressureless balls. The players might perceive these parameters through a combination of sound, vibration and perception of ball speed off the racket.

Soccer Ball Lift Coefficients via Trajectory Analysis.

We performed experiments in which a soccer ball was launched from a machine while two high-speed cameras recorded portions of the trajectory. Using the trajectory data and published drag coefficients, we extracted lift coefficients for a soccer ball. We determined lift coefficients for a wide range of spin parameters, including several spin parameters that have not been obtained by todays wind tunnels. Our trajectory analysis technique is not only a valuable tool for professional sports scientists, it is also accessible to students with a background in undergraduate-level classical mechanics.

High-speed observations of football-boot-surface interactions of players in their natural environment

A protocol has been developed to obtain two-dimensional kinematic shoe data of football players in their training environment through high-speed video analysis. Such kinematic data can provide an understanding of how to better replicate the boundary conditions of football movements when simulated using mechanical traction and penetration test devices. As part of a pilot study, 11 youth academy players from a Premiership football club performed football-specific movements which were filmed at 1000 frames s-1. The protocol required minimal set-up time and the area of the pitch to be filmed could be positioned in any part of the playing area, causing low disruption to the players. This aimed to ensure that the movements performed were representative of those carried out during competitive play. Results in this study are concerned with the kinematics of the shoe during contact with the ground for movements identified to be important in terms of injury risk and loss of performance (slipping). Shoe velocities and orientations were measured for subjects wearing shoes of different stud types (traditional round studs versus contemporary bladed studs) on two surfaces (artificial turf, in-filled with rubber and sand, versus a natural surface). All the parameters measured from the relatively small population of subjects had high variances and therefore few significant effects of studs and surface could be found. The data does however provide insight into the appropriate boundary conditions to be used in mechanical test devices. For example, in the forefoot push-off movement it can be seen that test devices should measure the traction forces when the shoe first starts to move, as this is when the player would lose performance, as opposed to the maximum traction which can occur after significant displacement of the shoe through the surface. Analysis of the orientation and velocity path of the studs just before contact with the ground shows that the studs could be aligned to enhance their penetration into the surface and optimise the traction properties of the studs. In order to determine the orientation and velocity of the shoes at crucial phases in movements force-plate data obtained in the laboratory should be utilised in future studies.

Science of synthetic turf surfaces: investigating traction behaviour

The traction forces produced between an athlete’s footwear and the playing surface are a crucial factor influencing a player’s performance. Four primary factors affecting traction have been identified from literature: the sports specific movement, the footwear, the playing surface, and the environment. Many authors have investigated traction behaviour mechanically, using a variety of shoe and surface types, concluding that the traction generated at the shoe–surface interface is dependent on each shoe–surface combination (see work by Gheluwe et al., Cawley et al., and Villwocket et al., details given in main text). There has been little attempt in the literature, however, to try and explain the behaviour of the surface from the traction resistance measurements that were observed, perhaps owing to the complex number of variables involved. Furthermore, the variety of methodologies used in past research makes it difficult directly to compare datasets. This paper presents datasets comparing the traction behaviour of several carefully prepared surface systems and states, using three mechanical test procedures, and investigates the factors influencing traction resistance. Results highlight that properties of synthetic turf carpets (fibres and tuft spacing) and the density state of the crumbed rubber infill component and the stud size and configuration influence the maximum traction forces generated at the shoe–surface interface. The magnitude of stud penetration under controlled vertical loading is also presented. The findings of this study further demonstrate the importance of understanding the detail of the surface system under test, and that the infill state has a measurable effect. The design and operation of mechanical traction measurement equipment also demonstrates influences on the traction values measured, and the largest influence is the normal load applied. The FIFA standard test is shown to be less sensitive to infill state than other tests. Recommendations are made for more robust testing methods for future research.

The development of an apparatus to understand the traction developed at the shoe–surface interface in tennis

The traction developed at the shoe–surface interface can have a significant influence on a player’s injury risk and performance in tennis. The purpose of this study was to investigate shoe–surface traction on a dry acrylic hard court and two artificial clay court tennis surfaces in dry and wet conditions. A laboratory-based mechanical test rig was developed to measure the traction force developed at the shoe–surface interface. Linear regression analysis was used to examine the relationship between normal force and three measures of traction: initial stiffness, peak traction force and average dynamic traction force. The normal force did not significantly influence the initial stiffness for the shoe–surface system on the acrylic hard court but did on the artificial clay surfaces. The infill particle size and the addition of moisture influenced the traction developed on the artificial clay surfaces. Small, dry particles developed greater traction and with a sufficiently high applied normal force will provide traction comparable to that on an acrylic hard court. However, increased particle size and/or the presence of moisture generally reduced traction. Strong and significant positive linear relationships were found between peak traction force and average dynamic traction force for all surface types and conditions. This study improves the understanding of the influence surface characteristics have on shoe–surface traction mechanisms. Once traction mechanisms are understood, surface properties and/or footwear can be effectively changed to maximise performance and/or minimise injury risk.

In vivo measurement of skin surface strain and sub-surface layer deformation induced by natural tissue stretching.

Stratum corneum and epidermal layers change in terms of thickness and roughness with gender, age and anatomical site. Knowledge of the mechanical and tribological properties of skin associated with these structural changes are needed to aid in the design of exoskeletons, prostheses, orthotics, body mounted sensors used for kinematics measurements and in optimum use of wearable on-body devices. In this case study, optical coherence tomography (OCT) and digital image correlation (DIC) were combined to determine skin surface strain and sub-surface deformation behaviour of the volar forearm due to natural tissue stretching. The thickness of the epidermis together with geometry changes of the dermal-epidermal junction boundary were calculated during change in the arm angle, from flexion (90°) to full extension (180°). This posture change caused an increase in skin surface Lagrange strain, typically by 25% which induced considerable morphological changes in the upper skin layers evidenced by reduction of epidermal layer thickness (20%), flattening of the dermal-epidermal junction undulation (45-50% reduction of flatness being expressed as Ra and Rz roughness profile height change) and reduction of skin surface roughness Ra and Rz (40-50%). The newly developed method, DIC combined with OCT imaging, is a powerful, fast and non-invasive methodology to study structural skin changes in real time and the tissue response provoked by mechanical loading or stretching.