Andy R. Harland

Nonperturbing measurements of spatially distributed underwater acoustic fields using a scanning laser Doppler vibrometer

Localized changes in the density of water induced by the presence of an acoustic field cause perturbations in the localized refractive index. This relationship has given rise to a number of nonperturbing optical metrology techniques for recording measurement parameters from underwater acoustic fields. A method that has been recently developed involves the use of a Laser Doppler Vibrometer (LDV) targeted at a fixed, nonvibrating, plate through an underwater acoustic field. Measurements of the rate of change of optical pathlength along a line section enable the identification of the temporal and frequency characteristics of the acoustic wave front. This approach has been extended through the use of a scanning LDV, which facilitates the measurement of a range of spatially distributed parameters. A mathematical model is presented that relates the distribution of pressure amplitude and phase in a planar wave front with the rate of change of optical pathlength measured by the LDV along a specifically orientated laser line section. Measurements of a 1 MHz acoustic tone burst generated by a focused transducer are described and the results presented. Graphical depictions of the acoustic power and phase distribution recorded by the LDV are shown, together with images representing time history during the acoustic wave propagation.

Application and assessment of laser Doppler velocimetry for underwater acoustic measurements

Abstract The majority of traditional methods for making underwater acoustic pressure measurements involve placing all or part of a measurement transducer in the acoustic field. A variety of optical metrology techniques have been developed in an attempt to reduce or remove any perturbing effects. An example of this is the use of laser interferometry which has been developed as the primary method of calibrating hydrophones in the frequency range 500 kHz – 20 MHz at the National Physical Laboratory (NPL). This technique involves suspending a thin Mylar pellicle in the acoustic field and recording the displacement of the pellicle surface using a Michelson Interferometer. This study details a comparison of a Laser Doppler Velocimeter (LDV) with the NPL Laser Interferometer, which gives a good correlation where agreement is within approximately 4% and 7% for two different power levels from a 500 kHz plane piston transducer and within 2.5% and 1% for the same power levels from a 1 MHz plane piston transducer. A novel, non-perturbing method of recording temporally resolved acoustic pressure distributions in water using an LDV is also described. The technique is shown to benefit from the consistent frequency response of the LDV detection system, such that the measured output resembles the drive voltage input to the transducer more closely than a similar hydrophone measurement. For the experimental arrangement described, the LDV system is shown to be sensitive to minimum pressure amplitudes of nominally 18.9 mPa /√ Hz .

Experimental and Numerical Analysis of Damage in Woven GFRP Composites Under Large-deflection Bending

Textile-reinforced composites such as glass fibre-reinforced polymer (GFRP) used in sports products can be exposed to different in-service conditions such as large bending deformation and multiple impacts. Such loading conditions cause high local stresses and strains, which result in multiple modes of damage and fracture in composite laminates due to their inherent heterogeneity and non-trivial microstructure. In this paper, various damage modes in GFRP laminates are studied using experimental material characterisation, non-destructive micro-structural damage evaluation and numerical simulations. Experimental tests are carried out to characterise the behaviour of these materials under large-deflection bending. To obtain in-plane shear properties of laminates, tensile tests are performed using a full-field strain-measurement digital image correlation technique. X-ray micro computed tomography (Micro CT) is used to investigate internal material damage modes – delamination and cracking. Two-dimensional finite element (FE) models are implemented in the commercial code Abaqus to study the deformation behaviour and damage in GFRP. In these models, multiple layers of bilinear cohesive-zone elements are employed to study the onset and progression of inter-ply delamination and intra-ply fabric fracture of composite laminate, based on the X-ray Micro CT study. The developed numerical models are capable to simulate these features with their mechanisms as well as subsequent mode coupling observed in tests and Micro CT scanning. The obtained results of simulations are in agreement with experimental data.

The aerodynamic performance of a range of FIFA-approved footballs

Much discussion surrounds the flight of a football, especially that which is perceived as irregular, and is typically done so with little understanding of the aerodynamic effects or substantive evidence of the path taken. This work establishes that for a range of FIFA-approved balls there is a significant variation in aerodynamic performance. This paper describes the methods used for mounting stationary and spinning footballs in a wind tunnel enabling accurate force data to be obtained, and the analysis techniques used. The approach has been to investigate a number of scenarios: the non-spinning Reynolds sweep, unsteady loads, orientation sensitivity (yaw sweep), and the spinning Reynolds sweep. The techniques are applied to a number of footballs with differing constructions and the results reported. To put the aerodynamic data into context, the results are applied in a flight model to predict the potential differences in the behaviour of each ball in the air. This paper concludes that, although the drag characteristics are different for each different ball tested, the simulation suggests that this has only a limited effect on the flight of the ball. It is also shown that the unsteadiness of the aerodynamic loads is unlikely to be responsible for unpredictable behaviour. However, it is also shown that there are significant differences in the lateral aerodynamic forces for a range of FIFA-approved match balls, and that these aerodynamic differences have a significant effect on the flight path for both spinning and slowly rotating balls.

Ball Launch Characteristics for Elite Rugby Union Players

The role played by a team’s kicker in determining the outcome of a rugby union match is becoming increasingly important. However, unlike in other sports, there is no existing data regarding the kicking and passing abilities of elite rugby players. The objective of this study was to determine the launch characteristics of a place kick, drop kick, spiral kick (kick to touch) and spin pass. Testing was carried out at senior English League rugby union clubs, and data from 14 elite kickers were evaluated including current international players. The subjects were asked to perform the different kicks on a specially marked rugby ball at a distance of 60 m from the posts. Each skill was performed until they had achieved five ‘good’ strikes or passes. A high speed camera (NAC 500), operating at 500 frames per second was used to record the ball velocity, spin and launch angle. The data presented shows that players are able to achieve velocities of 38.1 m/s whilst imparting 405 rpm to a rugby ball (drop kick). The maximum spin rates seen in the other types of kick are considerably lower. The study of the spin pass has shown that whilst players impart considerably lower levels of velocity to the ball (18.3 m/s), they are capable of achieving spin rates similar to those seen for a place and spiral kick.

Computational modelling of manually stitched soccer balls

Abstract Soccer is the major ball sport, which attracts both viewers and participants worldwide. The primary equipment requirement is the ball, and the soccer ball market is as competitive as the game itself. The global interest of the sport provides an ideal international stage for the marketing of sports equipment such as soccer balls, boots, gloves, shin guards, and apparel. Multinational sports equipment manufacturers strive for superior product performance to enable commercial advantage, and the design and development of soccer balls is a crucial activity in achieving this. This work details the development of a finite-element (FE) model of a manually stitched textile-reinforced 32-panel ball used in an elite competition. A basic icosahedron FE model was produced to describe a thin-walled isotropic homogeneous shell, and this was subsequently enhanced to include a skeletal-like stiffer stitching region. Material testing was carried out to determine the mechanical properties of the bladder, outer panel, and stitching materials and prescribed within each model by a hyperelastic strain energy potential equation. A stiffness proportional damping coefficient was also used to describe kinetic energy loss characteristics. Each model was validated by an experimental impact testing under dynamic conditions that are representative of play, evaluation of coefficient of restitution, contact time, and deformation. It was found that the advanced soccer ball model accurately represented the deformation behaviour of the ball throughout impact.

Analysis of Nonlinear Shear Deformations in CFRP and GFRP Textile Laminates

Carbon fibre-reinforced polymer (CFRP) and glass fibre-reinforced polymer (GFRP) woven composites are widely used in aerospace, automotive and construction components and structures thanks to their lower production costs, higher delamination and impact strengths. They can also be used in various products in sports industry. These products are exposed to different in-service conditions such as large tensile and bending deformations. Composite materials, especially ±45° symmetric laminates subjected to tensile loads, exhibit significant material as well as geometric non-linearity before damage initiation, particularly with respect to shear deformations. Such a nonlinear response needs adequate means of analysis and investigation, the major tools being experimental tests and numerical simulations. This research deals with modelling the nonlinear deformation behaviour of CFRP and GFRP woven laminates subjected to in-plane tensile loads. The mechanical behaviour of woven laminates is modelled using nonlinear elasto-plastic as well as material models for fabrics in commercial finite-element code Abaqus. A series of tensile tests is carried out to obtain an in-plane full-field strain response of [+45/-45]2s CFRP and GFRP laminates using digital image correlation technique according to ASTM D3518/D3518M-94. The obtained results of simulations are in good agreement with experimental data.

Analysis of nonlinear deformations and damage in CFRP textile laminates

Carbon fibre-reinforced polymer (CFRP) textile composites are widely used in aerospace, automotive and construction components and structures thanks to their relatively low production costs, higher delamination and impact strength. They can also be used in various products in sports industry. These products are usually exposed to different in-service conditions such as large bending deformation and multiple impacts. Composite materials usually demonstrate multiple modes of damage and fracture due to their heterogeneity and microstructure, in contrast to more traditional homogeneous structural materials like metals and alloys. Damage evolution affects both their in-service properties and performance that can deteriorate with time. These damage modes need adequate means of analysis and investigation, the major approaches being experimental characterisation, numerical simulations and microtomography analysis. This research deals with a deformation behaviour and damage in composite laminates linked to their quasi-static bending. Experimental tests are carried out to characterise the behaviour of woven CFRP material under large-deflection bending. Two-dimensional finite element (FE) models are implemented in the commercial code Abaqus/Explicit to study the deformation behaviour and damage in woven CFRP laminates. Multiple layers of bilinear cohesive-zone elements are employed to model the onset and progression of inter-ply delamination process. X-ray Micro-Computed Tomography (MicroCT) analysis is carried out to investigate internal damage mechanisms such as cracking and delaminations. The obtained results of simulations are in agreement with experimental data and MicroCT scans.

Damage in woven CFRP laminates subjected to low velocity impacts

Carbon fabric-reinforced polymer (CFRP) composites used in sports products can be exposed to different in-service conditions such as large dynamic bending deformations caused by impact loading. Composite materials subjected to such loads demonstrate various damage modes such as matrix cracking, delamination and, ultimately, fabric fracture. Damage evolution in these materials affects both their in-service properties and performance that can deteriorate with time. These processes need adequate means of analysis and investigation, the major approaches being experimental characterisation and non-destructive examination of internal damage in composite laminates. This research deals with a deformation behaviour and damage in woven composite laminates due to low-velocity dynamic out-of-plane bending. Experimental tests are carried out to characterise the behaviour of such laminates under large- deflection dynamic bending in un-notched specimens in Izod tests using a Resil Impactor. A series of low-velocity impact tests is carried out at various levels of impact energy to assess the energy absorbed and force-time response of CFRP laminates. X-ray micro computed tomography (micro-CT) is used to investigate material damage modes in the impacted specimens. X-ray tomographs revealed that through-thickness matrix cracking, inter-ply delamination and intra-ply delamination, such as tow debonding and fabric fracture, were the prominent damage modes.

Advanced finite-element modelling of a 32-panel soccer ball

Abstract The current paper details the development of a finite-element model of a manually stitched, textile reinforced 32-panel soccer ball used in elite competition. The model included material anisotropy, a stiffer region representative of the polyester fibre based stitching, and a latex bladder membrane which was pressurized through inflation. A stiffness proportional damping coefficient was included within the material model to describe kinetic energy loss characteristics. The model was validated through experimental impact testing at speeds representative of play. It was found that the combined effects of material anisotropy, panel stitching, and panel configuration had a profound effect on the impact characteristics of the ball.