Visualising scattering underwater acoustic fields using laser Doppler vibrometry

Analysis of acoustic wavefronts are important for a number of engineering design, communication and healthrelated reasons, and it is very desirable to be able to understand the interaction of acoustic fields and energy with obstructions. Experimental analysis of acoustic wavefronts in water has traditionally been completed with single or arrays of piezoelectric or magnetostrictive transducers or hydrophones. These have been very successful, but the presence of transducers within the acoustic region can in some circumstances be undesirable. The research reported here, describes the novel application of scanning laser Doppler vibrometry to the analysis of underwater acoustic wavefronts, impinging on circular cross section obstructions. The results demonstrate that this new non-invasive acoustics measurement technique can successfully visualise and measure reflected acoustic fields, diffraction and refraction effects

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

Dynamic bending behaviour of woven composites for sports products: experiments and damage analysis

© 2015 Elsevier Ltd. Carbon fabric-reinforced polymer (CFRP) laminates employed in sports products are usually subjected to large-deflection quasi-static and dynamic bending deformations during service. Such loading conditions induce damage within the material affecting its strength, stiffness and energy-absorbing capability. To study this, mechanical behaviour of woven CFRP composites in on- and off-axis orientations is first quantified by carrying out large-deflection quasi-static bending tests followed by dynamic ones employing an Izod type impact tester. CFRP laminates of various orientations were tested at loads increasing up to failure to determine their energy-absorbing capability. On-axis laminates demonstrated better strength and stiffness whereas off-axis laminates exhibited good energy-absorbing capability. However, for applications demanding strength, stiffness and energy absorption as in sports products, a combination of both types of plies, as in a quasi-isotropic layup, is an optimum choice. Micro-computed tomography (micro-CT) analysis of the tested specimens showed that matrix cracking, delamination and tow debonding were the dominant damage modes at the specimen's impact location, whereas fabric fracture occurred at the bending location. Further, a catastrophic brittle fracture was observed in the on-axis laminates whereas the off-axis laminates exhibited pseudo-ductile behaviour thanks to matrix cracking and fibre trellising before their failure at higher energies

Characterisation of mechanical behaviour and damage analysis of 2D woven composites under bending

In this paper, flexural loading of woven carbon fabric-reinforced polymer laminates is studied using a combination of experimental material characterisation, microscopic damage analysis and numerical simulations. Mechanical behaviour of these materials was quantified by carrying out tensile and large-deflection bending tests. A substantial difference was found between the materials' tensile and flexural properties due to a size effect and stress stiffening of thin laminates. A digital image-correlation technique capable of full-field strain-measurement was used to determine in-plane shear properties of the studied materials. Optical microscopy and micro-computed tomography were employed to investigate deformation and damage mechanisms in the specimens fractured in bending. Various damage modes such as matrix cracking, delaminations, tow debonding and fibre fracture were observed in these microstructural studies. A two-dimensional finite-element (FE) model was developed to analyse the onset and propagation of inter-ply delamination and intra-ply fabric fracture as well as their coupling in the fractured specimen. The developed FE model provided a correct prediction of the material's flexural response and successfully simulated the sequence and interaction of damage modes observed experimentally

The effects of different delivery methods on the movement kinematics of elite cricket batsmen in repeated front foot drives

The aim of this paper was to examine differences in delivery characteristics and the resulting response exhibited by ten elite cricket batsmen when hitting repeated front foot drives against three different ball delivery methods; a bowling machine, a Sidearm™ ball thrower and a bowler. Synchronous three-dimensional Vicon motion capture technology and high-speed video were used to track batsman, bat and ball motion, and a range of discrete timing and kinematic variables were extracted from the resulting biomechanical model. Results showed significant differences in speed and ball release-to-impact time between the three delivery methods, thus questioning the validity of the bowling machine and Sidearm™ in the way they are currently used as true representations of batting against a real life bowler. Findings from the timing and kinematics of the subjects’ movements suggest a different technical response is also exhibited when facing the different delivery methods; for example batters were found to initiate movement earlier and have a lower maximum bat speed against the bowling machine, but initiate and complete their front foot stride earlier as well as moving their COM further forward in the Sidearm™ trials. © 2014 The Authors. Published by Elsevier Ltd

Evolution and interaction of damage modes in fabric-reinforced composites under dynamic flexural loading

In this paper, an experimental study is performed to characterise the behaviour of fabric-reinforced composites used in sports products under large-deflection bending in Izod-type impact tests. X-ray micro computed tomography (micro-CT) is used to investigate various damage modes in the impacted CFRP specimens. It revealed that matrix cracking, delaminations, tow debonding, and fibre fracture were the prominent damage modes. Three-dimensional finite-element models are developed to study the onset, progression and interaction of some damage modes such as delamination and fabric fracture observed with micro-CT. A damage modelling technique based on a cohesive-zone method, which is more efficient than continuum damage mechanics approach, is proposed for analysis of interaction of damage modes. The developed numerical models are capable to simulate the damage mechanisms and their interaction observed in the tests. In this study, the pattern of damage formation observed in specimens was front-to-back, unlike bottom-to-top one in drop weight impact tests. The effect of boundary conditions on the dynamic response and damage evolution of composite laminates is also investigated. © 2013 Elsevier Ltd

The effects of different delivery methods on the movement kinematics of elite cricket batsmen in repeated front foot drives

The aim of this paper was to examine differences in delivery characteristics and the resulting response exhibited by ten elite cricket batsmen when hitting repeated front foot drives against three different ball delivery methods; a bowling machine, a Sidearm™ ball thrower and a bowler. Synchronous three-dimensional Vicon motion capture technology and high-speed video were used to track batsman, bat and ball motion, and a range of discrete timing and kinematic variables were extracted from the resulting biomechanical model. Results showed significant differences in speed and ball release-to-impact time between the three delivery methods, thus questioning the validity of the bowling machine and Sidearm™ in the way they are currently used as true representations of batting against a real life bowler. Findings from the timing and kinematics of the subjects’ movements suggest a different technical response is also exhibited when facing the different delivery methods; for example batters were found to initiate movement earlier and have a lower maximum bat speed against the bowling machine, but initiate and complete their front foot stride earlier as well as moving their COM further forward in the Sidearm™ trials. © 2014 The Authors. Published by Elsevier Ltd

Movement and gesture recognition using deep learning and wearable-sensor technology

Pattern recognition of time-series signals for movement and gesture analysis plays an important role in many fields as diverse as healthcare, astronomy, industry and entertainment. As a new technique in recent years, Deep Learning (DL) has made tremendous progress in computer vision and Natural Language Processing (NLP), but largely unexplored on its performance for movement and gesture recognition from noisy multi-channel sensor signals. To tackle this problem, this study was undertaken to classify diverse movements and gestures using four developed DL models: a 1-D Convolutional neural network (1-D CNN), a Recurrent neural network model with Long Short Term Memory (LSTM), a basic hybrid model containing one convolutional layer and one recurrent layer (C-RNN), and an advanced hybrid model containing three convolutional layers and three recurrent layers (3+3 C-RNN). The models will be applied on three different databases (DB) where the performances of models were compared. DB1 is the HCL dataset which includes 6 human daily activities of 30 subjects based on accelerometer and gyroscope signals. DB2 and DB3 are both based on the surface electromyography (sEMG) signal for 17 diverse movements. The evaluation and discussion for the improvements and limitations of the models were made according to the result

A study of football footwear bending stiffness

For the past century, football boots have constantly changed, with new designs and materials being developed by sporting goods manufacturers to meet player demands. Investigating the effect of such changes on performance was important to further develop football boots. Bending stiffness has been demonstrated as an important performance parameter in running footwear; a concept with potential to investigate football footwear. As football boots consisted of an upper and an outsole, with the latter being manufactured from a far stiffer material than the upper, it could be speculated that the outsole limits the boot's flexibility. The aim of this paper was to create a testing device to quantify whether the whole shoe needed to be considered when designing football boots. The test device consisted of a simplified foot based on biomechanical factors linked to an Instron machine to quantify the load during bending. The bending stiffness of the uppers and boots were found to be non-linear, highlighting the need to specify the angle at which the stiffness was recorded. The upper was shown to have a significant effect on the bending stiffness of the overall boot construction, whilst different upper constructions were shown to affect the bending stiffness. With regard to the findings in this paper, it was concluded that the upper was an important consideration in boot design. © 2014 Published by Elsevier Ltd

Characterisation of ball degradation events in professional tennis

Tennis balls are acknowledged to degrade with use and are replaced at regular intervals during professional matches to maintain consistency and uniformity in performance, such that the game is not adversely affected. Balls are subject to the international tennis federation’s (ITF) ball approval process, which includes a degradation test to ensure a minimum standard of performance. The aim of this investigation was to establish if the ITF degradation test can assess ball longevity and rate of degradation and determine if there is a need for a new degradation test that is more representative of in-play conditions. Ball tracking data from four different professional events, spanning the three major court surfaces, including both men’s and women’s matches were analysed. The frequency of first serves, second serves, racket impacts and surface impacts were assessed and the corresponding distribution of ball speed and (for surface impacts) impact angle was determined. Comparison of ball impact frequency and conditions between in-play data and the ITF degradation test indicated the development of a new test, more representative of in-play data, would be advantageous in determining ball longevity and rate of degradation with use. Assessment of data from different surfaces highlighted that grass court subjected the ball to fewer racket and surface impacts than hard court or clay. In turn, this appears to influence the distribution of ball speed on impact with the surface or racket, suggesting a surface-specific degradation test may be beneficial. As a result of these findings a new test protocol has been proposed, utilising the in-play data, to define the frequency of impacts and impact conditions to equate to nine games of professional tennis across the different surfaces