Fabrication of auxetic foam sheets for sports applications

Auxetic materials have a negative Poisson's ratio, which can enhance other properties. Greater indentation resistance and energy absorption, as well as synclastic curvature, could lend auxetic materials well to protective sports equipment and clothing. Sheets of foam often form padding within protective equipment, but producing large homogenous auxetic foam samples is challenging. The aim of this work was to investigate techniques to fabricate large thin sheets of auxetic foam, to facilitate future production and testing of prototype sports equipment utilizing this material. A mold was developed to fabricate sheets of auxetic foam − with planar dimensions measuring 350 × 350 mm − using the thermo-mechanical process. The mold utilized through-thickness rods to control lateral compression of foam. Sheets of auxetic foam measuring 10 × 350 × 350 mmd were fabricated and characterized. Each sheet was cut into nine segments, with density measurements used to determine how evenly the foam had been compressed during fabrication. Specimens cut from corner and centre segments were subject to quasi-static extension up to 30% to obtain stress versus strain relationships, with Digital Image Correlation used to determine Poisson's ratio. Specimens cut from the corner tended to have a marginally higher density, lower stiffness and more consistent negative Poisson's ratio compared to those from the centre, indicating some inconsistency in the conversion process. Future work could look to improve fabrication techniques for large thin homogenous sheets of auxetic foam

Efficient modal identification and optimal sensor placement via dynamic DIC measurement and feature-based data compression

Full-field non-contact vibration measurements provide a rich dataset for analysing structural dynamics. However, implementing the identification algorithm directly using high-spatial resolution data can be computationally expensive in modal identification. To address this challenge, performing identification in a shape-preserving but lower-dimensional feature space is more feasible. The full-field mode shapes can then be reconstructed from the identified feature mode shapes. This paper discusses two approaches, namely data-dependent and data-independent, for constructing the feature spaces. The applications of these approaches to modal identification on a curved plate are studied, and their performance is compared. In a case study involving a curved plate, it was found that a spatial data compression ratio as low as 1% could be achieved without compromising the integrity of the shape features essential for a full-field modal. Furthermore, the paper explores the optimal point-wise sensor placement using the feature space. It presents an alternative, data-driven method for optimal sensor placement that eliminates the need for a normal model, which is typically required in conventional approaches. Combining a small number of point-wise sensors with the constructed feature space can accurately reconstruct the full-field response. This approach demonstrates a two-step structural health monitoring (SHM) preparation process: offline full-field identification of the structure and the recommended point-wise sensor placement for online long-term monitoring

Damage identification of wind turbine blades - a brief review

The increasing size of these blades of wind turbines emphasises the need for reliable monitoring and maintenance. This brief review explores the detection and analysis of damage in wind turbine blades. The study highlights various techniques, including acoustic emission analysis, strain signal monitoring, and vibration analysis, as effective approaches for damage detection. Vibration analysis, in particular, shows promise for fault identification by analysing changes in dynamic characteristics. Damage indices based on modal properties, such as natural frequencies, mode shapes, and curvature, are discussed

Damage identification in complex structures using vibration data

This study presents a damage identification procedure for complex structures based on analysing changes in vibration data between healthy and damaged conditions. The procedure involves calculating and comparing the first six mode shapes of both the intact and damaged structures using finite element analysis. The case study of 5Mega Watt National Renewable Energy Laboratory offshore wind turbine blade has been used to demonstrate the application of the procedure. The main objective is to utilise this procedure to identify and evaluate the severity of damage in different scenarios. Additionally, the procedure can be applied at various stages to detect and identify early signs of damage, serving as an early warning system

Effect of Surrogate Surface Compliance on the Measured Stiffness of Snowboarding Wrist Protectors

Wrist injuries have been reported to account for 35%−45% of snowboarding injuries. Snowboarding wrist protectors are designed to limit impact forces and prevent wrist hyperextension. The absence of a standard for snowboarding wrist protectors makes it hard to identify models offering an adequate level of protection. Wrist surrogates are well suited for testing and benchmarking wrist protectors. This study investigated the effect of introducing a soft tissue simulant onto an otherwise stiff wrist surrogate on the bending stiffness of snowboarding wrist protectors. A compliant surrogate (stiff core and 3 mm thick silicone layer) and a comparable stiff surrogate were fabricated. Two snowboarding wrist protectors were tested on each surrogate, under three strapping conditions, following a bend test to ~80° wrist extension. The introduction of a compliant layer to the wrist surrogate gave higher torque values for a given wrist extension angle, increasing protector effective stiffness, relative to a rigid surrogate

Prosumers Matching and Least-Cost Energy Path Optimisation for Peer-to-Peer Energy Trading

Potential benefits of peer-to-peer energy trading and sharing (P2P-ETS) include the opportunity for prosumers to exchange flexible energy for additional income, whilst reducing the carbon footprint. Establishing an optimal energy routing path and matching energy demand to supply with capacity constraints are some of the challenges affecting the full realisation of P2P-ETS. In this paper, we proposed a slime-mould inspired optimisation method for addressing the path cost problem for energy routing and the capacity constraint of the distribution lines for congestion control. Numerical examples demonstrate the practicality and flexibility of the proposed method for a large number of peers (15 – 2000) over existing optimised path methods. The result shows up to 15% cost savings as compared to a non-optimised path. The proposed method can be used to control congestion on distribution links, provide alternate paths in cases of disruption on the optimal path, and match prosumers in the local energy market

The sensitivity of 5MW wind turbine blade sections to the existence of damage

Due to the large size of offshore wind turbine blades (OWTBs) and the corrosive nature of salt water, OWTs need to be safer and more reliable that their onshore counterparts. To ensure blade reliability, an accurate and computationally efficient structural dynamic model is an essential ingredient. If damage occurs to the structure, the intrinsic properties will change, e.g., stiffness reduction. Therefore, the blade’s dynamic characteristics will differ from those of the intact ones. Hence, symptoms of the damage are reflected in the dynamic characteristics that can be extracted from the damaged blade. Thus, damage identification in OWTBs has become a significant research focus. In this study, modal model characteristics were used for developing an effective damage detection method for WTBs. The technique was used to identify the performance of the blade’s sections and discover the warning signs of damage. The method was based on a vibration-based technique. It was adopted by investigating the influence of reduced blade element rigidity and its effect on the other blade elements. A computational structural dynamics model using Rayleigh beam theory was employed to investigate the behaviour of each blade section. The National Renewable Energy Laboratory (NREL) 5MW blade benchmark was used to demonstrate the behaviour of different blade elements. Compared to previous studies in the literature, where only the simple structures were used, the present study offers a more comprehensive method to identify damage and determine the performance of complicated WTB sections. This technique can be implemented to identify the damage’s existence, and for diagnosis and decision support. The element most sensitive to damage was element number 14, which is NACA_64_618

Study of centrifugal stiffening on the free vibrations and dynamic response of offshore wind turbine blades

Due to their large and increasing size and the corrosive nature of salt water and high wind speeds, offshore wind turbines are required to be more robust, more rugged and more reliable than their onshore counterparts. The dynamic characteristics of the blade and its response to applied forces may be influenced dramatically by rotor rotational speed, which may even threaten the stability of the wind turbine. An accurate and computationally efficient structural dynamics model is essential for offshore wind turbines. A comprehensive model that takes the centrifugal stiffening effect into consideration could make rapid and accurate decisions with live data sensed from the structure. Moreover, this can enhance both the performance and reliability of wind turbines. When a rotating blade deflects in its plane of rotation or perpendicular to it, the centrifugal force exerts an inertia force that increases the natural frequencies and changes the mode shapes, leading to changes in the dynamic response of the blade. However, in the previous literature, studies of centrifugal stiffening are rarely found. This study investigates the influence of centrifugal stiffening on the free vibrations and dynamic response of offshore wind turbine blades. The National Renewable Energy Laboratory (NREL) 5 MW blade benchmark was considered to study the effect of angular speed in the flap-wise and edge-wise directions. The results demonstrate that the angular speed directly affects the modal features, which directly impacts the dynamic response. The results also show that the angular velocity effect in the flap-wise direction is more significant than its effect in the edge-wise direction

Dynamic responses analysis of a 5MW NREL wind turbine blade under flap-wise and edge-wise vibrations

A wind turbine is subjected to a regime of varying loads. For example, each rotor revolution causes a complete gravity stress reversal in the low-speed shaft, and there are varying stresses from the out-of-plane loading cycle due to fluctuating wind load. Consequently, wind turbine blade design is governed by fatigue rather than ultimate load considerations. Previous studies have adopted many different beam theories, using different techniques and codes, to model the National Renewable Energy Laboratory (NREL) 5 MW offshore wind turbine blade. There are differences, from study to study, in the free vibration results and the dynamic response. The contribution of this study is to apply the code written by the authors to the different beam theories used with the aim of comparing the different beam theories presented in the literature and that developed by the authors. This paper reports the investigation of the effects of deformation parameters on the dynamic characteristics of the NREL 5 MW offshore wind turbine blades predicted by the different beam theories. The investigation of free vibrations is a fundamental step in the analysis of structural dynamics, and this study cmpares different computational structural methods and investigates their effect on the predicted dynamic response. The modal characteristics of every model examined have been combined with strip theory to determine the dynamic response of the blade