Experimental characterisation of rate-dependent compression behaviour of fibre reinforced composites

Fibre reinforced polymers (FRP) materials are being increasingly used for aerospace and automotive structural applications. One of the critical loading conditions for such applications is impact, consequently, understanding of the composite behavior under such loads becomes critical for structural design. The analysis and design process for achieving impact-resistant composite structures requires rate-dependent constitutive models, which, in turn, requires material properties of the composite over a range of strain rates. It is, therefore, the objective of the research to investigate the strain rate-dependent behavior of fiber reinforced composites under compressive loads for a wide range of fiber orientations. Quasi-static (≈ 1e-3 s-1) and high loading (≈ 200 s-1) rates are considered for the experimental study. Accordingly, two different test setups are utilized, a screw-driven universal testing machine for quasi-static tests and a Split Hopkinson Pressure Bar (SHPB) system for dynamic tests. The stress-strain response of the composite is reported for the different fiber orientations and the strain rates, revealing the rate-dependent characteristics of the carbon fiber reinforced composite. From the test results, it is observed that, the dependency of the fracture strength on the loading rate is significant. The results are summarised in terms of the failure envelope in the transverse compression-in-plane shear σ22-σ12 plane for the two strain rates

Development and applications of Ion-selective electrodes

A new type of liquid state electrode assembly based on a natural rubber membrane and using a graphite rod as internal reference electrode, has been developed for use with organic salts which are soluble in organic solvent. Electrodes have been developed for determination of zinc(II)j perchlorate and tetrafluoroborate ions, using basic dye salts containing those arAons. In general, natural rubber sheeting is treated with a water immiscible organic solvent containing the salt; swollen sheeting is used as the membrane. These electrodes respond in a Nernstian manner, giving a potential-concentration slopes of 29.5,57.5 and 58.5 mV for zinc(II), perchlorate and tetrafluoroborate electrodes respectively. The slope is stable for a reasonable length of time, e.g. potential-concentration slope is decreased by 0.5 W after about three to five weeks, depending on the type of electrode. In the preparation of these electrodes, the choice of solvent and the kind of natural rubber has been carefully selected, as these factors influence the operation of the membrane. Effects of solution pH and the temperature variations on response of the electrodes, have also been studied. Further work also has been done on studying the Gmelin reaction (reaction of nitroprusside and sulphide ions) with a sulphide ionselective electrode. This reaction involves a study of the reaction between nitroprusside and sulphide ions, where nitrosyl group is modified to pentacyanothionitroferrate(II) imparting a purple colour to the solution. For determination of sulphide and nitroprusside by potentiometric titration optimum conditions have been developed. Also a colorimetric procedure for sulphide determination is recommended

A Wedge-DCB Test Methodology to Characterise High Rate Mode-I Interlaminar Fracture Properties of Fibre Composites

A combined numerical-experimental methodology is presented to measure dynamic Mode-I fracture properties of fiber reinforced composites. A modified wedge-DCB test using a Split-Hopkinson Bar technique along with cohesive zone modelling is utilised for this purpose. Three different comparison metrics, namely, strain-displacement response, crack propagation history and crack opening history are employed in order to extract unique values for the cohesive fracture properties of the delaminating interface. More importantly, the complexity of dealing with the frictional effects between the wedge and the DCB specimen is effectively circumvented by utilising right acquisition techniques combined with an inverse numerical modelling procedure. The proposed methodology is applied to extract the high rate interlaminar fracture properties of carbon fiber reinforced epoxy composites and it is further shown that a high level of confidence in the calibrated data can be established by adopting the proposed methodology

On the Rate-dependent Plasticity Modelling of Unidirectional Fibre-reinforced Polymeric Matrix Composites

Three different approaches to plasticity are investigated to model the experimentally-observed non-linear behaviour of unidirectional fibre-reinforced polymeric matrix materials. The first and simplest approach consists on assuming independent one-dimensional rate-dependent plasticity on in-plane (12) and through-thickness longitudinal (13) shear components of the Cauchy stress tensor. The second, employs a 3D extension of the plane stress Hill'48 anisotropic plastic surface. The third and the last is formulated as a quadratic yield function inspired by Puck's fracture initiation criterion. It searches for a plastic localisation plane in which a certain combination of normal and shear stresses is maximum. Numerical simulations are conducted to analyse the off-axis compression behaviour of carbon fibre reinforced epoxy composite under varying loading rates. The afore-mentioned three different approaches are explored with an aim to predict the experimentally-observed non-linear response of such composites. The model parameters are determined using a deterministic inverse modelling strategy employing an iterative domain reduction optimisation technique. As far as the experiments are concerned, the quasi-static and medium rate tests were carried out in universal testing machines, while the experiments at high rate were conducted in a Split Hopkinson Pressure Bar system. The effectiveness in terms of accuracy and robustness of the three approaches are discussed

Numerical predictions of the anisotropic viscoelastic response of uni-directional fibre composites

Finite Element (FE) simulations are conducted to predict the viscoelastic properties of uni-directional (UD) fibre composites. The response of both periodic unit cells and random stochastic volume elements (SVEs) is analysed; the fibres are assumed to behave as linear elastic isotropic solids while the matrix is taken as a linear viscoelastic solid. Monte Carlo analyses are conducted to determine the probability distributions of all viscoelastic properties. Simulations are conducted on SVEs of increasing size in order to determine the suitable size of a representative volume element (RVE). The predictions of the FE simulations are compared to those of existing theories and it is found that the Mori-Tanaka (1973) and Lielens (1999) models are the most effective in predicting the anisotropic viscoelastic response of the RVE

The valuation impact of gender quotas in the boardroom: Evidence from the European markets

\ua9 2023 The Author(s)We investigate stock market reactions to the announcement of the new, June 2022 European Union (EU) regulation on board gender diversity, which requires firms to appoint a minimum of 33% female directors (or 40% female non-executive directors). We find that the abnormal market returns surrounding the EU announcement are positive. We also note that the observed positive valuation effects are particularly strong for: (1) firms in countries with softer existing regulations on board gender diversity; and (2) firms with a larger gap between current levels of board gender diversity and the 33% gender quota. Our analysis of the EU legislation on gender quotas offers solid evidence that board gender quotas are perceived by investors as beneficial, particularly for firms exposed to a large gender imbalance

FinTech adoption in banks and their liquidity creation

\ua9 2024 The AuthorsUtilizing an innovative financial technology (FinTech) index based on media sources, we analyse the effects of FinTech adoption on bank liquidity creation for a sample of the top 300 United States banks from Q1 2015 to Q2 2021. Our findings reveal a consistent negative association between FinTech adoption and bank liquidity creation, even during the coronavirus disease (COVID-19) pandemic. This relationship remains robust after conducting multiple rigorous tests including propensity score matching and difference-in-differences tests to address endogeneity problems. Overall, these results underscore the transformative influence of FinTech on fundamental liquidity creation function within traditional banking

Detection and Elimination of Fake Access Points in WLAN using Multi Agents Sourcing MethodologyÂ

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Predictions of the mechanical properties of unidirectional fibre composites by supervised machine learning

We present an application of data analytics and supervised machine learning to allow accurate predictions of the macroscopic stiffness and yield strength of a unidirectional composite loaded in the transverse plane. Predictions are obtained from the analysis of an image of the material microstructure, as well as knowledge of the constitutive models for fibres and matrix, without performing physically-based calculations. The computational framework is based on evaluating the 2-point correlation function of the images of 1800 microstructures, followed by dimensionality reduction via principal component analysis. Finite element (FE) simulations are performed on 1800 corresponding statistical volume elements (SVEs) representing cylindrical fibres in a continuous matrix, loaded in the transverse plane. A supervised machine learning (ML) exercise is performed, employing a gradient-boosted tree regression model with 10-fold cross-validation strategy. The model obtained is able to accurately predict the homogenized properties of arbitrary microstructures