211,486 research outputs found
Mechanisms for surface potential decay on fluorinated epoxy in high voltage DC applications
Full text linkEpoxy resin has been extensively used for decades as an insulation material in high voltage transmission systems. However, this insulation material does suffer from bulk and surface charging when used as insulating spacer, mainly in high voltage DC applications. By applying fluorination treatment, the surface of polymeric insulation is chemically treated and so modifies charge transport characteristics of the material. In doing so, excellent surface properties can be obtained without compromising the bulk characteristics of the polymeric insulation. In this paper, the authors investigate the surface potential decay performance of non-fluorinated and fluorinated epoxy resin samples. The surface decay performance of insulating material is a crucial parameter in dissipating accumulation of surface and bulk charge that can lead to premature breakdown of the insulating material. The epoxy samples were characterised by Energy Dispersive X-Ray (EDX) analysis to determine the changes in chemical composition of the samples before and after fluorination treatment. Surface potential decay measurement using positive corona discharging was then performed, followed by bulk DC conductivity measurement to further explain the mechanisms which govern the surface potential decay. The existence of surface-fluorinated layer on the treated samples had been found to play a major role in dictating the movement of charges away from the surface during the decay process. The influence of fluorination treatment on the decay mechanisms was discussed
Astragalus wui M. Idrees & Z. Y. Zhang 2021, nom. nov.
No full textAstragalus wui M. Idrees & Z.Y. Zhang, nom. nov. Replaced name:— Astragalus sylvaticus Y.H. Wu (2015: 718), nom. illeg., non A. sylvaticus (Pall.) Willd. (1802: 1300). Type:— CHINA. Xinjiang: Yecheng Country, Sukepiya, in border forest, alt. 3000 m, 15 Aug. 1987, Exped. Qinghai-Tibet Wu Yuhu 1067 (holotype: QTPMB, not seen). Etymology:—The specific epithet honours Prof. Dr. Wu Yuhu (Northwest Institute of Plateau Biology, Chinese Academy of Science, Xining, China), author of the replaced name, who first described this new species.Published as part of Idrees, Muhammad & Zhang, Zhiyong, 2021, Astragalus wui, a new replacement name for A. sylvaticus Y. H. Wu (Galegeae, Papilionoideae, Fabaceae), pp. 210-211 in Phytotaxa 524 (3) on page 210, DOI: 10.11646/phytotaxa.524.3.6, http://zenodo.org/record/564936
Numerical modelling of composite laminates with through-thickness-reinforcements
Full text linkThe main objective of the present research study was to develop numerical models to investigate the mechanical properties and effectiveness of z-fibre reinforced laminates. A survey of relevant literature on through-thickness reinforcements (TTR) was undertaken and z-fibre pinning was chosen as the main topic of study. The development of numerical tools was mainly based on the finite element (FE) method and was carried out at different model scale levels.
At a micro-mechanical level of analysis, two models were presented. Firstly a unit cell FE model based on the actual geometric configuration of a z-pinned composite was used. Calculations were performed to understand how the through-thickness reinforcement modified the engineering elastic constants and local stress distributions.
Secondly the study of an analytical micro-mechanical model was undertaken. The model simulated a z-fibre bridging a delamination crack tinder mixed-mode loads. A constitutive law relating the z-pin bridging forces with the crack displacements was defined as the "bridging law". Numerical examples for z-fibre bridging laws under Mode I and Mode II loads were computed along with design evaluations of the effect of several micro-mechanical parameters on the bridging laws. This analytical model was then implemented into a MATLAB code specifically written by the author. The code generated constitutive relationship for interface elements simulating the bridging laws of a single z-pin to be used in a FE analysis.
A detailed numerical study of the mode I interlaminar fracture of composite laminates with z-pins was then carried out. AFL• model of a double cantilever beam (DCB) was developed. The numerical analysis focused on the large scale bridging (LSB) caused by z-pins mechanics, which increased the laminate resistance against delamination growth. The numerical results were validated against experimental data. Computational curves for the energy balance and energy rates were also determined showing that the LSB process consumed a significant amount of irreversible energy. The assumption made by the linear elastic fracture mechanics (LEFM) that all energy dissipations were included in the fracture energy and confined within the damage front, was not valid for z-pinned laminates.
The FE analysis was then extended to study a curved single-lap shear joint, to prove the effectiveness of TTR against debond failure of the joint. The presence of TTR was shown to delay the propagation of the debonding and generally to enhance the load carrying capability of the joint. TTR is proved to be more effective in reducing the Mode I component of debonding driving force than that of the Mode II.
Finally a global-local approach was proposed to implement the TTR elements into large composite stnictural FE models. Possible future studies for TTR numerical modelling were also addressed
On the delamination suppression in structural joints by Z-fibre pinning
Full text linkThe main objective of this paper is to investigate the benefits of Z-fibre
pinning to improve the bonding strength of composite joints. The problem is
addressed from a design point of view in order to develop a simulation
methodology that can be employed to predict the strength of Z-fibre pinned
joints. Firstly, an efficient and accurate computational approach is presented
using the well established finite element method in conjunction with a
constitutive model of Z-fibre response behaviour under mixed mode loading
condition. The Z-fibre bridging model previously developed by the authors is
summarised in the paper. Secondly, the computational approach is demonstrated
via the analysis of two structural joints, namely a conventional T-joint and a
novel cruciform joint. Comparison with test data confirms the model’s predictive
capabili
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