1,721,043 research outputs found
Measuring strains of LDPE films: The strain gauge problems
No full textLow-density polyethylene (LDPE) films are the most widely used plastic covering materials of greenhouses in Mediterranean Europe. Research concerning these plastic films is an important issue in order to formulate technical suggestions for a reliable design, to make more efficient use of the film and to achieve a longer useful lifetime of LDPE films. Along this line, the elastic mechanical behaviour of LDPE films has been investigated experimentally and numerically. In the course of the experimental work, different approaches were used to measure the strain at selected points of the LDPE film under pressure and also in attempting to measure the Poisson's ratio. In the present work difficulties leading to failure in the measurement of strains of LDPE films are presented along with the analysis concerning the failure of commercial strain gauges for the measurement of strain of LDPE plastic films. © 2002 Elsevier Science Ltd. All rights reserved
Analysis and design of low-density polyethylene greenhouse films
No full textIn the present paper, the mechanical behaviour of low-density polyethylene (LDPE) films under various combinations of pre-tension and uniform pressure schemes is investigated experimentally and numerically using the finite element method of analysis. The behaviour of the film is simulated by means of numerical models and with the material properties obtained in the laboratory by using standard testing methods. The finite element models used include both a commercial finite element program and a recently developed research non-linear finite shell element, capable of modelling membrane behaviour. The numerical analysis results obtained under appropriate boundary conditions and different analysis Options are compared with experimental results obtained from a specifically designed experimental arrangement. For the cases tested experimentally, the two numerical approaches gave results in a good agreement with the experiment results, in the linear elastic region. Subsequently, using the research finite element model, design criteria are developed for the reliable design of LDPE greenhouse films. © 2003 Silsoe Research Institute. All rights reserved. Published by Elsevier Science Ltd
Radiometric and thermal properties of, and testing methods for, greenhouse covering materials
No full textIn this paper, the most critical physical properties for the greenhouse covering materials are reviewed. The
physical properties considered include the radiometric properties (transmissivity, re#ectivity and absorptivity)
over several radiation bands and also the K-value (the overall heat transfer coecient
for greenhouse covering materials. Also new testing methods should be elaborated, and specially designed for
greenhouse covering materials
Using Video Podcasts to Enhance International Student Collaboration in Biosystems Engineering
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Design and Use Criteria of Netting Systems for Agricultural Production in Italy
No full textAt the moment, there is a large number of agricultural net typologies on the market characterised by different texture features such as threads, texture, dimensions of fibres and meshing, physical properties such as weight, colours, shading factor, durability, porosity, air permeability and mechanical characteristics such as stress, strength at break and elongation. Protection from hail, wind, snow, or strong rainfall in fruit-farming and ornamentals, shading nets for greenhouses, or nets for moderately modifying the microenvironment are the most common applications. A systematic review of the current state-of-the-art of structural parameters, standard and regulations, most common agricultural applications of nets and their supporting structures has been developed by means of a literature study, technical investigations, interviews with permeable covering producers, specialised greenhouse builders and growers, who are familiar with permeable cladding applications. The interviews were based on questionnaires concerning characteristics, use and disposal of nets. As a result, the survey highlighted that in many cases different, not even similar, net typologies were adopted for the same application and the same cultivations by various growers. Results show that neither growers nor net producers have clear ideas about the relationship between the net typology optimisation for a specific function and the construction parameters of the net. The choice still depends often on empirical or economic criteria and not on scientific considerations. Moreover, it appears that scientifically justified technical requirements for nets used in specific agricultural applications have not been established yet
Going Beyond Counting First Authors in Author Co-citation Analysis
Get PDFThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Plastic Nets in Agriculture: A General Review of Types and Applications
No full textAt the moment, there are a large number of agricultural net types on the market characterized by different structurla features such as type of material, type and dimensions of threads, texture, mesh size, porosity / soliddity and weight; by radiometric properties like color, transmissivity/reflectivity/shading factor; by physical properties like air permeability and several mechanical characteristics such as tensile stress, strength, elongation at break, and durability. Protection from modifying the microenvironment for a crop are the most common applications. A systematic review of the current state-of-the-art of structural parameters, standard and regulations, most common agricultural net applications, and their supporting structures has been developed by means of a literature study, technical investigations, concerning characteristics and use of nets. As a result, the survey highlighted that in many cases different, not even similar, net types were adopted for the same application and the same cultivations by various growers. Results show that neither growers nor net producers have clear ideas about the relationship between the net typology optimization for a specific application and the construction parameters of the net. The choice often depends on empirical or economic criteria and not on scientific considerations. Moreover, it appears that scientifically justified technical requirements for nets used in specific agricultural applications have not been established yet
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