D. Briassoulis

Equivalent orthotropic properties of corrugated sheets

Abstract The analysis of corrugated shells (plates) is based on the assumption that they can be analyzed as thin, equivalent orthotropic shells of uniform thickness. The analytical expressions for the equivalent rigidities of orthotropic thin shells given in the literature are reviewed. The results of a finite element analysis of a corrugated sheet subjected to constant strain states reveals an inadequacy in some of the classical expressions in use today. These equivalent orthotropic properties are improved with the derivation of new expressions. In addition, expressions for the localized stress concentrations developed at the ridges of the corrugations are derived.

Plastic Nets in Agriculture: A General Review of Types and Applications

At the moment, there are a large number of agricultural net types on the market characterized by different structural features such as type of material, type and dimensions of threads, texture, mesh size, porosity / solidity 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 hail, wind, snow, or strong rainfall in fruit-farming and ornamentals, shading nets for greenhouses and nets moderately 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.

Standard testing methods for mechanical properties and degradation of low density polyethylene (LDPE) films used as greenhouse covering materials: a critical evaluation

LDPE films are currently the most widespread greenhouse covering material in the countries of the Mediterranean region. Taking into account the size of this market, the effect of the material performance on the greenhouse production as well as the related environmental impact problems arising from its disposal, the standardization of the testing of these materials should have already been achieved. However, there are no standard methods available in the European Union for testing LDPE greenhouse films. Furthermore, for predicting the useful lifetime of such films both the critical effect of the various climatic conditions and the effect of the harsh greenhouse micro-environment on their properties should be taken into account. In this paper the various methods for testing the mechanical properties of greenhouse polyethylene films will be presented and discussed critically. Also, the factors affecting ageing of LDPE used as greenhouse covering are presented, including methods for inducing ageing and testing methods in order to probe ageing.

Review, mapping and analysis of the agricultural plastic waste generation and consolidation in Europe.

A review of agricultural plastic waste generation and consolidation in Europe is presented. A detailed geographical mapping of the agricultural plastic use and waste generation in Europe was conducted focusing on areas of high concentration of agricultural plastics. Quantitative data and analysis of the agricultural plastic waste generation by category, geographical distribution and compositional range, and physical characteristics of the agricultural plastic waste per use and the temporal distribution of the waste generation are presented. Data were collected and cross-checked from a variety of sources, including European, national and regional services and organizations, local agronomists, retailers and farmers, importers and converters. Missing data were estimated indirectly based on the recorded cultivated areas and the characteristics of the agricultural plastics commonly used in the particular regions. The temporal distribution, the composition and physical characteristics of the agricultural plastic waste streams were mapped by category and by application. This study represents the first systematic effort to map and analyse agricultural plastic waste generation and consolidation in Europe.

Adaptation and harmonisation of standard testing methods for mechanical properties of low-density polyethylene (LDPE) films

Abstract In the framework of a research project carried out at a European level, a preliminary phase with intercomparisons between measured critical mechanical properties and methods has been completed. The aim was to achieve a satisfactory degree of harmonisation in the testing procedures used by the participants and adapt appropriately the corresponding standard testing methods. The recommended methods for testing the mechanical properties of greenhouse polyethylene films are presented. Following the procedure described in a relevant standard for tensile properties, it was shown that it is very important to define appropriately the whole set of parameters involved in the test. In addition, special adaptation of the equipment used is required. The problems encountered during this work led to the redefinition and modifications of several parameters involved in the procedure of measuring specific tensile properties. Harmonisation of the testing methods for impact and initial tear resistance proved to be more readily obtained. However, some parameters entering the corresponding measuring procedures had to be adapted. In general, harmonisation has been achieved regarding the measurement of the specific mechanical properties.

Finite element analysis of a cylindrical silo shell under unsymmetrical pressure distributions

Abstract This paper deals with the analysis of the behaviour and the state of stress developing in a silo shell under real asymmetric pressure distributions concerning both storing and discharge. The results obtained suggest that the design of such structures may not neglect the asymmetric features of the real pressures developed by the stored material.

The C 0 shell plate and beam elements freed from their deficiencies

Abstract The degenerated shell element is considered to be the most efficient shell element formulation. However, this element, as well as the corresponding C 0 plate and beam elements, exhibit several deficiencies, expressed by locking phenomena with thin-element applications and in many cases, by spurious kinematic modes when reduced integration is used to alleviate those locking phenomena. The present paper proposes a generally applicable new formulation which removes all shear and membrane locking mechanisms from the finite element equations of the structural C 0 shell, plate and beam elements. Employment of full integration with the proposed formulation eliminates also the zero energy modes problem, and/or softening effects, associated with the use of the technique of reduced integration in C 0 plate and shell element applications. The formulation proposed is obtained through a simple and cost effective scheme.

On the basics of the shear locking problem of C0 isoparametric plate elements

Abstract The flexual and shearing behaviors of the Mindlin plate and degenerated shell element, in flat element applications, are analyzed with respect to shear locking. The shear locking phenomenon is found to be strictly associated with the flexural behavior of the element and to be directly or indirectly associated with the development of spurious shear strain by the element, subjected to relatively high order Kirchhoff fields. The effects of the integration scheme, order of the element, finite element discretization and element distortion are investigated.

Analysis and design of low-density polyethylene greenhouse films

In 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.

Monomial test: Testing the flexural behavior of the degenerated shell element

Abstract Shear locking of the degenerated shell element is a result of the development of spurious shear strain by the element, when subjected to high order Kirchhoff displacement fields. The shear locking phenomenon is analyzed in this paper using an analytical test. The effects of the integration scheme, order of the element, order of the modeled Kirchhoff field and the element distortion are analyzed explicitly and quantitatively for the four-node, eight-node and nine-node degenerated shell elements.