The automated manufacturing of several complex shape composite parts simultaneously by compression and transfer molding method faces the problem of poor elastic properties of synthetic fibers, what leads to the insufficient depth of formed part and fiber breakage. Alternative to synthetic fibers, vegetable fibers have higher elasticity and lower impact on environment, but due to the lower fiber bulk integrity and orientation vegetable fibers have lower strength. However, fiber pre‐tension processes involved in various composite manufacturing routes could enhance the fibrous reinforcement orientation and increase tensile strength. Nevertheless, the influence of this method commonly is used to increase the flexural strength of the composites and its influence on the tensile properties of the composite was not studied. Thus, the aim of this work was to investigate vegetable fiber pre‐tensioning influence on the tensile and flexural properties of the composites. It was obtained that significant enhancement of flexural performance is caused by the residual compressive forces propagated in the matrix polymer, achieved only at high values of pre‐tension (approximately at 50–70% of the ultimate strength of the reinforcement), while even minimal pre‐tension load significantly increases the tensile strength of the composite. Thus, the pre‐tension technique is particularly effective in the case of vegetable fibers reinforced composite, whose properties after pre‐tension are comparable with the properties of glass fiber composites. POLYM. COMPOS., 34:1533–1537, 2013. © 2013 Society of Plastics Engineers

The aim of this work was to investigate the barrier and comfort properties of protective uncoated and coated-impregnated three-layered knitted fabrics with different arrangements of special yarns, such as conductive yarns and yarns with different filament cross sections. Depending on content (7.5-30%) of conductive PES yarns with carbon core filaments and PES/stainless steel spun yarns, fabrics were grouped into A and B. In order to achieve multifunctional barrier and comfort properties, high porosity polyurethane and fluorocarbon resin coatings were applied. At the beginning of the research, the fabrics of groups A and B were coated with commercially available micro-porous polyurethane foam Tubicoat Õ MB according to the crushed foams method and impregnated with fluorocarbon water-repellent agent Tubiguard Õ 270. The achieved functional, barrier (water and oil repellency, resistance to water penetration, and electrostatic shielding), and comfort (air permeability, water vapor permeability, water retentivity, and drying intensiveness) properties were determined.

In the fiber-reinforced composites industry together with the promotion of environmental friendly production, synthetic materials are attempted to be replaced by renewable, biodegradable and recyclable materials. The most important challenge is to improve strength and durability of these materials. Matrix that supports the fiber-reinforcement in composite generally is brittle and deformation causes fragmentation of the matrix. Pre-tension of reinforcement is a well-known method to increase tensile strength of woven material. The current study develops the idea to use pre-tension of woven fabric in order to improve quality and strength properties of the obtained composite. Natural (cotton fiber) and synthetic (glass fiber) woven fabrics were investigated. The pressure forming operation was carried out in order to study clamping imposed strain variation across the surface of woven fabric. The uniaxial tension test of single-layer composite specimens with and without pre-tension was performed to study the effect of pre-tension on strength properties of composite. The results have shown that pre-tension imposed by clamping is an effective method to improve the quality of shaped composite parts (more smoothed contour is obtained) and to increase the strength properties of composite reinforced by woven natural fabric. After pre-tension the tensile strength at break increased in 12 % in warp direction, in 58 % in weft direction and in 39 % in bias direction.

http://dx.doi.org/10.5755/j01.ms.17.1.250

Purpose -The purpose of this paper is to optimise parameters of digital image analysis to investigate the deformation behaviour of woven sample and to detect the onset and variation of wrinkling that occurs due to bias-tensioned fabric buckling. Design/methodology/approach -Using models of predescribed shape, the relationship between the digitized gray scale intensities and wrinkles of the surface are analysed and conditions of specimen illumination and filtering procedures are chosen. Findings -It is proposed to convert acquired images to binary to record the onset of buckling and to estimate critical buckling parameters of stretched woven samples. The threshold value is determined as mean value of approximated histogram of stretched specimen centre line. It is defined that profile curve and gray scale disperse presented by parameter CV can be used to obtain additional information and to compare behaviour of different samples during bias tension. Research limitations/implications -Proposed image analysis technique allows detection of the onset of buckling wave formation and evaluation of surface waviness changes in woven samples different in colour and weave type tension. However, the behaviour of fabric samples with sharp multicoloured and complicated patterns cannot be assessed by gray scale imaging. Originality/value -The proposed approach can be adjusted to investigate different wrinkling problems -buckling during simple shearing or picture frame test, seam puckering, draping.

Woven textiles have ability to obtain spatial shape surfaces undergoing complex deformations. Formability of woven textile material is referred to shear deformations in macro scale when angle between two yarns systems are changed at crossover points. When form stability limit of sheet material are overstepped, complex deformations arises and the material buckles. This research analyses buckling phenomenon of deformed fabric during bias extension and uniaxial concentrated load tension. Image analysis method was adjusted to commit critical buckling parameters: critical load P cr and critical elongation ε cr . To obtain fabric buckling during common used uniaxial tension method it is possible only in bias direction, when concentrated load tension has shown that fabric buckles not only in bias direction but in principal directions (weft and warp) as well. The buckling takes place among intersections of distinct deformation zones between two stress concentrators and is related with the uniformity of deformation distribution in these zones.