Research On The Test Method For Tensile Properties Of Biaxial Warp Knitted Fabrics

In this paper, biaxial warp knitted fabrics applied for geotextiles are investigated on test method for their tensile properties in order to provide some advices for further study which aims to make a proper standard of test method for biaxial warp knit fabrics' tensile properties.Working out an effective way to prevent specimen from slipping in the clamp when the specimen is under uniaxial tension, describing lateral contractions of tricot biaxial warp-knitted specimen when it is under lengthwise tension, investigating how the lateral contractions influence the maximal strength of specimen are the main targets for this paper. Also the factors which influence the ratio of lateral contractions and the repeatability of tensile test are analyzed.First, singled warp insertion and biaxial warp knit specimen covered by different kinds of materials are being stretched in order to get the best way to preventing specimen from slipping in the clamp by comparison. It turns out that sticking two strips of medical adhesive plaster on the place where the specimen is clipped by the clamp and also letting it be covered by sand paper can not only prevent the strength of specimen from being improperly reduced or resulting in stress concentrations but also prevent the specimen from slipping effectively.Second, tricot biaxial warp knit fabrics with warp insertion in different fineness or different density are being observed in their tensile failure process. It turns out that the impact of lateral contractions on the maximal strength of specimen depends on the strength comparison between warp insertion and ground yarn. If there is not big difference between the strength of warp insertion and that of ground yarn, lateral contractions will be obvious in the tensile failure process. Under this circumstance, the basic stitch will play an important role in bearing the stress and with the splitting and separating of the basic stitch, the strength of specimen will decline to breaking strength. So the unsimultaneity of filament break-up will be aggravated and the usage factor of the strength of singled warp insertion will be inclined. Under the contrary circumstance, lateral contractions are not obvious in the tensile failure process, the warp insertion will bear the major part of stress and the basic stitch will just ladder in small range. Specimen reaches its maximal strength by reason that most filaments in warp insertions breaks simultaneously. The usage factor of the strength of singled warp insertion is very high.Then the ratio of lateral contractions of specimens in different width and specimens with warp insertion in different fineness and specimens stretched at different speed are compared. It turns out that the ratio of lateral contractions are influenced by the width of specimen, the fineness of warp insertion, the stretching speed and the structure of basic stitch. With the increase of stretching speed or the decrease of specimen's width, the ratio of lateral contractions is becoming bigger. On condition that the density is the same, the finer the warp insertions are, the more serious lateral contractions are. If there is no underlap between wales of basic stitch, like pillar, then there will be no lateral contraction on the specimen when being stretched.The factors, which have an effect on the repeatability of tensile test, such as stretching speed, specimen's width, preload and the fineness of warp insertions are being analyzed. It turns out that in order to get goodrepeatability in tensile tests, 50mm of specimen's width and 20mm/min of stretching speed is preferred.The relationship between the width of specimen and the maximal strength is described by a regression equation made by SPSS in order to forecast the strength of biaxial warp knit geotextiles under actual use according to the strength obtained by tensile test. Both a standard linear and non-linear three elements mechanical model are used to imitate the tensile properties of biaxial warp knit specimens in this paper. It is found that the standard linear three elements mechanical model is much better. For high-strength PET tricot biaxial warp knit fabrics, a straight line is more suitable for describing their tensile properties.