Research On Mechanical Behavior Of Composites Reinforced By Warp-knitting Fabrics Under Bi-axial Loading

Warp knitted fabric reinforced composite is a kind of further developmentmaterial on the basis of fiber reinforced composites which own both goodmechanical performance and economic cost and has been the hot spots of researchand application since the late1970s. This kind of material has the advantages oflight weight, low cost, corrosion resistance, impact resistance, high axial strength,high in-plane strength, flexible design, etc., make it be widely used in aerospace,shipbuilding, automobile, building and so on many industries. As new candidatematerial of bearing structure, its mechanical performance has been the focus of theresearch at home and abroad. This article makes a forecast on the progressivedamage and carries out both single loading and biaxial loading experiment study ofthe symmetric layer warp knitted fabric composites.This paper design the form as while as the size based on the cross sample underloading cases of biaxial tension, biaxial compression, biaxial tension andcompression. With stress concentration factor and stress dispersion coefficient asquantitative optimization goal, based on sensitive parts of the stress concentrationfor scheme design, this article determines the sample form and size under threeloading cases by means of numerical simulation and optimization theory. Finallyverify that the center stress state on the measuring section can reflect loadproportion.A method of getting material mechanical parameters is proposed and theproblem of measuring stress in the biaxial experiments is solved. The accuracy ofthe method is verified by the uniaxial loading experiments. This approach is used tostudy the material mechanical behavior in uniaxial and biaxial loading experiment.Results show that the material presented obvious dual extension-compression elasticmodulus under tensile-compressive biaxial loading cases. Moreover, materials givefull play of the stretching and compression performance under the ratio of3:1loads.The failure mode under the complex stress state is analyzed through fracturemorphology and the stress state of the sample. The material fails due totensile/compressive or shear crack of the substrate, and then to fiber tensile/shearcut in the±45°layer, fiber compressive/shear cut in the90°layer and fiber tensilecut in the0°layer.Make a prediction on the gradual damage of warp knitted fabric compositeusing the MCT micromechanical theory. Of material failure process is usuallysubstrate tensile crack first, then fiber tenso-shear or compression-shear damage. It belongs to brittle material. The destruction of the substrate is the cause of the bentphenomenon in a stress-strain curve. The performance of the fiber in each layercontrols the ultimate failure state of the material. MCT, meanwhile, combined withfinite element software determines the initial damage of the materials and the finalbreakdown strength.