Compressive Behaviors And Failure Mechanism Of Biaxial Warp-knitted Composite Material Under Various Strain Rates

Bi-axial warp-knitted (BWK) fabric is made by lapping straight yarns and binding them by awarp-knitted stitch which improves the stability of the structure. The fabric has high tensilestrength and stiffness along the yarn direction because of the straight yarns. Biaxial warp knittedfabric can be used as reinforcement for composites used for aerospace, wind power generation andother fields. This research establishes a simplified microstructure model to study compressionperformance and failure mechanism of BWK composite using the finite element method based oncompression test.The main contents are as follows.(1) Impact compression test: The compressive properties of bi-axial warp-knitted compositecoupons under various strain rates were tested on a MTS material testing machine and amodified split Hopkinson pressure bar (SHPB). The average stresses vs. strain curves and the finalfracture morphology were obtained. The results showed that BWK composite is sensitive to strainrate and the failure stress increases with the increase of the strain rate while the failure straindecreases. The main failure mode was the interlaminar shear along the out-of plane direction,while along the in-plane direction, the delamination was the main failure mode.(2) The simplified microstructural model of the BWK composite: The mechanical propertiesof the knitting yarns were added to the original resin to form‘equivalent resin’. The representativevolume element (RVE) of equivalent resin was identified based on the geometric model of the knitstructure. At first, the RVE was subdivided into some sub-volumes whose fiber volume fractionwas the same as that of the RVE. The local stiffness matrix of the sub-volume was from themicromechanical model in the local system and transformed to the global one. The overall global stiffness of RVE was obtained through the volume average method. The simplified BWKcomposite contains ‘equivalent resin’and straight yarns. The simplified microstructural model ofthe BWK composite was established using a commercial finite element analysis (FEA) softwareABAQUS.(3) Analysis of calculated results: The quasi-static and dynamic impact compressionbehaviors were simulated with ABAQUS. A good agreement between experimental and FEAresults demonstrated that the simplified method is applicable of modeling the high strain-ratebehaviors. Observation of stress wave propagation in yarns and resin offers a better understandingto the compression damage mechanisms. The main failure mode of BWK composite along the outof plane direction was the interlaminar shear failure. During the compression process, shear bandformed and propagated at an angle approximately45°toward the compression axial. The fiberswhich were not along the shear plane were broken and the composite shattered into several parts.Consequently the large plastic deformation of BWK composite was induced. Along the in-planedirection, the main failure mode of BWK composite was delamination. Resin cracked graduallypropagating across the plane and fibers bore the impulsive loading. Hence, the basalt filamenttows without interlacing led to delamination because of buckling.