Damage Mechanism Of Biaxial Warp-knitted Flexible Composite Subject To Low-velocity Impact

The flexible membrane composites are composed of fabric and flexible matrix,which are widely used in membrane structure buildings.The flexible membrane composites are more vulnerable to breakage due to stones in the external environment,which leads to local structural damage such as holes and cracks.In addition,the flexible membrane material composites exhibit nonlinear and anisotropic mechanical response characteristics due to complex composition.The special yarn arrangement makes its impact resistance,in-plane strength and energy absorption performance significantly improved in the biaxial knitted fabric.It has great application potential in the fields of textile composites,aerospace,engineering and construction.In this paper,biaxial warp knitted fabric was used as the reinforcement of flexible membrane composite to investigate its quasi-static mechanical properties.The failure characteristics and damage mechanism of biaxial warp-knitted flexible composites under low-velocity impact were studied by means of experimental and finite element method.Firstly,the quasi-static mechanical properties of biaxial warp-knitted flexible membrane composite are studied.The off-axis tensile experiment,tearing experiment and stabbing resistance experiment of different structure flexible membrane composites are tested by universal material testing machine.And the failure mechanism of the flexible membrane composite is analyzed.It is found that different off-axis angles have an important effect on the tensile properties of the flexible membrane composites.The tensile strength firstly decreases and then increases form 0° to 90° directions,and the tensile strength in the direction of 90° is lower than that in the direction of 0°.The failure modes of the samples are tensile fracture failure and tensile-shearing failure.The vertical tearing stress of biaxial warp-knitted flexible membrane composite was almost higher than the horizontal tearing stress.The failure modes are the matrix debonding and yarn fracture damage.The stabbing resistance of flexible membrane increases with the increase of high tenacity polyester linear density and inserted yarns density.Then,the low-velocity impact property of flexible composites made of polyurethane as matrix and biaxial warp-knitted fabric as reinforcement has been investigated by using the drop hammer impact instrument.The effects of different initial impact energy,different high tenacity polyester linear density,different inserted yarns density and different impactor shapes on the impact energy absorption performance of flexible composites are analyzed.The results show that the initial impact energy has minor effect on the energy absorption performance of the biaxial warp-knitted flexible composite under low-velocity impact.The main structure of the sample is destroyed under the action of impact load due to thin thickness.The impact resistance of the biaxial warp-knitted flexible composite is positively correlation with high tenacity polyester linear density and inserted yarns density.The damage morphology of flexible composite materials is completely different under different impactor shapes.The main failure modes of biaxial warp-knitted flexible composite are matrix failure,fiber fracture and interfacial debonding between matrix and fiber.At last,this paper uses ABAQUS to perform low-velocity impact finite element simulation on biaxial warp-knitted flexible composite.Solidworks is used to model the simplified structure model of biaxial warp-knitted flexible membrane composite,and the impact behavior of the simplified structure model is simulated under different impactor shapes.The results show that the damage morphology of the biaxial warp-knitted flexible composite is basically similar in the finite element simulation and the experimental with three different impactor shapes.The load-time curve and energy absorption-time curve obtained from the finite element simulation were compared with the experimental results.It was found that the overall trend of the finite element simulation results was basically consistent with the experimental results.