Investigation Of Damage Model Of Stitched Laminates Subjected To Low-velocity Impact

The impact resistance performance and interlaminar toughness of stitched laminates is improved due to the introduction of the suture in the thickness direction,which has begun to be applied in aerospace structures.In maintenance,it is easy to have tool and ladder collision.Most of them do not consider the effect of stitches on the in-plane mechanical properties of the stitched composites.In addition,there is no comparison of the failure criteria applied in the damage determination.So it has significant application value to study the issue of low-velocity impact of stitched laminates.This paper has done research on the prediction of low-velocity impact damage of stitched laminates.And the main contents and conclusions of this paper are as follows:Firstly,a method for predicting the in-plane mechanical properties of a stitched single-layer board is proposed.The sutured monolayer cell model was determined.The effect of sutures on the volume content of the fibers in the cell surface was taken into account.The fiber volume content expression at any point in the fiber-resin mixed region was deduced,and any point within the unit cell was solved.The elastic constants and in-plane strength parameters are derived,and a uniform periodic boundary condition expression is deduced.Based on the Hashin failure criterion under the plane stress state,the in-plane mechanical properties of a stitched single-layer panel are solved.Secondly,a method of predicting gradual damage of stitched laminates at low-velocity impact based on the prediction of the performance of stitched veneers is proposed.In terms of material properties,the in-plane mechanical properties of stitched laminates based on the unit cell model have been developed with the adoption of the in-plane mechanical properties of stitched veneers to give the material special features.In the aspect of the model,the finite element model of stitched laminates considering the suture and the interface between the layers is established.The mechanical properties of the suture in the thickness direction are simulated by the bar element which only bears the tensile stress.Considering damage assessment,three failure criteria were combined for damage analysis.Meanwhile,an analytical procedure is developed based on finite element software.Thirdly,gradual prediction and verification of suture T300/QY9512 laminates the low velocity impact damage is done.Three failure criteria were combined to determine the damage.It was concluded that the prediction error of the damage area using LaRC03-Weng combination criterion was within 6.8 percent of the experimental value,which verified the rationality method for gradual damage prediction for low-velocity impact of stitched laminates.The delamination damage caused by low-velocity impact on the sewn laminate exhibits a small and large distribution in the thickness direction.The damage spreads from the impact surface to the bottom along the thickness direction,forming a horn shape;low-velocity impact causes fiber and matrix compression on the surface of the sewn laminate.Damages and cracks appear on the bottom surface of the suture.The degree of damage on the bottom surface of the sutured laminate is higher than that on the impact surface.The suture absorbs impact energy.If there is a suture near the impact location,the impact energy can be absorbed to reduce the damage.It can suppress the crack propagation on the bottom surface of low-speed impact test pieces.Finally,a static load test was carried out on a one-way plate with a 5×5mm stitch density between a one-way plate with T300/QY9512 material and a one-way plate with a 5×5mm stitch density.The in-plane mechanical properties of suture 5×5mm single-layer plates are predicted.Good agreement was found.It verifies the rationality of the analytical method for the mechanical performance of stitched veneers.The rationality of the method for predicting the in-plane mechanical properties of the stitched composites was verified.The results of the test of the mechanical properties of the seamless and stitched unidirectional plates were compared.The following conclusions were obtained:[0°]₁₆ One-way board static performance after stitching Decreased;[90°]₁₆ unidirectional plate tensile strength increased after sewing.