Study On Multiple Low Velocity Impact Properties Of Stitched Foam Sandwich Composites

Traditional foam sandwich composites have the advantages of light weight、high strength、high specific strength、high specific modulus、sound insulation and good impact resistance,and are widely used in aerospace,marine engineering,medical equipment and other fields.However,in practical applications,products are inevitably affected by repeated low-speed impacts,such as sea waves,hail and gravel impacts,which easily highlight the defects of low impedance and easy delamination of the foam sandwich structure,which seriously affects the service life and safety of products.performance.Therefore,the introduction of stitches in the thickness direction improves the interlayer performance and impact toughness of the product.In this paper,repeated low-velocity impact tests at different energies,non-destructive testing of damaged panels,and mathematical formula fitting of inflection point curves are carried out for stitched/unstitched foam sandwich composites.The main research contents of this paper include:（1）Research on low-velocity impact test of stitched/unstitched foam sandwich composites under different impact times.The stitched/unstitched carbon fiber foam sandwich composites with ply[45⁰/-45⁰/0⁰/90⁰]_2s were prepared by vacuum-assisted transfer molding（VARTM）process,and the same type of specimens were subjected to different impact times The impact test under different impact times was carried out by Micro-CT and depth measure,and the non-destructive testing of the samples under different impact times was carried out to reveal the stitched/unstitched low-speed impact damage and structural dynamics evolution state under different impact times under 15J impact energy.The damage impedance of multiple impact stitched/unstitched foam sandwich composites under 15J impact energy was studied.The results show that under the impact energy of 15J,the main damage modes of the interlaminar damage of the panel are different from the impact surface to the interior of the material,and the damage area is increasing.maximum.In addition,the stitched resin column can effectively improve the impact resistance of the foam sandwich structure.Compared with the non-stitched carbon fiber foam sandwich structure composite,the maximum residual impact load of the stitched carbon fiber foam sandwich composite material is increased by 33.4%～48.1%under the same number of impacts.（2）Experimental study on impact behavior,damage impedance and damage tolerance of stitched/unstitched foam sandwich composites under different impact energies.10J,15J,20J impact energy was used to study the impact behavior of composite materials under different impact times,and the impact load and time,displacement history curve,absorbed energy and time curve,pit depth and interlayer under different impact times were obtained.The damage area was obtained;and the relationship between the depth of the pit,the damage area between layers and the number of impacts under different impact energies was obtained.The action mechanism of the suture resin column on the foam sandwich structure under different impact energies is analyzed.The results show that under the impact energy of 10J,except for the 7th to 8th interlaminar layers near the foam,the damage area is the largest,and the damage area between the layers in the fiber panel gradually decreases.The damage area of the stitched fiber panel is larger than that of the unstitched fiber panel.structure;under the impact energy of 20J,the damage area between layers of the carbon fiber foam sandwich structure increases gradually from the surface to the bottom,the damage area of the stitched structure is larger than that of the unstitched structure,and the difference in damage value between different layers is small.With the increase of energy,the damage rate of fiber panels and interlayer damage increases,and after breaking the inflection point,the difference in pit depth of the stitched/unstitched structure increases.