Study On K-cor Sandwich Structure Mechanics Performance And Influence On Thehygrothermal Environment

K-cor sandwich is a new kind of foam sandwich reinforced by Z-pinning techniques which can adapt to the needs of aerospace grade better. But now the domestic and overseas work are rarely reported about K-cor structure which encountered the environmental impact in the process of the engineering application. Based on the K-cor structure machanical properties changed under the hot and wet, low speed and the mixed environment, this article research the enhancement mechanism of Z-pin and the damage mechanism and failure mode of the structure in order to design the excellent comprehensive performance of K-cor sandwich structure.Studied on the influence of K-cor structure moisture absorption when changed the implanted density of Z-pin and the angle of facesheet layer, the results indicate that Z-pin improves the dimensional stability of the samples by changing the implanted density in sandwich structure, but it does not affect the moisture absorption rate. And when the angle of facesheet layer was smaller changges under the larger water vapor concentration, the moisture absorption rate of the structure becomes lower.Using the three point bending and compression performance test to study the changes in the machanical properties of the core in the K-cor structure under hot and wet conditions, experimental results showd that the sample which increased the implanted density of Z-pin can significantly improve the three point bending strength and compressive strength under dry and wet conditions, and the samples have a higher strength retention rate. The three point bending strength of K-cor with Z-pin density of 12mm×12mm in wet condition improve by 11.1%, and the compressive strength of K-cor with Z-pin density of 8mm×8mm in wet condition improved by 17.5% which compared with blank sandwich structure in dry condition. It shows that K-cor sandwich structure has excellent wet and heat resistance. The samples which reduced implantation angle of Z-pin can significantly increase the compressive strength under dry and wet conditions. The compressive strength of K-cor sample with Z-pin of 0 degree in wet condition was as same as Z-pin of 40 degree in dry condition. But this factor has a little influence on three point bending strength.In order to research the shock resistance of K-cor structure, this article used drop harmmer to test the K-cor sandwich structure under low velocity, and combined with infrared non-destructive testing and the compression-after-impact(CAI) strength test. Experimental results indicated that the thickness of core material in the K-cor sandwich structure is thicker, then the impact damage area is larger, but the compression strength retention is higher. Increasing the bending length of Z-pin under the premise of not more than implanted spacing can significantly reduce the impacted damage area of K-cor structure and enhance the compression-after-impact strength. In addition, it is more advantagrous to improve the bending length of Z-pin than the implanted density which can reduce the damage area and improve the compression-after-impact strength and the strength retention of the sample under the condition of the same core material density. What's more, it's more serious impacted damage when the drop hammer's parameters were 10mm/1.28 Kg rather than 12.5mm/2Kg.Considered the effects of mechanical properties in K-cor structure under hot and wet environment and low speed impact environment, experimental program designed that K-cor structure were subjected to drop harmmer which impacted at low speed after hot and wet treatment, then measured the damage area, damage depth and the compression-after-impact strength. The results showed that increased the implanted density of Z-pin can effectively reduces the negative effects of damage area and damage depth in the K-cor sandwich structure under hot and wet conditions, and alleviate descending degree of the residual strength when specimen changed the failure mechanism in the lateral pressure mode so that the shock resistance of K-cor structure became more stable.