Experimental Investigation of Helium Gas Leakage Behavior of CFRP Laminates with Partially Thin-Ply Laminates under Biaxial Stress at 20 K

Abstract

Carbon fiber reinforced plastic (CFRP) has superior specific strength and stiffness compared to aluminum alloys. Therefore, it is a promising material for cryogenic propellant tanks of rockets and hydrogen aircrafts. One of the technical challenges in realizing a cryogenic tank made of CFRP is the prevention of gas leakage due to microscopic damage evolution. To prevent gas leakage, applying thin-ply CFRP (ply thickness is 0.1 mm or less) is devised. It is reported that thin-ply CFRP is effective in suppressing microscopic damage and reducing gas leakage. However, since the cost of thin-ply CFRP increases due to the increase in the number of layers, methods to apply thin-ply CFRP to only some of the layers (partial thin-ply) have been proposed. Earlier study by Hamori et al. Composite Structures 2019 reported that inserting thin-ply CFRP in the middle of the stacking sequence improved onset strain of leakage and leak rate. They focused only on cross-ply laminates, and the effect of off-axis (±45°) thin-plies on gas leakage behavior has not been investigated. The purpose of this study is to experimentally investigate the leakage behavior of off-axis partial thin-ply under cryogenic environment and biaxial stress. The specimens were [04/904/04/904]s cross-ply laminates with [45/-45]s and [45/0/-45]s offaxis partial thin-ply inserted. The specimens were subjected to biaxial tensile loading using bi-axial loading rig while the center of the specimen was locally cooled to -246 ±5°C by a GM type cryogenic refrigerator. The helium gas leak rate in the thicknessdirection was then measured at room temperature (25°C) with no load. The results showed that there was small difference in the onset strain of leakage and leak rate between the cross-ply and off-axis partial thin-ply. We assumed that the reason for this was the reduction of damage at the crack intersection and the decrease in crack length due to the thinning.