(A) Study on the improvement of fracture toughness of stainless steel adhesive joints at cryogenic temperature

Abstract

The CCS (Cryogenic Containment System) of LNG (Liquefied Natural Gas) has primary and secondary barriers, in order to prevent LNG leakage with high reliability at cryogenic environment of -163oC. The secondary barrier is composed of adhesively bonded thin austenite stainless steel sheet (SUS 304 or 304L). Since the thin stainless steel sheets are not suitable for welding or other mechanical joining without leakage, thin stainless steel sheets are adhesively bonded using epoxy adhesive which has relatively low CTE (Coefficient of Thermal Expansion) and high strength adhesion characteristics. However, as adhesives become quite brittle at the cryogenic temperature, the fracture toughness of adhesive joint is relatively low at the cryogenic temperature. Therefore, in order to improve the reliability of secondary barrier, the adhesives should be reinforced with fibers or fillers to increase their fracture toughness. In this work, a film-type epoxy adhesive was reinforced with E-glass, polyester and aramid fiber mats to improve the fracture toughness of adhesive joint composed of austenite stainless steel adherends at the cryogenic temperature of -150oC. To evaluate the fracture toughness and crack resistance of the stainless steel-epoxy adhesive joints, cleavage tests on the DCB (Double Cantilever Beam) adhesive joints were performed with respect to the crack length and fiber volume fraction of the reinforcements at the cryogenic temperature. From the experiments, it was found that the crack propagated in the stable mode with significantly increased fracture toughness when the film-type epoxy adhesive was reinforced with the E-glass, polyester and aramid fiber mats. Also, optimum volume fractions of fibers were suggested for the film-type epoxy adhesive in the adhesive joint at the cryogenic temperature.