Investigation on strength and fracture mechanism of aluminum plate-fin structures at cryogenic temperature

Abstract

In order to improve the service life of aluminum plate-fin heat exchangers and prevent them from thermal stress damage at cryogenic temperature, the strength and fracture mechanism of their plate-fin structures is studied at cryogenic temperature based on axial tensile test and scanning electron microscope (SEM) observation. Through axial tensile test, the mechanical properties and stress-strain curve can be obtained for plate-fin structures. Compared to the performance of plate-fin structures at room temperature, the yield limit, tensile strength and strain interval length (from yield limit to tensile strength) increased by 42.9 %, 49.5 % and 23.6 % for plate-fin structures at cryogenic temperature, respectively. This shows that plate-fin structures at cryogenic temperature have greater toughness and plasticity. Through SEM observation, the differences and commonality of fracture mechanism are compared and analyzed for the typical zones of plate-fin specimens. The results can be concluded that the fracture mode of plate-fin specimens belongs to quasi-cleavage fracture, and mainly shows the morphological characteristics of small cleavage sections, tearing ridges and dimples. Meanwhile, the fracture process and specific morphological characteristics of plate-fin specimens at cryogenic temperature are different from those at room temperature. Under the influence of thermal stress at cryogenic temperature and stress concentration, the fracture area of plate-fin specimens at cryogenic temperature is only located in the brazing zone. There is a significant displacement in the horizontal direction for specimens at cryogenic temperature when their temperature returns to room temperature after fracture. Therefore, the fracture in the brazing zone can be prevented for plate-fin structures at cryogenic temperature by adding brazing filler metal and improving the brazing process under process permission. The slow heating/cooling mode can be adopted for aluminum plate-fin heat exchangers to prevent them from thermal stress damage in the start-stop and operation stages.