1. Introduction

Heat exchanger tubes are hollow cylinders that transport liquids between devices placed in different positions, and commonly composed of cast iron, steel, copper, aluminum, and rubber 1. Regarding tube materials, although copper has popularly been used, aluminum has currently been considered copper’s replacement due to its cheaper production cost and high anticorrosive property, light weight and high productivity, compared to copper. Aluminum tubes are one-fourth times cheaper and 35% lighter than copper tubes2, 3. In the operation of air conditioners, aluminum tubes are not subject to high loads or high stresses. However, notches and other surface defects occur frequently when the tubes undergo various heat-exchanger manufacturing processes. After the heat exchanger products are manufactured and installed in air conditioners, the tubes become exposed to long-term vibration and thermal loads. Thus, they must be inspected for fatigue failure in order to enhance their overall structural reliability4, 5.
In previous studies, Gerber et al. claimed that fatigue tests must be conducted in consideration of actual operating conditions because the fatigue life can be affected when the material properties change under varying conditions (high temperature or high pressure) even if pressure boundary components are conservatively designed6. Choi et al. performed bending fatigue tests on standard specimens extracted from steel tube products against actual tube products, and found that the actual tubes had a significantly shorter lifetime7, 8. Fatigue tests on standard specimens are generally performed from a material perspective9-11. However, in order to determine the effects of the surface finishes, heat treatments, and operating environments on fatigue lives, several studies conducted fatigue tests on actual tube products12, 13.
Standard specimens are different from actual tube products in terms of the material distribution in the inner/outer components and external appearance depending on the manufacturing process. For instance, tubes used in the heat exchangers of air conditioners are obtained by the extruding and drawing of raw materials. These processes result in differences in material uniformity, which subsequently affect various properties including surface roughness and hardness14-16. Furthermore, when heat exchanger tubes are eventually installed to the final air conditioner products through further processes such as bending, cutting, and welding, they cause defects such as notches and grooves on the tube surface as well as non-uniform material properties. It leads to the tubes to have different fatigue characteristics from those of standard specimens. Therefore, less accurate fatigue characteristics are obtained when using standard specimens, and it can be difficult to design products that satisfy external operating requirements based on these results17, 18. When examining the effects of these defects and the external environments on the fatigue lives of aluminum tubes used in heat exchangers of air conditioners, it is necessary to conduct a quantitative analysis using the actual tubes instead of standard specimens.
The purpose of this study is to analyze how fatigue characteristics and material properties are influenced by the defects arising due to the processing and assembling of Al3003-O aluminum tubes for heat exchangers in air conditioners. This study also considers the environmental factors affecting heat exchanger tubes. Notches such as scratches and dents on the tube surface significantly reduce the fatigue life of tubes. However, because it is possible to reduce such defects by employing materials with high hardness, the correlation between changes in grain size and the hardness of aluminum tubes due to the number of re-weldings that could occur during installation into air conditioners. Fatigue tests were performed on Al3003-O aluminum tubes instead of standard specimens, and the resulting stress-life(S–N) curves for the actual tubes were obtained. Furthermore, the effects of high-temperature (125 °C) and notches on the fatigue lives of aluminum tubes that could occur during the manufacturing and operation of heat exchangers were assessed.