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.