Effects of external operating environments on fatigue of heat-exchanger copper tubes

摘要

The purpose of this study is to analyze the effects of surface defects (eg, notches) and external environment conditions (eg, operating temperature, the number of re-weldings) on the static strength and fatigue of C1220T-O copper tubes used in the heat exchangers of air conditioners. Instead of using standardized specimens, as is done in general rotary bending fatigue tests, special specimens were fabricated in this study by inserting metal plugs on both ends of the copper tubes to perform fatigue tests on the actual tube product, and then the fatigue characteristics were evaluated using stress-life (S-N) curves. Regarding the welding conditions (maximum 1000°C and 10 seconds), the grain size grew (grain size number decreased), and the hardness decreased as the number of re-weldings increased. The effects of the operating temperatures on the fatigue life were examined at a room temperature of 25°C and a heat exchanger operating temperature of 125°C, resulting in the same fatigue limit (70.21 MPa) at both room and operating temperatures. However, the fatigue limit of 37.46 MPa measured in the notched specimens (radius of 3 mm, depth of 0.2 mm) was lower than that obtained from those without notches. The material constant (1.07) used in the Peterson equation was then computed from the fatigue notch factor (1.87 = 70.21/37.46), and the stress concentration factor (2.18) of the notched tube specimens was obtained from the structural analysis. This material constant can be used to predict a decrease in the fatigue limit over varying notch sizes in copper tubes (C1220T-O).