Fatigue Limit Evaluation Of Metals And Welding Spot Structure Using An Infrared Thermographic Technique

The fatigue analysis plays an important role in determining the durability of vehicles. As the vital component of fatigue analysis, fatigue testing is the key in appraising process. In determining the fatigue limit, it requires an extremely high accuracy, as well as practicality and economy. The traditional experimental method correlates the fatigue life of a smooth specimen under constant amplitude stress conditions. It costs 18 samples and 30days to plot the Wolher curve. Even by using a step to step method, this procedure needs to circulate some samples ten million times.The cause of fatigue damage is plastic deformation which leads metallic microstructure to irreversible changes, such as point defection, structure dislocation and quantity change. These changes could form surface defections. In irreversible thermodynamic point, these changes of microstructure delivery a large amount of heat during fatigue process. In microscopic view, inner friction is the cause of heat. Otherwise, in macroscopic view, heat dissipating in the area surround crack trip makes the surface temperature rising quickly. There is a correlation between fatigue damage and temperature increments. Because of the tiny dissipation, temperature measurement needs a high preciseness. With the development of temperature measurement technology, infrared thermographic system is becoming a leading method to record dissipation, and it satisfies all requirements of temperature measurement.The paper aims to illustrate a new methodology of fatigue limit evaluation of metals and welding spot by analyzing the intrinsic dissipation of the external surface during the application of limited cycle by cycle loading. Compared to traditional testing techniques, there are some advantages of infrared thermograph technique as a non-destructive, real-time, rapid and low- expends technique.