Thermal fatigue is a serious problem in todays power plants. A large amount of damages around the world circles around that subject. The damage can come from the mechanism of two flows mixing with different temperatures in a T-junction. It is a fundamental prerequisite that these T-junctions are frequently tested for damages. An amount of criterias steers the rate of testings. This guide/document will provide the process of rating the T-junctions resistance with valid information and ease the course. If a T-junction is stated as very sensetive for thermal fatigue, it will be graded with a low damage index. This value is reliant of the temperature difference between the two flows and can be changed with different actions, for example by adding a mixer to the T-junction. The mixer will blend the two flows so the thermal fatigue reduces or doesn’t occour at all. The T-junction can be tested with several methods, ultra sonic testing or eddy-current testing. The ultra sonic testing method is the most used method and can test the inside of the T-junction pipe without access of the inside.