Thermographic Non-Destructive Testing of Fatigue Strength in Operating Conditions

The authors propose a thermographic method of non-destructive testing of fatigue strength that includes the following steps. First, individual fatigue limits are determined for a batch of samples of a certain manufacturing level using a pre-developed technique, followed by the determination of the control stress equal to the statistically established lower fatigue limit for the sampled parts calculated with a pre-set confidence factor. Subsequently, the critical entropy increment per oscillation cycle is calculated as the upper value of the specific entropy increment for those sampled parts that have the fatigue limit equal or exceeding the control stress. Fatigue strength testing resides in testing of parts at the control stress level. During testing, a part is brought to the beginning of the second stage of cyclic loading (relative stabilization stage), and the actual entropy increment per cycle is obtained after several loading cycles. If it is less or equal to the critical entropy increment, the part is considered fit for use and vice versa. When passing through all the testing stages, the acceptable parts largely retain their cyclic life.

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