Thermal fatigue cracking may initiate at a tee pipe where high and low temperature fluids flow in. The mixing flow causes fluid temperature fluctuation near the pipe wall and may result in fatigue crack initiation. The fatigue crack is caused by the thermal strain fluctuation at the pipe inner surface. Authors has reported the effect of the loading sequence on thermal fatigue in a mixing tee in the previous study. The thermal strain at the pipe inner surface was obtained by thermal hydraulic tests using a novel test section for measuring wall temperature. It showed that the strain fluctuation became large locally around the hot spot, which was heated by the hot jet flow from the branch pipe. The fatigue damage at failure of the specimen was calculated using Miner’s rule and the obtained strain around the hot spot. The calculated fatigue damage around the hot spot became less than unity and the minimum value was 0.18. Therefore, Miner’s rule predicted non-conservative fatigue life. Since the strain around the hot spot had waveforms with periodic overload, loading sequence with periodic overload caused reduction of the fatigue life around the hot spot. In this study, the effect of a single overload on the fatigue crack growth rate was investigated in order to clarify the reduction of the fatigue life at the mixing tee due to strain with periodic overload. In addition, the prediction method of the fatigue life for the variable thermal strain was discussed. Plate specimens with a notched crack were used to measure the change in fatigue crack growth rate after an overload. The specimen was made of austenite stainless steel which was same as used in the previous study. A single overload was applied when crack length reached 3 mm by loading constant strain amplitude. The crack growth rate increased after an overload for both cases of tensile and compressive overloads. In next step, the crack opening strain was also obtained by the measured stress-strain curve and unloading elastic compliance method. The change in the effective strain amplitude, which was defined as the difference between the maximum strain and the crack opening strain, was observed. The effective strain amplitude increased after the application of a single overload. It means that the reduction of the crack opening strain caused the increment of the effective strain amplitude and the acceleration of the crack growth rate. The crack growth rate for strain with an overload was correlated with the effective strain intensity factor range rather than the strain intensity factor range. The fatigue damage for the variable strain around the hot spot at the mixing tee was recalculated using Miner’s rule and modified fatigue curve. The modified fatigue curve was made by the relationship between the effective strain amplitude and the fatigue life of constant amplitude. It implies that the crack opening strains at all cycles were assumed to be equal to the minimum strain of each cycle. The calculated fatigue damage became more than unit except a few cases. The fatigue life could be assessed conservatively for variable strain at the mixing tee using fatigue curve which considered the effective stain amplitude.


Fatigue Life Prediction for Variable Strain at a Mixing Tee by Use of Effective Strain Amplitude

Division / Sponsors: 
Pressure Vessels & Piping (PVP)


Paper Proceedings: https://asmedigitalcollection.asme.org/PVP/PVP2020/volume/83815

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