PSI - Issue 60

P.A. Jadhav et al. / Procedia Structural Integrity 60 (2024) 631–654 P. A. Jadhav et.al. / Structural Integrity Procedia 00 (2019) 000 – 000

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Figure 5: DHC velocity plotted as a function of applied stress intensity factor K I at a constant temperature.

The rate of crack growth is also affected by the solubility curve of hydrogen in zirconium, which exhibits hysteresis. This can be seen in Fig. 3. The influence of the thermal history on the solubility of hydrogen can be understood with the help of Fig. 6 given in IAEA-TECDOC-1410 (2004). For example, consider a specimen of Zr-2.5Nb alloy that has a bulk hydrogen concentration of 60 ppm. Initially, the specimen is at room temperature (S), and it is heated along the path S-A-B on the TSSD curve. As the temperature increases, the hydrogen concentration in the specimen also increases. When the specimen is cooled down after reaching point B, the hydrogen concentration does not change until it reaches point C, which lies on the TSSP curve. On further cooling, the concentration decreases along the path C-D on the TSSP curve. If the temperature is increased again after reaching point D, the concentration remains the same until it reaches point A. This illustrates that the concentration of hydrogen in the alloy can be maintained by following a specific thermal path. This is useful for measuring the DHC velocity at different temperatures, as the highest velocity is typically achieved when all of the hydrogen is dissolved at the peak temperature of the test. To get the maximum DHC velocity at a temperature T, the hydrogen concentration H conc in the specimen should be at least equal to the corresponding TSSP concentration and the specimen should be heated above the temperature corresponding to this concentration on TSSD curve. This is shown in Fig. 7. If for example, the DHC velocity is required to be evaluated at T Test = 250 O C, then the minimum concentration of hydrogen in the specimen should be at least equal to H specimen = 65.7 ppm. The specimen should be heated to a temperature of T Heat = 310 O C and subsequently cooled to T Test . If the specimen is heated to 250 O C and cooled, the DHC crack growth velocity observed will be very low.