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

637

7

0 1 2 3 4 5 6

ln(TSSP) ln(TSSD)

-3 -2 -1

ln(concentration of H in ppm)

1

1.5

2

2.5

3

3.5

1000/T K -1

Fig. 4. The TSSP and the TSSD curve are plotted after re-arranging the equation 1 and 2

The process of hydrogen migration and the formation of hydride platelets is followed by the cracking of these platelets, which leads to the formation of cracks. These cracks will then grow through a process called "DHC crack growth." While the process of crack initiation is important, it is typically studied after the growth and velocity of cracks are determined experimentally. This is because the experiments that are conducted to measure crack growth velocity also provide information about the conditions under which cracks initiate. The growth of a crack can be linked to the applied stress intensity factor (SIF). Fig. 5 shows how the velocity of crack growth varies with the SIF. When the SIF is below a certain critical value of stress intensity factor ( ) , no crack growth occurs. Once the SIF exceeds this critical value, stable crack growth takes place, as indicated by the plateau in the figure. Eventually, the size of the crack becomes large and the SIF exceeds the material's fracture toughness, leading to unstable fracture. The velocity of crack growth in the plateau region is strongly influenced by temperature. This temperature dependence is due to two processes that occur before crack growth: the transport of hydrogen to the crack tip, which is temperature-dependent, and the amount of hydride needed to cause cracking, which is also temperature-dependent. The transport of hydrogen is influenced by the amount of hydrogen in the bulk material, the diffusion coefficient, and the force driving the movement of the hydrogen. The velocity of crack growth decreases as the temperature decreases. The temperature dependence of DHC velocity ( DHCV ) is given by Arrhenius equation as shown in eq. 3.

(3)

Q





exp RT A pre-exponential constant Q activation energy         DHCV A