Conventionally, the process of Stage II fatigue crack growth is governed by Paris law with less microstructural effect. When fatigue crack growth test was carried out for pearlitic steels ( both eutectoid and hypereutectoid steel), sometimes the main crack ( macro-crack is of some mm) was arrested, a new crack nucleated nearby the crack tip ( about 5 to 10 micrometers) and connected to the main crack after a number of cycles. Should I understand that not SIF but microstructural plays a role in this phenomenon? Are there any suggestions to reveal the reason for this crack growth behavior?
Thank you so much for your time and response!
Dear Thao Phuong Bui,
Interesting question and observation. From the outset, I have to say that discontinuity in the course of stage II fatigue crack propagation would not be outlandish in all cases. As you mentioned in the description, one of the most important points that should be considered about this stage of crack propagation is that the dependency of the crack on the microstructure is considerably less than the preceding stage. However, we should consider that after the transition from stage I to stage II, the crack is still dependent on microstructural features and evolutions. This assertion is practically proved despite a bit of deviation from the textbooks.
About the cracks arresting, initiating new ones, and subsequent coalescence of them, one probability would be the crack bifurcation phenomenon. Considering the given dimensions (some mm for crack length vs sub-micrometer for displacement between former and the new crack), it is quite probable that you cannot precisely detect the crack at the early stage of bifurcating.
As far as crack bifurcation is concerned, it is worth mentioning that there are controversial views about the effect of this phenomenon on crack propagation behavior; in stage I, crack branching can reduce the energy of the main crack, thereby playing a role in the decrement of fatigue crack growth rate (FCGR). On the other hand, in stage II, crack branching would remarkably increase the FCGR.
Best regards,
Amirsalar
Dear Thao Phuong Bui,
Interesting question and observation. From the outset, I have to say that discontinuity in the course of stage II fatigue crack propagation would not be outlandish in all cases. As you mentioned in the description, one of the most important points that should be considered about this stage of crack propagation is that the dependency of the crack on the microstructure is considerably less than the preceding stage. However, we should consider that after the transition from stage I to stage II, the crack is still dependent on microstructural features and evolutions. This assertion is practically proved despite a bit of deviation from the textbooks.
About the cracks arresting, initiating new ones, and subsequent coalescence of them, one probability would be the crack bifurcation phenomenon. Considering the given dimensions (some mm for crack length vs sub-micrometer for displacement between former and the new crack), it is quite probable that you cannot precisely detect the crack at the early stage of bifurcating.
As far as crack bifurcation is concerned, it is worth mentioning that there are controversial views about the effect of this phenomenon on crack propagation behavior; in stage I, crack branching can reduce the energy of the main crack, thereby playing a role in the decrement of fatigue crack growth rate (FCGR). On the other hand, in stage II, crack branching would remarkably increase the FCGR.
Best regards,
Amirsalar
Dear Dr. Seyed Amirsalar Anousheh
It would be great to get a detailed answer about this phenomenon and your opinion about the practical approach of microstructural effect and Stage II fatigue crack growth.
Moreover, I would compare this behavior in ferritic and pearlitic steel and fully pearlitic steel as well. In ferritic pearlitic steel, I could observe crack arrested and new crack initiated from the main crack ( behind the crack tip) while in fully pearlitic steel, as mentioned-above a new crack initiated in the fracture process zone ( ahead of the crack tip). Though after a number of cycles, both of them looked like a crack bifurcation ( crack branching), the process in these two cases are different and their effects on FCG rate also different.
I am sorry if my description may be difficult to understand. If you have any doubt, please kindly let me know.
As you suggested, I am studying more about the crack birfucation process to understand the microstructural effect on this behavior.
Thank you and sincerely,
Thao
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