Papers by Keyword: Threshold Stress Intensity Factor

Abstract: In this paper we first review the inertial effect coefficient model for fatigue crak growth (FCG) under constant amplitude (CA) loading based on static fracture mechanics with thinking about the inertial effect coefficient and its impact to crack tip, and then discuss the relation between threshold stress intensity factor (K_th ) and fatigue crak growth rate, which is in good agreement with the result by analysing the relation between dynamical coefficient and fatigue crak growth rate. Comparison of the present analysis with experimental data taken from the literature demonstrates that threshold stress intensity factor (K_th) has a greater impact to the fatigue crack propagation.

Abstract: It has been recognized that the threshold stress intensity factor range Kth of a short crack is lower than that of a long crack. The short crack behavior in plain specimen has been studied by many researchers. However, the behavior of a short crack at the root of a long notch is not yet clear. The crack closure behavior is considered to be affected by the constraint at notch root and it is dependent on the length and the root radius of notch. In this study, fatigue tests on specimens with short pre-crack at long notch were done and the difference in crack closure behavior was studied. As a result, short crack effect appeared in any notch root radius. In a sharp notch, the crack opening point easily reached its stable condition after a small amount of crack extension. On the contrary in a dull notch, the opening point was lower than the stable condition and consequently short crack effect lasted in relatively wide range of crack extension. The small crack effect of notched specimen was discussed based on crack closure behavior.

Abstract: Hydrogen is considered to be a possible energy source in the coming future. However, it has been recognized that hydrogen has a detrimental effect on the fatigue strength of metal. The fatigue crack growth characteristic is an important property for the integrity assessment of hydrogen utilization machine. In this report, the effect of hydrogen on the fatigue crack propagation characteristic was studied using low alloy steel, carbon steels and A286 alloy. Especially in this study, very short pre-cracked specimen as small as 0.03 mm deep was used and the near threshold fatigue crack behavior was studied. As a result, materials whose Vickers hardness was higher than 300 were found to be susceptible to absorbed hydrogen.

Abstract: In this research, corrosion fatigue crack initiation tests using 13 Cr stainless steel were conducted in synthetic seawater to investigate the corrosion fatigue crack initiation life (Nc). In addition, a corrosion fatigue crack initiation and propagation model was suggested also. Until now, corrosion fatigue crack initiation life was treated ambiguously. The important point of this model is the suggestion of the moment of the corrosion fatigue crack initiation life at which the critical size of the corrosion pit changes to the crack. As the crack which emanates from the pit is usually small, accordingly it is treated as a small crack. In addition, the observation of the corrosion fatigue fracture surfaces using SEM were conducted. And the fracture mechanics analysis using an intrinsic crack model was conducted for the treatment of the small crack. Finally, the following were obtained. In the case without a clear stress concentration point which seems to fall into a corrosion fatigue crack initiation, as the meaning to define the number of cycles, at which the fatigue crack propagation rate becomes faster than the corrosion pit growth rate so that the fatigue crack initiates from the pit and its propagation starts in earnest, as the reasonable and universal corrosion fatigue crack initiation life (Nc) has been clarified.

Abstract: The threshold stress intensity factor (ΔKth) of small crack is affected by various factors, especially by material hardness, stress ratio and crack size. Test results showed that harder material had higher ΔKth and smaller crack had lower ΔKth. The crack closure measurement on a very small crack was done to make clear the root cause of those effects. Most of those effects could be explained by the peculiar behavior of crack closure for crack deeper than 100µm. However, everything could not be understood only by the crack closure behaviour. In addition to the difference in crack closure, (ΔKeff)th itself was also dependent on crack size when the crack depth was shallower than 100µm. Another remarkable phenomenon was experienced in this study. Unusual decrease in ΔKth was found in some case. The large decrease of ΔKth occurred under the conjunction of three factors, that is, extremely high stress ratio (R) higher than 0.8, small crack and hard material. This kind of large reduction in ΔKth in high R region is of much importance in turbo machinery that operates under high mean stress with small vibratory stress.