An experimental investigation of transient fatigue crack growth phenomena under elevated temperature conditions in superalloy 718 and titanium Ti-1100. Ph.D. Thesis
Two transient crack growth phenomena are investigated in high temperature structural alloys. The first phenomenon examined is the growth behavior of small cracks under elastic-plastic conditions in Alloy 718 at 650 C. The second phenomenon to be investigated is the mechanism of the creep-fatigue crack growth in a new near-alpha titanium alloy, Ti-1100. Understanding these phenomena is essential for accurate fracture mechanics based residual life component management techniques. The first part of the dissertation is an experimental study of the elastic-plastic fatigue behavior of small surface cracks in Alloy 718 at 650 C conducted under conditions of total strain control. During cycling, the crack growth was continuously monitored using a direct current potential drop technique while the influence of crack closure was monitored using a laser interferometry technique measuring the crack mouth opening displacement. The crack tip plastic zone size was also measured using a post-test delta phase decoration technique. Results show that the growth rates of the small cracks correlate well with long crack data when using an appropriate elastic-plastic driving force parameter. The anomalous crack growth rates observed in some experiments were found to be experimental transients dominated by the crack initiation fracture and do not represent an intrinsic behavior of Alloy 718. The second part of this document deals with a series of crack growth experiments performed on the near-alpha titanium alloy, Ti-1100, to determine the mechanism of the creep-fatigue interaction. Based on pure creep crack growth results, the increase in the creep-fatigue crack growth rate is not amenable to separate contributions of creep crack growth and fatigue crack growth. A mechanism has been proposed to account for the increase in creep-fatigue crack growth rate based on the planar slip of titanium alloys which results in the formation of dislocation pileups at the prior beta grain boundaries.
- Research Organization:
- Rhode Island Univ., Kingston, RI (United States)
- OSTI ID:
- 237318
- Report Number(s):
- N--96-23095; NIPS--96-07648
- Country of Publication:
- United States
- Language:
- English
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