A direct current potential drop (DCPD) calibration was developed for an open-hole specimen to characterize fatigue crack growth (FCG) behavior of cracks initiating from a small corner flaw and transitioning to a through-thickness crack. A single Mode I stress intensity (KI) solution was derived covering the entire range of crack growth. The purpose of using this specimen geometry was to: 1) capture short crack behavior for specific crack length intervals of damage tolerance criteria and 2) use the data to evaluate predictive FCG methodology. Driven by these objectives, a non-standard specimen was chosen that better simulated the geometry and stress state of the component of interest: an aircraft wing design detail. Thin panels of selected 7XXX and 2XXX aluminum alloys were prepared with a centrally located open-hole with a corner notch of 0.13 mm (∼0.005 in.) to serve as a crack initiation site. Testing was conducted in high-humidity air under constant and variable amplitude loading. Short crack results agreed well with existing, closure-corrected, long crack data. Duplicate tests of the 7XXX alloys confirmed repeatability. The 2XXX alloys exhibited greater variability than the 7XXX alloys in the short crack regime. Evaluation of the predictive methods is beyond the scope of this paper and will be published separately.

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Public reporting burden for this collection of information is estimated to average 1 hour per response, including the time for reviewing instructions, searching existing data sources, gathering and maintaining the data needed, and completing and reviewing this collection of information. Send comments regarding this burden estimate or any other aspect of this collection of information, including suggestions for reducing this burden, SPONSOR/MONITOR'S ACRONYM(S) SPONSOR/MONITOR'S REPORT NUMBER(S)Approved for public release; distribution is unlimited. SUPPLEMENTARY NOTES ABSTRACTThe near-threshold fatigue crack growth behavior of a 7075-T651 aluminum alloy was studied in laboratory air, vacuum, and an aqueous 3.5% NaCl solution. Results obtained indicate that raising the stress ratio R enhanced the near-threshold fatigue crack growth with a greater crack growth rate da/dN and smaller threshold stress intensity range AK" in laboratory air and an aqueous 3.5% NaCl solution. However, the reverse was observed in vacuum. The near-threshold fatigue crack growth was most sluggish with the smallest da/dN and greatest AK^, values in vacuum, intermediate with an intermediate da/dN and AK^, in the aqueous 3.5% NaCl solution, and fastest with the greatest da/dN and smallest AK," values in laboratory air. In laboratory air and aqueous 3.5% NaCl solution AK", initially decreased with increasing R until a value of 0.5 was reached, and then leveled off or decreased slightly. The AK ft values for these two environments appear to converge at a higher R. On the other hand, in vacuum, the AKth increased linearly with increasing R. In addition, at lower R, a greater resistance to nearthreshold fatigue crack growth was detected in the aqueous 3.5% NaCl solution than in laboratory air. This is attributable to crack closure, induced by corrosion product at the crack tip. 15. SUBJECT TERMS fatigue, near-threshold fatigue crack growth, stress ratio, vacuum, laboratory air, 3.5% NaCl solution, threshold stress intensity range, maximum stress intensity 16. Abstract:The near-threshold fatigue crack growth behavior of a 7075-T651 aluminum alloy was studied in laboratory air, vacuum, and an aqueous 3.5% NaCl solution. Results obtained indicate that raising the stress ratio R enhanced the near-threshold fatigue crack growth with a greater crack growth rate da/dN and smaller threshold stress intensity range AK t h in laboratory air and an aqueous 3.5% NaCl solution. However, the reverse was observed in vacuum. The near-threshold fatigue crack growth was most sluggish with the smallest da/dN and greatest AK t h values in vacuum, intermediate with an intermediate da/dN and AK t h in the aqueous 3.5% NaCl solution, and fastest with the greatest da/dN and smallest AK t h values in laboratory air. In laboratory air and aqueous 3.5% NaCl solution AK t h initially decreased with increasing R until a value of 0.5 was reached, and then leveled off or decreased slightly. The AK t h values for these two environments appear to converge at a higher R. On the...

Post yield design methodology using Ductile Failure Damage Indicator (DFDI) for well tubulars was proposed and has been used for tubulars and connections life assessment. The tubular design assessment model incorporates a connection strain localization factor (SLF) to assess the fatigue life of the tubulars. Critical strain, a material-dependent parameter essential for DFDI, is obtained using the uniaxial stress-strain tests (i.e., strained to failure uniaxial tests). Understanding the impact of accumulated cyclic damage on critical strain is essential to the post-yield design approach. This paper aims to validate and evolve the low cycle methodology by 1) quantifying the effect of accumulating cyclic plastic strain on the critical strain through a series of post-yield axial and thermal strain fatigue experiments, and 2) applying the post-yield design approach to assess tubulars and connections. Low cycle fatigue experiments demonstrating the critical strain measurement and its dependency on the thermal and axial-strain cycles will be discussed in the paper. Critical strain (K55 and L80) from monotonic tests is compared to critical strain obtained from cyclically preconditioned samples. Effect of cyclic plasticity on critical strain is established quantitatively. Coupons are also subjected to the post-yield axial and thermal cycles to failure and compared to critical strain-based DFDI design predictions. Since connections are known to be the weakest link in the casing system, the impact of connection thread-forms on the strain localization factor is demonstrated using a series of finite element models and the experimental material responses. Axial strain-controlled loading would be applied on the tubulars and connections to estimate the damage using the DFDI approach. A systematic approach to delineate the dependency of critical strain on cyclic straining validates the effectiveness of DFDI in thermal well design. Further, the quantification of SLF for integrating connection into thermal well design provides a complete solution.