Dissertation interim review
- 1. Interim status review Shelby Highsmith Mechanical Engineering 11 May 2007
- 2. Proposal review The questions: Will Ti-6-4 show similar tensile-shear modal crack growth transition as seen in aluminum, steel? What is Mode III effect? Change Mode I-II transition mixity? Increase transition likelihood (change mechanism)? What driving force controls crack growth rate and trajectory under complex 3D mixed mode loadings?
- 3. Proposal review The approach: Design / validate 2 novel mixed-mode FCG specimens BEM of initial flaw configurations SIF/mode mixity distributions Mixed-mode crack growth testing Tension-torsion Various combinations of static/cyclic Modes I/II/III Post-test quantitative fractography Crack angle measurement FCGR estimation Develop / validate crack path criteria 114mm D = 19mm PREFLAW CONFIGURATION 2a a 114mm D = 19mm PREFLAW CONFIGURATION
- 4. Progress: SIF modeling Extensive FRANC3D modeling of two specimen configs to generate closed-form SIF solutions Paper in review for Engineering Fracture Mechanics
- 5. Specimen validation in PMMA Original promise to validate SIF solutions of through-crack specimen not panning out Getting quick final fracture from only 1 or 2 of the 4 crack tip extrema
- 6. Specimen saga Original ETA on specimens: April ‘06 Specimens received: late October ‘06 All specimens essentially EDM rough-cut blanks from forging Out of round, out of tolerance on diameter, ungrippable Trial specimens reworked Feb ‘07 Precracking in March problematic First test April failed in grips Program: funded extension to June, no-cost extension to August. Funding for specimen re-work and new specimens improvised.
- 7. Reworked specimen test Delay between compressive precracking & testing to analyze effect of vertical crack from hole-sink kt Test in tension-torsion failed on first cycle due to massive crack initiated at grip adapters Adapters being beveled & precrack cycles being tweaked for second specimen - TBD
- 8. Update on the “state of the art” An additional year to beat the bushes for papers Three main papers skirt the I-II-III transition question Chao & Liu - Mode I-II transition based on MHSC vs. MSSC Liu et al extended MHSC vs. MSSC to Mode I-III transition
- 9. Update on the “state of the art” Buchholz & Richard et al tackled I-II-III but only in context of crack path accdg to MHSC and their specimen generated modest K II , miniscule K III Still no treatment of shear-mode transition under I-II-III, nor published solutions of 3-mode MSSC criterion probably in a race against Liu
- 10. Plan B: verify, not quantify Small batch of decent specimens just to look for Mode III effect Al 2024 or steel 4140 4 each of two crack angles: with & without Mode III Repeat Mode I-II mixities with/without Mode III Just enough for duplicate results to show IF there’s shift in shear-mode I-II transition due to Mode III, but not enough to fully characterize failure envelope
- 11. Circumferential-crack specimens 0-degree specimens fabricated -- shallow pre-flaws, high loads required to pre-crack likely to suffer same grip failures 45-deg specimens pending -- deeper EDM pre-flaws may enable low enough loads to avoid grip failure Torsion vs. no-torsion tests could produce the same effect vs. no-effect qualitative data on Mode III loading Increment torsion until shear-cracking dominates Time permitting
- 12. Additional external data NASA Marshall testing mixed-mode FCG in thin-walled cylinders of INCO-718 Their initial attempts at 3-mode testing unsuccessful -- thin-wall specimens prohibited angled precrack Focus now on effect of different waveforms in Modes I & II
- 13. NASA data: new questions Vast majority of mixed-mode studies are on fracture, not FCG Unexplored questions: Is it K max mixities? K mixities? Energy release rate? What about static K’s? NASA matrix looks at IP, OP, and static-cyclic waveforms in 2 modes Have already generate shear-mode growth in this specimen under IP loading
- 14. Summary Two specimen configs thoroughly analyzed and submitted for publication Most hopeful salvage of original hypothesis in small batch of Al or steel through-crack specs More qualitative than quantitative Anticipated circumferentially cracked (45) specimens may provide additional data As yet unfab’d, more adapters ($) req’d, time short NASA currently churning out Mode I-II data that can add an interesting, but tangential, chapter on effect of waveforms