Probing Fatigue Failures In Additively Manufactured Titanium Alloys Using Combined Statistical Aand Fractographic Techniques
Open Access
- Author:
- Wietecha-Reiman, Ian
- Graduate Program:
- Materials Science and Engineering
- Degree:
- Master of Science
- Document Type:
- Master Thesis
- Date of Defense:
- January 26, 2023
- Committee Members:
- Todd Palmer, Thesis Advisor/Co-Advisor
Albert Eliot Segall, Committee Member
John Mauro, Program Head/Chair
Jay Keist, Committee Member - Keywords:
- Failure analysis
image analysis
fatigue
titanium alloys
additive manufacturing
failure analysis
image analysis
fatigue
titanium alloys
additive manufacturing
statistics
statistical modeling
processing defects
x-ray computed tomography
XCT
fractography
fractal
topology
general linear model - Abstract:
- Fatigue failures are one of the most common mechanisms by which mechanical and structural components fail. While fatigue behavior for wrought and cast materials have been characterized and designed for, the increased complexity and uncertainties in additively manufactured materials makes much transference of knowledge and strategies dubious, and potentially hazardous. Considering the complexities both in processing and fracture of additively manufactured materials, a meta-analysis of fatigue data has been performed and accompanied by a novel quantitative fractography technique capable of high throughput, high statistical power measurements. Complex and varied materials such as L-PBF Ti-6Al-4V can produce ambiguous fracture surfaces, potentially with mixed-mode mechanisms. To improve on the drawbacks of qualitative fractography, conventional quantitative fractography methods, and current fractal-based fractography methods, a technique was developed that implements both pseudo-3D fractal geometry and topology. Based on electron microscopy, the technique can collect and analyze fracture information across an entire fracture surface. As applied to Ti-6Al-4V fracture surfaces, the fractography technique was able to distinguish between surface, sub-surface, and internal fatigue crack initiations. Measurements of the fractal dimension, or crack tortuosity, were crucial in identification, especially as multiple initiations were present for surface and sub-surface failed specimens. In-vacuum regions of internal crack propagation and in-air crack propagation could be differentiated based on both fractal and topological metrics. However, topology metrics showed sensitivity in discerning between stage IIa, IIb, and IIc fatigue crack propagation when present. Using these metrics in conjunction provides valuable insight as to how a fatigue crack forms and grows.