Measuring small fatigue crack growth with the aid of marker bands in recrystallized annealed TI6AL4V

icaf2023 Tracking Number 123

icaf2023 presentation

Recrystallized annealed Ti6Al4V (RA) has a small grain size typically with equiaxed alpha surrounded by remnant beta. This material and microstructure is used in a large number of structural parts for aircraft, and now, additively manufactured parts that may have similar microstructures. In all cases, such structures need to be durable and resist fatigue damage. One aspect of this type of microstructure is that fatigue cracks can remain very small for a long part of a component's life. This may not be mostly due to nucleation, rather, under variable amplitude spectra, loaded cracks tend to nucleate quickly and grow very slowly for much of the time. For this and other reasons, it is important to understand the growth of small natural fatigue cracks that may occur in aircraft critical structures. A known method to measure crack growth is through the addition of loading cycles to variable amplitude spectra for quantitative fractographic analysis. To some extent, the marker loads chosen depend on the requirements of the test and material being tested. While aluminium alloys are relatively easy to mark, the fine-grained Ti6Al4V alloys are difficult to mark at small crack sizes. Typical markers may be over- and/or underloads, high loads, higher than the largest spectrum load, or constant amplitude blocks with varying mean and/or maximum stresses, etc. Many test programs containing such markers for aluminium alloys can be found in the literature. However, examples for Ti6Al4V RA under spectrum loading are scarce, due to the fine mixed-phase microstructure that does not lend itself well to marking from nucleation to fracture. It is with this intent that specific markers have been developed for a coupon truncation study using Ti6Al4V RA, with a focus on understanding small to long crack growth in this material. In this paper, the process used to develop these markers and their effectiveness; the results of crack growth curves measured for a wing root bending moment spectrum; and, some observations on the nature of cracking over the first 250 microns (‘crack initiation life’) will be discussed, with comparisons to cracks produced in another common Ti6Al4V form: beta annealed material.