Tech Briefs: What got you thinking there might be a correlation between the first cycle of stress and ultimate failure?
Professor Tresa Pollock: It wasn't something we were looking for initially, but we had new techniques, which were very labor intensive. We have to put a sample in our scanning electron microscope and strain it in situ, which takes a great deal of time and energy. So, we had a tendency to just look at the early stages of cycling — to gather information about what was happening early on. But then later we did some very long-term testing because we were able to test at very high frequencies. When we were looking at the data, we realized that there was a direct connection from the earliest to the later stages. So, we didn't start with that hypothesis, but discovered it just by the way we designed the experiments.
Tech Briefs: How long did the test take? What kind of frequencies do you use?
Pollock: The fatigue tests that go to a billion cycles usually take about a week because we're doing them at 20 kilohertz. If you were doing that under conventional conditions, you couldn't even get to that many cycles — because it's a resonance technique you can get there very quickly — so that's been one of several tools that have helped us figure out what's going on.
Tech Briefs: Can you tell me about the instruments you use.
Pollock: There are two, maybe three instruments. First, we have a big infrastructure inside the scanning electron microscope where we do some very specialized versions of digital image correlation while we strain. And then in many cases we take those samples and use femtosecond lasers to slice the samples micron by micron, by ablation, to construct 3D data sets. You can see what's going on at the surface, but you want to know what's going on just beneath. The ultrasonic fatigue system is separate — It's something we've been working on for about a decade. The innovation there, is the controls for the instrument, which make sure you can keep samples from heating in an uncontrolled way. We've also had a lot of collaboration with the University of Michigan, where I used to be before I came to Santa Barbara. So, it's another technique that we've been working on for many years.
Tech Briefs: You’ve said that the phenomenon has to do with slip bands — what are they?
Pollock: Anytime you plastically deform something — say, take a bar and bend it — the way that you accommodate that plastic deformation process is by the motion of line defects that are called dislocations. If you have a whole bunch of dislocations following each other on a single plane, then you get this very localized slip band. When you get that very localized deformation, it ultimately creates a crack. That's been known for a long time, but there's not been anything known about it as quantitatively as we've been able to look at it. Being able to analyze the amplitude of the displacements that occur in those slip bands has led us to better understand the problem.
Tech Briefs: You’ve said that some slip bands disappear when you return the metal to its original shape, and some don't.
Pollock: Yes, that's right, and that's always been one of the big mysteries about fatigue, that when you strain something in tension it will deform by motion of those dislocations in one direction, and then when you go back into compression they can reverse and go the other way. But only a fraction of them are able to retrace their path. This inability to retrace the path is what often ends up being the critical phenomenon that leads to a crack forming — that's known as irreversibility. There have been some things known about irreversibility, but only at a much more macroscopic level. Now we're able to trace that back to very individual slip events, individual slip bands.
Tech Briefs: Any theory about why some reverse and some don't?
Pollock: In many of the materials we're interested in, it has a lot to do with the fact that you strengthen the materials with second phases inside the material. So, once you deform those phases to move through them, then it's difficult to go backward. These defects are not always confined exactly to a single slip plane, so if they, for mechanical reasons, go out of the plane, then then they can't retrace their steps. There are a variety of features, but most of the features that result in them not being reversible are associated with very high strength materials — and of course we always want high strength materials. But that doesn't necessarily make them fatigue resistant.
By the way, many of these early fatigue studies were done at NASA. There was a big effort at NASA in the 1970s when fatigue and fracture mechanics became much better understood from the mechanics point of view.
Tech Briefs: You’ve written that the three-dimensional structure of the atoms has some relationship with this phenomenon. What accounts for the different arrangements of the atoms?
Pollock: So, unless you do something special like grow a single crystal, all materials are composed of lots of little, tiny grains and they're all stuck together at different orientations relative to one another. You have one little crystal next to another little crystal and the slip planes and the orientations of the slip planes are different as you go from one grain to the next. It’s the details of this polycrystal structure that give you the rare events that kick off the slip bands, which then eventually make cracks.
Tech Briefs: Is that arrangement just random?
Pollock: It can be, it has a lot to do with the way that you process the material. Our interest in this has always been connected to aircraft engines. The polycrystalline turbine discs that hold everything together are usually processed to have fairly fine crystals (grains). The way they make those materials Is such that the structure is, from the point of view of the orientations of the grains, fairly random. The sizes of the individual crystals will vary and the way that the orientations change where they connect at the boundaries, will be different from grain to grain. Most of the materials that we've been looking at are fairly clean from the point of view of extrinsic defects, so to speak, because of the way they're made.
Early in my career I worked both at what is now Rolls Royce and also at GE Aviation. So, some of my earliest exercises in engineering were related to some of these problems.
Tech Briefs: What does it mean when you say the correlation is linear across materials?
Pollock: That was a surprise, that we would have strength in a linear relationship between fatigue strength and localization amplitude. We tested a wide variety of materials, under varying testing conditions, like zero to maximum stress, then back to zero; or zero into compression, then back to zero and with different grain sizes — lots of variables and yet, in spite of all that, everything fell on a straight line.
Tech Briefs: Do you have any idea of what accounts for that linearity across all different materials?
Pollock: We think this is fundamentally related to irreversibility. All high strength materials have a tendency to localize deformation and form slip bands. It seems to suggest that the highest strength materials are more prone to irreversible deformation. Some of the details of microstructure of the materials seem to matter less than expected.
Tech Briefs: Are you interested in figuring out why that's so?
Pollock: Yes, we would like to spend more time looking at the details experimentally. Also, we're doing some modeling now with colleagues here to determine the set of mechanical conditions that assist in the formation of slip bands. We take our 3D data sets and put structural information into mechanical models and go through the loop in really trying to understand what details affect the slip localization— there's a PhD student working on that now.
Tech Briefs: What are your next steps?
Pollock: We would like to look at a broader set of materials, and development recommendations for what changes in structure that can be achieved during material processing can make materials more fatigue resistant. What can an aircraft engine manufacturer like GE do to make that turbine disc more durable? I should point out that GE supported a lot of this research, they like students/future employees to understand the basics of how fatigue limits materials.