Hiroyuki Akebono: Humans and Metals

2022/10/12

I have been conducting research on "fatigue" from my student days until today. That is to say, it is not human "fatigue," but the "fatigue" of metallic materials. Just as we humans get fatigued, metallic materials also experience "fatigue." The greater the load and the longer the duration of the load, the more fatigue damage progresses. Humans and metals are the same in this regard. The only difference is whether or not recovery is possible. We humans know how to recover. Eating delicious food, getting plenty of sleep... the methods vary from person to person. In the case of metallic materials, it is extremely difficult to recover from fatigue damage, and in most cases, fatigue simply continues to accumulate. And eventually, they "fracture." Sometimes, fracture accidents caused by metal fatigue lead to catastrophic results, leading not only to economic and social losses but even to the loss of human life. Unfortunately, with current technology, it is impossible to realize "fatigue-free metal = unbreakable metal." However, technology for creating "metals that are difficult to break" is progressing steadily, step by step. To that end, it is important to understand the mechanism of why things break. Driven by this motivation, I continue my daily exploration of metal fatigue.

To elucidate the mechanism of metal fatigue, it is important to observe the damage behavior that accumulates due to fatigue, but it is extremely difficult to accurately observe damage on the order of several microns occurring inside metallic materials. However, one day more than ten years ago, a student in my laboratory showed me an amazingly beautiful three-dimensional image of internal damage. I was told that they polished a cross-section near the damaged area, photographed the damage with a microscope, then polished it by 0.01 mm and photographed the damage on the new cross-section. They explained that it was an image created by three-dimensionally stitching together a vast number of two-dimensional images obtained by repeating this process endlessly. The progress of observation equipment in recent years has been remarkable, and we are now in an era where we can observe things that were completely impossible to see just a decade or so ago. However, every time I look at the 3D image created by that student, I feel as though I am being reminded that even without using such expensive, state-of-the-art equipment, one can sometimes achieve results that surpass those of the latest expensive observation devices by working steadily and sparing no effort.

The "fatigue" required to obtain this single image is immeasurable. I am glad that we humans have the ability to recover from fatigue.