I would like to surprise the world
through the development of
new testing methods
that solve society's problems

Associate Professor
Department of Materials Processing,
Tohoku University
Yoshikazu Ohara

© School of Engineering, Tohoku University

Making defects inside materials visible in 3D
Promising application to concrete infrastructure

There are many people who still remember the collapse of the Sasago Tunnel on the Chuo Expressway in December 2012. The cause was the aging of metal bolts that secured the ceiling panels in place. After the incident, the government made the inspection of tunnels and bridges every five years mandatory. It also introduced the concept of "preventive maintenance" wherein repairs and other measures are taken before failures occur in the function or performance of a facility. “But even so, the reality is that testing is only done by visual testing and by hammering (hitting concrete surface with a hammer and determining whether the concrete is good or bad based on sound). In the case of concrete, there is still no established method of testing its interior,” says Associate Professor Yoshikazu Ohara of the Department of Materials Processing of the Graduate School of Engineering at Tohoku University whose research is centered on “ultrasonic nondestructive evaluation.”

Nondestructive evaluation is the testing of the interior of a material or structure without destroying it. Associate Professor Ohara explains the significance of his research this way: “When we probe into the causes of collapsed bridges and other structures, we see that they are caused not only by surface cracks; oftentimes they are caused by defects deeper inside. Even though we can now make detailed nondestructive tests of the interior of metals, it is very difficult to test concrete due to its complex internal structure. It is the most difficult problem left in the field of nondestructive evaluation.”

In September 2020, Associate Professor Ohara presented the development of a “high-resolution ultrasonic imaging method” that can make defects inside materials visible in 3D. He explains, “Ultrasonic waves, which are harmless to humans and are highly sensitive, are widely used as a way to measure internal defects. The technology used to turn information obtained through ultrasonic waves into images has become widespread, but such images are limited to 2D or low-resolution 3D. However, many internal defects have complex 3D shapes. With this new method, I succeeded in creating 3D images of stress corrosion cracking (a type of crack) with complicated branching, which is a problem in power plant pipes and other equipment. Going forward, I would like to apply this method in many fields where metals and composite materials are used. Ultimately, I would like to further my research so that the method can also be used to test concrete infrastructure such as bridges, expressways, and tunnels.”

The world’s first imaging of a “closed” crack
An achievement that astounded the world


Ever since he became a researcher, Associate Professor Ohara has been working on internal testing methods using ultrasonic waves. An achievement of his research that astounded the world was “SPACE” (Subharmonic Phased Array for Crack Evaluation) made public in 2007. Although internal measuring using ultrasonic waves was already being used in practical applications, in the case of “closed” cracks wherein a crack's faces are in contact with each other or sealed, ultrasonic waves simply passed through those cracks and was unable to take measurements, which was an obstacle that couldn’t be overcome for a long time. He explains, “My method uses large-amplitude ultrasonic waves to jiggle open and shut the 'closed' crack itself, and then utilizes the unique scattered waves generated at the crack, especially the subharmonic waves that produce excellent resolution and contrast. Through SPACE, which is a fusion of large-amplitude ultrasonic generation technology and phased array (technology that uses an array sensor with multiple elements and its controller to make images of the interior through electronic scanning), I succeeded in imaging a 'closed' crack for the first time in the world.”

Associate Professor Ohara still cannot forget the first time he presented his doctoral thesis “SPACE” at an international conference. He says, “The response was so great that a crowd formed in front of my poster; and many researchers with excited expressions on their faces asked me all kinds of questions. Since then, I've had more opportunities to lecture overseas. When I present innovative research, I can see the sparkle in people's eyes even mid-way through my presentation. During Q&A sessions, a lot of hands are raised, and even after the lecture ends, many researchers continue the discussion. These are the times when I feel the happiest as a researcher.

Up until now, Associate Professor Ohara has chosen more urgent issues for his research, but going forward, he also wants to work on research that will change the world. He says, “Solving society's problems is important, and it is the foundation of engineering research. If we continue doing such research, we may come up with new ideas that can significantly change the future and aren't limited to solving just one problem. Just as smartphones and the Internet have changed society, I believe that a thrilling part of research is being able to try out innovative ideas that can change the world. While tackling ongoing issues, I would also like to work on the kind of research that can change the world even though it may take a long time.”

東北大学 工学研究科・工学部 Driving Force 明日を創るチカラ INTERVIIEW REPORT

Behind the growth of a researcher is an encounter with a role model

Associate Professor Ohara acquired his master's degree at the Graduate School of the Nagoya Institute of Technology, and came to the Graduate School of Tohoku University for his doctoral program. That was because one of his former teachers at the Institute in Nagoya retired. He had the option of going to a university in the United States or Canada, but he came to Tohoku University because he was drawn to Professor Kazushi Yamanaka (currently a professor emeritus at Tohoku University) who was researching nondestructive testing at the Graduate School of Tohoku University. He explains, “I attended an academic conference where Professor Yamanaka was speaking, and he looked like he was so full of life. His mind was so sharp, and even after his presentation during discussions, I felt his intelligence was of the kind I'd never encountered before. At that moment, I knew I wanted to do research with him and under his guidance. In the course of a researcher's growth there is often an encounter with an admirable person who then becomes your role model. This is precisely what happened to me when I met Professor Yamanaka.”

When Associate Professor Ohara was in the first year of his master's degree, he received an opportunity to attend an academic conference in the United States. It was the first time he rode an airplane, and he also experienced making an oral presentation. “I studied English on my own, and prepared for my presentation thoroughly. Although the content of my research received a certain level of recognition, the language barrier was higher than I thought. Other researchers from different universities abroad seemed to have a very good time talking with each other. Seeing this, I thought that someday, I wanted to be part of their circle and do joint research work with them. After returning to Japan, I tried all kinds of things to improve my English. I memorized a lot of words, I practiced English composition so that I could speak English spontaneously, and I also worked on the basics of pronunciation because I realized it's hard to catch what someone was saying if I couldn't pronounce the words that they were saying. In 2010, an opportunity came where I could test the results of my effort.”

The “luxury” of overcoming stagnation in research
under the guidance of world-class professors

In 2010, Associate Professor Ohara participated in an international conference hosted by Dr. Paul Johnson of the Los Alamos National Laboratory in the United States for the first time. The conference differed from a typical international conference in that there were only 30–40 participants. It was unique in that each participant's presentation lasted a long 40 minutes and it was designed in a way that all the participants developed the discussion further together as a group. Associate Professor Ohara looks back at that time saying, “I was the only Japanese person, and there were many participants whose names I recognized from international, authoritative scientific journals.” Participating in this conference gave him a major leap in his subsequent research. He explains, “Dr. Johnson, whom I admired, took an interest in my research and he gave me several opportunities to stay at the Los Alamos National Laboratory. Aside from my work with Los Alamos, I am currently conducting joint research with other researchers abroad. Dr. Johnson's conference gave me the chance to do those things, and I’ve been attending his conference for 10 consecutive years and continue to receive inspiration from it.”

His participation at the international conferences in Los Alamos reminded Associate Professor Ohara of the importance of discussion, which he is conscious of when he is teaching his students. He says, “If a student's research isn't going well, I make the student think hard about the result, discuss it with others, and think about the reason for failure before they make the next move. I make sure they spend a lot of time on this. There are many world-class professors at Tohoku University. I think that thinking logically, building up a solid hypothesis, and overcoming stagnation in research through multiple verifications under the guidance of such world-class professors is a “luxury” only Tohoku University can offer.”

Associate Professor Ohara's hobby is running. He usually runs 5 to 6 kilometers on weekdays and 15 kilometers on weekends. “I wake up at 5:00 and work before I go running from 6:00 am. Running after working organizes my thoughts, and it helps me come up with ideas,” he says. Seeing the invisible, and making the invisible visible – this is the essence of Associate Professor Ohara's research. One day while running, he might just come up with ideas for structures and industrial products that meet safety and reliability needs, and even ideas that might change the world.