A new book about hole-drilling as a residual stress measurement tool on the way

Interview with Gary Schajer

Gary Schajer signing his previous book of Practical Residual Stress Measurement Methods for Lasse Suominen, President of Stresstech Oy.

Gary S. Schajer is Professor of Mechanical Engineering at the University of British Columbia, Vancouver, Canada. He has been the recipient of numerous awards for teaching and research including the Lazan Award of the Society for Experimental Mechanics and the Association of Professional Engineers and Geoscientists of British Columbia Teaching Award for Excellence. Professor Schajer is an eminent researcher in hole-drilling measurements of residual stresses, the use of Electronic Speckle Pattern Interferometry (ESPI) and Digital Image Correlation (DIC) and X-Ray CT Log Scanning. He has published numerous papers and book chapters. He is the editor of the well-known reference book Practical Residual Stress Measurement Methods, published by Wiley. He is currently writing a new book with another residual stress measurement specialist, Philip Whitehead, on the hole-drilling method for measuring residual stresses.

Professor Schajer, it’s our privilege to have you here in Finland at Stresstech Oy, would you tell us the motive of your travel to here?

I have come to Stresstech to focus on the writing of my new book on residual stress measurements using hole-drilling method. I wanted to come here to have more information about practical application of the residual stresses in industrial projects. I have a very friendly relationship with Stresstech and we share the same research and development areas of residual stresses and hole-drilling with ESPI.

You have been teaching residual stress related subjects at the University of British Columbia. How did you first become interested and why did you choose to work in this area?

As part of the PhD qualifying procedure when I was a student at the University of California, Berkeley, I was asked to do a small project on a different subject than my own research area. The idea was to demonstrate your ability by conducting an independent research on an unfamiliar subject. The student advisor at Berkeley suggested the residual stress topic to me, which turned out to be a good idea because I happened to have some contacts regarding residual stress research from my previous job. The work went well and I was lucky that the paper that I wrote about it for the Journal of Engineering Materials and Technology won their Best Paper Award. So, by accident I came to residual stresses.

How important are the residual stresses for the industry?

It’s easy to ignore the residual stresses because they are not visible nor are they associated with any external loads. For many cases, they can be considered as the hidden killer, especially for dynamically loaded components, where their influence on fatigue properties can be substantial. Residual stresses could be the main reason of catastrophic failures; they have even brought down large bridges. Generally, residual stresses are the integral part of engineering design.

Are there conferences about residual stresses?

Yes, there are several conferences specifically featuring residual stresses in engineering materials. The ICRS- International Conference on Residual Stresses and ECRS- European Conference on Residual Stresses are organized in every four years. These events are typically attended by academics rather than industry people. Conversely, at the Residual Stress Summit most participants are engineers from industry.

There are also other events organized by SEM- Society of Experimental Mechanics with special residual stress related sections. Lastly, the Denver X-ray Conference features residual stress measurement methods.

What will be the role of residual stresses in our future?

Residual stresses will be always with us and their importance will be more significant in the future. The mechanical design of the components getting more sophisticated than ever. With a very conservative design, one could ignore the effect of some stresses, but modern design tends to use more modest load factors. Thus, the effects of residual stresses no longer get absorbed among other things are so they need to be considered more carefully in their own right.

When we talk about the future trends, what is the relationship of new manufacturing methods with residual stresses?

As an example, additive manufacturing becomes more affordable and capable day by day and we start to see many 3D printed metal components. In 3D printing, liquid material is being added on top of the solid material. This is a recipe for residual stresses. With plastic materials, residual stresses may not be as serious as for metals, but if they become large they will deteriorate the material properties.

You are well-known for hole-drilling and the ESPI method. Can you describe this method?

The hole-drilling method involves drilling a small hole, 1-2 mm in diameter, and measuring the resulting deformation in the surrounding material. From those measurements, it is possible to calculate the residual stresses that originally existed at the hole location. Traditionally the measurements were made using strain gauges, but now the ESPI method provides a more convenient and flexible measurement technique. ESPI stands for Electronic Speckle Pattern Interferometry. It is a non-contact optical technique capable of measuring very small surface movements at high resolution. The advantage is that the method can make very fast residual stress depth profile measurements with little or no surface preparation.

Lastly, what are your recommendations to future engineers?

I suggest them to approach their challenges with a good sprint. Being open minded, open hearted and aware of what’s going on around them will help them greatly.

As it happens, a student of Professor Schajer, Juuso Heikkinen, is also with us and I would like to ask him a question as well.

Juuso, you have been working at Stresstech Oy and now working with Prof. Schajer. Can you tell us how did this become possible?

I was an exchange student in UK where I took a course about ESPI and later I became interested to work on this subject. After returning to Finland, I expressed my interest to Stresstech to work on a ESPI related subject. Stresstech then introduced me to Prof. Schajer and I went to Canada to work on his lab to conduct my MSc thesis research, which was funded by Stresstech. After completing my thesis, I worked at the residual stress measurement laboratory of Stresstech here in Finland. As of September 2016, I have been a PhD student at Prof. Schajer’s lab at the University of British Columbia. My research aim is to develop a Digital Image Correlation method to ease to use of current ESPI methodology.

Juuso Heikkinen and Gary Schajer making a measurement with Prism (ESPI hole drilling equipment).

Conducted by: Murat Deveci

Place: The “Karri” room, which was named after Prof. Schajer, at Stresstech, Finland.

Time: 29.06.2017. 16:00 -16:35