An interview with President Lasse Suominen of Stresstech Oy

(?) MFN: What is Stresstech Oy?

(!) L. S.: Stresstech Oy is a company based in Finland, specialized in the stress measurement business. The company was founded in 1983 to manufacture equipment for measuring stresses. Nowadays we also provide measurement services around the world. Stresstech’s wide selection of stress measuring methods covers almost all materials - not only metals but also composites, ceramics, plastics, glass etc.

We have an efficient worldwide organization that, in addition to the head office in Finland, includes offices in Germany, the USA and India. These offices are strong centers that support our customers locally. We also have representatives in more than 20 countries worldwide, who carry our products in their selection.

Stresstech Group is well-known around the world among our customer base. Most of our customers belong to transportation industries, e.g. to car, truck, heavy equipment and aircraft manufacturers. Recently also the energy sector has become more important for us. The newest interesting field is welded structures made with high strength steels.

(?) MFN: You claim to be the leading stress testing house in the world, on what grounds?

(!) L. S.: We have systematically expanded our product line in the stress measurement field. We started more than 25 years ago with the magnetic method called Barkhausen noise, which method we commercialized. In 1990 we added X-ray diffraction to our expertise, and now we have also the hole-drilling method in our selection. All of these methods have unique features which no one else has ever applied before.

As measuring stresses has been our sole task, we have during the years gained a lot of experience with stresses and quality control. It is a very well-known fact that you can’t cover all stress measurement needs with one method; X-ray can be applied only to crystalline materials, Barkhausen noise only for ferromagnetic materials and hole-drilling is not sensitive to surface stresses, just to mention a few restrictions.

The quality of their products is always the key issue for our customers. A very important part of this quality is the residual stress generated in the production process. We have invested a lot of resources to make it easier to perform stress measurements by the means of easy-to-use software and automated measurements, which mean fewer user-generated side effects. All our products and new ideas are tested in our measurement laboratory, where we do also service measurements for customers who need only occasional measurements or want to know if our methods can solve their needs.

(?) MFN: How do you serve and work with your customers?

(!) L. S.: Our main goal is to help our customers to improve the performance of their products. We offer our customers tools to measure stresses by providing them instruments to easily study stresses in their products, so that they can get positive results in their own manufacturing processes. With the help of our products our customers can easily spot the weak points in their production or process or verify that the process works the best possible way and that their products meet the set quality demands. Defects in their products or problems in their processes can cause remarkable costs or irreplaceable damage to their image.

We work very closely with our customers either with the help of our representatives or by ourselves, as especially our sensors and stands are usually custom-made according to the specific needs of the part or surface to be measured or, for example, of the automation demands.

(?) MFN: What means do you have to measure stresses?

(!) L. S.: Our instruments measure stresses based on three different methods, which are all environmentally-friendly, using no acids or other harmful chemicals. The Barkhausen noise method is a magnetic method that is very suitable for even 100% production control, since it’s fast and able to separate defective parts from good ones. With the X-ray diffraction method you can find absolute values for stresses. X-ray is a static measurement, which is why it is most often used in production development, e.g. for finding correct shot peening parameters. The hole-drilling method is typically applied in cases where the X-ray method does not work. Such situations are, for example, on castings, extruded aluminium or on non crystalline materials.

(?) MFN: You manufacture Barkhausen noise instruments, how do they measure stresses?

(!) L. S.: The Barkhausen Noise Analysis (BNA) method, also referred to as the Magnetoelastic or the Micromagnetic method, is based on a concept of inductive measurement of a noise-like signal, generated when a magnetic field is applied to a ferromagnetic sample. Ferromagnetic materials consist of small magnetic regions resembling individual bar magnets called domains. Each domain is magnetized along a certain crystallographic easy direction of magnetization. Domains are separated from one another by boundaries known as domain walls. AC magnetic fields will cause domain walls to move back and forth. In order for a domain wall to move, the domain on one side of the wall has to increase in size while the domain on the opposite side of the wall shrinks. The result is a change in the overall magnetization of the sample.

If a coil of conducting wire is placed near the sample while the domain wall moves, the resulting change in magnetization will induce an electrical pulse in the coil. How easily the domain walls move depends on stresses and microstructure of the sample.

(?) MFN: What is special in your X-ray equipment?

(!) L. S.: Our X-ray stress analyzers measure the stresses imposed on crystalline material by X-ray, based on the well-known phenomenon known as Braggs Law. Our X-ray system works by direct detection of X-rays by silicon-based detector technology. This makes our system simple and versatile.

Another unique development area is our centerless goniometer, of which the latest model is the robot goniometer used in our Xstress Robot. In this equipment all goniometer movements that are needed such as tilting, rotating and mapping are handled by the robot. This field is strongly developing as being easy to use and more accurate. An example of a complicated part benefiting of the robot application is the crankshaft, which is now much easier to measure than before.

(?) MFN: You also make hole-drilling instruments. What are they and how do they measure stresses?

(!) L. S.: Hole-drilling is one of the most common methods for the measurement of residual stress. In the traditional method, strain gages are used to detect surface deformation resulting from the hole-drilling. Removing material from a part changes its stress equilibrium. Stress rearrangement leads to a characteristic displacement of the surface around the hole, which carries information about the stresses that were present in the volume drilled out. Our hole-drilling equipment (Prism) measures deformation around the hole by an optical method. This method determines surface displacements directly. Electronic Speckle Pattern Interferometry uses coherent laser light, in the form of an illumination and a reference beam that interfere. This allows sensing displacements much smaller than the wavelength of the light. The whole area around the hole is mapped, pixel by pixel, and this experimental displacement map is then fitted by one generated using three independent stress components and three rigid-body movement variables that can account for any possible shifting of the part during the measurement. The measurement is fully computer-controlled. The benefit of the optical method is that there is no need for surface preparation, no centering demand of the drilling, and the method gives full field information, whereas the traditional method with strain gages typically gives only three values. All this means that the optical method is clearly faster than a method based on strain gages.

(?) MFN: Your instruments are quite valuable. Do you offer your customers any way to determine stress values other than buying the equipment?

(!) L. S.: Indeed, our instruments are worth their value, and can save a lot of money and trouble when put into use in time, before permanent damage has been done. For those who are not ready to buy the instruments, we also offer measurement services in our laboratories with any of our products. Measurement services are often used to study capabilities of the methods for the specific applications, or for some temporary projects.

(?) MFN: Making stress measurements can be time-consuming and troublesome. Can you offer some shortcuts or helping tools for that?

(!) L. S.: Making the measurement as easily as possible without compromises regarding the results is our main goal. According to the customer’s specifications, we can provide a wide variety of stands, from simple manual stands to fully-automated measurement stations with robotized loading and unloading of the samples. These we can offer for both our Barkhausen noise as well as X-ray instruments. For example, our new Xstress Robot improves the measuring convenience a lot by making separate X-Y and rotation units as well as floor stands unnecessary. With our new equipment, previous hard-to-reach areas can now be reached. The new XTronic software also makes the measuring and calculations easy to do.

(?) MFN: What is the business situation in the stress measurement field?

(!) L. S.: The stress measurement business is expanding strongly. Customers are specifying the allowable stress levels in the parts they manufacture or buy. If this is done, you have to verify that stresses are what are specified to be. Another trend, which is increasing the demand for stress measurements, is the demand to increase the strength of the materials, as a lighter structure means less energy consumption, especially in the transportation industry. Higher strength often means that the structure is more sensitive to all kinds of defects such as wrong residual stress levels. An example of such an application is welding high strength steels. Weld seams are typically hammered or shot peened to produce compressive residual stresses, which have to be controlled by some method. Business looks positive in our field.

We would like to thank Lasse Suominen for this interview!

For Information:
Stresstech Oy
Tikkutehtaantie 1, 40800 Vaajakoski
Finland
Tel. +358.14.333.000
Fax +358.14.333.0099
E-mail: info@stresstech.fi
www.stresstechgroup.com