History of Stresstech

Barkhausen noise starts the business

Even though the German Professor Barkhausen had discovered the concept of inductive measurement of a noise-like signal, generated when magnetic field is applied to a ferromagnetic sample as early as in 1919, the method’s usability for grinding burn quality control was only discovered in 1980’s. The method had gained attention for industrial applications, when a young Finnish scientist Seppo Tiitto studied the it at the Department of Electrical Engineering, University of Oulu and at the Electronics Laboratory, Technical Research Centre of Finland, Oulu, in 1970’s, and published his doctorate thesis about it in 1977*. Commercial serial production of the analyzer utilizing the above method had been initiated in 1976.

The early marketing efforts in Finland were, however, unsuccessful. Seppo Tiitto moved to USA, where he together with his wife Kirsti Tiitto, Doctor of Science (Technology) from University of Oulu, too, started to market the method and instrument, with better success.

The early days of American Stress Technologies, Inc. (AST) – the first company of the group known today as Stresstech Group – were indeed very modest and unassuming. AST was incorporated in 1983 and had its first offices in Seppo and Kirsti Tiitto’s apartment and the basement of their home in Bethel Park, PA for the first 5 years. Stresstech Oy in Finland was founded in 1984 to manufacture the instruments sold in USA. The first Rollscans were put together modestly in a garage.

Most of the marketing efforts from 1983 to the middle of 1985 were directed at 2 markets – utilities and steel company rolling mill departments, latter resulting the instrument name Rollscan. The first Rollscan was sold in 1983 in Canada to a power generating company.

In 1985 with the cooperation of a North American crankshaft manufacturer, Barkhausen noise was used to detect grinding damage in hardened steel parts. Later in 1985 the first production floor Barkhausen noise system (Rollscan 100 and inspection stand) was installed for detecting grinding damage and an entire nital etch line was taken off of the production floor.

* Tiitto, Seppo: “On the influence of microstructure on magnetization transitions in steel”. Acta Polytechnica Scandinavica. Ph 119. Helsinki 1977.

Patented sensor technology

The key to the diversity of Barkhausen noise applications is the patented sensor technology. To get the best results, the sensor needs to be constructed according to the measurement surface. There are only a few practical limits to the size and shape of the sensors. As a result, part as small and complex as semiconductor lead frames or as large as rolling mill rolls can be inspected.

Sensors suitable for static and dynamic, manual and automated inspection containing one or multiple channels have been made for customers in automotive and in aviation industry.

During the Stresstech Group operating years more thousands of sensors have been specially designed and built for the applications reaching from the most common camshaft and gear applications to special applications such as knifes and saw blades. Sensors for hard to reach internal surfaces of small holes and e.g. the grooves of drill bits have also been manufactured. It can be said that the sensors are each custom made – however, there are a few general purpose or multi-purpose or so frequently asked sensors that are not so sensitive to the shape of the surface that can be kept available from stock.

From manual to automated measurements

As its simpliest Barkhausen noise measurements can be performed manually by using only a sensor and a Rollscan. Today this fulfills the demands of the quality departments very seldom either being too slow, sensitive to the operator skills and leaving no permanent trace of the quality of the part.

From quite early on Stresstech Group has provided complete systems, which allow manual or fully automated measurements, and recording and saving the measurement data easily and automatically with the help of the ViewScan data acquisition software.

During its operation Stresstech Group has sold and installed about a more than 1000 Barkhausen noise systems world-wide. Today the applications delivered to customers range from handheld universal testers to automated multi-channel in-line systems.

Early Rollscan models have already retired, and new, more efficient and sophisticated models have stepped into their role. Today there are three different Rollscan models to choose from, carefully designed for different uses.

Barkhausen noise method acknowledged

The first “formal” organization to accept Barkhausen noise for grinding burn detection was the Department of the Air Force, who did it In October 1986. After the endorsement of BNA inspection by the SAE and FAA authorities, the automotive manufacturers soon followed the trend.

Also in 1986 and early 1987 the McDonnell Douglas Corporation successfully used Barkhausen noise for measuring compressively overloaded 300 m steel in landing gears. Their research lead to McDonnell Douglas and the US Navy buying many Rollscan systems for this application.

1987 several organizations studied the use of Barkhausen noise to test for residual stress, grinding burns and heat treat defects through chrome plating. This lead to a publication by AST, followed by an Aerospace Recommended Practice # ARP 4462 in 1992 followed by Boeing’s Process Specification BAC 5653 entitled Barkhausen Inspection For Thermal Damage In Steels in 1997.

1991 found Split Ball Bearings working with Barkhausen noise for the detection of subsurface tensile stress in an aircraft engine main shaft bearing. This work lead to General Electric’s specification # P1TF82 entitled Process Control Of Residual Stress In M50 Bearing Raceways in 1992.

Barkhausen noise method has reached wide interest and acceptance among the scientists and industry. It is in active use in several universities and has resulted in many postgraduate and doctorate thesis also after the beginning.

Barkhausen noise method also has its own discussion forum, the International Conference on Barkhausen noise and Micromagnetic Testing, ICBM (www.icbmconference.org). The ICBM has already had 11 conferences on several continents.

Production range expands with X-ray instrument

In 1991, Stresstech added X-ray diffraction to its product range. The measurement technology was first developed in the Helsinki University of Technology in the Laboratory of Materials Science in 1990’s. For commercializing and developing the instrument, the company Mexpert was founded, from which the technology was bought to Stresstech. The first X-ray instrument was called AST X2000 and it was sold and installed in Mexico by AST in 1991. Model AST 2002 followed soon after, and was sold in North America in 1991.

Development of the technology by Stresstech produced two years later model AST X2001, which was easier to take out of laboratory than precedent models. The first goniometer model G1 was also quite soon replaced with models G2 or with rotation G2R, which are still in production.

Customers in Asia and North America with a few ones in Europe adopted this X-ray diffraction technology without prejudice. The first system sold in Europe was to France. European markets opened for Stresstech Group’s X-ray systems in 1996.

Development towards today’s portable X-ray unit produced the fourth generation and easily portable Xstress 3000 system in 1996. New goniometer G3/G3R was introduced to market in 2005.

Since then a robotized X-ray system known as Xstress Robot, has been developed and brought to market. The Xstress Robot system is a turnkey solution for measuring residual stresses. It uses an industrial robot as a goniometer, thus bringing more flexibility and automation to X-ray measurement processes. With it many obstacles to make residual stress or retained austenite measurements on difficult surfaces and parts were removed.

Prism takes Group towards a full-service stress testing house

In 2006 Stresstech Group expanded its residual stress testing technologies more as AST bought a new type hole-drilling method from its developer Hytec, Inc. This method overcomes many of the challenges of X-ray diffraction for measuring residual stresses. Stress analysis by X-ray diffraction (XRD) has severe limitations for materials with relatively large grains or significant crystallographic texture. Like other hole-drilling techniques, Prism is much less sensitive to these factors and can deliver meaningful results where XRD can’t.

Prism marries the time-proven hole-drilling technique invented in the 1930s with the relatively new ESPI (Electronic Speckle Pattern Interferometry) with roots in 1970, which has become much more practical through the rapid development of computers and digital photography. Hole-drilling releases residual stress by removing a small volume of material at known depth range. ESPI is used to measure the resulting distortion of the surface, which then allows the calculation of the stresses that were responsible for the relaxation measured.

Prism increases the speed of data acquisition over the traditional strain gage hole-drilling technique. It eliminates the strain gage, and the entire drilling, data acquisition, and stress analysis functions are integrated into a single system. Stress depth profiles are produced in about half an hour. The traditional strain gage method requires applying the gage, soldering leads, and reading strains, followed by analysis of the strains to determine residual stress. This process can take between one and four hours.

Stresstech Group can now offer instruments and services for stress measurement by several different methods, each serving a different range of applications.

Measurement services

In 1993, Stresstech Group addressed the need for those customers, who only occasionally needed a stress value or who lacked manpower or capital to invest into the necessary hardware for more regular measurements. It started a testing services group in Pittsburgh, PA office to provide residual stress, retained austenite and Barkhausen Noise testing for a fee. Today measurement services are offered from all three companies.

In 2016 Stresstech Oy's measurement laboratory has been qualified for accredited laboratory status for residual stress and retained austenite measurements by X-ray diffraction as the first laboratory in Finland. This is recognized with FINAS accreditation, code T294.

Born global

Globalisation is the word of today. Markets in one country are hardly enough for any modern high-tech company. Stresstech Group can claim having been global already from its birth, because its markets were first opened in USA but the products were made in Finland.

Expansion to other markets was not easy and took its toll. First efforts to open a sales office in Europe were not succesful, so it was decided to start the business in Europe and Asia with agent basis representation. From 1988 on Andrzej Wojtas marketed the products in the European main markets finding and training local representives to many of them and supporting them in their first steps. Asia was first taken care by AST from USA, before local representatives were found and hired to India, China, Japan, Korea, and later Malaysia, Singapore and Australia.

This special business in non-destructive measuring field attracted also bigger companies so that in 1991 Seppo and Kirsti Tiitto sold AST, Inc. and Stresstech Oy to Outokumpu Instruments Oy. Three years later the business was taken over by Metorex International Inc. through a MBO sale. Ownership of the companies has changed twice since then; first in 1997, when four personnel members and Midinvest Oy bought the companies from Metorex. Latest MBO happened in 2006 so that the Group companies are now owned by four of their managers, who are Robert M. Fix, Lasse Suominen, Dominik Dapprich and Juha Siiriäinen.

By 1999 the methods and instruments made by the Group had gained so much acknowledgement from the market that a demand to open an own office in Germany was grounded. Stresstech GmbH was founded and has been run from that on by Dominik Dapprich having established itself to the heart of Germany in Rennerod.

Later on Stresstech Oy has expanded to Asia by establishing a company to serve the customers in India, Stresstech Bharat Pvt. Ltd.