About Us

Barkhausen noise starts the business

Even though Professor Heinrich Barkhausen discovered the phenomenon of inductive measurement of a noise-like signal generated when a magnetic field is applied to a ferromagnetic sample as early as 1919, researchers did not recognize the method’s suitability for grinding burn quality control until the 1980s. The method gained attention for industrial applications when a young Finnish scientist Seppo Tiitto studied it at the Department of Electrical Engineering, University of Oulu and at the Electronics Laboratory, Technical Research Centre of Finland, Oulu, in 1970s, and published his doctorate thesis about it in 1977*. Commercial serial production of the analyzer based on the above method began 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, started to market the method and instrument with better success.

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

Founding of American Stress Technologies and early commercialization

The early days of American Stress Technologies, Inc. (AST) – the first company of the group known today as Stresstech – were indeed very modest and unassuming. After incorporating AST in 1983, Seppo and Kirsti Tiitto established the company’s first offices in their apartment and the basement of their home in Bethel Park, Pennsylvania, where they remained for the next five 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.

From 1983 to mid-1985, the company focused most of its marketing efforts on two markets: utilities and steel company rolling mill departments, latter resulting the instrument name Rollscan. The company sold the first Rollscan instrument to a power generation company in Canada in 1983.

In 1985, AST worked with a North American crankshaft manufacturer to use Barkhausen noise for detecting grinding damage in hardened steel parts. Later that year, AST installed the first production-floor Barkhausen noise system (Rollscan 100 and inspection stand) for grinding damage detection, enabling the manufacturer to remove an entire nital etch line from the production floor.

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, Barkhausen noise can inspect parts ranging from small, complex semiconductor lead frames to large rolling mill rolls.

We design and manufacture sensors for the automotive and aviation industries, supporting static and dynamic, manual and automated inspections with one or more channels.

Since its founding, Stresstech has designed and built thousands of sensors for applications ranging from common camshaft and gear inspection to special uses such as knives and saw blades. We have also manufactured sensors for hard-to-reach internal surfaces, including small holes and the grooves of drill bits. In practice, each sensor is custom-made; however, we also maintain a small number of general-purpose and multi-purpose designs in stock to meet frequent demand for sensors that are less sensitive to surface shape.

From manual to automated measurements

As its simplest, Barkhausen noise measurements can be performed manually with only a sensor and a Rollscan. Today, manual measurements rarely meet the demands of quality departments since they are slow and sensitive to the operator’s skills.

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

Over the years, Stresstech Group has sold and installed thousands of Barkhausen noise systems worldwide. Today, the solutions delivered to customers range from handheld universal testers to automated multi-channel in-line systems.

Early Rollscan models have already been 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, which did so in October 1986. After the endorsement of BNA inspection by the SAE and FAA authorities, the automotive manufacturers soon followed the trend.

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 led to McDonnell Douglas and the US Navy buying many Rollscan systems for this application.

In 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.

Growth in applications, standards, and research community

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.

The Barkhausen noise method has gained wide interest and acceptance among scientists and industry professionals. Several universities actively use it, and researchers have included it in many postgraduate and doctoral theses.

Barkhausen noise also has its own discussion forum, ICBM, the International Conference on Barkhausen noise and Micromagnetic testing.

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 1990s. For commercializing and developing the instrument, the company Mexpert was founded, from which the technology was brought to Stresstech. AST sold and installed the first X-ray instrument, called AST X2000, in Mexico in 1991. AST soon followed with the AST 2002 model, which it sold in North America in 1991.

Two years later, Stresstech’s continued development of this technology resulted in the AST X2001 which was easier to take out of the laboratory than previous models. The company soon replaced the first diffractometer model G1 with the G2 and G2R (rotation) models, which it still produces today.

Customers in Asia, North America, and Europe soon adopted X-ray diffraction technology. European markets opened for Stresstech Group’s X-ray systems in 1996, with the first system sold in Europe going to France.

Development towards today’s portable X-ray unit produced the fourth generation highly portable Xstress 3000 system in 1996. Stresstech introduced the new G3/G3R diffractometer to the 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 diffractometer, thus bringing more flexibility and automation to X-ray measurement processes. The system removes many obstacles to measuring residual stress or retained austenite on complex surfaces and components.

Prism takes Group towards a full-service stress testing house

In 2006, Stresstech Group expanded its residual stress testing technologies 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 measures the resulting surface distortion and then enables the calculation of the stresses responsible for the measured relaxation.

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 of those customers who only occasionally need a stress value or who lack manpower or capital to invest into the necessary hardware for more regular measurements. Stresstech started a testing services group in its Pittsburgh, PA office to provide residual stress, retained austenite, and Barkhausen Noise testing. Today, we offer measurement services through all three companies.

In 2016, Stresstech Oy’s measurement laboratory was qualified for accredited laboratory status for residual stress and retained austenite measurements by X-ray diffraction as the first laboratory in Finland.

Born global

Globalization is the word of today. Markets in one country are hardly enough for any modern high-tech company. Stresstech Group has had a global reach from the very beginning, as it first entered the USA market while manufacturing its products in Finland.

Expansion to other markets was not always easy. Early attempts to open a European sales office failed, so the company expanded through agents in Europe and Asia. From 1988 onwards, a sales agent marketed products in key European markets, trained local representatives, and supported their early steps. AST in the USA initially managed the Asian market. Later, the company hired local representatives in India, China, Japan, Korea, Malaysia, Singapore, and Australia.

This special business in the non-destructive measuring field also attracted bigger companies so that in 1991, Seppo and Kirsti Tiitto sold AST, Inc. and Stresstech Oy to Outokumpu Instruments Oy. Three years later, Metorex International Inc. acquired the business through a management buyout (MBO). Ownership of the companies has changed twice since then; first in 1997, when four personnel members and Midinvest Oy bought the companies from Metorex. In 2006, four managers—Robert M. Fix, Lasse Suominen, Dominik Dapprich, and Juha Siiriäinen—completed a management buyout and took ownership of the Group companies.

By 1999, the market had widely acknowledged the Group’s methods and instruments, creating demand for a new office in Germany. The Group founded Stresstech GmbH and has managed the company since its establishment in Rennerod, located in central Germany.

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

In 2017 Nova Instruments LLC, a Boston, Massachusetts, USA  acquired Stresstech Oy.