Fatigue analyses at thyssenkrupp Automotive Systems Test Center
24 hours a day, seven days a week: With individually designed test stands, the thyssenkrupp Automotive Systems test center performs high-end fatigue analyses for demanding automotive customers.
Essen, Münchener Strasse 100. In the heart of the Ruhr region lies thyssenkrupp Automotive Systems’ test center, a laboratory with state-of-the-art equipment for fatigue testing of chassis components and systems. This is where the automotive future is put through its paces. 24 hours a day, seven days a week – until design life expectancy is reached or tests are broken off because of component failure.
When vehicles drive over potholes and obstacles, loads can occur that are two to three times higher than normal. The damage caused can significantly reduce the fatigue strength and thereby the lifetime of components and systems. Before cars are launched on the market, they are therefore subjected to intensive test procedures to check the fatigue strength of components and identify weak points in advance. This is done with real prototypes on test tracks, in computer simulations and on test stands in laboratories such as the thyssenkrupp test center in Essen.
Expertise in the automotive, railroad and machine-building sectors
Here, 25 employees test the durability and life expectancy of systems and components for the automotive industry and other sectors. Customers include major auto manufacturers and their suppliers. The center cooperates closely above all with thyssenkrupp sister companies from the chassis businesses like the shock absorber, springs and stabilizer or steering divisions. “Fatigue analyses for the automotive sector are our main focus,” says Dr.-Ing. Jan R. Kurzok, head of the test center since 2005. “But we also have a great deal of expertise in other sectors such as railroad equipment or machine building.”
In the field of railroad equipment, for example, the center is currently testing components of rubber-sprung railroad and tram wheels. Dr. Kurzok: “We simulate wear and other fatigue phenomena to validate the effectiveness of product optimizations. However, the facilities available at the test center also allow testing of larger railroad components subject to higher loads, such as complete wheels.”
The test center’s expertise is also in demand from other areas, e.g. the machinery sector. For example, the Essen team collaborated with a manufacturer of power plants and generators to develop a procedure to test high-voltage bushings and analyze their fatigue behavior. A test stand developed in-house produced the same failure behavior as in real-world operation. As a result, the bushings were optimized on the test stand before being used for decades in the power plant.
Parts on the test stand experience the same conditions as on the test track
In the 2,500 square meter test center, Jan Kurzok and his team simulate fatigue phenomena that occur in very many applications involving not only vibrations but also climatic stresses. “We mainly run load cases for chassis systems in our labs,” explains the doctor of engineering. “That’s not something a normal car driver usually does with his car,” he smiles. “It’s not uncommon for our customers’ development engineers to attend our tests and be amazed at how accurately the wear patterns reflect what they are familiar with from real-world operation or from the test track. It’s also not uncommon for important findings to come to light that have nothing to do with fatigue analysis. It’s not usual for our customers to experience their systems and components in extreme driving situations this way, in isolation from other components.”
Each test stand is individually designed
The forces acting on the parts to be tested – which include axles, shock absorbers and truck cab mounts – are simulated on the test stand by hydraulic cylinders and pneumatic or, increasingly, electric actuators. Influences such as drive torques, braking and steering forces as well as extreme temperatures are also taken into account. Various sensors, from strain gauges to load cells, measure the loads, displacements, accelerations and forces. “In theory, this can involve any number of sensors,” says Dr.-Ing. Michael Oeter, responsible for quality assurance and scheduling at the test center. Dipl.-Ing. Roland Pötzsch, who completes the management trio and is responsible for the center’s test equipment adds: “A complex axle test stand, for example, has over 70 measuring points”. Uniquely, the team doesn’t use standardized test stands but designs each stand individually.
The greatest challenge is to apply the correct forces to the parts to be tested. Because the actuators influence each other, especially in multi-axial test stands, a complex procedure is required. “Each actuator introduces part of the excitation functions and may have couplings with other load application points, so we can’t just take the signal from the outside and apply it,” explains Michael Oeter. The test engineers calculate the correct, synthetic exitation signals, which are further refined in an iterative process. Roland Pötzsch: “For complex test stands, we start by applying only 20 percent of the defined target load to the system and then analyze the result. Step by step, we then increase the target load for all actuators until the signals on the test stand and potential responses correspond as closely as possible to the target specifications.” For particularly complex projects, such as the testing of truck cab mounts, this iteration process can take up to three weeks.
Testing new limits with curiosity
Technical tasks like these are a constant challenge to the test center team, as are constant advances in technology. Jan Kurzok: “We always ask ourselves what will be the demands of our customers tomorrow and the day after tomorrow? What are the next challenges awaiting our customers and how can we support them?”
With innovative systems such as “steer-by-wire” and “eSuspension”, the customers of the test center are playing a big role in shaping the automobile of the future. The growing importance of electronic and mechatronic components and systems presents the engineers in Essen with new challenges every day. Roland Pötzsch: “In the past, a shock absorber was a shock absorber: a piston in a cylinder pushed back and forth by a bit of oil. Nowadays, for example the characteristics of our customers’ dampers can in some cases even be adjusted in advance depending on driver preferences and the road situation. So today we no longer just have to simulate the wheel movements but also supply the appropriate control current so the mechatronic damper works on the test stand just as it does in the vehicle.”
Test stand work becoming increasingly important
Against a background of increasingly complex systems and ever-shorter development cycles in the automotive industry, test stand work coupled with computer simulation is becoming more and more important. “Customers come to us, for example, when they realize that a system environment is too complex to test a product on their own test stands or they simply lack the capacity,” explains Michael Oeter. “We offer enormous flexibility – both in terms of space and the number of control and measurement channels we can handle to simulate more system environments.”
Even for very expensive rough-road tests, it is worthwhile for customers to work with thyssenkrupp’s test center. Before vehicles are sent out on the test track, components and systems can be pretested on the test stand to a point where the test results are ideally only confirmed in the vehicle. That way, customers can save time and costs. “Road tests on the test track are not only expensive and time-consuming,” says Jan Kurzok, “they also put an extreme strain on test drivers and are dependent on the weather. I remember one winter on the test track in Papenburg. We always chose the time slots after the track had been used by other cars so we knew the surface would be dry and we could achieve sufficient lateral acceleration. We got our results, but it was very time-consuming, with a lot of waiting in nasty weather. It’s much nicer in the lab,” he laughs
“The most important thing we have are our employees”
For their demanding work in the laboratory, the test engineers in Essen can rely on high-end equipment: state-of-the-art control technology, extremely powerful software and high-performance computers. “We have a lot of cool equipment here,” says Roland Pötzsch, “but the most important thing we have are our employees. Our people have a lot of know-how. They are our capital.” Jan Kurzok is certain: “When it comes to chassis system testing, we and our team are one of the very top labs.”