Pretty close to hell: Material testing under extreme conditions

Using four MTC test benches developed by IABG, the German Aerospace Center (DLR) in Augsburg undertakes important basic research for future generations of engines. It tests a wide variety of materials under precisely reproducible mechanical, thermal, and chemical conditions – at temperatures of up to 1,400°C, pressures of 20 bar, and with synthetically mixed exhaust gases. PC-based control, EtherCAT, and precision measurement technology from Beckhoff are responsible for generating these extreme test conditions, monitoring the test sequences, and recording all measurement results.

As part of the innovation partnership with IABG in Ottobrunn, which has been in place since 2020, four different MTC test benches (a planar biaxial hydraulic test bench, a uniaxial hydraulic test bench, creep test bench, and a 1,000 Hz very-high-cycle fatigue test bench) have been developed together with the German Aerospace Center (DLR, Deutsches Zentrum für Luft- und Raumfahrt) for testing materials under simultaneously acting mechanical, thermal, and chemical loads (MTC). “By using these test benches, we can test material samples under engine-like conditions to assess their long-term performance,” explains Dr. Matthias Wimmer, Project Manager at IABG. As a result, the test benches help to achieve more reliable and precise predictions of the service life of engine components subjected to multiphysical loads.

Engine-like conditions

On all four test benches, the material samples can be exposed to a mixture of various clean gases (N₂, O₂, CO₂, NO, CO, SO₂) and water vapor inside a pressure chamber during testing. “This allows DLR researchers to realistically simulate an engine’s exhaust gases as well as the incoming air and to observe the effects on the material,” says Andreas Keil, Project and Product Manager at IABG. In addition, each test bench is equipped with a laser-based heating system that can precisely heat the samples to temperatures of up to 1,400°C. “Such a combination of mechanical, thermal, and chemical exposure to exhaust gases and moist air has not been seen before,” says Dr. Laura Cordes, Head of Department at the DLR Institute of Test and Simulation for Gas Turbines (SG). The respective material tests are conducted under defined environmental conditions. These tests typically include cyclic sinusoidal loading and testing sequences, standard Wöhler tests, or crack propagation studies. “The results will be incorporated into the development of the next generations of gas turbines and engines,” emphasizes Dr. Wolfgang Müller, Head of the Mechanical-Thermal-Chemical Testing Group. In addition, the measurements help to validate existing numerical simulation models for materials and alloys that are being developed as part of the virtual engine project at the DLR Institute.

Measurement technology, safety, and automation in one system

For automation, test room monitoring, and measurement technology, IABG relies on the Beckhoff portfolio and EtherCAT. “PC-based control is the ideal platform for us to implement such complex systems,” says Andreas Keil, “while remaining high-performing, open, and flexible.” Thomas Gabler, who is responsible for automation, controller, and software development at IABG, adds: “The ability to easily exchange safety and non-safety data in particular made implementation much easier for us – especially with measurement technology as an integral part of the system.”

For Andreas Keil, openness is essential when dealing with such heterogeneous test benches that involve many different equipment suppliers. “With PC-based control, we know from our experience in many projects that the integration of a wide variety of protocols and interfaces actually works. This includes the integration of components via PROFINET RT and TCP protocols.” EtherCAT is used for communication with the laser controller, whereas the safety components are wired in the traditional manner. The position of the laser heads is monitored by safety switches. The temperature of the laser traps can be measured using thermocouples, TwinSAFE SC, and EL3314-0090 EtherCAT Terminals.

According to Raphik Shahmirian from Sales at Beckhoff Munich: “The positive experience with PC-based control also prompted the laser supplier to use the C60xx ultra-compact Industrial PCs and TwinCAT as the general control platform for their own devices going forward.” The door interlocks for the individual test rooms are, in turn, integrated via EtherCAT P and FSoE. “This option saved us a lot of wiring effort and made it easier to integrate the system into the safety circuits,” emphasizes Stefan Glauer, Technical Manager for Test Bench Automation at IABG.

Since test benches are constantly being expanded with additional equipment to meet new requirements, a communication system like EtherCAT – which can be adapted flexibly to these requirements in terms of performance and topology – is virtually ideal. In addition to several EtherCAT segments and EtherCAT P technology, IABG uses EtherCAT Couplers for other fieldbuses. “The extensive modular system of EtherCAT Terminals and other components is one of Beckhoff’s strengths and is very valuable to us,” emphasizes Andreas Keil.

Standard sensors used for safety

One example of this diversity is EtherCAT Terminals with TwinSAFE SC (Single Channel), which are used to monitor the safety-critical temperatures of the laser traps and laser beam containment tubes. These are measured using standard thermocouples and PT100 resistors and digitized via the corresponding EtherCAT Terminals (EL3314-0090 and EL3214-0090). “TwinSAFE SC enabled us to safeguard the performance level determined by prior risk assessment,” says Andreas Keil. This is because some of the experiments run for several weeks and must operate reliably even when unsupervised. Raphik Shahmirian adds: “TwinSAFE SC is a very elegant solution for safely evaluating temperature inputs.”

In analog measurement technology, EtherCAT demonstrates another of its strengths as a fast, synchronized measurement technology bus: The signals from the thermocouples and all other sensors are digitized with high precision near the test chamber and transmitted to the controller. “This decentralized I/O approach enables short, cost-effective sensor cabling to be used while ensuring extremely precise synchronization in the sub-µs range throughout the entire system,” adds Christian Lindemann, I/O Product Manager at Beckhoff. The ELM3704 multi-functional terminal is used on the test benches as a universal measuring device in the decentralized control cabinets.

On the MTC test benches, the forces, positions and displacements, and, in some cases, torques are read from the controllers of the load frames via their analog outputs. “We had no choice, because there was no possible synchronized digital interface between the controller of the load frames and the Beckhoff controller that would have met the requirements of the DLR,” says Christian Lindemann, explaining the reasoning behind this solution. For this reason, the standardized signals (±10 V) from the load frames are acquired with high precision at a sample rate of 10 ksamples/s via 4-channel ELM3004 Terminals for voltage measurement. “At a PLC cycle of 1 ms, a tenfold oversampling rate is perfectly sufficient,” explains Stefan Glauer. Oversampling with EtherCAT enables highly dynamic data acquisition at moderate cycle times, thereby reducing the performance requirements for the IPC.

The test benches are controlled, monitored, and configured from a central control room using a LabVIEW™ user interface, which also allows the EtherCAT Terminals to be configured via TwinCAT ADS (Automation Device Specification) and CoE (CAN over Ethernet). To this end, the Interface for LabVIEW^TM (TF3710) was further developed in accordance with the requirements to enable flexible configuration of the EtherCAT Terminals from the control room and to coordinate the measurements.

CX2043 and CX5140 Embedded PCs are used as controllers for the test benches; they are connected via an EL6692 EtherCAT Bridge and collect all process values – such as temperatures, pressure readings, and gas composition, as well as load conditions and device status information – in sync with all measurement channels. “Despite different sample rates, TwinCAT consolidates all measured values based on the precise timestamps of the EtherCAT telegrams,” says Christian Lindemann. The complete process image is then available for analyzing the test series. “We prefer a traditional architecture for the test benches and keep the system controller separate from the measurement technology,” adds Andreas Keil. In the future, MATLAB^® and Simulink^® models developed by DLR scientists are also expected to run in parallel on the measurement technology IPC. Thanks to this clear separation, any modifications to the measurement technology will be without impact on the control technology.

Looking ahead, the DLR and IABG still have many ideas: They are considering using OPC UA not only to transmit information from the test benches to the DLR, but also to use this interface to take control actions. “In principle, it would be possible to adjust the load cycles during operation and, for example, to replicate the conditions inside a turbine during flight in real time on the test bench,” says Dr. Wolfgang Müller. Similarly, the researchers could run a simulation model against the test bench and use the data to compare it with real-world physics. Based on the open approach and flexibility of PC-based control, developers and researchers can also implement these next stages of development.