In order to ensure product properties, suitable test methods for more advanced maturity levels are required in addition to materials testing. IWU has extensive expertise in the design and operation of such testbenches. This ranges from the characterization of semi-finished products and the replication of actuator and sensor concepts to the testing of complete systems according to customer specifications.
SMA actuators and sensors are characterized by numerous influencing factors, such as the usable stress and strain, cyclical thermal and mechanical loads, various environmental influences and the control power. The complex interactions between these factors often lead to considerable experimental effort in the experimental investigations, which is not necessarily justified.
For this reason, designers and manufacturers of SMA semi-finished products and systems have a need for special test rigs that differ in terms of design, testing technologies and the influencing variables investigated.
IWU has a large number of testbenches that can be used for the analysis of SMA semi-finished products and actuators. These range from microsystems with forces in the milli-Newton range to small actuators or springs in the Newton range and high-load actuators in the kilo-Newton range.
Cyclic test benches can be used to carry out quality, reliability and service life tests on semi-finished wire products or specific actuator configurations, including control electronics.
SMA wires show a distinct run-in and degradation behavior, which is why their functional properties change during operation. For the targeted design of actuators, it is necessary to know this behavior and to take it into account in the design.
For this purpose, Fraunhofer IWU developed a testbench and a methodology that enable the functional degradation of actuator wires to be characterized and evaluated. Actuator wires can be cyclically loaded with individual load and activation parameters on a total of three tracks. The load parameters are categorized into the actuating stroke and the actuating force or the elongation and mechanical tension as well as the number of cycles.
The actuating force is applied to the wires using specially manufactured weights to simulate a constant load. The displacement or elongation can be set individually using a device and is measured and recorded using a laser displacement sensors. The wires are activated by means of time-controlled current signals and can be adapted to the semi-finished wire product being tested. External measurement equipment allows the wire resistance to be recorded during cycling and the ambient temperature to be monitored.
The aim is to generate measurement data with as little experimental effort as possible, which is then approximated using mathematical regression models. These models allow the functional degradation in the workspace to be predicted.
Performance and measurement accuracy
Variable load simulation
Variable activation parameter
In addition to functional fatigue, structural fatigue - i.e. the mechanical damage to the SMA actuator - is a second key design parameter. Particularly in the case of electrically activated and tension-loaded SMA wires, the mechanical load and the strain used play a decisive role in achieving high cycle numbers. With the structural fatigue test bench, SMA wire semi-finished products can be subjected to cyclic loading until breakage under various load scenarios.
On the one hand, actuator systems including control electronics can be simulated and validated, and on the other hand, basic investigations into the load limits of semi-finished wire products or the influence of batch fluctuations can be carried out.
Life cycle analyses
Performance and measurement accuracy
Flexible activation
In addition to electrically activated wires, SMA springs are another class of actuators that have been used in a wide range of applications for decades. In contrast to wires, SMA springs are usually activated by the surrounding medium, such as air, water or oil, and not by electric current. The cyclic testing of SMA springs is therefore very time-consuming due to the long activation and deactivation times. With the SMA spring test bench, such tests are nevertheless possible thanks to the fully automated test procedure.
Performance and measurement accuracy
Operation modes/measurement methods
The most powerful class of SMA actuators in terms of achievable force are the so-called SMA high-load actuators. These are actuators with a large effective actuator surface to realize the large forces and at the same time large lateral surfaces and optimum thermal design to improve the dynamics. A high-load testbench with high rigidity and precise measurement technology is available for the investigation of such actuators, on which fully automated metrological investigations can be carried out.
Performance and measurement accuracy
Flexible activation
Variable load simulation
Long-term tests
Fraunhofer Institute for Machine Tools and Forming Technology