Additive manufacturing is currently attracting growing interest in many areas of mechanical, plant and vehicle engineering. In order to better control the reliability of components manufactured in this way, the Fraunhofer Institute for Structural Durability and System Reliability LBF has set up a new laboratory called "AM FATIGUE LABS". This laboratory develops methods that simulate stresses for additively manufactured components, which in turn serve as the basis for design recommendations. These realistic simulations can be used to determine accurate design parameters for the design of such components. They also ensure reliable design validation by taking into account the influence of all relevant process parameters, operational stresses and, depending on the application, environmental influences. Specially developed load simulators increase the precision and reproducibility of the measurements. This provides an insight into the cyclical material and component behavior that is almost impossible to achieve with conventional testing methods.
In order to exploit the advantages of additive manufacturing in terms of lightweight construction for safety-relevant components, numerous challenges must be overcome in the interplay between component geometry, production, operational stresses and environmental influences. Depending on the component geometry, the exposure strategy and the material used, among other things, almost any property gradient can be set in the component. However, these can also lead to geometrically identical components having significantly different operating behavior and ultimately different lifetimes under the same load.
Local phenomena drive component fatigue
The realization that the fatigue of components is driven by local phenomena is becoming increasingly important, especially for additively manufactured components. "The new degrees of freedom in component development require a new design concept in order to leverage the potential of this manufacturing technology for cyclically stressed, safety-relevant components," explains Dr. Rainer Wagener, under whose leadership the new laboratory was set up at Fraunhofer LBF. The manufacturing process induces geometric defects in the form of pores, inclusions or rough surfaces on the one hand, and on the other hand the locally very limited heating leads to the formation of significant property gradients.
In addition to the parameters of the exposure strategy or the process gas, which can be controlled directly by the user, the build direction and the design of the required support structures also play a significant role in the formation of the material microstructure and thus the local properties, including the defect distribution.
Optical strain measurement provides new insights
In the AM FATIGUE LABS, the Fraunhofer LBF team uses various optical strain sensors whose measurement signals have the necessary real-time capability. This enables strain control in failure-relevant component areas, for example. At the same time, the Darmstadt scientists can derive information about the local damage mechanism from the load-synchronous measurement of local strain fields. This information can be used to optimize components. In addition, it can also be used to increase material utilization by taking into account the defect-oriented material behaviour in the early design phases. "The derivation of dedicated design concepts and testing methods creates a level of application reliability for additively manufactured components that cannot be achieved with currently available regulations, which are all based on conventional manufacturing technologies," emphasizes Dr. Wagener.
Fraunhofer LBF would like to thank the German Federal Ministry of Education and Research for funding the joint project "Fatigue strength of additively manufactured components - BadgeB", which enabled part of the AM FATIGUE LABS to be realized.
