Fatigue Behavior and Lifetime Prediction

The reliability of components is a central aspect in the qualification of new manufacturing processes with resource-efficient material use. In this context, the focus is on fatigue properties, the experimental verification of which is often the bottleneck for the transition from development to series production. With its modern high-frequency testing laboratory, Fraunhofer IWS offers a wide range of possibilities for a rapid strength assessment of materials and components and thus creates a previously unattainable reduction in the necessary testing times.

Load transmission elements can be found in almost all industrial applications, regardless of the drive concepts used. For this reason, the operationally stable design of these components is a constantly topical issue. At Fraunhofer IWS the focus is on welded, rotationally symmetric components that are subject to superimposed torsional and axial loads. Both the simulation-based design and the experimental fatigue strength verification for the complex stress conditions in the joint area represent an intensive field of research at Fraunhofer IWS.

Fatigue fracture in materials and components is mostly based on notch effects resulting from defects or microstructural irregularities. A load-compliant design of structural components is therefore only possible if these strength-reducing features are taken into account. This aspect increases in importance for additively built or joined structures. Therefore, at Fraunhofer IWS service life estimates are always based on experimental correlations between the fatigue strength and the crack-inducing elements of a structure. The focus here is not necessarily always on “as good as possible“, but much more often on “as good as necessary“.

The use of natural fiber materials in structurally relevant applications such as bridges and other load-bearing structures requires reliable lifetime prediction that also accounts for mechanically repeated loading. The Materials and Component Reliability research group at the Fraunhofer IWS focuses on the reliability assessment of natural fiber materials. The sustainable material class of wood and wood-based connections provides an ecological alternative to conventional fiber-reinforced composites and is gaining increasing importance across various industrial sectors due to its sustainability.

The objective of the research activities at Fraunhofer IWS is to evaluate the mechanical properties, with particular emphasis on material behavior under cyclic mechanical loading. For this purpose, various testing methods are available, including four-point bending test setups and in-situ measurement techniques (e.g., thermography and digital image correlation for strain and displacement measurements). In addition, application-specific testing methods are being developed to determine practice-relevant characteristic values for quasi-static strength and fatigue behavior.

A central aspect of this work is the investigation of damage evolution. Only when failure-critical material regions can be identified is it possible to implement strength-enhancing countermeasures. In this way, risks can be minimized and operational safety can be improved.

The insights gained are directly incorporated into the development of new, high-performance natural fiber products. Through this work, Fraunhofer IWS makes an important contribution to the establishment of sustainable material solutions in industry.