Approaches to fatigue life assessment applied in the very high cycle regime

Designers and calculation engineers are becoming increasingly interested in the latest results on very high cycle fatigue (VHCF). Often, the influence of loading with a very high number of cycles on component behaviour is estimated conservatively, but the exact safety margin is unknown. This paper gives an overview of failure mechanisms in the HCF‐ and VHCF‐regions and of material and component related influences, which have to be considered in the fatigue life assessment. The state of the art of design codes, recommendations from the literature and initial investigations on variable amplitude loading in the VHCF‐regime are presented. This review indicates that further research activities are necessary to improve fatigue life assessment in order to allow a reduction of safety margins.     

Erweiterte Prüfmethoden zur sicheren Rad- und Fahrwerksverschraubung der E-Mobilität

Changes towards electrified vehicles lead to an overall increase of loads in chassis components, particularly in wheel attachments. Especially wheel attachments are facing increased dynamic loads, caused by increased axle loads, wheel dimensions, cornering loads due to tire grip and center of gravity improvements…to name some examples. To prevent fastener failures in future, new measurement and analysis procedures are being presented, enabling a complete and accurate verification of superimposed dynamic stresses caused by dynamic axial and bending loads in wheel & chassis fasteners at its critical failure area. Furthermore load-specific S−N-curves are required to accomplish an appropriate damage accumulation. This detailed understanding of loads occurring at fasteners itselfs enables improved safety in all chassis fasteners, provided these loads are known and taken into account at an early stage of a development process.     

Behaviour of cellular materials under impact loading

The paper describes experimental and computational testing of regular open‐cell cellular structures behaviour under impact loading. Open‐cell cellular specimens made of aluminium alloy and polymer were experimentally tested under quasi‐static and dynamic compressive loading in order to evaluate the failure conditions and the strain rate sensitivity. Additionally, specimens with viscous fillers have been tested to determine the increase of the energy absorption due to filler effects. The tests have shown that brittle behaviour of the cellular structure due to sudden rupture of intercellular walls observed in quasi‐static and dynamic tests is reduced by introduction of viscous filler, while at the same time the energy absorption is increased. The influence of fluid filler on open‐cell cellular material behaviour under impact loading was further investigated with parametric computational simulations, where fully coupled interaction between the base material and the pore filler was considered. The explicit nonlinear finite element code LS‐DYNA was used for this purpose. Different failure criteria were evaluated to simulate the collapsing of intercellular walls and the failure mechanism of cellular structures in general. The new computational models and presented procedures enable determination of the optimal geometric and material parameters of cellular materials with viscous fillers for individual application demands. For example, the cellular structure stiffness and impact energy absorption through controlled deformation can be easily adapted to improve the efficiency of crash absorbers.     

Beyond HCF – Is there a fatigue limit?

The paper gives an overview to the present state of research on fatigue strength and failure mechanisms at very high number of cycles (N>10⁷). Testing facilities are listed. A classification of materials with typical S‐N curves and influencing factors like notches, residual stresses and environment are given. Different failure mechanisms, which occur especially in the VHCF‐region like subsurface failure, are explained. There microstructural inhomogeneities and statistical conditions play an important role. Investigated materials are different metals with body‐centred cubic lattice like low‐ or high‐strength steels and quenched and tempered steels but also materials with a face‐centred cubic lattice like aluminium alloys and copper.