PSI - Issue 53

Rainer Wagener et al. / Procedia Structural Integrity 53 (2024) 161–171 Author name / Structural Integrity Procedia 00 (2019) 000–000

162

2

Nomenclature a

width of a rectangular cross-section height of a rectangular cross-section engineering strain amplitude engineering total strain amplitude engineering strain

b e

e a

e a,t

ɛ

true strain

ɛ a

true strain amplitude true total strain amplitude peak-to-valley altitude

ɛ a,t Δ h K R Δ l N f

peak-to-valley height to length ratio peak-to-valley distance in measuring direction

number of cycles to failure

S

engineering stress

S a

engineering stress amplitude

σ

true stress

σ a

true stress amplitude

1. Introduction An on-going enforcement of the additive manufacturing technology even for cyclically loaded safety relevant metallic structures requires methods and tools to assess the cyclic material behavior already within early design stages of the component development process. Keeping the short and almost purely virtual development process of additively manufactured structures in mind, numerical methods should be used to exploit the full lightweight potential. On the one hand, within a fatigue approach the damage contribution of load time histories has to be assessed considering macroscopic elastic stress-strain behavior and amplitudes related to the High Cycles Fatigue regime as well as elastic plastic stress-strain behavior of amplitudes related to the Low Cycle Fatigue regime. Finally, and with respect of service life extensions, damage mechanisms related to the Very High Cycle Fatigue regime have to be taken into account. On the other hand, compared to the well-established manufacturing technologies, the number of parameters which can have an impact on the microstructure including defects, and thus on the structural durability increase drastically in case of additive manufacturing. The Wire Arc Additive Manufacturing (WAAM) is no exception. In order to perform a high-quality numerical fatigue analysis, it is important to consider these influencing factors in an adequate manner. Apart from the mean stress and notch geometry, influences caused by the production process, such as, gradient material properties along the cross section or the influence of load frequency should be considered for a proper fatigue estimation. To take advantage of the lightweight potential as well as the design freedom of Wire Arc Additive Manufacturing the local component-related material behavior should be characterized. Furthermore, the industrial needs of reducing the number of material properties and increasing the economic relation between numerical fatigue accuracy and experimental effort must be considered. Therefore, an increased knowledge of the component related material behavior is required to improve the conventional experimental procedure to derive cyclic material properties as well as the numerical fatigue approach methods especially in case of additively manufactured structures, including also the multitude of influences on the fatigue behavior of those components. With respect to an optimized degree of utilization and maybe a service life extension under service loading conditions the damage mechanisms from Low Cycle Fatigue up to the Very High Cycle Fatigue regimes have to be considered. To perform a fatigue approach of cyclically loaded components a variety of more or less different methods exists. They can be stress- or strain-based and they differ in the assumed material behavior. The stress-based fatigue approach concepts use linear-elastic stress-strain behavior and presume a homogeneous property distribution. On the other hand, the basic idea of strain-based fatigue approach concepts is to describe the local material behavior assuming an identity between the material behavior of homogeneous loaded cross-sections of finite dimension and an infinitesimal small, and therefor homogenously loaded material volume at the notch root. This implies the use of polished specimens to