PSI - Issue 53

Rainer Wagener et al. / Procedia Structural Integrity 53 (2024) 151–160 Author name / Structural Integrity Procedia 00 (2019) 000–000

152 2

Nomenclature b

cyclic strength exponent

b j

slope of the elastic strain life curve of the j

th -regime, with j = 1 to 3, of the Fatigue Life Curve

c cyclic ductility exponent HBV highly stressed volume HCF High Cycle Fatigue K’

cyclic strengthening coefficient

k

slope of the SN-curve (before the knee point) slope of the SN-curve after the knee point

k 2 K t n’ N i

stress concentration factor

LCF

Low Cycle Fatigue

cyclic strengthening exponent number of cycles to crack initiation

R

load ratio

RSE

Representative Structural Element

VHCF Very High Cycle Fatigue ε a,e elastic strain amplitude ε a,p plastic strain amplitude ε a,t total strain amplitude ε f ’ cyclic ductility coefficient σ a stress amplitude σ f ‘ cyclic strength coefficient

In this context it is above all production-related properties, taking into account the industrial boundary conditions, that means constraints of economic and time expenditure. Furthermore, the number of required properties with respect to their transferability should be limited, but the mean influences on the structural durability have to be considered. Finally, the numerical effort should be manageable, so that the manufacturing and test cannot be carried out in less time than the numerical simulation. For the discussion of the cyclic material and component behavior of additively manufactured structures, due to its high Technology Maturity Index according to Ampower (2019) compared to other technologies, Powder Bed Fusion Laser Beam of metallic structures (PBF-LB/m) is the chosen manufacturing processes and will therefore be considered in the following. Today several fatigue approach methods are available. These methods have been developed to enable a fatigue approach of conventional manufactured components in order to ensure their structural durability. Therefore, these methods could be distinguished in different families of approach methods like stress- or strain-based methods and, depending on their point of view, more global or local ones. At the end, all methods need an assumption for the stress strain behavior, e.g., linear elastic or elastic-plastic stress-strain behavior, as well as force- or strain-SN curves. On the one hand and due to the nature of the force-based fatigue approaches, a reference SN-curve, normally a fully reversal tension-compression (R = -1), of unnotched specimens at lab conditions is used. To consider the influence of, e.g., notches, mean stresses or ambient temperatures, on the fatigue sensitivities of influence factors, which must be derived experimentally, are required to shift the reference SN-curve. Due to this procedure, the numerical simulation effort can be limited, provided that a derivation of the individual influencing factors is possible, and no mutual influences takes place. On the other hand, the strain-based fatigue methods reduce the experimental effort at the expense of the numerical simulation effort. Starting with the so-called cyclic stabilized material behavior of the sound material it has to be distinguish whether an influence depends on a different microstructural or is related to a geometrical effect, e.g., surface roughness or pores. Within the first case another set of cyclic material properties is needed and, in this second one, the geometry of the defects must be integrated into the geometry model, which requires a fine mesh and increases the time to solve the FE model.