PSI - Issue 19

Rainer Wagener et al. / Procedia Structural Integrity 19 (2019) 380–387 Author name / Structural Integrity Procedia 00 (2019) 000 – 000

383

4

increasing amplitudes. Within the presented examination, the test started with the maximum amplitude and from reversal point to reversal point the amplitude was reduced by an increment of 5% of the maximum amplitude until the amplitude was equal to 0. Following the minimum, the amplitudes were increased again by the same increment until the maximum amplitude was reached. The resulting test blocks were repeated until crack initiation occurred. In order to compare the so-called cyclically stabilized stress-strain curve, the block at 50% of the number of cycles to crack initiation was evaluated. The basic idea of local fatigue concepts is to describe the localized fatigue behavior at the notch root by means of an infinitesimal small and therefore homogeneous material volume. In case of significant different microstructures and property gradients, another set of material properties, representing possible inhomogeneities is required. With additively manufactured structures in mind, it is very difficult to define an infinitesimal material volume as well as specimens for the experimental derivation of the material properties even by the use of sub-size specimen geometries, because additively manufactured structures contain many different microstructures. With respect to numerical fatigue approach, the effort will increase intensively. Having a closer look on additively manufactured structures, Fig. 6, different microstructures are recognizable. Due to the exposure strategy, different microstructures in the contour and the core exist. Pores of different sizes are mainly located inside the inner half and a rough surface characterizes the contour. Even after the removal of support structures an influence on the microstructure due to different mass concentrations and resulting cooling conditions remains. The microstructure of the core material is characterized by the scanning pattern, because the orientation of the hatching, which is rotated layer by layer, can be identified. The transitions can be interpreted as a metallurgical notch effect, which leads to a subsurface stress concentration and reduces the fatigue life. 2.3 Stress-strain system

Figure 6: Microstructure of additively manufactured AlSi10Mg by laser powder bed fusion

Due to the surface conditions, it is difficult to measure a cross-section in order to define a nominal stress even in the case of unnotched test specimens. On the other hand with reproducible surfaces as a function of the exposure strategy and support structures it is sufficient to define a structure element with a technical surface. The resulting material or better structure properties contain the influence of these specified surface layer conditions. The classical way to consider irregularities like pores, surfaces or different microstructures caused by the manufacturing process during a fatigue approach is to start with a homogeneous material followed by influencing factors to adjust the fatigue strength or life. Normally, the scope of a fatigue approach is not the local material behavior but the structural behavior. Keeping this in mind a transition from local stresses and strains to stresses and strains of a structure element should be useful for a fatigue approach of an additively manufactured structure. Due to this reason, the stresses and strains as well as the Young’s modulus have to be interpreted as a resulting stress, strain or stiffness of structure elements. Well knowing that due to stress concentrations and irregularities the local stresses and strains could be higher.