PSI - Issue 46

L. Lücker et al. / Procedia Structural Integrity 46 (2023) 94–98

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Lukas Lücker / Structural Integrity Procedia 00 (2021) 000–000

1. Introduction and state of the art Today's metal forming is not only used for changing the shape of materials, but also for the precise adjustment of product properties and capability (Tekkaya et al. (2017)). The features of formed steel parts are dependent on forming induced damage, strain hardening and residual stresses. At present, only strain hardening and residual stresses are considered in the design of forming parameters. Damage in metals is not a failure, but it describes the decrease of the load-bearing capacity due to the appearance and evolution of voids (Lemaitre (1985)). Consequently, damage nucleates and evolves long before failure occurs. Therefore, a damage-controlled forming process provides safer parts with higher failure tolerance or even lightweight potential. For damage-controlled forming, damage has to be known, preferentially determined in non-destructive measurements. Initial studies underlined that ductile pre-damage in form of pores and micro-cracks correlate well with micromagnetic measurements (Teschke et al. (2020), Borsutzki et al. (2010)). However, micromagnetic values capture several microstructural properties changing with cyclic loading (Tschuncky (2016)). Thus, the conclusion on ductile damage evolution is not unambiguously possible with pure micromagnetic measurements, coupled measurement methods and approaches are mandatory to determine ductile damage quantitatively. Electrical measurements have already been successfully correlated with pore volumes in aluminum (Koch et al. (2020)). In order to fully understand ductile damage evolution and its influence on fatigue behavior, practice-oriented three-point bending fatigue tests were performed. To the knowledge of the authors, no investigations have been made so far on the forming-induced pre damage influence on bending fatigue behavior. 2. Experimental procedure 2.1. Material Cold-formed 16MnCrS5 (AISI 5115, 1.7139) steel parts were investigated with a chemical composition shown in Table 1. For cold forged parts, the forming process parameters were chosen with the aim to keep strain hardening and residual stresses constant, while pore size and number differ due to pre-damage.

Table 1. Chemical composition of the material investigated, all data in wt%. Material C Si Mn

S (max)

Cr 0.8

Fe

16MnCrS5

0.14

< 0.4

1.1

0.02

bal.

Using the forward rod extrusion forming process, there is almost a constant strain on the centerline at the same extrusion strain. By varying the shoulder opening angle, different stress states are generated, which lead to different ductile pre-damage levels. This has already been quantified in SEM (Hering et al. (2020)). The change of shoulder opening angle has no detectable influence on the strain in the area around the central axis. Consequently, a comparable texture and grain size can be expected. Due to the preparation of specimens and the reduction during the ejection process, it can be assumed that the influence of residual stresses can be neglected.

Previous studies of Samfaß et al. (2020) have shown that the difference in ductile damage is particularly noticeable at a shoulder opening angle 2α = 30° and 2α = 90° and a constant extrusion strain φ = 0.5. Therefore, forward rod extrusion formed components with same forming parameters were compared in these investigations. From each component (Fig. 1(a)), three flat specimens with a thickness of t = 1 mm were extracted from the center axis by means of wire erosion (Fig. 1(b)). The specimen

(b)

(a)

Fig. 1. (a) Forward rod extrusion parts with different shoulder opening angle and resulting area of voids (A void ), (b) Location of specimen extractions from parts.