PSI - Issue 42

Lukas Lücker et al. / Procedia Structural Integrity 42 (2022) 368–373 Lukas Lücker / Structural Integrity Procedia 00 (2019) 000 – 000

369

2

Keywords: Non-destructive damage characterization; DCPD; AISI 5115

Nomenclature A Area A Void

Void area DCPD Direct current potential drop f Frequency N Number of cycles R Electrical resistance R  Load ratio SEM Scanning electron microscope  R

Relative change in electrical resistance

φ

Extrusion strain

2 α

Shoulder opening angle

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 stress state. Usually, only strain hardening and residual stresses are considered in the selection of forming parameters. Damage in metals is not failure, but it describes the decrease of the loading capability due to the amount and evolution of void, Lemaitre (1987). Consequently, damage nucleates and evolves before failure occurs. Therefore, damage-controlled forming provides safer parts with higher failure tolerance or even lightweight potential, i.e. 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 correlates well with micromagnetic measurements, Teschke et al. (2020) and Borsutzki et al. (2010). However, micromagnetic values capture several microstructural properties changing with loading history, Tschuncky et al. (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 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, instrumented tests were performed. In other studies, the fatigue behavior of steel was described with the means of electrical resistance measurements, Starke et al. (2018). To authors knowledge, no investigations have been made so far on the forming-induced pre-damage influence on fatigue behavior captured by electrical resistance measurements. 2. Material and experimental procedures 2.1. Material The investigated cold-rolled AISI 5115 (16MnCrS5, 1.7139) steel has a chemical composition shown in Table 1 and were treated with a ferrite-pearlite annealing (+ FP). The steel bars were turned from a diameter of 40 mm to 30 mm and cut into parts. These parts were then formed by means of forward extrusion to components. With the aim to keep strain hardening and residual stresses constant, the forming process parameters of these cold forged parts were chosen while pore amount and size differ due to pre-damage.