PSI - Issue 42

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

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Until 50% fatigue lifetime, a slight  R increase is noticeable for both states. However, the increase is greater in the less pre-damaged 30° specimen from 25% to 50%. This observation is attributed to earlier microcrack formation. A significant increase occurs from 50% to 75% lifetime, for 30° to 2.0% and 90° to 1.6%. In this range the increase is comparable for both states. Failure occurs when a defined value of total damage is accumulated. The total damage is a sum of forming induced pre-damage and fatigue damage, i.e. less forming-induced pre-damaged 30° specimen allows more fatigue damage compared to 90° specimen. Thus, a correlation between the electrical resistance and the fatigue damage can also be demonstrated here. 3.3. Microstructure investigations In order to prove that the difference in electrical

Fig. 4. Relative change in electrical resistance as function of the normalized fatigue lifetime for specimens with shoulder opening angles of 30° and 90°.

resistance can be attributed to damage only, microstructure investigations were performed. In the upper area of Fig. 5 for a forward rod extrusion formed component with 30° shoulder opening angle a SEM micrograph of a cross section of the central axis and an EBSD scan in cross section is shown, in the bottom area images for 90° shoulder opening angle.

In the SEM images (left) a higher amount of pores can be detected for 90° shoulder opening angle, in the EBSD scans (right) a grain elongation or anisotropic alignment according to the flow direction. As expected, the microstructural properties of the formed parts with 30° and 90° are comparable, the average grain size is 10 µm. A grain size reduction by a factor of two has been observed due to the forming process and hardness measurements in the cross-section result in 190 HV1 for both conditions. With these investigations, it could be shown that the microstructural properties affecting the electrical resistance are comparable for both states. Therefore, it can be assumed that the measured differences are due to the damage in the form of pores. 4. Summary and outlook

Forming-induced pre-damage has a great influence on the loading capability of formed parts. A reliable, non-destructive and efficient test method is needed. In this study, forward rod extrusion formed components with shoulder opening angles of 30° and 90° were investigated with a self-developed experimental setup for DCPD measurements. The Fig. 5. SEM images (left) and EBSD scans (right) of cross sections of the central axis of forward rod extrusion formed components with a shoulder opening angles of 30° (top) and 90° (bottom).