PSI - Issue 13

Meike Funk et al. / Procedia Structural Integrity 13 (2018) 279–284 Author name / Structural Integrity Procedia 00 (2018) 000 – 000

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Steps near the edge have less influence than a step in the middle. Compared to specimens with similar sized steps but different positions, specimens with the step in the middle of the specimen have an up to 20 % higher cyclic lifetime. Specimens with a step near the surface have lifetimes comparable to specimens without steps. The detection of the first cracks with the DC-potential drop method depends on the stress scenario. The appearing crack in Figure 3 is identifiable with a suddenly rising potential compared to that of the non-damaged specimen. The same increase of the potential was found after 2 % of the cyclic lifetime in experiments without an overload ( σ max = 80 MPa). This clearly shows that the cracks are initiated within the first cycles. However, the formation of a continuous crack front happens after 50 to 90 % of the lifetime. Thus, the main part of the cyclic lifetime depends on the location of the crack initiation sites. This part determines the cyclic lifetime as well as the scatter of lifetime data. The calculation of the cyclic lifetime up to the formation of a continuous crack has to consider the position of the crack initiation sites and the interaction between the cracks emanating from these sites. Therefore, statistical methods have to be used for a description of this part. Further investigations can link these results with the corresponding stress integral of the notch shape. The size of the stress equivalent surface can possibly predict the probability of the incipient crack sites, whereas the sharpness of the notch plays a role. Sharp notches with a small stress equivalent surface shall effect less and smaller steps and a smaller standard derivation of the cyclic lifetime is estimated. Fatigued specimens were investigated in terms of different crack initiation scenarios and their effect on the cyclic lifetime. Different types of pre-damages vary the amount and position of the incipient crack sites. This investigation shows the following main points: (1) The amount and position of incipient cracks in the notch root has a strong influence on the crack path and the coalescence of cracks and with that on the specimen’s lifetime. (2) A linear correlation between the height of the step respectively the length of the step and the cyclic lifetime can be shown. (3) A single overload in the first cycles reduces the cyclic lifetime significantly. The crack initiation due to the overload can be measured with the DC-potential drop method. (4) Forcing crack initiation by laser cuts in the notch root leads to a higher reduction of the cyclic lifetime compared to crack initiation with an overload. This effect can be contributed to the missing plastic deformation in front of the crack tip. 5. Conclusions Bär, J., Wilhelm, G., 2014. Propagation of Fatigue Cracks in Notched Specimens of EN AW 7475-T761. Key Engineering Materials Vols. 592 593, 789-792. Bär, J., Tiedemann, D., 2017. Experimental Investigation of Short Crack Growth at Notches in 7475-T761. Procedia Structural Integrity 5, 739 800. Hu, W., Liu, Q., Barter S., 2009. A Study of Interaction and Coalescence of Micro Surface Fatigue Cracks in Aluminum 7050. ICAF 2009, Bridging the Gap between Theory and Operational Practice, 643-659. Johnson, H. H., 1965. Calibrating the Electric Potential Method for Studying Slow Crack Growth. Materials Research & Standards, 442 – 445. Tiedemann, D., Bär, J., Gudladt, H.-J., 2014. The Crack Propagation Rate According to Notches and Overload Levels. Procedia Material Science 3, 1359-1364. References