PSI - Issue 7

M. Wicke et al. / Procedia Structural Integrity 7 (2017) 235–241

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M. Wicke et Al./ Structural Integrity Procedia 00 (2017) 000–000

4. Experimental Results 4.1. Long Crack Threshold

In order to measure the long-crack threshold, three experiments on specimens of both materials in LS- and TS orientation were performed according to the method described previously. The resulting da/dN vs. Δ K-curves of two experiments is shown in Fig. 2. Left to the dashed lines, representing the threshold values of Δ K th = 1.34 MPa√m for material 6PA-LS (Fig. 2a) and of Δ K th = 1.30 MPa√m for material 6PA-TS (Fig. 2b), respectively, the cracks stopped repeatedly as schematically illustrated by the circles on the x-axis, so that the stress amplitude had to be increased by 5 - 10 %. After reaching the threshold value, the cracks grew stably at a constant stress amplitude and showed long crack behavior.

Fig. 2. da/dN vs. Δ K-curve with threshold Δ K th determination:a) 6PA-LS; b) 6PA-TS.

Reaching the threshold further leads to a change of the crack growth rate of at least one order of magnitude. While the crack growth rate is in a range between 10 -11 and 10 -10 m/cycle below the threshold, it increases to values of 10 -10 to 10 -9 m/cycle after passing the threshold. This notable transition of the crack growth rate, which already indicates that there is a certain minimum crack growth rate required for continuous crack extension as discussed in [Stein et al. (2017)] in detail, was also observed in the additional experiments. An overview of the determined threshold values is given in table 1.

Table 1. Threshold data for material 6PA and 6OA. 6PA-LS 6PA-TS

Δ K Δ K Δ K Δ K ����

6OA-LS

6OA-TS

th , 1 th , 2 th , 3 th

1.34 1.13 1.38 1.28

1.30 1.51 1.32 1.38

1.10 1.17 1.20 1.16

1.13 1.33 1.14 1.20

The variance in the threshold values already points out the dependency of this material parameter on both, the measurement method and microstructure. Results indicate that the peak-aged material has superior near-threshold fatigue crack propagation resistance compared to the overaged material, which is seen in terms of higher values of ΔK th . The shift of ΔK th towards lower values with increasing aging is in agreement with the results presented in the literature for various aluminum alloys (e.g. [Vasudevan and Suresh (1982), Lafarie and Gasc (1983), Suresh et al.