PSI - Issue 2_B

Patrick Mutschler et al. / Procedia Structural Integrity 2 (2016) 801–808 P. Mutschler, M. Sander/ Structural Integrity Procedia 00 (2016) 000–000

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4

a)

b)

R = 0.1

R = 0.5

Fig. 3. (a) Mean curves of the crack growth tests for R = 0.1, and (b) for R = 0.5

a)

b)

R = 0.1

da/dN

( da/dN ) cr

ΔK

eff

Fig. 4. (a) Schematic illustration of crack growth regimes according to Petit et al. (1999); (b) Mean curves normalized by the Young’s moduli

Fig. 3 displays mean curves of the crack growth data for R = 0.1 and R = 0.5 for the investigated temperatures. Both R -ratios show that the crack growth rates in the Paris-area increase for increasing temperatures. For lower cyclic SIF values the Paris-line slightly bends to lower crack growth values. This is noticeable especially at T = 400 °C and R = 0.1 (Fig. 3a). A further reduction of the cyclic SIF relates to a kink in the Paris-line. This effect is much more distinctive at R = 0.1 (Fig. 3a) than at R = 0.5 (Fig. 3b). The slope of the Paris-line is observable lower in this area. However, this effect is much more striking at the single crack growth tests than at the mean curves. In the threshold region a double “s-shape” occurs for temperatures higher than T = 300 °C. Further it is noticeable that the threshold values in Fig. 3a of the elevated temperatures are significantly lower than the threshold value of the tests at room temperature. Within the higher temperatures the trend that higher temperatures lead to lower threshold values is not existent. This is also visible for the threshold values at R = 0.5 (Fig. 3b). However, the R -ratio dependence of the threshold value decreases at the temperatures between T = 300 - 600 °C as compared to the threshold values at room temperature significantly. Petit et al. (1999) divide the fatigue crack growth of long cracks in polycrystalline metallic materials in three phases (Fig. 4a). The intrinsic phase occurs under vacuum. Under environmental influence the adsorption assisted phase appears. For these two phases is considered by Petit et al. (1999) that the crack growth is proportional to ( Δ K eff / E ) 4 . Below the critical crack growth rate ( da / dN cr ) of about da/dN = 10 -5 mm/cycle hydrogen assisted crack