PSI - Issue 23

Tomáš Oplt et al. / Procedia Structural Integrity 23 (2019) 101–106 Tomáš Oplt / Structural Integrity Procedia 00 ( 2019) 000 – 000

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That implies, that in the vicinity of the free surface, crack driving force is even smaller and thus, crack front curvature due to the closure effects combines together with free surface effect, resulting into bigger curvature than at high asymmetry.

Fig. 3 Distribution of SIFs through the thickness for load ratio R = 0

In order to make the comparison of closure for various load ratios R and various load range ∆ K possible, it is convenient to use unified parameter U established by Elber (Elber 1970).The parameter U was defined as the ratio between effective and total stress intensity factor range (or similarly for different loading parameters F or σ ): = Δ ΔK = − − = − − . (4) Similarly as previous calculation, another were performed for load ratios R = -1,-0.5 and 0.1. All results are presented together on Fig. 4, where only closure ratio U in the middle of the body and at the free surface are plotted. It can be seen that results in the middle of the body estimated for LDU scheme are in very good agreement with Newman+Wanhill (Oplt et al. 2019) empirical estimation and Pokorny’s experimental work (Pokorný et al. 2017) . Slightly more conservative results bring the LDULU scheme, whose advantage is in its low mesh sensitivity in contrast to classic LDU (Oplt et al. 2019). At the free surface, crack was closed significantly earlier then in the middle of the body for all load ratios. That implies that in all cases, initial straight crack front will tend to be curved close to the free surface during its fatigue growth.

Fig. 4 Comparison of closure values in the middle of the body and at the free surface for various load ratios R