PSI - Issue 16

Grzegorz Lesiuk et al. / Procedia Structural Integrity 16 (2019) 51–58 Grzegorz Lesiuk et al. / Structural Integrity Procedia 00 (2019) 000 – 000

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The tests for mixed mode (I+II) were carried out on a MTS 809 servo-hydraulic axial/torsional test machine. The different mixity level was created due to changing the load angle θ (Fig. 4a), The tests for mixed mode (I+III) were performed on the fatigue test stand MZGS-100 enabling to carry out cyclically variable and static (mean) loading (Rozumek et al. (2012)). Different mixity level was achieved due to changing the ratio of bending moment to torque. Crack growth was observed on the lateral specimen surface with the optical method. The fatigue crack increments were measured with a digital micrometer located in the portable microscope with magnification of 25 times and accuracy 0.01 mm. At the same time, a number of loading cycles N was recorded. After this part, the stress intensity factors were calculated using numerical method as well as using analytical formulas. Technical details are availab le I previous Authors’ papers (Rozumek et al. (2012, 2018), Ferreira et al. (2018)). Typical KFFD for mode I (18G2A-S355 grade steel) is presented in Fig. 5. For mixed mode stress intensity factor equivalent, Rozumek et al. (2018) proposed, according to the Huber-Mises criterion for proportional loading, the following equivalent stress intensity range:

2 II 2 I eq K K 3 K      .

(3)

Fig. 5. Fatigue crack growth rate diagram for S355 (18G2A) steel (R = 0.1).

Similar kinetic fatigue fracture diagrams for mixed mode (I+II, I+III) for different R-values are presented below in Figs. 6 – 7. As it is noticeable, for linear  K approach or nonlinear  J approach, the influence of R-ratio is observed. On the other hand, the novel approach for the description of FCGR is based on the new introduced in the paper of Lesiuk (2019), the nonlinear fracture mechanics parameters describe better correctly the kinetics of fatigue crack growth in elastic-plastic range. Based on the energy approach, a new CDF (Crack Driving Force) is proposed as: (4) The possible extension of such parameter for mixed mode loading condition can be represented as follows (Lesiuk (2019a)): * S J J     max

*     S

max

J J

,

(5)

eq

eq eq

where:  J eq + – an equivalent positive range of the J-integral obtained for mixed mode loading conditions, J equivalent maximal value of the J-integral obtained for mixed-mode loading conditions.

eq (max) – an