PSI - Issue 5

D. Rozumek et al. / Procedia Structural Integrity 5 (2017) 896–903 D. Rozumek et al. / Structural Integrity Procedia 00 (2017) 000 – 000

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a)

b)

Fig. 2. Fatigue test stand for mode a) I+II, experimental setup: 1 – 100 kN MTS load cell, 2 – clevis, 3 – CTS specimen holder, 4 – light source with polarized filters, 5 – DinoLite microscope, 6 – integrated measurements system operated by the PC and FlexTest console, b) I+III, MZGS 100, where: 1 – bed, 2 – rotational head with a holder, 3 – specimen, 4 – holder, 5 – lever (effective length = 0.2 m), 6 - motor, 7 – rotating disk, 8 - unbalanced mass, 9 – flat springs, 10 – driving belt, 11 – spring actuator, 12 – spring, 13 – hydraulic connector.

3. Experimental results and discussion

During experimental tests lengths of fatigue cracks growth were measured and number of cycles were registered. Next, on the basis of experimental results in the range of linear-elastic, the stress intensity factor (SIF) range  K was analytically (numerically) calculated for mode I, mode II and mode III loading, and equivalent range of stress intensity factor was calculated according relations (1) and (2). The equations (3) and (4) were suitable only in initial stage of CTS specimen testing. However, they are not valid for kinked cracks (Fig. 3) observed in experiments. This was also confirmed in papers Lesiuk et. al. (2016) and (2017). Therefore, based on the experimental crack paths, the SIF’s were computed using Boundary Element Methods (BEM) in FRANC2D environment. Based on this, for each point of the crack tip the K I , K II were computed using VCCT method (Virtual Crack Closure Technique). The real exemplary crack path, discrete model and stress state in the last stage (with large scale yielding) of fatigue fracture process (load angle 45°) is presented in Fig. 4. The tests were done under controlled loading from the crack occurrence to the specimen fracture. Starting point of crack initiation at notch root was observed of the specimen. For CTS specimen, the crack length was monitored with the travelling polarization microscope DinoLite® with automated measurement algorithm. The crack length was measured in x and y directions (see Fig. 3). The origin of the coordinate system has been localized at the end of the pre-crack. The x-axis was oriented along the mechanical notch direction. After the experiments, the crack lengths were calculated by the correlation of the periodically registered pairs of the (x, y) crack tip coordinates. a) b)

Fig. 3. Initial fatigue crack paths and measured crack initiation angle for CTS specimen loaded by the a) θ = 30 ° angle, b) θ = 45 ° angle.