PSI - Issue 39

R. Yarullin et al. / Procedia Structural Integrity 39 (2022) 364–378 Author name / Structural Integrity Procedia 00 (2021) 000–000 to 0.3 were used. The geometric parameters of the specimen test section and of the growing crack are presented in Fig. 1. In this figure, is the current crack length, with the crack front approximated by an elliptical curve with major axis 2 and minor axis 2 . The crack length is obtained by measuring the distance between the advancing crack break through point and the notch break through point. The depth of the initial curvilinear edge notch is denoted by . 367 4

Fig. 1. Details of the hollow specimen geometry and initial notch.

The rate and path of fatigue crack growth were determined as a function of the loading conditions, evaluating crack growth for pure cyclic tension, cyclic tension with superimposed cyclic torsion, and pure cyclic torsion. Complex stress state tests were performed by using the following equipment: Servo-hydraulic Axial Torsion Test System, Model Bi-00-701 with axial capacity +/-100kN and torque +/- 2000N*m; displacement gage (crack mouth opening displacement - CMOD) Model Bi-06-203, and optical microscope MBS 10 (Fig.2). All tests were carried out at room temperature with a frequency of 10 Hz and a stress ratio R=0.1 .

Fig. 2. Test equipment for complex stress state.

A variation of the stress ratio value (from 0.1 to 0.5) was applied several times, for a few cycles, in order to highlight the current crack tip position, namely, beach marks were produced by increasing the applied stress ratio from 0.1 to 0.5 and keeping constant the maximum cyclic nominal stress; such variation was applied for a number of cycles sufficient to determine an increase of the surface crack length equal to b ≈ 0.1 mm. With such procedure, the beach mark loading did not induce load history effects or overload retardation. The typical fracture surface marks are shown in Fig. 3 for different loading conditions.