PSI - Issue 13

Tatyana Tretyakova et al. / Procedia Structural Integrity 13 (2018) 1774–1779 Author name / Structural Integrity Procedia 00 (2018) 000–000

1776

3

The temperature fields are measured by the infrared thermography, that is, the infrared camera Flir SC7700 with a resolution of 640×512 pixels was applied. It is important to notice that the software and hardware synchronization of the testing system and measurement equipment was carried out with the DAQ device NI USB-6251. Both surfaces of specimens were prepared in order to improve accuracy. The preparation of surface from the side of the Vic-3D system included spraying the black/white speckle pattern (set of random black dots on the white matt base). From the side of the IR-camera the surface was covering with a black matt coat to reduce reflections from the surface. 3. Results and discussions 3.1. Growing of initial fatigue cracks Growing of initial cracks was carried out at the cyclic loading on tension-compression with different parameters of cycles. To minimize the area of plastic deformation in the crack tip region, the amplitude and the peak-to-peak load value were decreased. The resulting lengths of preliminary fatigue crack in flat specimens with different gauge length are shown in Table 2. In Fig. 2, failure surfaces of specimens with the central notch and the preliminary fatigue crack also presented. The l c is a length of fatigue crack, 1 — the area of fatigue crack, 2 — the area of crack propagation during uniaxial tension test (see Fig. 2).

Table 2. Length of preliminary fatigue crack in flat specimens with different gauge length Specimen Gauge length [mm] Crack length right/left of the hole 2-50-H 50 2.51/1.93 2-100-H 100 2.39/1.77 1-150-H 150 1.73/1.52 1-250-H 250 3.39/3.29

Fig. 2. Photo of failure surfaces of specimens with the central notch and the preliminary fatigue crack.

Fig. 3. Load versus displacement curves for specimens with different gauge length: 1-250-H ( 1 ), 1-150-H ( 2 ), 2-100-H ( 3 ) and 2-50-H ( 4 ).

3.2. Stable crack growth in bodies with concentrators under various stiffness of loading system Figure 3 shows the load versus displacement curves for specimens with four different gauge lengths. The curves were obtained by built sensors of the testing system. As can be seen, the degree of realization of the postcritical deformation stage depends on the length of the specimen. The 50 mm length specimen (curve 4, see Fig. 3) has a higher stiffness of the loading system relative to the crack path area compared to the longer specimens. The crack opening v versus load P (see Fig. 4 (f)) and versus time t (see Fig. 4 (b)) curves obtained from the DIC-analysis of displacement and strain fields, and also by using of the ‘Vic-3D’ software module ‘virtual extensometer’, for four different gauge lengths of specimen: 250 mm ( 1 ), 150 mm ( 2 ), 100 mm ( 3 ) and 50 mm ( 4 ).