PSI - Issue 7

Igor Varfolomeev et al. / Procedia Structural Integrity 7 (2017) 359–367 Igor Varfolomeev et Al./ Structural Integrity Procedia 00 (2017) 000–000

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2.3. Specimen FZ1 This specimen was tested by applying 7 load blocks, each consisting of 6,000 cycles, while alternating the maximum stress from 600 MPa (odd blocks) to 800 MPa (even blocks). In the 8 th block at 800 MPa, a considerable increase of the deformation was registered after 300 cycles, so that the specimen was subsequently fatigue loaded at 600 MPa until fracture. The defect field in FZ1 has an elliptical shape (Fig. 3b) with a somewhat larger extension than derived from the respective EFBH value. Assuming = 1.3 mm and the aspect ratio of ⁄ = 0.4 (ratio of the minor to major axis of the ellipse), as suggested in FKM (2009), the minor axis of the ellipse is estimated according to Eq. (1) as 2 = 1.6 mm. In some extension from the initial defect field, the fracture surface of the specimen FZ1 exhibits light marks with a shape close to an elliptical one. Two of these marks corresponding to load blocks 3 and 5, both with the maximum stress of 600 MPa, are indicated in Fig. 3b. Their width increases from about 80 µm to 150 µm, respectively, which correlates with accelerating crack propagation at increasing crack size. Fractographic examinations at higher magnification (see Fig. 3b) revealed fatigue striations with a width of about 0.4 µm in the transition area from block 2 to block 3. This is an indication that the initiation of fatigue crack growth from the initial defect field occurred in the 2 nd load block at maximum stress of 800 MPa, whereas the whole 1 st block and likely a part of cycles of the 2 nd block did not contribute to crack propagation. 2.4. Specimen FZ2 This specimen with the biggest cross-section was intended to be tested in the same regime as the specimen FZ1. However, the high tensile force in the 2 nd loading block with the maximum stress of 800 MPa caused plastic deformations and fracture of a loading pin. For this reason only 580 cycles at 800 MPa were applied, whereas the rest testing was achieved at 600 MPa. To produce beach marks, the stress ratio was periodically changed from = 0.1 to = 0.6 for about 6,000 cycles. Overall, the specimen FZ2 was subjected to about 33,000 load cycles excluding those in the beach mark regime. The fracture surface of FZ2 (Fig. 3c) shows three defect fields: a major defect field at the center of the cross section and two smaller fields at the surface. Those defect groups grew quite independently from each other until the onset of final fracture. Similar to other specimens, the defect fields consist of aluminum and calcium oxide particles sized between 50 to 400 µm. The major defect field has a kidney-shaped geometry with the dimensions of the envelope curve about 5.4×8.7 mm². This is somewhat larger than the defect size derived from the EFBH value listed in Table 1. Accordingly, assuming = 2.7 mm, Eq. (1) yields an estimate of the minor ellipse axis of about 2 = 3.4 mm (see the elliptical contour in Fig. 3c). In contrast to the specimens FZ1 and FZ3, the crack initiation in FZ2 occurred already in the first loading block at the maximum stress of 600 MPa. This can be attributed to a relatively large extension of the major defect group for this specimen. 2.5. Defect statistics As indicated in Fig. 3a, fractographic examinations of the fracture surfaces included measurements of the size of and distance between neighboring non-metallic inclusions. Those data were evaluated for the specimens FZ1 and FZ3 in terms of respective distribution functions. An example in Fig. 4 shows the probability density functions for the specimen FZ1. Both the size (an average of the two principal dimensions, cf. Fig. 3a) and the distance between the defects can be described by a three-parameter Weibull distribution with the median values of about 120 µm and 260 µm, respectively. These are larger than the corresponding parameters for FZ3: 100 µm and 160 µm, respectively. It should also be noted that, comparing to FZ1, the specimen FZ3 exhibits a considerably larger defect density (number of defects per unit volume or unit area).