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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2.2. Specimen FZ3 The test program was started on the specimen FZ3. At first, 20,000 cycles with the maximum stress of 600 MPa were applied. Since no crack initiation or propagation were indicated by means of the clip gage, the maximum stress was subsequently increased to 800 MPa, and the test was continued during further 19,900 cycles until a rapid increase of the deformation was registered. In the both test stages, the stress ratio was recurrently changed from = 0.1 to = 0.5 for 1,000 to 2,000 cycles to produce beach marks at the fracture surface. After the second stage, the specimen was fatigue loaded with the maximum stress of 600 MPa until fracture. Fig. 3a shows the fracture surface of FZ3 containing a defect field at which the crack nucleation and propagation occurred. Single defects represent aluminum and calcium oxide particles of the size between about 30 and 300 µm. The diameter of the major defect field is about = 2 mm which corresponds to the EFBH value of 1.6 mm from the ultrasonic measurement. The correlation between the EFBH value and the defect size is given by (FKM, 2009) = 2 = 2 ∙ √ 10 , (1) thus resulting in = 1.0 mm with being a radius of an equivalent circular crack. Against expectation, no beach marks were observed at the fracture surface of FZ3, which suggests that short-term alterations of the stress ratio did not produce sufficient crack growth and thus contrast crack front marks for the material under investigation. Consequently, the testing regime was adapted for the remaining two specimens.

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a) specimen FZ3

b) specimen FZ1

c) specimen FZ2

Fig. 3. Fracture surfaces of the specimens FZ3 (a), FZ1 (b) and FZ2 (c). Red numbers in b) indicate load blocks.