PSI - Issue 7

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L.L. Liu et al. / Procedia Structural Integrity 7 (2017) 174–181 L. L. Liu et al. / Structural Integrity Procedia 00 (2017) 000–000

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Fig.3 (a) the relationship between the experimental lifetime and the mean grain size after dimensionless, (b) the relationship between the experimental lifetime and the δ phase content after dimensionless.

3.2 Fractography Fatigue fracture surface was then observed following the fracture of test specimens as shown in Fig.4 (a) and (c). Obviously, the GH4169 fatigue fracture can be divided into three regions: fatigue source region A, crack propagation zone B and instantaneous fracture zone C. After comparing the area of each part, we may conclude that the cracks propagate very slowly after initiation and then quickly at the later stage of fatigue lifetime. The cracks initiate from inclusions near surface or the surface of specimen, as shown in Fig.4 (b), (d). In most cases, multiple LCF cracks initiate from different site at the specimen surface. This is because the grains on the free surface are not restrained by other grains which are prone to produce plastic deformation, and there are scratches, corrosion and notches on the surface of the material which are easy to cause stress concentration. In addition, the bonding force between the inclusions and the matrix is weak, and the crack initiations are often caused by the interfacial separation [17] . As shown in Fig.4 (e), we can see a smooth cross-section and obvious fatigue striations within the stable crack growth regime from SEM images indicating that under transgranular failure mechanism. The arrow in the figure points to the direction of crack propagation. A large number of dimples in instantaneous area whose center are partially filled with inclusions can be observed from Fig.4 (f), reflecting the toughness fracture of the microporous aggregation type. For smaller grain size specimens, when it is subjected to external force, plastic deformation is more uniform and the stress concentration reduce by plastic deformation dispersing in more grains.

3.3 LCF lifetime prediction

3.3.1 SWT model At present, the local strain method based on Manson-Coffin formula is widely used to estimate the low cyclic fatigue lifetime of compressor disc [18,19] . It is a fact that the fatigue load such as thermal stress, centrifugal stress of the compressor disc is almost asymmetric, while the traditional Manson-Coffin model is often found in symmetrical LCF lifetime prediction. Therefore, it is necessary to modify the data of fatigue performance, and the results are significant different because of different mean stress amending method.