Statistical Evaluation of the Fatigue Life of Fine-Grained Ti6242

Kurzfassung

The scatter in fatigue life of a forged, fine-grained Ti-6Al-2Sn-4Zr-2Mo-0.1Si (Ti6242) has been studied as a function of the height of the maximum stress level by testing 15 specimens per stress level. As expected the scatter in fatigue life increases at decreasing maximum stress. The smallest scatter in fatigue life is found for 15 specimens tested at the largest applied maximum stress level of 1000 MPa. This scatter can be expressed with the ratio maximum life to minimum life Nf,max/ Nf,min measuring 3.2, whereas the largest scatter for the 800 MPa data determined this way measures Nf,max/ Nf,min = 8.2. A statistical analysis of all fatigue life data making use of a two-parameter Weibull distribution shows that the shape parameter m clearly indicates an increase of the scatter (m = 3.4 at 1000 MPa and m = 1.8 at 800 MPa) at decreasing maximum stress. The present investigation suggests that the variation in microstructure contributes to the life variability to a great extend. Characteristic for the investigated material is the small grain size with alpha-grains of 1-4 µm and even smaller beta-grains, but also with areas in which the microstructure is locally textured with beta-platelets of up to 5 µm in length. Microscopic inspection of the fracture initiation sites reveals that the scatter is the result of nuclei for crack initiation of different types and/or dimensions hidden in the microstructure. These nuclei form crack-like-discontinuities, CLDs 1), in the early phase of crack initiation. At low stresses and maximum life the CLD appears to be controlled by the alpha-grain size while the larger CLD responsible for the shorter fatigue life appears to be rather influenced by the extension of the beta-platelets. These findings generally also apply for the fracture initiation sites produced at higher stresses, although the microstructural details are less pregnant.