PSI - Issue 82

Juraj Belan et al. / Procedia Structural Integrity 82 (2026) 119–124 Juraj Belan et al. / Structural Integrity Procedia 00 (2026) 000–000

123

5

The initial state of the alloy without an a -case layer and after oxidation annealing with an a -case layer of varying thickness, corresponding to isothermal holding, are compared. The curves of the maximum bending stress s o clearly show the significant influence of the formation of the a -case layer on the fatigue life of the samples. The decrease in fatigue life compared to the initial state is » 46% for a 0.5 h hold and » 42% for a 1 h hold. The decrease in fatigue life is mainly caused by the presence of cracks of various sizes on the surface. Their occurrence is almost impossible to predict, which is the main reason why the S-N curves does not show the condition with an isothermal hold of 1.5 hours. In this case, the samples failed unpredictably, i.e., it was not possible to determine a combination of static preload and dynamic load such that the samples would reach the specified number of cycles to failure – run-out. The influence of the alloy microstructure (coarse lamellar microstructure in the initial state vs. fine needles a¢ - martensite) on fatigue life in this case can be considered negligible, because fatigue damage initiates from the surface and, if any cracks occur, they become a significant stress concentrator and thus accelerate the process of fatigue crack initiation and the speed of crack propagation itself.

a)

b)

c)

d)

Fig. 4. Failure surfaces of Ti6Al4V alloy after three-point bending fatigue test; a) initiation site of starting stage, b) fatigue crack propagation area with fatigue striations observed, c) initiation site after oxidation annealing at 1050 °C/ 1 h, note the significant surface crack in � -case layer, d) fatigue crack propagation area with a number of secondary crack and brittle fracture stage – intercrystalline mechanism, SEM.