PSI - Issue 2_B

O. Plekhov et al. / Procedia Structural Integrity 2 (2016) 2084–2090 Author name / Structural Integrity Procedia 00 (2016) 000 – 000

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4. Conclusion

The titanium and Grade 4 with different grain sizes (from 40 to 0.15 micron) were tested under cyclic loading. The study shows the inefficiency of the classical approach for fast determination of the fatigue limit of SMC metals based on the self-heating test (Risitano technique). The results of the experiments can be summarized as follows: 1. The conventional grain titanium exhibits an ordinary two steps behavior. The temperature rises sharply after the transition through the fatigue limit. The temperature kinetics for SMC titanium under self-heating conditions is linear up to the failure (the failure stress amplitude increases by 35-50 %). It leads to the inapplicability of the standard self-heating test for determination of the fatigue limit of materials with complex structure. 2. The dissipation ability of the SMC material is higher than that of the conventional grained material at small stress amplitude. Taking into account the fact that grain crushing leads to the formation of SMC nonequilibrium structures containing high density of lattice and grain boundary defects with long range elastic stress fields, we can conclude that SMC titanium has a unique structural mechanism for energy dissipation and can use this mechanism at both small and high stress amplitudes. The results of the ultrasonic fatigue test are as follows: 1. At the stress amplitude close to the fatigue strength determined during the 10 7 loading cycles, the material in a SMC state has a much longer lifetime than the material with coarse grains. The difference in the lifetime of coarse-grained and SMC materials decreases with decreasing stress amplitude. 2. The characteristic (mean) grain size is not a unique parameter determining the fatigue strength of titanium. This fact was proved by the comparative study of titanium samples with very close mean grain sizes and different states of grain boundaries. It has been found that the material with homogeneous structure and high grain misorientation (SMC1) has higher durability than the material with dislocation texture in the longitudinal direction and anisotropic grains shape (SMC2). The results of the structural investigation are as follows: 1. The increase of the number of cycles leads to the dilatation titanium samples. A change in the density caused by deformation was ∆ρ/ρ=8*10 -4.The dilatation can be caused by initiation of new dislocations, micro voids and cracks. 2. The ECAP leads to a degradation of Young`s modulus from the initial value of 106.48 GPa to 102.95 GPa. This reduction can be caused by the generation of defects and dilatation areas in the samples undergoing severe plastic deformation. Young`s modulus decreases during deformation in the gigacycle fatigue regime from 102.95 GPa to 102.72 GPa.

Acknowledgements

The work was funded by RFBR according to the research project No.14-01-00122, No.14-01-96005.

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