Abstract: Based on grey theory, the characteristics on low cyclic fatigue of DD3 Ni-based single crystal alloy under multi-axial non-proportional loading are researched. Degree of grey incidence of factors influencing fatigue life, such as equivalent strain range, temperature, strain path angle, tension/torsion phase angle, axial strain ratio, is analyzed. A damage parameter Q is introduced to express asymmetric load cyclic characteristic and tension/torsion multi-axial effect. A low cyclic fatigue life prediction model GM(1, N) with parameter Q, equivalent strain range Δεe and Mises stress range Δσe as damage parameters is built. The results show that degree of grey incidence of influencing factors on multi-axial low cyclic fatigue life，from maximum to minimum，are equivalent strain range , temperature and strain path angle for the first level, tension/torsion phase angle for the second level, and axial strain ratio for the third level. Absolute degree of incidence of test life and predictive life for fatigue model GM(1, N) at 680 ℃ and 850 ℃ are 0.97, 0.86, respectively, and mean relative errors are 4.9%, 6.0% respectively. Almost all test data of above-mentioned two temperatures were fall into the factors of 1.93 and 2.13 scatter band, respectively.

Key words: grey prediction, multiaxial fatigue, Ni-based superalloy, non-proportional loading, single crystal