An improved unified viscoplastic model (UVM) is proposed to simulate the cyclic viscoplastic behaviors of Alloy 709 under low-cycle fatigue (LCF) and creep fatigue interaction (CFI) loadings. And the effects of strain amplitudes and hold times on the model parameters as well as the relationship between the static recovery parameter and the accumulated inelastic strain are considered to reflect the evolutionary behavior of strain-stress hysteresis loops, the varying stress relaxation and cyclic hardening behavior. Moreover, an efficient numerical implementation is proposed using Voigt notation and implicit radial return method with Newton-Raphson iteration, and the consistent tangent operator (CTO) for the improved UVM can be simply solved using matrix operations. The proposed model accurately depicts the mechanical behavior of Alloy 709 at 750°C.
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