A B S T R A C T Because of their simplicity, many isotropic damage models have been used to approximately predict the fatigue life of metallic engineering components. However, experimental observations confirm that the anisotropic damage evolves at probable failure sites even for isotropic materials. In this study, a model of microstructure of boom-panel is constructed to simulate a representative volume element (RVE), and the anisotropic damage of the RVE is described by the independent isotropic damage of boom and panel. Firstly, the constitutive equation of the RVE in terms of stiffness of boom-panel is deduced by the principle of deformation and static consistency. Then the expressions of damage-driving force for boom and panel based on the principle of thermodynamics are introduced, and the damage evolution equations are constructed. The parameters of boom and panel are identified from fatigue test data of uniaxial tension and pure torsion, respectively. Finally, the aforementioned method is applied to predict the fatigue life of two structures: one is Pitch-Change-Link, which is a kind of structure in helicopter, and the other is a specimen under tension-torsion. The prediction results all fit well with the experimental data.Keywords anisotropy; boom-panel model; fatigue damage; life prediction; structural components. N O M E N C L A T U R ED 0 = the initial damage extent D e , D d , D p = the damage extents of edge boom, diagonal boom and panel, respectively E e , E d = the elastic moduli of edge boom and diagonal boom, respectively G p = the shear moduli of panel 2l = edge length of the RVE N = the number of cycles N e , N d = the axial forces of edge boom and diagonal boom, respectively k e , k d , k p = stiffness of edge boom, diagonal boom and panel without damage, respectively R = stress ratio RVE = representative volume element T = the shear force of panel W = the strain energy density Y = the damage driving force α, β, m, Y th , η, γ = fatigue parameters in damage evolution equation Δ e , Δ d = elongations of edge boom and diagonal boom, respectively Δ p = the displacement of panel due to shear deformation ε th = the threshold of strain φ e , φ d , φ p = the continuity of edge boom, diagonal boom and panel, respectively σ th = the threshold of stress
Spin-torque oscillator (STO) with reference layer (REF) has been proposed to be an efficient way to excite oscillation at low current density [1][2] . A well-controlled STO is the center part of microwave assisted magnetic recording (MAMR); however, in the recording process, the STO is exposed in an alternative external head field in addition to the applied current [3][4] . To assist the writing in MAMR, the STO should switch and change the oscillation chirality when the head pole flips, and the switching time needs to be shorter than 0 .2 ns so as to follow the write field flipping . In this work, we would like to find the suitable design of Co/Pt REF to achieve this target . The micromagnetic model is built up to study a STO consisting a Co/Pt REF [2] with a total size of 30nm×30nm×t and a field generation layer (FGL) with a size of 30nm×30nm×10nm . The dynamics of STO is obtained by solving the LLG equations including the Slonczewski spin transfer torque term [5] . The interfacial anisotropy K u of Co/Pt REF layer is sensitive to the Co layer thickness . The thickness and K u of Co layer of three samples are A: 0 .4nm, 2 .2×10 7 erg/cc; B: 1nm, 1 .1×10 7 erg/ cc; C: 3nm, K u ≈0 [6], respectively . The rise time of the external field is 0 .1ns and 0 .2ns . The external field is 10 kOe perpendicular to the film . The switching dynamics of STO A, B and C are compared in Fig . 1 respectively, with 0 .2ns rise time of head field . The current density is 120 MA/cm 2 . It can be obtained that the switching time of REF in sample B is shorter than other situations, and the FGL oscillation is least disturbed, which is suitable for STO in MAMR . The switching time of STO will not be shorter with 0 .1ns rise time of head field, because the switching time of STO is mainly determined by the compensation of switch speed between the FGL and REF . For sample A, since the K u of REF is large, the switch of REF is much slower than the FGL, as a result, after the REF switch, it takes a long time to re-excite the FGL to the stable oscillation angle (SOA) . For sample C with zero anisotropy REF, the switch is slower and SOA of FGL is not stable . Therefore, the most suitable design of Co layer in REF of MAMR STO is sample B .
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