Effect of phase angle on the cyclic behavior of AISI 410 alloy

Multiaxial strain-controlled fatigue tests were conducted on AISI 410 according to ASTM 2207-E08 [1] under proportional (phase angle, θ = 90°) and non-proportional (phase angle, θ = 90º) loadings. Axial strain amplitudes in the range of 0.2%-0.35% are paired with shear strain amplitudes in the range of 0.25%-0.5%. Comparison was made between the hysteresis loops under both loading modes. It was found that peak stresses are higher under nonproportional loading, but with less plastic deformation. Additional stress hardening is exhibited by the alloy, resulting in the tip of the hysteresis loop under nonproportional loading to be rounded. Stress evolution with the number of fatigue cycles, mainly show cyclic stress softening except in axial stress under nonproportional loading where stress stabilization is largely the case after initial hardening. Fatigue life under nonproportional loading is generally lower than that of proportional loading.

On Elastoplastic Buckling Analysis of Cylinders Under Nonproportional Loading by Differential Quadrature Method

The paper investigates the elastoplastic buckling of thin circular shells subjected to nonproportional loading consisting of axial tensile stress and external pressure. The governing equations of buckling for cylindrical shells derived by Flugge serve as the basis of analysis. To capture the elastic/plastic behavior, two plasticity theories are considered; the flow theory and the deformation theory of plasticity. Plastic buckling pressures for cylinders with various combinations of boundary conditions are presented for which no analytical solutions are available. The results obtained from the flow and deformation theories confirm that, under over-constrained kinematic assumptions, the deformation theory tends to provide lower values of buckling pressure and the discrepancies in the results from the two plasticity theories increase with increasing thickness-to-radius ratios, tensile stresses, boundary clamping and E/[Formula: see text] ratios. The plastic buckling results obtained by means of the differential quadrature method are compared with carefully conducted FEA results for both the flow and the deformation theory of plasticity. The reasons underlying the apparent plastic buckling paradox are thus investigated for a vast class of boundary conditions and loads.

Higher-Order Thermomechanical Gradient Plasticity Model With Energetic and Dissipative Components

The thermodynamically consistent framework accounting for the thermomechanical behavior of the microstructure is addressed using the finite-element implementation. In particular, two different classes of the strain gradient plasticity (SGP) theories are proposed: In the first theory, the dissipation potential is dependent on the gradient of the plastic strain, as a result, the nonrecoverable microstresses do not have a value of zero. In the second theory, the dissipation potential is independent of the gradient of the plastic strain, in which the nonrecoverable microstresses do not exist. Recently, Fleck et al. pointed out that the nonrecoverable microstresses always generate the stress jump phenomenon under the nonproportional loading condition. In this work, a one-dimensional finite-element solution for the proposed strain gradient plasticity model is developed for investigating the stress jump phenomenon. The proposed strain gradient plasticity model and the corresponding finite-element code are validated by comparing with the experimental data from the two sets of microscale thin film experiments. In both experimental validations, it is shown that the calculated numerical results of the proposed model are in good agreement with the experimental measurements. The stretch-passivation problems are then numerically solved for investigating the stress jump phenomenon under the nonproportional loading condition.

The Effects of Nonproportional Loading on the Elastic-Plastic Crack-Tip Fields

In this paper, the loading path effects on the plane strain elastic-plastic crack-tip stress field are investigated computationally. Three different loading sequences include one proportional loading and two non-proportional loading paths are applied to the modified boundary layer (MBL) model under small-scale yielding conditions. For the same external displacement field applied at the outer boundary of the MBL model, the mode I K field and T-stress field combined as the different loading paths are applied to investigate the influence of the nonproportional loading. The results show that for either the compressive or tensional T-stress, the loading path which applied K field followed by T field generates the lower crack-tip constraint. There is only slightly difference between the proportional loading path and that with the T-stress field following by K field. The results show that it is very important to include the load sequence effects in fracture analysis when dealing with nonproportional loading conditions.