Papers by Keyword: Cyclic Plasticity

Abstract: Advanced light metals have recently attracted the interest of the aerospace and automotive industry. The need for accurate description of their cyclic inelastic response under various loading histories becomes increasingly important. Cyclic mean stress relaxation and ratcheting are two of the phenomena under investigation. A combined kinematic isotropic hardening model is implemented for the simulation of the behavior of Aluminum and Titanium alloys in uniaxial mean stress relaxation and ratcheting. The obtained results indicate that the model can perform well in these cases. This preliminary analysis provides useful insight for the evaluation of the models capabilities.

Abstract: The accumulation of cyclic plasticity in bond coat (BC) is a key factor controlling the displacement instability of the thermally grown oxide (TGO) in thermal barrier systems. The cyclic plasticity is affected by the component material properties, which vary observably with the service temperature. A numerical model with the behavior of creep and thermal growth in TGO under thermal cycling is used to explore the effect of temperature-dependent properties on cyclic plasticity in BC. The influence of temperature-dependent Young's modulus of thermal barrier coating (TBC), TGO, BC and substrate, thermal expansion coefficient of TBC, BC and substrate, and the yield strength of BC on cyclic plasticity in BC is discussed respectively.

Abstract: The fatigue crack growth behavior of one compact tension specimen of 16MnR steel under high-low sequence loading was investigated. The symmetric half finite element model under plane-stress state was used to calculate the elastic-plastic stress-strain responses, in which the Armstrong-Frederick type cyclic plasticity model was implemented as a user material subroutine UMAT of ABAQUS. A recently developed dynamic crack growth model was used to simulate the effects of high loading step on the successive low loading step. The detailed evolution process of the crack closure and cyclic plastic zone within the retardation region of fatigue crack growth was obtained. The extend of the crack closure, the size of cyclic plastic zone and the stress gradient have significant influence on the fatigue crack growth rate. The predicted fatigue crack growth rate is in good agreement with the experimental data.

Abstract: The cyclic deformation including the ratcheting of TA16 titanium alloy was investigated experimentally at room temperature. Experimental results under symmetrical strain-controlled cyclic loading with various strain amplitudes show that the responded stress amplitude keeps almost unchanged with the increasing number of cycles. It is concluded that TA16 titanium alloy can be regarded as a cyclic stable material. Remarkable ratcheting was also observed under asymmetrical stress-controlled cyclic loading, i.e., ratcheting strain increases with the increasing number of cycles. The ratcheting strain strongly depends on the stress level and increases with the increase of applied mean stress, stress amplitude and stress ratio. These findings are useful to reasonably model the cyclic deformation of TA16 titanium alloy.

Abstract: In sheet metal forming, the anisotropy and the Bauschinger effect of sheets affect greatly their formability. This paper discusses how the planar anisotropy and cyclic plastic behavior (the Bauschnger effect and cyclic workhardening characteristics) correlate with the crystallographic texture based on the crystal plasticity analysis on A5052-O sheet. The analytical predictions of r-values and the cyclic stress-strain responses are compared with the experimental observations (S. Tamura et al., Materials Trans, 52-5 (2011), pp.868-875).

Abstract: Based on the characteristic of cyclic softening of the quenched and tempered Carbon steel 45, a cyclic plastic constitutive model was proposed to describe the cyclic behavior under symmetrical and unsymmetrical strain cycling with different strain amplitudes. In this model, the phenomenon of the decrease of the up yield limit stress with the increase of strain in the initial 1/4 cycle was taken into account. The proposed evolution equations of the yield size and backstress can simulate the cycling softening under symmetrical and unsymmetrical strain cycling well. The results indicated that either the simulated shape of the cyclic softening hysteresis loops or the evolution of stress amplitude with the increase of the cyclic number during the low cycle fatigue coincides with the experimental ones very well.

Abstract: To date the analysis of fatigue crack growth from an open hole subjected to contained cyclic plastic deformation is problematic due to considerable computational effort involved. This paper presents the integration of a new efficient method for calculating the elastic-plastic notch stress field under cyclic loading into a crack growth analysis tool. The anticipated crack path is defined along a line emanating from an open hole, and the line is discretized into a series of elements on which the elastic-plastic stress distribution is solved for the uncracked body. This solution is then used in conjunction with a weight function approach to determine the stress intensity factors used in the crack growth analysis. This technique has been incorporated into a Defence Science and Technology Organisation proprietary crack growth analysis tool based on plasticity-induced crack closure. Results are presented for cases involving service load spectra from F/A-18 and F-111 aircraft and are compared to those obtained from a commonly used crack growth code that does not account for the effect of contained notch plasticity. It is hoped that this generic approach will lead to improved predictions for crack growth in plasticity-affected zones.

Abstract: Smooth specimens made from austenitic-ferritic duplex steel were subjected to constant stress amplitude loading with positive mean stresses. Hysteresis loops were recorded during the fatigue life and plastic strain amplitude and cyclic creep rate were determined. Fatigue hardening/softening curves, cyclic creep curves and cyclic stress-strain curves for different positive mean stresses were evaluated. Typical dislocation structures developed in both phases of the duplex steel were identified using TEM, compared with the saturated plastic strain amplitude and correlated with the decrease of the cyclic creep rate during cycling and the slope of the cyclic stress-strain curve.

Abstract: The numerical simulation of crack closure is employed to assist on the prediction of crack growth rate. Under fatigue load, the stress-strain response of metals is altered due to cyclic loading. For this reason, the material properties characterization is of prime concern as an input parameter to obtain reliable results. From numerical simulations, it was observed that simple material models do not provide accurate data for long crack lengths. In this paper, the effect that different hardening models have on the opening response of a cracked component when it is subject to variable amplitude loading is analyzed. The interaction effects (crack arrest/acceleration) for long crack length simulation are specially highlighted. For this purpose, a 6082-T6 aluminium alloy was analyzed experimentally and numerically in order to measure crack closure, and then, those data were used to predict fatigue crack growth rate under different patterns of overload. The Paris equation and the Elber crack closure concept were employed. The results showed that small variations in the opening stresses obtained from different material models produce high overestimated simulations of crack growth rate. Also, it was proved that the crack closure mechanism is able to take into account interaction effects due to variable amplitude loading.

Abstract: The stresses, strains, and deformations produced by repeated, two-dimensional non-steady state rolling-sliding contact were analyzed using an elastic-plastic finite element model. An advanced cyclic plasticity model was used. The non-steady state rolling contact was restricted to a harmonic variation of the normal Herztian contact pressure. Repeated rolling and sliding were simulated by multiple translations of a set of varying normal and tangential surface tractions across an elastic-plastic semi-infinite half space. The non-steady state loading considered results in a wavy contact surface profile. The surface displacements and wave depth of the wavy deformation increase with increasing rolling passes, but the increases in wave depth per rolling pass (ratchetting rate) decay. The residual stresses and strains near the wave trough of the residual wavy deformation are higher than those near the wave crest. The results are in agreement with the experimental observations. The tangential force has a greater influence on the residual strains than on the residual stresses.