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The effect of monotonic and cyclic deformation on the evolution of back stress vis-a-vis Bauschinger effect due to different amounts of unidirectional plastic deformation in annealed OFHC copper has been investigated. It is observed that for the same amount of prestrain the back stress evolution and the corresponding Bauschinger phenomenon depend upon the mode of deformation. It is found that higher back stress develops in case of monotonic deformation as compared to ratcheting deformation for same amount of unidirectional prestrain. It is also found that the effect of stress rate on the change of Bauschinger parameters is comparatively less than the effect of increasing prestrain in both types of deformation.

The present investigation has been carried out to understand the effect of mean stress on room temperature ratcheting behaviour of solution annealed AISI 304 stainless steel at constant stress amplitude. It is found that the ratcheting strain to failure linearly increases with increase of mean stress and there is no significant difference in the accumulated ratcheting strain with change in solution annealing temperature vis-a-vis grain size. While the number of cycles to complete failure increases exponentially with mean stress irrespective of solution annealing temperature, the ratcheting life is found to increase when soltionin annealing is done at lower temperature. This variation is believed to be associated with the tensile yield strength of the specimens solutionised at two different temperatures.

Fretting is of a serious concern in many industrial components, specifically, in nuclear industry for the safe and reliable operation of various component/system. Comprehensive experimental and numerical studies have been carried out to understand the mechanics and mechanisms involved under fretting conditions. Experiments were performed on a first-of-a-kind fretting machine, with the capabilities of simulating different fretting regimes under ambient and high vacuum (10–5mbar) conditions and, temperatures up to 873K. Studies were made on self-mated stainless steel (SS) and stainless steel mated against coated surfaces. Chromium carbide with 25% nickel chrome binder coatings using plasma spray and High Velocity Oxy Fuel (HVOF) processes on SS were investigated in detail. As evident from the experimental studies, the initiation or nucleation of damage in self-mated SS is due to severe plastic deformation. Ratcheting has been observed as the governing damage mode under cyclic tangential loading condition. Once the crack has been initiated under ratcheting, the propagation is controlled by fatigue. The orientation of crack propagation has been observed to be governed by the contact conditions prevailing at the contact interface. Numerical analysis result shows that the propagation of the subsurface cracks is controlled by strain energy release rate.

As part of a project within the Swedish nuclear industry with the objective to investigate the performance of different constitutive models in ratchet simulation, an extensive experimental program has been conducted on pressurized tube specimens. In total 30 test specimens made of two different materials, 316L and P235, have been manufactured and tested. In order to determine material properties, monotonic tensile load and internal pressure experiments have been performed. The remaining test specimens have been used for ratcheting experiments. The experimental results show ratcheting in the hoop direction when the tube is subjected to certain combinations of internal pressure and cyclic axial strains. The higher the pressure is and the larger the strain ranges are, the higher the ratcheting response becomes. Measured ratcheting strains are compared to numerical simulations using different constitutive models. In this paper the interrelated models of Prager, Armstrong-Frederick and Chaboche are investigated. In addition to these, the Besseling model is investigated. Based on the result from this investigation, recommendations on how to conduct ratcheting simulation of pressurized equipment subjected to cyclic secondary loading are presented.

The fatigue behavior at high number of cycles in elastic-plastic field of quenched and tempered carbon chromium steel was experimentally investigated for high performance reciprocating compressors application. Fatigue tests on un-notched specimens were performed both under load and strain controls, by imposing different levels of stress/strain and for each of them different values of stress ratios R, especially high values. Stress and strain trends have been monitored, during the fatigue life, and either ratcheting or relaxation, respectively, was evident. The stress control tests have resulted into fatigue fractures only for low values of R with significant ratcheting and an increasing rate during the final part of the test, thus the fracture could be considered as a synergy between fatigue damage and plastic failure. On the contrary, the ratcheting stabilized for high values of R and the tests were finalized without any fracture. Within an intermediate region, for medium/high values of R, a minor ratcheting and the fracture transition have been found. Similarly, for the tests under strain control, low values of R showed fatigue fractures despite a considerable relaxation, conversely for high values of R, the relaxation was limited without any fracture. After reporting the tests on the Haigh plane, the Smith-Watson-Topper equation (SWT) provided the best prediction of the fatigue strength, at least until the intersection with the ultimate stress line, both under stress and strain control loadings. The cyclic behavior of the material was then investigated through several static and cyclic tests on plain specimens. A kinematic hardening Chaboche model, with three parameter couples, was proposed and the values of these parameters derived and discussed. Finally, other tests have been conducted on notched specimens with C geometry and blunt radius, again at high R values. FE analysis allowed the prediction of the stress evolution during the loading cycling, implementing the Chaboche model, and observing a combined effect of ratcheting and relaxation at the notch tip. The stabilized stresses were finally reported on the Haigh diagram and the results were found in agreement with the plain specimen fatigue line.

With an objective of understanding the cyclic fracture, fatigue crack growth and ratcheting behaviour of Type 304LN stainless steel piping components used in primary heat transport system of nuclear power plants, subjected to various operational, loading and environmental conditions, experimental studies were carried out on totally 36 straight pipes and eight elbows. The pipe dimensions were in two categories: 170 mm OD and 15 mm thickness or 324 mm OD and 25 mm thickness. These studies on actual size piping components of power plants have resulted in very valuable fatigue behaviour related data, which would form important inputs in assuring safety and damage tolerant design of power plants. An overview of the studies is presented in this paper.

When materials are subjected to cyclic plastic loading with nonzero mean stress, strain usually accumulates in the direction of mean stress with an increasing number of cycles. This kind of strain accumulation is called cyclic creep or ratcheting. The phenomenon strongly depends on many factors and its accurate numerical modelling is problematic, especially under non-proportional loading. Two modifications of AbdelKarim-Ohno cyclic plasticity model were proposed for better ratcheting simulations. The developed model was implemented into the commercial FE software ANSYS using a radial return method. In this paper, the model capability is verified on a set of axial/torsional loading paths taken from Chen et al. The model predicts ratcheting strain reasonably well for almost all test cases., Kada su materijali podvrgnuti cikličnom plastičnom opterećenju sa ne-nultim srednjim naprezanjem, deformacija se obično akumulira u smjeru srednjeg naprezanja sa sve većim brojem ciklusa. Ova vrsta akumulacije deformacije zove se ciklično puzanje ili akumulacija plastične deformacije (ratcheting). Ovaj fenomen ovisi o mnogim čimbenicima i teško je napraviti njegovo točno numeričko modeliranje, posebno u uvjetima neproporcionalnog opterećenja. U svrhu postizanja boljih simulacija akumulacije plastične deformacije predložene su dvije modifikacije AbdelKarim-Ohno modela ciklične plastičnosti. Dobiveni model je implementiran u komercijalni FE software ANSYS korištenjem metode radijalnog povratka. U ovom radu su potvrđene mogućnosti modela pomoću niza aksijalnih/torzijskih opterećenja (Chen i ostali). Model prilično dobro predviđa akumulaciju plastične deformacije za skoro sve probe.

This thesis develops a new FEM based algorithm for shakedown analysis of structures made of elastic plastic bounded linearly kinematic hardening material. Its concept can be briefly described as: Hardening law is simulated using a two-surface plastic model. One yield surface is the initial surface, defined by yield stress sigma_y, and the other one is the bounding surface, defined by ultimate strength sigma_u. The initial surface can translate inside the bounding surface without changing its shape and size. The subsequent yield surface is bounded by one of the two following conditions: (1) it always stays inside the bounding surface, or (2) its centre cannot move outside the back-stress surface, where the back-stress surface is defined by pi = sigma_u - sigma_y. Both ways of bounding are equivalent. The subsequent yield surface may touch the bounding surface, it means ratchetting occurs and the benefit of hardening is quite clear; or it may not touch the bounding surface, it that means alternating plasticity occurs, and there is no effect of hardening. If sigma_y = sigma_u, the two-surface model becomes perfectly plastic model, and if sigma_u >= 2*sigma_y, the model becomes an unbounded kinematic hardening model. Since the two-surface model bases only on yield stress and ultimate strength, so it does not depend on the hardening curve, consequently it is a linearly kinematic hardening model. Direct methods lead to plastic limit and shakedown bounds directly. They help to reduce considerably computing costs and numerical errors, and make the solution simpler. Mathematically, the shakedown problem is considered as a nonlinear programming problem. Starting from upper bound theorem, shakedown bound is the minimum of the plastic dissipation function, which is based on the von Mises yield criterion, subjected to compatibility, incompressibility and normalized constraints. This constraint nonlinear optimization problem is solved by combined penalty function and Lagrange multiplier methods.

We consider a material having a constant true activation volume and estimate the apparent activation volume that it displays in cyclic creep or ratcheting. We show that cyclic stresses have an effect on the measured value, and that this can be accounted for in data interpretation. (C) 2011 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.