PSI - Issue 14

Bimal Das et al. / Procedia Structural Integrity 14 (2019) 619–626 Das et al./ Structural Integrity Procedia 00 (2018) 000 – 000

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1. Introduction

The increase in power consumption and to increase the efficiency of has significantly affected the operating temperature and pressure of power plants. The ultrasupercritical steam generation plants operating at temperature above 600 0 C led to the development of chrome and nickel based alloys. Among them 9Cr- 1Mo martensitic steel possessing high thermal conductivity and low thermal expansion coefficient is considered to be candidate material for the fabrication of components of steam generator and piping systems. These structural components are subjected to symmetric and asymmetric cyclically varying loads and thus cyclic plastic deformation becomes inevitable. Thus the study of elastic – plastic stress – strain response of the material becomes important in the design and structural integrity assessment of the structures. Various types of stress raisers in the structural components results in cyclic plastic phenomena such as ratcheting and cyclic stress relaxation due to the evolution of plastic strains [2,3]. It is well recognized that mean stress has a significant effect in the cyclic stress – strain response and fatigue life. The cyclic dependent variation of mean stress i.e. mean stress relaxation must be taken into account for the accurate prediction of fatigue life for safe design [3]. Thus in the present study effect of mean tensile strain in the strain controlled loading response of P91 steel is studied. Mean stress relaxation is defined as cycle dependent decrease in the absolute value of mean stress under constant strain amplitude. The rate and amount of relaxation of mean stress depends on the material characteristics and loading conditions [4]. The mean stress relaxation response of the ASTM A-516 Gr. 70 carbon low alloy steel analyzed by Ellyin [2] depicted an initial decrease in the mean stress in early life prior to stabilization thereafter increases due the hardening characteristic of the material. Lee et al. [5] conducted uniaxial asymmetric strain controlled tests at room temperature to investigate the relaxation behavior of S32750 super duplex stainless steel. The tensile mean stress was found to relax rapidly in first few cycles and thereafter stabilizes to a certain value. The influence of strain amplitude on the rate of relaxation is more significant than the imposed mean strain. Paul et al. [3] showed that the mean stress decreased progressively with increase in cycle till failure of the specimen. They also showed that specimen preloaded with tensile prestrain followed by the symmetrical strain controlled loading of same amplitude as of asymmetric strain cycling revealed similar nature of cyclic stress response. The asymmetric strain controlled behavior of X12CrMoWVNbN10-1-1 steel at 873 K were analyzed by Wu et al. [6]. The material depicted cyclic softening response for both symmetrical and asymmetrical loading. The variation of mean stress and stress amplitude with the number of cycles were observed to be of similar nature. Also, the cyclic softening was found to be independent of applied mean strain under asymmetric strain controlled loading. Colin et al. [7] studied the effect of mean strain on the strain controlled response of 304LN stainless steel. The relaxation of mean stress was very less in the first few cycles due to considerable hardening in the material. The rate of relaxation at higher strain amplitudes was found to be rapid than at lower strain amplitudes. The prediction of the cyclic stress-strain response of material is very essential in regard to the conservative and safe design of structural components. Though numerous constitutive models are proposed to simulate the symmetric strain controlled behavior of various structural materials. Very few works has been reported to capture the relaxation of mean stress under strain controlled histories. Khutia et al. [8] modified the dynamic recovery term of Armstrong Frederick model by incorporating a fading memory stress function in the kinematic hardening to simulate the mean stress variation subjected to asymmetric strain controlled loading in SA333 C – Mn steel. Wu et al. [6] introduce a parameter , a function of both maximum applied plastic strain and accumulated plastic strain in the modified the Abdel-Karim kinematic hardening model to successfully capture the mean stress relaxation phenomena of 9-12% Cr steel. In another work reported by Lee et al. [5], the dynamic recovery term of Chaboche model is modified by a radial evanescence tem of Burlet and Cailletaud rule to control the activation of stabilized critical state. This modification improved the accuracy of prediction of asymmetric strain and stress controlled loading stress-strain response of the duplex stainless steel. From the above literature it can be understood that the mean strain can significantly affect the materials stress- strain response and thereby affecting the fatigue life. Therefore the present work aims to address the cyclic stress strain response of P91 steel under asymmetric strain controlled loading by experiment and cyclic plasticity modeling.