Triphasic ratcheting strain prediction of materials over stress cycles

2012

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This study intends to compare ratcheting response of 42CrMo, 1020, SA333 and SS304 steel alloys over uniaxial stress cycles evaluated by a parametric ratcheting model and Bower's hardening rule. The parametric ratcheting equation was formulated to describe triphasic stages of ratcheting deformation over stress cycles. Mechanistic parameters of mean stress, stress amplitude, material properties and cyclic softening/hardening response of materials were employed to calibrate parametric equation. Based on the framework of cyclic plasticity theory, the modified Armstrong–Frederick nonlinear hardening rule of Bower was employed to assess ratcheting response of steel alloys under uniaxial stress cycles. Bower's model was chosen mainly due to simplicity of the model and its lower number of constants required to predict ratcheting strain over stress cycles as compared with other hardening rules. Ratcheting strain values predicted by Bower's model showed good agreements over stage I of stress cycles as compared with experimental values of ratcheting strain. Beyond of stage I stress cycles, Bower ratcheting strain rate stayed constant resulting in an arrest in ratcheting process. The predicted ratcheting strains based on the parametric equation were found in good agreements over three stages of ratcheting as compared with those of experimentally obtained.

Section: Parametric Triphasic Ratcheting Equationmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

Ratcheting assessment of steel alloys under uniaxial loading: a parametric model versus hardening rule of Bower

Ahmadzadeh

2012

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“…[18][19][20][21][22][23] Ratcheting progress over asymmetric stress cycles was found as a result of cyclic plastic strain accumulation in materials extended over distinct stages with various rates and magnitudes. [18][19][20][21][22] Kang et al 24 reported macroscopic evidences of ratchetting deformation over uniaxial loading cycles in 316L stainless steel coinciding with dislocation activities over stress cycles. Microscopic examinations have revealed that progressive deformation over these stages was associated with plastic slip, dislocation interactions, and number of active dislocations.…”

Section: Ratcheting Mechanisms and Stagesmentioning

confidence: 99%

“…Parametric equations have been structured based on the applied mean and amplitude of stresses, cyclic hardening or softening behaviours, and number of cycles. 18,21,184 Analogous to creep phenomenon, the stages of ratcheting as illustrated in Figure 2 expand over life cycles. Experimental evidences have supported triphasic response of ratcheting in engineering alloys.…”

Section: Figure 18mentioning

confidence: 99%

Ratcheting in pressurized pipes and equipment: A review on affecting parameters, modelling, safety codes, and challenges

Nayebi

2018

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The current study has attempted to comprehensively review ratcheting response of materials involving various influential parameters such as loading spectra, thermal cycles, stress levels, stress raisers, strain rate, and visco‐plasticity and material types with a focus on pressurized pipes and equipment. The mechanism of deformation, types, and the rate of progress over stages of ratcheting were discussed. Safety design codes and procedures were highlighted for reliable design of pressurized pipes against progressive ratcheting and to safeguard the system against catastrophic failure at which both mechanical and thermal ratcheting were integrated. Boundaries and demarcation of ratcheting‐shakedown zones developed based on Bree's diagram were employed to assess plastic deformation accumulation over stress cycles. Shakedown and ratcheting boundaries were discussed through methods developed on the basis of Melan's theorem over last few decades. Ratcheting response of materials was reviewed through descriptions of parametric models and kinematic hardening rules involving various variables and parameters. Interaction of ratcheting with creep and low‐cycle fatigue has promoted progressive damage in materials. Pressurized pipes subjected to thermal cycles and external bending cycles were evaluated by numerical solutions along with kinematic hardening rules to assess ratcheting over stress cycles.

“…2,6 During last stage, ratcheting process is typified as ratcheting strain rate increases uncontrollably in successive cycles resulting in the cross-sectional area reduction. 8 Ratcheting deformation has been intensively discussed for various materials in the literature. 2,4,6,7 The triphasic ratcheting response of materials over stress cycles with positive s m /s a ratios has been formulated in a recent paper by the present authors.…”

Section: Introductionmentioning

confidence: 99%

Ratcheting assessment of materials based on the modified Armstrong–Frederick hardening rule at various uniaxial stress levels

Ahmadzadeh

2013

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The present study evaluates ratcheting response of materials by means of the Armstrong–Frederick (A–F) hardening rule, the modified A–F rule (Bower's model), and further modifications of the hardening rule based on new introduced coefficients. The implemented modifications on the A–F‐based hardening rule aims to address stages of ratcheting over stress cycles. The modified hardening rule predicts the ratcheting strain rate decay over stage I and the constant rate of strain accumulation during stage II. The modified hardening rule consisted of the coefficients of the hardening rule controlling stress–strain hysteresis loops generated over stress cycles during ratcheting process (Bower's modification on A–F rule) plus the coefficients controlling rates over stages of materials ratcheting deformation. Stress–strain‐dependent coefficients in the modified rule are responsible to compromise overprediction of ratcheting of A–F during stage I and the premature plastic shakedown beyond stage I induced by Bower's model. Ratcheting strain rate coefficients improved the hardening rule capability to calibrate and control the rate of ratcheting in stages I and II and enabled the modified hardening rule to predict ratcheting strain over a prolonged domain of stress cycles. The modified hardening rule was employed to assess ratcheting response of 304, 42CrMo, 316L steel and copper samples under uniaxial loading conditions. The predicted ratcheting values based on the modified hardening rule and the experimental ratcheting strains were found in good agreements.