The present study is aimed at experiments/analyses on monotonic and cyclic fracture behavior of C-Mn steel at temperatures (T) varying from 28 degrees C to 300 degrees C. The fracture tests have been performed on standard compact tension specimens under different crack mouth opening displacement (CMOD) rates and load ratios (R: 0, -0.5, and -1) in a dynamic strain aging (DSA) regime. The fracture toughness in terms of () curves are compared for different test variables (T, R, and CMOD rate). The least fracture resistance curve has been observed at 300 degrees C for fast CMOD rate under R = -1. The fracture resistance curves are also plotted using the proposed to account for additional damage under cyclic fracture. The proposed considers conventional J and Delta J to account for damage owing to tearing and fatigue, respectively. The fracture toughness in terms of and are compared for test conditions. Proposed curves are approximately independent load ratios of 0 and -0.5 for a given temperature and CMOD rate. Further, fractographic investigations have been performed to bring out the individual effect of temperature, load ratio, and loading rate on fracture toughness curves.

The present study is aimed to investigate the effect of temperature, especially in dynamic strain ageing (DSA) regime, on the fatigue crack growth rate (FCGR) behaviour of low C‑Mn steel. The FCGR tests have been performed on standard compact tension specimens with crack surrounded by small-scale-yielding and largescale-yielding conditions. The scanning electron microscopic studies have also been performed to correlate the fatigued surface features with continuum length scale observations in terms of fatigue crack growth rate and fracture mechanics-based crack driving force parameter. The test fatigue crack growth rate behaviour nearly remains unaffected by temperature and crack tip yield conditions.

The present study focuses on application of Digital Image Correlation technique (DIC) to low cycle fatigue testing for localized strain measurement and capturing crack initiation event at relatively lower value of strain amplitude. Low cycle fatigue tests have been conducted on unnotched as well as notched tube specimen under different cyclic loading conditions. The validation exercise of the DIC systems has been carried out by conducting tests on unnotched tube specimens subjected to completely reversible loading under extensometer-controlled condition. This is followed by conducting strain-controlled test on notched tube specimen with a single sided through thickness transverse hole in the gauge region of the tube and subjected to remote cyclic axial and torsional loading conditions. During the test, measurement of the localized strain field information as well as capturing the crack initiation event at/ahead of notch have also been carried out on the outer surface of the tube specimen. Further, the applicability of the DIC measurement technique for curved geometry under such low strain amplitude and complex loading scenario has been evaluated by comparing the DIC measurements with their corresponding strain gauge measurements. DIC measurements are in good agreement with the strain gauge results.

It is well understood that the fatigue crack initiation life of a structural component having a notch/discontinuity is primarily governed by two parameters: (i) the (i) magnitude of the localised peak equivalent strain amplitude at the notch tip and (ii) the extent of the strain gradient at the notch at/ahead of the notch tip. However, there hardly exists any standard test procedure for conducting notch fatigue tests. The current test practices existing in literature tend to bring out the combined effect of peak equivalent strain and strain gradient on fatigue life instead of the individual effects. The present study aims to develop a new test methodology where the individual effects of (i) peak equivalent strain and (ii) strain gradient on fatigue crack initiation life have been brought out explicitly by conducting fatigue tests on tubes of C-Mn steel. Axial/torsion strain-controlled tests have been performed on a tube having a one-sided, through-thickness circular hole at the centre of the gauge region. Different hole diameters representing different strain gradient conditions have been investigated. The test methodology makes use of the pretest finite element analyses in such a way that it results in a common peak equivalent strain amplitude for different gradient conditions (different hole diameter) or different peak equivalent strain amplitudes for a common hole diameter. The remote strain/relative displacements outcome from the pretest finite element analyses has been used as a controlled parameter in the actual test to get the desired peak strain amplitude at the hole tip. The elastic-plastic pre-test FE analyses use three decomposed Chaboche material models and have been calibrated from the saturated hysteresis loops obtained from uniaxial low cycle fatigue tests on solid unnotched specimens.

The present work is aimed at numerical assessment of dissimilar metal welds under pure axial and pure torsion cyclic loading conditions. Finite element analyses have been performed to quantify the strain gradient on hot-wire narrow-groove DMWs of SA508 Gr.3 Cl.1 and SS304LN with SS308L filler material. The elastic plastic finite element analyses have been performed using three-decomposed Chaboche material models of these segments (two parent materials and one weld material). The remote strain is controlled to attain maximum total von-Mises equivalent strain amplitudes of 1%, 0.75%, 0.5%, and 0.35% in the gauge region of weldment. The quantifications of strain gradients under different strain amplitudes and remote loading conditions have been performed in radial and axial directions. Fatigue life estimation is performed for different equivalent strain amplitudes, utilizing material specific strain-life equations.

Present study aims to investigate individual effects of strain gradient and peak equivalent strain amplitude on fatigue life of notched tubes of C-Mn steel. Remote pure axial and pure torsion strain-controlled tests have been performed for different strain gradients (owing to different hole diameter in tube specimen) subjected to same peak equivalent strain amplitude at hole tip. During the tests, local strains ahead of notch were measured using digital image correlation/strain gauge techniques. Measured strains have been compared with simulated strains using detailed finite element analyses. Additionally, fatigue life assessment has also been carried out using equivalent strain amplitude at (i) peak strain location and (ii) different distances from peak strain location. A critical distance parameter (dc), in present study, has been deduced based on least scatter between predicted and test fatigue lives.

The present study aims to present experimental results on notched tubes of C-Mn steel under proportional and non-proportional axial-torsion cyclic loading conditions. Tubes with two different sizes of through-thickness circular holes in the gauge region of tube specimens were used for fatigue experiments. These tubes with circular holes have been subjected to remote in-phase (proportional) and out-of-phase (non-proportional) axial-torsion strain-controlled conditions. During the test, localized strain fields ahead of the hole have been measured using digital image correlation technique. The test fatigue lives under proportional and non-proportional loading conditions have been compared for the same set of remotely controlled axial and torsion strain amplitudes. The experimental fatigue life under non-proportional conditions comes out to be higher than that of corresponding proportional loading conditions for such tubes with holes and same remote loading conditions.

An essential component of ensuring the structural safety of mechanical components is the assessment of damage caused by fatigue in its operating temperature range. Conventionally, fatigue crack growth is attributed due to crack driving force range derived from classical fracture mechanics approach. However, mechanistically it is caused due to cyclic plastic deformation ahead of the crack tip. The nuclear power plant piping steel, SA333 Gr.6 employed in this study, is prone to dynamic strain aging in its operating temperature range. The tensile and low cycle fatigue tests conducted at temperatures ranging from 75 °C to 375 °C in literature, indicate that the temperature range for the dynamic strain aging phenomenon is between 175 °C and 300 °C.The present study investigates the cyclic plastic zone ahead of the crack tip, both at room temperature and at a temperature of 300 °C. Two-dimensional finite element analyses are conducted on full compact tension specimens with the Chaboche nonlinear kinematic hardening material model. In order to demonstrate the effect of crack sizes, a total of 12 distinct crack sizes are simulated. The study aims to analyze the behaviour of ratcheting at the maximum triaxiality point and at the midpoint of the cyclic plastic zone for a given crack size. The impact of the load ratio is also examined on the ratcheting behaviour. It has been shown that negative load ratios result in early shakedown.

ABSTRACTThe present study is aimed at analyzing the fatigue crack growth rate data on compact tension (CT) and three‐point‐bend (TPB) specimens of C–Mn steel under different positive load ratios. Detailed elastic–plastic finite element analyses have been performed using nonlinear kinematic hardening rule of Chaboche material model. The numerically calculated plastic zone ahead of crack tip has been compared with measured plastic zone using digital image correlation technique. The fatigue crack growth rate curves have been analyzed with respect to different crack driving forces such as (i) single‐parameter stress intensity factor (SIF) range, (ii) two‐parameter‐based SIF (), and (iii) cyclic plasticity zone–based models. A new crack driving force has been proposed considering cyclic plastic zone and representative plastic strain accumulation within this zone. The parameter and proposed model resulted in improved assessments for different load ratios and constraint geometries (CT and TPB).

The cyclic hardening/softening behaviour of C-Mn piping material has been studied in detail. The material exhibits cyclic hardening-hardening-hardening (H-H-H) at higher strain ranges, softening-softening-hardening (S-S-H) at lower strain ranges. Quantitative investigations of isotropic transformations show strain range dependent variation of cyclic yield strength. Higher translations of centre of yield surface have been observed at higher strain ranges. Present work has explored cyclic hardening/softening primarily due to variation of isotropic softening. The material exhibits additional hardening under multistep asymmetric strain cycling. New multi-objective plasticity model has been developed, results in excellent prediction of cycle-cycle behaviour, H-H-H, S-S-H, additional hardening till failure.

The present study aims to bring out the individual effect of peak equivalent strain and strain gradient on the fatigue life in presence of notch/discontinuity. Tests have been conducted under remote pure axial and pure torsion conditions on tubes of C-Mn steel with single circular hole. Tubes having different size holes have been taken to study the effect of strain gradient. The remote strain amplitude has been so adjusted that it results nearly same peak equivalent strain amplitude (using pre-test FE analyses) for different size holes. This test methodology brings out the individual effects of fatigue damage in terms of fatigue crack initiation life as a function of strain gradient (owing to different hole size) and peak equivalent strain amplitude at hole tip.Further, the localized strains ahead of hole have been measured using digital image correlation and strain gauge techniques. The fatigue life has been predicted using point based critical distance and critical plane models. The predicted fatigue life is compared with test fatigue life for different hole sizes and remote pure axial/ pure torsion conditions.

The present study validates recent as well as extensively used critical plane (CP)-based fatigue life assessment models for 10 ferrous and 7 non-ferrous materials. Fourteen CP models, including recently reported in-house developed model, have been analyzed. These models use resolved stresses, strains, strain energy density, and combinations of stresses and strains to identify critical plane and quantify fatigue damage. The loading conditions cover various proportional and non-proportional axial-torsion strain paths with different phase shift angles with (or without) mean stress/strain component(s), loading waveforms, and asynchronous axial-torsion stain paths. The comparative study brought out the goodness of various models by quantifying the extent of scatter and computational time.Some of the strain energy density-based models (including in-house model) have been shortlisted, which produced accurate fatigue life assessments. Among the shortlisted models, in-house model is free from the calculation subjectivities of resultant shear and performs faster fatigue damage calculations.

In the present work, a cyclic plasticity model based on Ohno-Wang kinematic hardening rule and Tanaka nonproportionality parameter, is proposed to simulate the cyclic stress strain response of three materials. The proposed model has been validated with respect to reported test results on two grades of steel (SA 333 Gr. 6 and E355) and one grade of austenitic stainless steel (X5CrNi18-10) under asynchronous axial-torsion loading conditions with a wide range of frequency ratios. The proposed model resulted in a comparable assessment of axial and shear stress-strain response with respect to the test loops for all three materials.

In this work, a prominent cyclic plasticity model based on modified AbdelKarim-Ohno kinematic hardening rule and Calloch isotropic hardening is implemented and validated w.r.t. test results of low C-Mn steel. In reference to the shortcomings of the model, an improved model based on modified Ohno-Wang and Tanaka non -proportionality parameter is introduced. Since the precise assessment of stress-strain response is an essential precondition for carrying out fatigue life analysis, therefore hysteresis loop responses under various multiaxial LCF loading having different strain paths are compared. The simulation result with the proposed model resulted in improved assessment for non-proportional strain paths.

Present work is aimed at performing fatigue tests on notched C-Mn steel tubes. Tests were conducted on different sizes of notches under remote axial strain-controlled conditions. Individual effects of peak equivalent strain amplitude and strain gradient are brought out on experimental fatigue life. The localized measured strains are compared with corresponding outcome of FE analyses. Fatigue life for notched specimens was predicted based on peak equivalent strain amplitude/existing critical plane model considering stress/strain information at peak/characteristic distance locations. The characteristic distance location with critical plane model and peak equivalent strain amplitude results in improved fatigue life predictions.

In this present work, a damage-coupled cyclic plasticity model has been developed for more accurate ratcheting–fatigue life estimation under strain and stress controlled ratcheting. Ratcheting–fatigue damage behavior under uniaxial multistep strain-controlled ratcheting shows that the incremental mean ratcheting strain deteriorates the elastic slopes cycle by cycle, by means of ratcheting damage. Severe ratcheting strain accumulation rate has been observed in tertiary region under uniaxial stress controlled ratcheting. The proposed damage-coupled model has been constructed which incorporates both fatigue damage and damaging effect of the accumulated mean plastic strain. The proposed model incorporates a critical fatigue damage parameter which can predict effects of early fatigue crack nucleation due to combined ratcheting and fatigue damages. The performance of the proposed damage-coupled model has been investigated in the present study based on the critical fatigue damage parameter. The proposed model is calibrated on experimental data of SA333 Gr. 6 carbon steel and SA508 Gr. 3 steel. The proposed formulations have been applied in user material subroutine UMAT of finite element software, ABAQUS. The proposed model has been validated by comparing predicted ratcheting behavior with experiments for the two different steels. All the predicted number of cycles to failure are located within 0.5 times error band. The proposed damage-coupled model has demonstrated excellent capabilities of predicting ratcheting–fatigue life under cyclic loading with ratcheting damage.

In this work, a phenomenological unified model has been developed to characterize the cyclic plastic deformation response including hardening, softening and non-Masing characteristics of SA333 Gr.6 low C-Mn steel under various fatigue loading conditions. The unified proposed model embedded in the ABAQUS platform through user defined subroutine is based on the framework of the modified Ohno–Wang kinematic hardening rule and memory stress-dependent isotropic hardening formulation. The solid and tubular low cycle fatigue (LCF) specimens and also a pressurized primary heat transfer (PHT) straight nuclear pipe of this material under different loading conditions have been analyzed with this unified model using single set of material parameters derived from the experimental results of uniaxial LCF. The solid and tubular specimens have been loaded with uniaxial tensile-compressive low-cycle fatigue loading and multiaxial in-phase tension–torsion. For component-level analysis, three-point and four-point bending loads have been applied along with constant internal pressure in the PHT straight pipe. The predicted elastic–plastic response from the proposed model is correlated well with the corresponding experiment response under uniaxial tension–torsion and multiaxial in-phase tension–torsion loading. The major findings include the comparison of simulated hysteresis loop area, von Mises stress and variation of stress amplitude with the experiment response under strain-controlled LCF loading. The prediction of load-dependent hardening/softening and non-Masing characteristics using a unified proposed model is attributed to the continuous evolution of kinematic hardening and isotropic hardening utilizing the concept of memory stress. Further, the proposed model has improved the simulation of the circumferential strain ratcheting of the piping component under the combined three-point or four-point bending loads with constant pressure.

The present study is aimed at understanding the saturated cyclic stress-strain material behaviour of low C-Mn steel under pure axial, pure torsion, proportional axial-torsion and non-proportional axial-torsion test conditions. The axial/shear stress-strain hysteresis loops have been investigated vis-à-vis classical cyclic material modelling rules/criteria available at continuum length scale. It has been observed that non-linear loading arms of pure axial, pure torsion and proportional axial-torsion loops are comparable on von-Mises equivalent stress and strain axes for corresponding equivalent strain amplitude. This indicates the need of von-Mises yield criterion for uniaxial and proportional conditions. The nature of axial and torsion loops under proportional conditions closely resembles with associative flow rule. Therefore, von-Mises yield criterion, Prandtl–Reuss flow rule, Armstrong–Frederick family of non-linear kinematic hardening rules are required to simulate uniaxial and proportional conditions. However, axial and shear test hysteresis loops under non-proportional conditions indicated non-associative nature of plastic flow. This test observation under non-proportional cyclic condition is scarcely reported. Most of the advanced cyclic plasticity material models use associative flow rule as it is simple to apply computationally on evolving yield function. This study is an attempt to highlight the key issues related to material constitutive equations under non-proportional loading conditions.

The present study is aimed at validation of notch stress/strain estimation schemes such as classical Neuber, Hoffmann–Seeger, and recently developed Ince–Glinka method for nuclear piping material (low C–Mn steel). The study has considered different constraints, loading conditions, and various hole sizes to accommodate strain gradient variations and equivalent peak strains. The notch stress field evaluated using these schemes is compared with corresponding stress using elastic–plastic finite element (FE) analyses. The comparisons have brought out that the Hoffmann–Seeger scheme results in reasonably accurate assessment of stress localization nearly for all constraint geometries, loadings, and strain gradients. However, the classical Neuber scheme is more suitable for low constraint geometries and intermediate constraint geometries, whereas it results in underestimation of maximum principal stress for high constraint geometries, thereby leading to overprediction of fatigue life. Further, the suitability of energy equivalence equations of Ince–Glinka model for individual stress components has been reviewed.

The present study is aimed at validation of notch stress/ strain estimation schemes such as classical Neuber, Hoffmann-Seeger and recently developed Ince-Glinka method for Nuclear piping material (low C-Mn steel). The study has considered different constraints, loading conditions, various hole sizes to accommodate strain gradient variations and equivalent peak strains. The notch stress field evaluated using these schemes is compared with corresponding stress using elastic-plastic Finite Element (FE) analyses. The comparisons have brought out that the Hoffmann-Seeger scheme results in reasonably accurate assessment of stress localization nearly for all constraint geometries, loadings and strain gradients. However, the classical Neuber scheme is more suitable for low constraint geometries and intermediate constraint geometries whereas it results in under-estimation of maximum principal stress for high constraint geometries, thereby leading to over-prediction of fatigue life. Further, the suitability of energy equivalence equations of Ince-Glinka model for individual stress components, has been reviewed.