Round Bar Specimens Research Articles

Effect of direct-quenching and tempering on hydrogen-induced delayed fracture resistance of high-strength bolt steel

This investigation attempts to explore the potential significance of controlled forging, i.e., direct-quenching at two different finish-forging temperatures of ∼930 °C (B8H sample) and ∼780 °C (B8L sample), and tempering at 600 °C in improving the hydrogen-induced delayed fracture (HIDF) resistance of a 1300-MPa-grade high-strength bolt steel by constant load tensile testing using circumferentially notched round bar specimens and hydrogen thermal analysis. The microstructural characteristics of the tested samples were analyzed and their influences on HIDF were discussed. The results showed that the B8L sample demonstrated a considerably increased HIDF resistance expressed by critical notch delayed fracture stress of 1825 MPa and a decreased hydrogen embrittled index of as low as 21% compared with those of 1675 MPa and 27% for the B8H sample, although the former exhibited slightly higher strength and absorbed hydrogen content. Scanning electron microscopy observation of the fracture surface demonstrated that the B8L sample prevented the brittle intergranular fracture with smaller quasi-cleavage facet and decreased area fraction of the brittle crack initiation zone compared with the B8H sample. The increased resistance to HIDF of the B8L sample is primarily because of its fine microstructure with increased number of finely distributed V-rich nanoscale MC precipitates and decreased dislocation density. It is proposed that controlled forging coupled with V-microalloying is a feasible and economical way to further increase the HIDF resistance of high-strength bolt steel.

Cyclic Void Growth Model Parameter Calibration of Q460D Steel and ER55-G Welds after Exposure to High Temperatures

When high-strength steel is heated to high temperatures and then cooled naturally, its ductility decreases. In earthquake-prone areas, it is necessary to evaluate the ultra-low cycle fatigue fracture (ULCF) behavior of high-strength steel structures after a fire if these structures are used continuously. However, the ULCF fracture model of high-strength steel subjected to high temperatures followed by natural cooling has not been deeply studied. In view of this, twelve notched, round bar specimens fabricated from Q460D steel and ER55-G welds were heated to 900 °C followed by natural cooling and then cyclic loading experiments and finite element analyses (FEA) were performed on these specimens. The fracture deformation obtained from the experiments was used in the FEA to calibrate the damage degradation parameter of a Cyclic Void Growth Model (CVGM) of Q460D steel and ER55-G welds under this condition. The calibrated values were 0.30 and 0.20, respectively. The calibrated CVGM was employed to predict the number of cycles and the force and displacement at the fracture moment of the notched round bar specimens. The predicted results aligned closely with the experimental results, indicating that CVGM is effective in predicting the fracture of Q460D steel and ER55-G welds following exposure to 900 °C and subsequent natural cooling.

The cyclic GTN model for ULCF fracture analysis of Q355 steel

The cyclic GTN (C-GTN) model, built upon the GTN model, considers the influence of cyclic loadings on the evolution of microvoids, enabling the prediction of ULCF fracture in materials. In this study, the C-GTN model was calibrated for Q355 steel based on tests on notched round bar specimens reported in the literature. Following existing four tests on single-edge notched plate (SENP) specimens subjected symmetric cyclic loadings, additional four ULCF tests were conducted on SENP specimens with asymmetric cyclic loadings. ULCF fracture was observed in these tests and ULCF lives of these SENP specimens were obtained. Mesh sensitivity was investigated for the ULCF fracture analysis of the SENP specimens with the C-GTN model. After balancing the computational accuracy and cost, the ULCF fracture analysis of the SENP specimens were conducted using finite element models with a mesh size of 0.2 mm at the notch tip. ULCF lives predicted by the C-GTN model agree very well with test results for all SENP specimens. The agreement validated the applicability of the C-GTN model for the ULCF fracture analysis of Q355 steel. The increase of the void volume fraction and the degradation of its critical value played dominant roles in the ULCF fracture of V-notched and U-notched SENP specimens respectively.

Cyclic performance study of Q460D steel and ER55-G welds after high temperature

Coupon test specimens with the gauge length part of Q460D high-strength steel or ER55-G weld respectively were heated up to high temperature of 900 °C and then cooled in water. Uniaxial tensile tests were conducted on those cooled specimens. The engineering stress-engineering strain curves were obtained. Meanwhile, twelve smooth round bar specimens made of Q460D high-strength steel and ER55-G welds were heated up to high temperature of 900 °C and then cooled naturally or cooled in water. Cyclic loading tests with two different loading protocols, namely constant amplitude and variable amplitude, were conducted on these specimens at different strain levels. The engineering stress-engineering strain curves of each specimen were obtained, enabling to compare and analyze the differences of the results in cyclic mechanical properties of each specimen in terms of damage morphology, hysteresis performance, and ductility characteristics. The test results show that the different cooling methods after the high temperature of 900 °C have a significant impact on the ultimate strength and elongation of Q460D high-strength steel and ER55-G welds. The ultimate strength of the specimens after water cooling was significantly higher than that of the specimens after natural cooling, while the elongation result was opposite. It was obvious from the test results that water cooling and natural cooling after high temperature led to a decrease in ductility and energy absorption capacity of the base metal and welds to various degrees, while their energy dissipation capacity was significantly improved at low strain level.

Post-fire mechanical properties and constitutive model of Q690 high-strength structural steel

Fire hazard is one of the main threats to the integrity of steel structural buildings. Developing an accurate post-fire constitutive model for structural steel is crucial for enhancing the failure assessment of steel structures exposed to fire. In this study, the post-fire mechanical behavior of Q690 high-strength structural steel is investigated. Eighteen round bar specimens made of Q690 steel were heated with temperatures ranging from 20 °C to 900 °C. Tensile tests were conducted on these specimens to obtain the post-fire stress-strain curves and associated mechanical parameters of the material. To accurately characterize the post-fire behavior of Q690 steel, a temperature-dependent elastoplastic constitutive model combined with its numerical implementation algorithm is proposed. This model consists of four components: a temperature-dependent elasticity criterion, the von Mises yield criterion, an associated flow rule, and a temperature-dependent isotropic hardening law. The proposed constitutive model has a notable feature, i.e. the material parameters in the model can be defined as a function of temperature according to the test results. As a result, when utilizing this model for post-fire assessments of steel members, only three routine parameters, i.e. elastic modulus at room temperature, Poisson's ratio, and the temperature experienced by the material, are required in the numerical simulation to generate the material’s stress-strain relationships. To verify the proposed constitutive model, the new model was introduced into the numerical simulations of the tested round bar specimens after exposure to various temperature conditions. Both the load-displacement curves and local deformation of each round bar specimen obtained from the numerical simulations are compared with the test results to validate the accuracy of the new model. The constitutive model presented in this study provides a useful tool for assessing the post-fire behaviors of steel components.

Continuous damage model for structural steels and weld metals under ultra-low-cyclic loading

For the fracture problem of structural steel under strong earthquakes, a continuous damage model which comprehensively considers the influence of stress triaxiality and Lode parameter was proposed in this paper. According to the variation law of Mises stress under cyclic loading, this model decomposes the effective cumulative plastic strain into isotropic hardening and kinematic hardening components, assuming a linear proportional relationship between the fatigue damage induced by these two components. The basic mechanical properties and constitutive relationships of five common Chinese structural steels and four corresponding weld metals were investigated. The fracture failure tests for both notched round bar specimens and pure shear specimens subjected to monotonic tensile and ultra-low-cyclic loading were thoroughly examined on these materials. The continuous damage model parameters were calibrated by means of test results and complementary finite element simulations. Experimental and numerical simulation results indicate that Q690 high strength steel exhibits cyclic softening behavior, whereas the other steels show cyclic hardening characteristics. For pure shear specimens, plane strain specimens and shear energy dissipation cover plates, the continuous damage model was adopted to simulate and analyze the damage development and fracture failure process of the specimens. The predictions for crack initiation, crack propagation and fatigue life are consistent with experimental results, lending further credibility to the model. The calibrated continuous damage model of various steels provides an effective means for analyzing the ULCF fracture failure of structural steel under intense earthquakes.

Finite-element method for the analysis of surface stress concentration factor and relative stress gradient for machined surfaces

Abstract. Surface topography is an important parameter for evaluating the quality of surface machining, and the stress concentrations produced at notches can have a profound effect on the fatigue life of notched components. The stress concentration factor (SCF, Kt) and relative stress gradient (RSG, χ) are important parameters used to quantitatively characterize stress concentration. In this study, a finite-element (FE) method was used to evaluate the surface SCF and RSG and determine the effect of microscopic surface topography on machined surfaces. An FE simulation of the static tension test of V-notched round-bar specimens was performed, and the stress due to the local surface topography was investigated. The FE method was used to analyze the stress concentration of round-bar specimens with Kt=1, and the reliability of the results was verified using a perturbation method. The FE method was used to calculate the surface SCF and RSG with high accuracy. The surface SCF and RSG values increased with the surface roughness, and the local maximum values of the surface SCF and RSG were at the bottom of the local topography. Therefore, the SCF and RSG could be estimated based on a linear relationship involving average roughness.

A cyclic GTN model for ultra-low cycle fatigue analysis of structural steels

Ultra-low cycle fatigue (ULCF) is a critical concern in the seismic design of steel structures due to its adverse effects on the ductility and energy dissipation capacity of steel connections. The Gurson-Tvergaard-Needleman (GTN) model, a well-accepted micromechanical fracture model, cannot be applied directly to ULCF analysis, as it requires proper estimation of the effects of cyclic loadings on the ductile fracture of materials. In this paper, a cyclic GTN (C-GTN) model was proposed for the ULCF prediction of materials, in which the evolution of microvoids during ULCF loadings was addressed for tensile and compressive half load cycles, respectively, and the effect of the cumulative plastic strains on the material’s resistance to ductile fracture was considered by the reduction of the critical void volume fraction for void coalescence. Then, a VUMAT subroutine was programmed for the C-GTN model, in which the cyclic hardening behavior of materials was simulated with the Voce-Chaboche model. Finally, the C-GTN model was calibrated for the G20Mn5QT cast steel based on previous tests on notched round bar specimens of the material. The applicability of the C-GTN model and the VUMAT subroutine were validated in the ULCF analysis of double-hole plate specimens of the G20Mn5QT cast steel.

Effect of stress triaxiality on the hydrogen embrittlement micromechanisms in a pipeline steel evaluated by fractographic analysis

Hydrogen gas, as an energy carrier, is expected to play an important role in the storage and transport of energy produced by renewable sources. The transport of hydrogen gas can be managed through the use of existing pipeline networks. However, the uptake of hydrogen in steel structures is known to cause a degradation of mechanical properties, a phenomenon called hydrogen embrittlement (HE). In this study, the hydrogen-assisted ductility loss of an API 5L X70 pipeline steel is investigated. The influence of hydrogen on mechanical behavior is evaluated by tensile tests on uncharged and hydrogen charged specimens. These tests are performed on notched round bar specimens with different notch radii, allowing for a range of positive stress triaxialities to be examined. Notched samples are more embrittled than unnotched specimens after hydrogen charging. Hydrogen signatures in the form of fisheyes and quasi-cleavage are detected on the fracture surface. Fisheyes initiate primarily at segregation bands in the steel for all specimen geometries. Fisheyes can also be linked to different inclusion types present in the steel, depending on notch geometry. For unnotched as well as the least sharply notched specimens tested in this study, the embrittlement can be correlated with the area fraction of fisheyes on the fracture surface.

Flow stress curves for 980MPa- and 1.5GPa-class ultra-high-strength steel sheets weakened under high-stress triaxiality

The weakening behavior of ultra-high-strength steel(UHSS) remains unknown, even though it is a requirement for the accurate prediction of strain localization and ductile fracture in automobile applications. In this study, we, therefore, revealed the stress–strain curves of UHSSs up to their ductile fracture under different stress triaxialities for the first time. Smooth and notched tiny round-bar specimens cut from UHSSs with 1.6 mm thickness were subjected to tensile tests, during which the forces and neck diameters of the specimens were measured. Notably, the evaluated flow stresses in the 1.5 GPa-class notched UHSS specimens exhibited weakening of up to 3%. This was in contrast to the 980 MPa-class UHSS specimens, which did not exhibit notch-induced weakening. The absence of weakening during stress measurements, as confirmed by synchrotron X-ray diffraction (XRD) analysis, suggested that weakening was caused by material damage. However, the weakening behavior did not follow the previously developed damage models based on microvoid formation. The volume fraction of the microvoids, as observed by X-ray computer tomography, was extremely small (at 0.2%) and cannot account for the 3% material weakening indicated by the flow stress measurement results. A new damage mechanism, associated with unobservable small lattice vacancies, was implied in UHSSs deformation under high stress triaxialities.

Numerical modeling of cyclic softening/hardening behavior of carbon steels from low- to high-cycle fatigue regime

The aim of this study is to characterize the stress–strain behavior of three construction steels (SM490, SM570, and F18B) through both experimental and numerical investigations. The material performance was evaluated by conducting tests on round bar specimens subjected to monotonic, fatigue, and incremental step fully reversed loading conditions. The experimental campaign was conducted to provide valuable information on the mechanical performances of the steels and data for calibrating the material constants required for numerical analyses. The numerical simulations aimed to demonstrate the effectiveness of the proposed unconventional plasticity model, the Fatigue SS model (FSS), in describing the non-linear behavior of the materials under a broad range of loading conditions, including stress states below and beyond the macroscopic yield condition. This aspect is a significant advantage of the FSS model, as conventional elastoplastic theories fail to provide a phenomenological description of inelastic material deformation under stress states within the yield condition. The good agreement between the experimental and numerical results confirms the validity of the calibration of the material constants and the reliability of the computational approach.

A new ductile fracture model for Q460C high-strength structural steel under monotonic loading: Experimental and numerical investigation

Tensile fracture static tests on nine specimens covering different stress states were performed to investigate the ductile fracture behavior of Chinese Q460C high-strength structural steel (HSS). Based on combining the Rice-Tracey model and Xue model, the new model considering the effect of stress triaxiality and Lode angle was proposed to predict ductile fracture of Q460C HSS. Furthermore, the stress softening criterion was incorporated into the new fracture model to simulate the fracture propagation of Q460C HSS better. The material parameters of the proposed fracture model for Q460C HSS were identified using MATLAB optimization code. The VUMAT subroutine was developed to perform ductile fracture simulations for each specimen, and the fracture simulation results show satisfactory agreement with the experimental results. In addition, the five existing ductile fracture models were also selected to compare the prediction accuracy of the new model. The comparative results show that the new model has better accuracy and is more suitable for predicting fracture initiation and propagation of Q460C HSS. Finally, the applicability of the new model in predicting the fracture of Q460 HSS was furtherly verified by successfully simulating the existing fracture tests of the notched round bar specimens and the bolted connections.

Anisotropic fatigue performance of directed energy deposited Ti-6Al-4V: Effects of build orientation

Directed energy deposition (DED), an important branch of additive manufacturing technology, has unique processes that attribute to the fatigue performance anisotropy associated with the build orientation of DED materials. However, the mechanism of fatigue performance anisotropy still remains unclear due to multiple factors of the defect and microstructure, which has limited further application of DED materials. This paper investigates the effects of build orientation on the anisotropic fatigue performance of DED Ti-6Al-4V treated with stress relief annealing, specifically the impact of defects and microstructures on fatigue performance anisotropy. The defects in the fully dense DED Ti-6Al-4V are non-directional porosity defects with smooth morphology and high sphericity; the microstructures are mainly characterized by prior-β columnar grains, α colonies and basketweave structures. To represent fatigue performance anisotropy of the material, standard round bar specimens were sampled from three build orientations, i.e., X, Y and Z directions, where Z-direction represents the build direction. These specimens were subsequently subjected to constant amplitude high cycle fatigue tests at the same loading condition and room temperature. The fatigue test results showed that the logarithmic mean fatigue lives of the X- and Y-direction specimens were close to each other, but significantly longer than those of the Z-direction specimens. Fractography and EBSD tests were conducted to explain the causes of fatigue performance anisotropy, suggesting that microstructures are more likely to explain the anisotropy caused by the build orientation than the porosity defects, with crystallographic orientation and α colony being two critical factors.

A unified creep life prediction method and fracture mechanism of high-temperature alloys under multiaxial stress

In this paper, a unified creep rupture life prediction method based on continuum damage mechanics under multiaxial stress was proposed. By optimizing the traditional skeletal point stress method, the number of models that need to be computed during the simulation was reduced by 5/6, and the generality of the method for different notch sizes was significantly improved. Creep tests on Ni-based superalloy round bar specimens with different notch sizes, which verify the accuracy of the method, were conducted. Furthermore, creep rupture mechanism analyses were conducted for different notch sizes by combining finite element simulations with experiments, stress and damage were analyzed, and void growth was explained based on triaxial stress states.

Real-time measurement of minimum cross-sectional area and radius of curvature of miniature plate specimens using a high-speed laser profiler for calculating true stress-true strain curves

True stress-true strain curves are fundamental for designing and analyzing structures such as fusion reactors. These curves are typically obtained by conducting tensile tests on round bar specimens. However, due to material dimension limitations, plate specimens are sometimes used instead of round bar specimens. Obtaining true stress-true strain curves experimentally from plate specimens can be challenging. To address this challenge, this study aims to obtain true stress-true strain curves of miniature plate specimens using both analytical and experimental methods. The analytical method involved inverse finite element method (FEM), while the experimental method utilized real-time measurement of the minimum cross-sectional area and radius of curvature of a miniature plate specimen with a high-speed laser profiler. Comparing the true stresses obtained from the analytical and experimental methods, we found that the difference was typically within 5%. These findings suggest that inverse FEM and laser profilometry are effective methods for determining the true stress-true strain of miniature plate specimens.

Fracture performance study on Q460D steel and ER55-G welds after high temperature

Uniaxial tensile tests were conducted on smooth round bar specimens made of Q460D high-strength steel base metal and ER55-G welds after a high temperature of 900 °C and natural cooling. Counterpart specimens at room temperature were also examined for comparison. The basic mechanical properties of each specimen were obtained, enabling quantification of the true stress-plastic strain curve of the material required for finite element analysis (FEA) in ABAQUS. The notched round bar specimens were heated up to high temperature of 900 °C, and then naturally cooled to room temperature. Uniaxial tensile tests were carried out on the notched round bar specimens. By combining test results with ABAQUS FEA and scanning electron microscope tests, the void growth model (VGM) parameters of Q460D high-strength steel and ER55-G welds after high temperature of 900 °C and natural cooling were calibrated. Finally, the calibrated VGM model was adopted as a fracture criterion in the ABAQUS subroutine VUMAT to simulate the uniaxial tensile tests on notched round bar specimens of Q460D steel base metal and ER55-G welds after high temperature of 900 °C and natural cooling. It is indicated that in most cases, the post-fracture path and the whole force-deformation curve can be simulated through the FEA with subroutine VUMAT with reasonable accuracy.

Constitutive model of duplex stainless steel S22053 and its weld under cyclic large plastic strain loading

This study investigated the mechanical properties and constitutive models of duplex stainless steel S22053 under cyclic large plastic strain loading. To this end, smooth round bar specimens of the base metal, deposited metal, and heat-affected mixing zone material were tested under monotonic tensile loading and cyclic loading. Furthermore, the failure modes and stress–strain curves of the specimens were analysed. Based on the test results of the smooth round bar specimens under cyclic symmetric constant amplitude and monotonic tensile loading, the N3L1 and S10 parameters of the Chaboche model for duplex stainless steel S22053 were calibrated, respectively. Then, the accuracy of the calibrated Chaboche model was verified against the test results of the smooth round bar and circular notched bar specimens. The results showed that the base metal and heat-affected mixing zone material have higher ductility, and the strength of the deposited metal is the highest. Furthermore, the N3L1 parameters can accurately simulate the behavior of smooth round bar specimens under cyclic medium plastic strain loading, while the S10 parameters are more suitable for simulating the mechanical response of circular notched bar specimens under cyclic large plastic strain loading. The accuracy of the Chaboche model depends on the matching of the loading strain range of the test data for calibrating the model with that of the simulation cases. The results of this study provide fundamental data for analysing the seismic and ductile fracture performance of duplex stainless steel and its welded structures.

Development of mechanochemical degradation induced slow crack growth model for high density polyethylene in cracked round bar specimen

Development of mechanochemical degradation induced slow crack growth model for high density polyethylene in cracked round bar specimen

Atomistic investigation on adhesive strength of coupling agents to aluminum

For the adhesion of aluminum, which is of great importance in multimaterialisation, this paper focused on silane coupling agents as one of dominant factors. Amino, mercapto and EP silanes were employed as typical examples, then the atomistic adhesion mechanism was clarified through first-principles calculations and verified in continuum mechanics by tensile tests and finite element analysis. First-principles calculations revealed that the interfacial adhesive strength is governed by stiffness and occurrence of necking, even in atomistic stress-strain relationships. It was also found that stiffness correlates with the deformation mode of the molecular structure, and that necking occurs between the weakest bonds with small bond population and are determined by the interatomic local strain. Interfacial debonding occurred in both first-principles calculations and tensile tests, and the first-principles calculated values of interfacial adhesive strength were in qualitative agreement with the interfacial maximum stress values obtained by finite element analysis based on the breaking load during tensile tests of butt-bonded round bar specimens. Based on these results, this paper summarized effectiveness, applicable conditions and challenges of upscaling method from quantum mechanics to continuum mechanics.

Corrosion fatigue damage mechanism and life evaluation of Inconel Alloy 690TT steam generator tube in high temperature pressurized water

Corrosion fatigue behavior of the actual Alloy 690TT steam generator (SG) tubes in high temperature water were systematically studied by using the designed boat-shaped specimen. Results demonstrated that the environmental fatigue correction coefficient of the SG tube in high temperature water was lower than that of round bar specimen in ANL model, which may be related to the internal surface oxide film rupture-persistent slip bands (PSBs) deformation zone preferential dissolution caused by the shape effect of SG tube. Moreover, the corrosion fatigue life of the actual SG tube was evaluated by the method of model correction.