Fracture Of Structural Steels Research Articles

Cyclic adaptive cohesive zone model to simulate ductile crack propagation in steel structures due to ultra‐low cycle fatigue

AbstractMicromechanics‐based continuum damage criteria have previously been developed to simulate the initiation of ductile fracture in structural steels under conditions with large‐scale plasticity where conventional fracture mechanics indices are invalid. Such models have been combined with methods to simulate ductile crack growth for monotonic loading. In this study, a micromechanics‐based adaptive cohesive zone model for simulating ductile crack propagation under monotonic loading is extended to handle cyclic loading. The proposed model adaptively modifies the cohesive traction–separation relationship for crack opening and closure, as the loading reverses between tension and compression. The approach is implemented into the finite element analysis platform WARP3D, and results of simulations that use the model are compared with data from coupon‐scale tests. The results demonstrate that the proposed model can accurately simulate the effect of crack propagation on specimen response, as well as other key aspects of observed behavior, including crack face closure and crack tunneling.

Analysis and simulation on fracture of structural steel at elevated temperatures based on extended finite element method

This paper presents numerical simulations of fracture behavior of ASTM A992 steels at elevated temperatures using the extended finite element method (XFEM). The XFEM approach has advantages of avoidance of calibrating many material parameters and re-meshing of elements. Two fracture criteria based on maximum principal true strain and stress were calibrated and validated against experimental results, respectively. The calibration and validation results show that the XFEM can predict the fracture mode and engineering stress-strain curves of structural steels at elevated temperatures with reasonable accuracy. The improvement in fracture simulation using XFEM is further verified by a comparison of predicted fracture behavior of steels between the conventional FEM and XFEM. Moreover, it is found from a sensitivity study that the engineering strain and stress of steels at fracture for a given temperature are not sensitive to the stress and strain criteria. Finally, generalized fracture criteria of ASTM A992 steels at elevated temperatures are proposed for numerically simulating steel fracture in ABAQUS by using the average values of calibrated principal true strain and stress at fracture based on test results.

The effect of triaxiality on finite element deletion strategies for simulating collapse of full-scale steel structures

Collapse prediction of steel structures should incorporate a finite element model that accounts for ductile fracture through material separation in critical structural members. Finite element deletion approaches have been used successfully in the past to account for fracture in steel members. However, the current common approach in collapse modeling of steel structures, a constant critical strain strategy (CS), typically requires recalibration when used with different structural configurations due to the fact that it does not account for triaxiality, which is a primary parameter in ductile fracture. To better predict structural response of steel structures undergoing collapse, it is important to study the effect of triaxiality on fracture in steel structures. A new finite element deletion approach that accounts for triaxiality was previously proposed, calibrated, and validated in small steel specimens for use in predicting collapse of steel structures. In this approach, fracture initiation is modeled using Void Growth Model (VGM) and the subsequent softening of the material to element deletion is modeled by a Hillerborg model. This paper describes the change of triaxiality, equivalent plastic strain, and other parameters during the duration of the loading, influencing the strategies implemented and provides explanation for the performance shown. In addition, the paper examines the effect of triaxiality on accurately predicting fracture in steel structures through comparison of VGM to CS strategy with validation in simulations of full-scale structural steel connection and frame tests without recalibration. The VGM strategy provided an accurate prediction based on calibration to test results that are most widely available for different types of structural steels, while CS strategy frequently provided less accurate results. The VGM strategy thus allows for an accurate collapse modeling of steel structures for use by researchers, code developers, and practitioners who address collapse of steel structures.

A modified micromechanics framework to predict shear involved ductile fracture in structural steels at intermediate and low-stress triaxialities

This paper employs a micromechanics framework to investigate the mechanisms and to predict the ductile fracture of structural steels at intermediate and low-stress triaxialities and under shear involved stress states. Unit cell-based micromechanical analyses are carried out to provide insights into the combined effects of triaxiality, Lode parameter, and shear stress component on the micro-mechanisms of ductile fracture. Based on the micromechanical analyses, an existing fracture model is modified to incorporate the influence of the shear stress component. Experimental investigations are carried out on three axisymmetric tension specimens, and a series of shear specimens made of Chinese Q460 steels to achieve uniaxial tension and shear dominated loading conditions, respectively. Finite element analyses are performed to evaluate the stress and strain fields in the tension and shear specimens. The modified model is calibrated by combining the experimental results with micromechanical analyses. Validation studies show that the predicted fracture initiation by the model agrees well with experimental results, implying a good performance of the proposed micromechanics framework for the prediction of the shear-dominated ductile fracture at intermediate and low-stress triaxialities.

Weibull stress analysis in local approach to fracture

As a measure of the probability of cleavage fracture, the Weibull stress within the framework of local approach has the potential capability to predict constraint effects on fracture of structural steels. This paper mainly analyzes Weibull stress considering constraint effect using constraint parameters T-stress and Q. Weibull stress is solved with constraint effects characterized by T-stress for elastic material and Q for elastic plastic material. These solutions are verified with existing solutions for T = 0 and finite element solutions including modified boundary layer models, contact tension models and single-edge bend models. Good agreement has been obtained in all cases. The Weibull stress solutions can be further adopted to predict scale fracture toughness.

A Simplified Analytical Solution for the Necking Semi-empirical Stresses based on Aramis System

To make comparison between classic necking stress solution Bridgman formula and ChenChi formula, the difference of necking surface profile assumptions in these two formulas were discussed theoretically. Then, the uniaxial tension experiment on 16 groups of carbon structural steels Q235 and Q345 were carried out at room temperature. Associate with the necking displacement measured by Aramis optical dynamic 3D strain measurement system, the parameter equation of necking surface profile was determined. The comparison results indicate that ChenChi formula is more accurate than Bridgman formula in calculating the necking stress distribution. Furthermore, to overcome the difficulty of measuring the radius curvature R in these formulas, an empirical formula of the geometric dimensions in the necking section was applied and the parameter was corrected based on the necking strain measured by Aramis system. As a result, the obtained equation in this study can provide an improved calculation method for the analysis of three-dimensional stress in necking and a reference for future research on the necking and ductile fracture of structural steels.

Theoretical Study of Ductile Fracture in Steel Structures in the Presence of Spatial Variability in Toughness

AbstractMicromechanical or local models are increasingly used for predicting microvoid-growth–induced ductile fracture in structural steel components. Methods to calibrate and apply these models pr...

A unified damage factor model for ductile fracture of steels with different void growth and shrinkage rates

AbstractMicromechanical fracture modelling is an effective method to predict ductile fracture in steel structures. This paper aims to establish a simplified and general fracture model for various loading conditions, which is convenient to calculate the instantaneous damage index. With the concept of the ductile damage factor, a unified ductile damage factor model considering the difference of rates between void growth and shrinkage has been proposed. Based on experiment results and finite element analysis, the model parameters for Q235B Chinese structural steel were calibrated. The ductile damage factor and equivalent plastic strain at fracture initiation were investigated. Comparison among the proposed model, experimental results, and the cyclic void growth model demonstrated the effectiveness and accuracy of the proposed model. A parametric study was conducted to investigate the influence of cyclic constitutive parameters on the accuracy of fracture prediction. The predicted results are acceptable while reducing those calibrated cyclic constitutive parameters by 20%.

A framework for statistical modelling of plastic yielding initiated cleavage fracture of structural steels

The well established consensus that cleavage fracture is preceded by plastic deformation in structural steels implies that plastic yielding is the threshold stress state for a volume element to incur cleavage fracture. An accurate compliance with this consensus underlies the normalisation of cumulative cleavage fracture probability and the justification of constraint effect on cleavage fracture. These understandings lead to the proposal of a framework for statistical modelling of cleavage fracture in structural steels. The framework takes the spatial microcrack distribution into account to formulate the cumulative failure probability model that allows for a pertinent physical interpretation of Weibull statistics, and derives the fracture probability of an elemental volume in conformity with the yielding condition from a set of commonly adopted microcrack size or strength distributions. Alternative approaches to calibrating model parameters are suggested based on frequency analysis of brittle particles as cleavage initiators and on statistical analysis of cleavage fracture stress. The strict adherence to plastic yielding as a prerequisite to cleavage fracture also reveals the probabilistic nature of notch brittleness and ductile-to-brittle transition behaviour.

A statistical model of cleavage fracture in structural steels with power-law distribution of microcrack size

Based on the well-accepted physical understanding that cleavage fracture of structural steels is preceded by plastic deformation, a critical flaw in the Beremin model and its modifications is proposed, and a new statistical model with the power-law distribution of microcracks is obtained to describe the cumulative probability of cleavage fracture. A set of cleavage fracture toughness data of a nuclear pressure vessel steel is used to highlight the difference between the new model and the Beremin model.

Measurement of Nonuniformity of Fracture in Structural Steels with Heterogeneous Structure

Different methods for measurement of nonuniformity of fracture in structural steels with heterogeneous structure are studied including the partition of space into Voronoy polyhedrons for measuring the differences in random sets of points in a plane, synthesis of a 3D-picture from several 2D ones (stereophotogrammetry) and indirect analysis of 3D-peculiarities from a 2D-shot, i.e., a “plane” image, for reconstructing scenarios of crack propagation, and solution of problems of linear regression with the use of the principle of maximum likelihood.

Brittle fracture in structural steels: perspectives at different size-scales.

This paper describes characteristics of transgranular cleavage fracture in structural steel, viewed at different size-scales. Initially, consideration is given to structures and the service duty to which they are exposed at the macroscale, highlighting failure by plastic collapse and failure by brittle fracture. This is followed by sections describing the use of fracture mechanics and materials testing in carrying-out assessments of structural integrity. Attention then focuses on the microscale, explaining how values of the local fracture stress in notched bars or of fracture toughness in pre-cracked test-pieces are related to features of the microstructure: carbide thicknesses in wrought material; the sizes of oxide/silicate inclusions in weld metals. Effects of a microstructure that is 'heterogeneous' at the mesoscale are treated briefly, with respect to the extraction of test-pieces from thick sections and to extrapolations of data to low failure probabilities. The values of local fracture stress may be used to infer a local 'work-of-fracture' that is found experimentally to be a few times greater than that of two free surfaces. Reasons for this are discussed in the conclusion section on nano-scale events. It is suggested that, ahead of a sharp crack, it is necessary to increase the compliance by a cooperative movement of atoms (involving extra work) to allow the crack-tip bond to displace sufficiently for the energy of attraction between the atoms to reduce to zero.

Mathematical Simulation of Cyclic Thermoviscoplastic Deformation and Fracture of Materials

Fundamental concepts and equations of the thermoviscoplasticity (inelasticity) theory are examined. Material functions are chosen, the basic experiment and identification method for the material functions, closing the thermoviscoplasticity theory, are formulated. Thermoviscoplasticity theory potentials for describing deformation and fracture of structural steels and alloys at different cyclic thermomechanical loads are demonstrated.

Thermoviscoplastic cyclic deformation and fracture of materials

The paper considers the main theses and equations of the modern theory of thermoviscoplasticity (inelasticity). The authors highlight material functions, formulate the basic experiment and method for identifying the material functions which enclose the theory of thermoviscoplasticity. The possibility of the theory are illustrated to adequately describe the processes of deformation and fracture of structural steels and alloys in a variety of modes of thermal power cyclic loading.

Effect of Strain-gradient Plasticity in Engineering Fracture Assessments

This study implements the conventional mechanism-based strain gradient plasticity (CMSG) in the engineering fracture assessment of structural steels, to estimate both the near-tip opening displacements and the probability of brittle fracture. The CMSG theory recognizes the dependence of the material hardening on both the strain and its gradient, for plastic deformations occurring at micron or sub-micron levels, through a material length scale. The CMSG presents a more realistic description of the stress, strain and displacement field in the immediate vicinity of the crack tip, than does the classical plasticity. This study therefore examines the near-tip opening displacement, commonly used in the assessment for ductile fracture in structural steels. This study also integrates the CMSG theory in calculating the microscopic crack driving force in a cleavage fracture assessment framework, namely the Weibull stress approach. The accuracy of the scalar Weibull stress relies significantly on the gradient- dependent, near-tip stress field, which subsequently impinges on the failure probability estimated using the Weibull stresses.

Finite element simulation of ultra low cycle fatigue cracking in steel structures

This paper proposes a new method of simulating ductile fracture in steel structures under large amplitude cyclic straining experienced in earthquakes. The method is developed based on an existing micromechanical model originally proposed for predicting crack initiation in ultra-low cycle fatigue, ULCF. It involves a step-by-step simulation of material degradation within the framework of conventional nonlinear FEM. The method is validated through simulating fracture in a structural detail (column-to-base plate connection) for which several cyclic tests has been previously conducted. It is found that the method can successfully predict the cracking site, its propagation path, the number of cycles corresponding to crack initiation, and also final fracture.

Parameter calibrations and application of micromechanical fracture models of structural steels

Micromechanical facture models can be used to predict ductile fracture in steel structures. In order to calibrate the parameters in the micromechanical models for the largely used Q345 steel in China, uniaxial tensile tests, smooth notched tensile tests, cyclic notched bar tests, scanning electron microscope tests and finite element analyses were conducted in this paper. The test specimens were made from base metal, deposit metal and heat affected zone of Q345 steel to investigate crack initiation in welded steel connections. The calibrated parameters for the three different locations of Q345 steel were compared with that of the other seven varieties of structural steels. It indicates that the toughness index parameters in the stress modified critical strain (SMCS) model and the void growth model (VGM) are connected with ductility of the material but have no correlation with the yield strength, ultimate strength or the ratio of ultimate strength to yield strength. While the damage degraded parameters in the degraded significant plastic strain (DSPS) model and the cyclic void growth model (CVGM) and the characteristic length parameter are irrelevant with any properties of the material. The results of this paper can be applied to predict ductile fracture in welded steel connections.

Experimental Study to Calibrate Monotonic Micromechanics-Based Fracture Models of Q345 Steel

Micromechanics-based facture models have been proved to predict ductile fracture in steel structures with good accuracy. The stress modified critical strain (SMCS) model and the void growth model (VGM) are suitable to predict fracture initiation under monotonic loading. In order to calibrate the parameters in these models for the largely used Q345 steel in China, material tests, scanning electron microscope tests and finite element analyses were conducted. The test specimens were made from base metal, deposit metal and heat affected zone of Q345 steel to investigate crack initiation in welded connections. The results of this paper can be applied to predict ductile fracture in welded steel connections under monotonic loading.

Versions of the theory of inelasticity

Several versions of the theory of inelasticity (thermoviscoplasticity), which is a generalization and development of the ideas contained in various versions of the theory of plasticity, creep, inelasticity, and damage accumulation, are considered. The ability of various versions of the theory of inelasticity to provide an adequate description of processes of inelastic strain and fracture of structural steels and alloys under various loading conditions are illustrated.

Thermal Elastic-Plastic Analysis Considering Temperature Rise by Rapid Plastic Deformation in Undermatched Joints

The characteristics of dynamic strength and fracture in structural steels and their welded joints particularly for pipelines should be evaluated based on the effects of the strain rate and service temperature. The temperature, however, rises so rapidly in structures due to the plastic work under the high strain rate such as ground sliding by earthquake when the effect of the temperature cannot be negligible for the dynamic fracture. It is difficult to predict or measure the temperature rise history with the corresponding stress-strain behavior, including the region beyond the uniform elongation, though the behavior at the large strain region after the maximum loading point is very important for the evaluation of fracture. In this paper, the coupling phenomena of the temperature and stress-strain fields under dynamic loading were simulated by using the finite element method. A modified rate-temperature parameter was defined by accounting for the effect of the temperature rise under rapid plastic deformation, and it was applied to the fully coupled analysis between the heat conduction and thermal elastic-plastic behavior. The temperature rise and stress-strain behavior, including the coupling phenomena, were studied including the region beyond the maximum loading point in structural steels and their undermatched joints, and then compared with the measured values.