Fracture Of Steel Research Articles

Mechanical Properties and Fracture of Austenitic Steel Fabricated by Selective Laser Melting

Mechanical Properties and Fracture of Austenitic Steel Fabricated by Selective Laser Melting

Effect of Mo and Cr on S-Induced Intergranular Fracture in γ-Fe

S is a common corrosion medium for austenitic stainless steels. The severe intergranular fracture of austenitic stainless steels occurs in sulfur environments. In this paper, the permeation of S at different atomic positions for three symmetric tilt grain boundary types, i.e., Σ5(210), Σ5(310), and Σ9(114) have been computed using first-principles calculations. S has the strongest segregation tendency in the Σ5(210) grain boundary. A high content of S at the grain boundary indicates harm to the grain boundary. Sulfur segregation in the grain boundaries can weaken the strength of the metallic bond. When Mo and Cr are present at the Σ5(210) grain boundary, the sulfur-induced embrittlement is inhibited. With increased S concentration at the grain boundary, the coexistence of Mo and Cr can suppress the intergranular fracture of S on the grain boundary. The reason why high-Mo stainless steel has excellent sulfur-induced intergranular corrosion resistance is explained at the atomic level.

Mixed type brittle fracture in 1.5 GPa dual-phase steel via {100} ferrite cleavage cracking

In the present work, we clarify the fracture mechanisms in a dual-phase (DP) steel consisting of ∼75% martensite and exhibiting tensile strength of 1.5 GPa. Generally, the DP steels are considered resistant to brittle fracture due to the crack-arresting behavior of the ferrite. However, contrary to conventional wisdom, we report a predominant brittle fracture in the DP steel and clarify the associated damage mechanisms. Crack initiation occurs via martensite cracking along the prior austenite grain boundaries. The crack propagation in ferrite primarily occurs via {100} brittle cleavage cracking. Occasional void coalescence is also observed in ferrite. The brittle fracture observed in martensite is a mixed mode fracture consisting of both intergranular fracture and transgranular cleavage cracking along {110} planes. Furthermore, it is observed that the morphology of the martensite crack (sharp and blunt crack) influences ferrite cleavage cracking. The ferrite damage mechanisms associated with the sharp and blunt crack have been elucidated. Finally, the micro mechanism for the occasional ductile ferrite fracture has been explained.

On the influence of specimen size and geometry on the fracture toughness of tungsten heavy metal alloys

We previously showed that commercial tungsten (W)-based heavy metal alloy (WHA) composites containing 90 to 97 wt% W, reinforced with 3 to 10 wt% of a NiFe ductile phase (DP), have room temperature (RT) maximum load elastic-plastic fracture toughness (KJm) values from 69 to 110 MPa√m. This is far higher than for monolithic RT W toughness (≈ 8 ± 4 MPa√m). In all cases, fracture took place by stable crack growth. However, these results were based on very small (16/3.3/1.65 mm) pre-cracked bend bar tests, which naturally raises the question of size effects on the measured KJm. While they are best thought of as being composites, here we experimentally assess size and geometry effects on the KJm of WHA, in the broad context of validity criteria that have been developed for both cleavage and ductile tearing, R-curve fracture in steels. Specifically, we examine the relation between KJm and a dimensionless elastic-plastic zone small scale yielding deformation limit criterion, M ≥ σobo/Jm. Here σo is the flow stress, Jm is the measured J at maximum load, and bo is the initial unbroken ligament dimension. Tests on specimens from 3x to 8x larger than the original small (1x) bend bars show only weak or no size effect up to 95W with KJm ≈ 82 ± 5 MPa√m. However, 3x 97W alloy fractured elastically at KIm ≈ 38 ± 4 MPa√m. The corresponding average for the 1x specimens was KJm 100 ± 15 MPa√m for the 90–95W and 69 ± 12 MPa√m for the 97W alloys. The approximately constant KJm for 3–8x specimen sizes suggests that the M required for constraint-related size independence is ≈ 200. However, DP fraction also plays a role, and there are additional effects of compact tension versus bend bar specimen geometries.

An investigation on the plane-strain fracture toughness of a water atomized 4130 low-alloy steel processed by laser powder bed fusion

A water atomized 4130 steel powder was processed by Laser powder bed fusion and investigated both in as-built condition and after quench and tempering thermal treatment. Analyses were focused on the different microstructures developed and on steel fracture behavior in terms of tensile fracture elongation, Charpy impact properties, and linear elastic fracture toughness. Comparisons were also drawn by testing a reference 4130 steel fabricated from a gas atomized powder. The slightly higher oxygen content found in the water atomized powder led to the formation of finely dispersed nano-size oxide particles in the steel matrix. It was found that these inclusions have a minor effect on the tensile properties, but a significant influence on the impact toughness response. The fracture toughness tests showed that the orientation leading to propagation of cracks along the inter-layer planes represented the most critical situation, and the steel toughness could be significantly improved after the quench and tempering treatment owing to the achievement of a more homogeneous microstructure. The results suggest that the investigated water atomized low-alloy steel powder feedstock can be considered as a suitable and cheaper alternative for structural parts produced by additive manufacturing, which could replace the more popular gas atomized steel grades.

Dynamic Hardening Behavior and Ductile Fracture of High-Strength Steel at Intermediate Strain Rates

The mechanical properties of constructional steels at different strain rates were the basis for the dynamic analysis of the progressive collapse of steel structures. The effect of strain rate on the hardening behavior and ductile fracture for the high-strength steel Q690 is investigated in this paper. Quasistatic and intermediate strain-rate experiments were performed. A smooth sheet and a V-shaped notched specimen were carefully designed to cover intermediate and high-stress triaxialities. The experimental results revealed that the effect of strain rate on the hardening behavior is sensitive to stress state. A novel dynamic isotropic hardening law was developed, taking advantage of the Cowper–Symonds and Johnson–Cook hardening models. Also, negative and positive strain-rate effects on the ductility of Q690 were observed in the smooth and notched specimens, respectively. An attempt was made to introduce a new strain-rate term into an uncoupled ductile fracture model to describe the differences of the strain-rate effects at various stress states. The new hardening law and the strain-rate-dependent ductile fracture model were validated with satisfactory accuracy.

A Simple Calibrated Ductile Fracture Model and Its Application in Failure Analysis of Steel Connections

A new fracture model is developed to predict the ductile fracture of structural steel under multiaxial stress states. First, the Lee–Mear void growth theory is used to establish the quantitative relationship between the stress triaxiality and material’s ductility. A stress triaxiality dependence function, which accounts for the material’s strain hardening, is derived from modifying the dilatation rate of a spherical void in a typical unit cell. Subsequently, the Tresca failure model is used in conjunction with the Swift hardening law to establish a Lode dependence of fracture strain. Then, the theoretical formula of the new fracture model is obtained by combining both stress triaxiality and Lode angle dependence functions. The proposed fracture model has a unique advantage: i.e., this model has only two material parameters. These two parameters can be easily calibrated through a simple standard coupon test, which significantly reduces the difficulty of model calibration work and facilitates its application in practical engineering. In order to verify the new fracture model, the test results of five types of Q460 steel specimens were used to calibrate the model parameters. The prediction accuracy of the new model is then checked by calculating the average error between the test results and the predicted fracture strain envelope. Finally, the new fracture model was applied in the numerical analysis of two types of steel connections. The validation of the proposed fracture model is verified by comparing the load–displacement curve and failure modes of the steel connections obtained from both test and numerical analysis.

Analysis of the Condition of Tendons in Prestressed Concrete Structures

Prestressing Steel Fracture Detection Bernd Hillemeier, Simon Knapp, Thomas Luther Abstract The earliest possible non-destructive location of a prestressing steel fracture is a challenge of our time. The endangerment of prestressing steels through stress cor-rosion cracking and hydrogen embrittlement is not visible from the outside. The estimated service life of existing prestressed concrete structures must take into account the construction plans as well as the construction year. The design criterion "crack before break" did not always have priority. Magnetic fields open up a view of the interior of the supporting structure. They show the condition of the prestressed reinforcement in a picture document. The method is practicable and has already developed so far that the DGZfP published the position paper „Magnetische Verfahren zur Spannstahlbruchortung" ("Magnetic methods for prestressing steel fracture location") in 2017. The know-how of prestressing steel fracture detection is to imprint magnetic fields into the prestressed reinforcement. This happens with electromagnets or permanent magnets. The smaller and more manageable the devices, the less their depth effect. The specific advantage of those devices lies in measuring prestressed concrete gird-ers with direct bond and low concrete cover. How do you guide the magnets, how do you interpret the data? Transverse tendons in carriageway slabs are tested at pe-destrian speed to a width of 3.5 m with a 3 t heavy self-propelled electromagnet, preferably at night due to traffic conditions. Despite the fact that our many years of practical experience from Germany, Switzerland, GreatBritain and the USA lead to the constant further development of the process steps, more and more specific measuring tasks are carried out more practically and economically. shows that the probability of failure without early announcement is low. The answer to the question of whether there are cracks in tendons and, if so, in what number and how they are spatially distributed, significantly increases the reliability of the in-formation concerning safety. The non-destructive testing method based on rema-nence magnetism localizes fractures in prestressing steels in a direct bond or in ducts. The presentation will show examples from the areas of prestressed concrete girders of bridge structures, carriageway slabs of road and motorway bridges with a tested area of around 4000 m² per day, floor slabs of parking garages, prestressed con-crete girders in swimming, sports and industrial halls, VT folds of hall structures and Tank container in prestressed concrete construction. Nevertheless, if the complexity of a structure with regard to the arrangement and concentration of the mild and prestressed concrete reinforcement is great, the labor-atory test with the simulated reinforcement situation is recommended. In the case of the shell roof of the town hall in Biel (Switzerland), in addition to the laboratory repro-duced structure in Berlin, the client manufactured another test element for experi-mental verification. The presentation also reports on this.

マルテンサイト鋼の水素ぜい性破壊

マルテンサイト鋼の水素ぜい性破壊

Experimental investigation on the low temperature fracture performance of Q690 steel for application to long-span high-speed railway bridges in Tibet harsh environment

Charpy impact test and three-point bending test on 32 mm and 50 mm thick Q690 high-strength steel were carried out according to the environmental features of plateau low temperature to assess the fracture toughness of long-span high-speed railway bridge steel in Tibet difficult environment. The results show that that the effect of temperature (T) on Charpy impact energy (AKV) and CTOD (δ) follows the Boltzmann function law. The ductile-brittle transition temperature of Q690 high-strength steel is determined using the Boltzmann function, and the mechanism of brittle fracture of high-strength steel caused by low temperature is revealed using a metallographic phase and microstructure of the target material. The nonlinear relationship between Charpy impact energy and CTOD is established based on the Roos-Kussmaul nonlinear empirical equation with the same thickness, which is basically consistent with the test data. As a result, the complex CTOD test, which is costly, can be determined by a simple low-cost Charpy impact test, providing guidance for the widespread application of Q690 high-strength steel in low temperature environments at high altitude or high latitude.

Hydrogen-induced delayed fracture of Cu-containing high-strength bolt steel

Hydrogen-induced delayed fracture of Cu-containing high-strength bolt steel

Phase-Field Description of Ductile Fracture in Structural Steel under Cyclic Loading

Phase-Field Description of Ductile Fracture in Structural Steel under Cyclic Loading

Modelling of the intergranular fracture of TWIP steels working at high temperature by using CZM–CPFE method

Grain boundaries (GBs) of metallic materials will become the weakest and most vulnerable place when the materials deform at high temperature. This could directly affect deformation flow, plastic strengthening, the initiation and propagation of microcracks, and finally the fracture failure of materials. In this research, to reveal the high temperature deformation mechanisms of twinning–induced plasticity (TWIP) steels, the crystal plasticity finite element method (CPFEM) was employed to model the mechanical response of grain interiors. The cohesive zone model (CZM) of the bi–linear traction separation law (TSL) was developed to describe the damage and failure of GBs. By combining CZM and CPFEM, an analysis method was proposed to investigate the effect of the microstructure morphology, orientation and size of grains, and temperature. Furthermore, the damage initiation, accumulation and fracture at GBs were represented by the representative volume element model based on the CZM–CPFE method to explore the influences of GB angle, grain size and local microvoids (including the location and size) on the GB cracking. The stress damage cloud maps, the distribution of quadratic stress damage initiation criterion (QUADSCRT) and the scalar stiffness degradation (SDEG) describing the damage and fracture along the GBs were used to articulate the relationships of GB angle, size effect, local microvoids and the expected failure strain. The results show that low angle grain boundaries (LAGBs), the small average grain size (<21.2 μm), microvoids at the horizontal GBs and microvoids with a diameter below 1.5 μm can delay the initiation of microcracks and fracture failure along the GBs. The predicted damage and failure of GBs and the intergranular cracking characteristics in plastic deformation of TWIP steels at high temperature were corroborated by microscopic in–situ SEM experiments, in which the deformation characteristics inside the grains, and microcrack initiation and propagation near GBs were observed at 500 and 750°C. This research enhances the understanding of intergranular fracture, mechanical response and microstructure evolution of TWIP steels worked at different temperatures, and also provides a new way and strategy for studying the deformation behaviours of TWIP steels considering GB properties.

Effects of Non-Metallic Inclusions and Mean Stress on Axial and Torsion Very High Cycle Fatigue of SWOSC-V Spring Steel

Inclusion-initiated fracture in high-strength spring steel is studied for axial and torsion very high cycle fatigue (VHCF) loading at load ratios of R = −1, 0.1 and 0.35. Ultrasonic S-N tests are performed with SWOSC-V steel featuring intentionally increased numbers and sizes of non-metallic inclusions. The fatigue limit for axial and torsion loading is considered the threshold for mode I cracks starting at internal inclusions. The influence of inclusion size and Vickers hardness on cyclic strength is well predicted with Murakami and Endo’s area parameter model. In the presence of similarly sized inclusions, stress biaxiality is considered by a ratio of torsion to axial fatigue strength of 0.86. Load ratio sensitivity is accounted for by the factor ((1 − R)/2)α, with α being 0.41 for axial and 0.55 for torsion loading. VHCF properties under torsion loading cannot appropriately be deduced from axial data. In contrast to axial loading, the defect sensitivity for torsion loading increases significantly with superimposed static mean load, and no inclusion-initiated fracture is found at R = −1. Size effects and the stress gradient effective under torsion loading are considered to explain smaller crack initiating inclusions found in torsion ultrasonic fatigue tests.

Effect of Oxide Metallurgy on Inclusions in 125 ksi Grade OCTG Steel with Sulfide Stress Corrosion Resistance.

The effects of Al deoxidation and Zr deoxidation on the microstructure and properties of sulfide stress corrosion resistant high-strength steel have been investigated. The feasibility of the Zr deoxidation instead of Al deoxidation was confirmed by the thermodynamic analysis of the deoxidation of various elements. The experimental results indicate that the average diameters of the inclusions in Al-Steel and Zr-Steel were 2.45 μm and 1.65 μm, respectively. The Al-Steel and Zr-Steel contained 22.38% and 68.77% inclusions per unit area, respectively, and the fraction of inclusions in the Al-Steel and Zr-Steel with diameters less than 2 μm was about 73.46% and 89.63%, respectively, indicating that the Zr deoxidation process could effectively refine inclusions and promote dispersion. The average diameters of austenite grain for the Al-Steel and Zr-Steel were about 9.1 μm and 8 μm, respectively. The fine particles in Zr-Steel could pin the austenite grain boundaries and clearly refine the grains. The average grain size of tempered martensite was 8.2 μm and 3.8 μm, respectively. The yield strength of the Al-Steel and Zr-Steel was 922 MPa and 939 MPa, respectively; the impact energy was 60 ± 6 J and 132 ± 6 J, respectively. Moreover, the fracture time of the NACE-A was from 28 h (Al-Steel) to 720 h (Zr-Steel) without fracture. The experimental steel deoxidized by Zr achieved a simultaneous improvement in strength, toughness and sulfide stress corrosion resistance, and the effect of inclusions on the fracture of the sulfide stress corrosion resistant high-strength steel can be explained by the Griffith theory.

The Role of Splitting Phenomenon under Fracture of Low-Carbon Microalloyed X80 Pipeline Steels during Multiple Charpy Impact Tests

The ambiguity of the splitting effect on X80 low-carbon microalloyed pipeline steels’ tendency towards brittle fracture prompted an experimental study of impact toughness scattering based on multiple Charpy impact tests in a temperature range from 20 °C to −100 °C. A fractographic analysis of a large number of fractured samples was carried out. The relationships between impact toughness, deformability and splitting characteristics were studied. A number of common features of three X80 low-carbon microalloyed pipeline steel fractures were revealed. It was experimentally established that the reason for the scattering of the impact toughness values during completely ductile fracture of specimens, as well as during fracture accompanied by the splitting formation, is the local inhomogeneity of plastic properties. The higher the susceptibility to the formation of splits for a particular steel, the lower the impact toughness. Using the electron backscatter diffraction (EBSD) technique, an uneven distribution of local plasticity in the plastic zone of impact-fractured specimens was established. A comparative analysis of specimens with equal impact toughness values at different test temperatures makes it possible to identify the mechanism of negative splitting influence compensation by the increased plasticity of certain specimen.

Influence of stress states on cleavage fracture in X70 pipeline steels

Stress state is a primary factor controlling the ductile fracture behavior of steels, which is typically represented as the combination of the stress triaxiality and Lode angle parameter. The cleavage fracture properties of pipeline steels at low temperatures are usually assessed under plane strain conditions, such as using the fracture mechanics experiments. In this study, the cleavage fracture properties of a X70 steel at liquid nitrogen temperature (−196 °C) are characterized over a broad range of stress states. A comprehensive experimental program is carried out by performing tensile tests using various flat specimens of different geometries immersed in liquid nitrogen, including shear, central hole, notched dog bone, and side grooved plane strain. Pronounced plasticity occurs prior to the final fracture within the tested range of stress states at the very low temperature. Anisotropy effects are considered by conducting tensile tests of fracture specimens along the rolling, diagonal and transverse directions. Finite element simulations of corresponding experiments are performed using an evolving quadratic plasticity model to extract the local stress state variables to establish the fracture criteria, which are formulated based on the critical values of plastic strain and maximum principal stress. The fracture strain of the investigated material at liquid nitrogen temperature is affected by the stress triaxiality, Lode angle parameter and loading direction.

Ductile Fracture Prediction of X80 Pipeline Steel Using Void Growth Model

In this study, the Void Growth Model (VGM) is employed to predict the ductile fracture of X80 pipeline steel. The X80 pipeline tends to be applied in challenging scenarios, such as extremely deep water and long-distance pipelines, which might cause a ductile fracture; however, the study of ductile fractures for pipeline steel is rare, especially for X80 pipeline steel. To understand ductile fractures of X80 pipeline steel, a hybrid numerical–experimental calibration method is used to determine the fracture parameter for the VGM model. The toughness capacity defined by the critical void growth index (VGI) in this study is determined to be 4.304. A shear-tension specimen is applied to verify the calibrated VGM. The results show that the calibrated VGM can predict the fracture initiation of the shear-tension specimen. In addition, the fracture of the shear-tension specimen initiates at the center of the section and propagates to the edge of the groove of the specimen. The initiation of fracture is identical to the testing observation.

Experimental and Analytical Research on Flexural Behavior of Concrete-Filled High-Strength Steel Tubular Members.

Using high-strength steel (yield strength fy ≥ 460 MPa) in concrete-filled steel tubes is expected to provide a superior bearing capacity by achieving light weight and efficient construction, but the existing design limitation on diameter-to-thickness (D/t) ratios for concrete-filled high-strength steel tubular (CFHST) members inevitably obstructs its wide application. In this study, aiming at the application of circular CFHST members using Q690 steel (fy ≥ 690 MPa), a total of 15 CFHST beams were examined using a three-point loading test to investigate the failure mode, bearing capacity and plasticity evolution. Subsequently, finite element models (FEMs) were established to analyze the full-range curves, composite effect, failure mechanism and influences of key parameters including material strengths, D/t ratios, and shear-span ratios. A simplified calculation method for bearing capacity was finally proposed and verified. The results indicate that the full-range performance of tested CFHST members with out-of-code D/t ratios have ductile behavior, though they fail through the mode of steel fracture and concrete cracks in the tension zone as well as through local buckling in the compression zone; out-of-code CFHST members (e.g., D/t = 120) can perform reasonable composite behavior because of contact pressure larger than 2.5 MPa, where a thin-walled steel tube experiences an arch failure mechanism similar to core concrete at a trussed angle of 45°; the simplified bearing capacity model achieves a mean value of 0.97, and can be accepted as a primary tool to perform structural design and performance evaluation.

Multi-scale three-dimensional analysis on local arrestability of intergranular crack in high-strength martensitic steel

The present study investigated the local arrestability of intergranular crack in high-strength martensitic steel through multi-scale three-dimensional (3D) analysis using X-ray computed tomography and focused ion beam machining (FIB)-scanning electron microscopy (SEM) serial sectioning combined with electron backscattering diffraction (3D EBSD). Macroscopic analysis using X-ray computed tomography demonstrated discontinuous propagation of the intergranular cracks, indicating local arrest of the crack propagation. An analysis of the opening displacement of each crack component revealed that the resolved normal stress was not the only factor determining the intergranular crack propagation path. The relationship between the microstructure and local crack-arrestability was microscopically analyzed using FIB-SEM serial sectioning. The 3D EBSD analysis clearly suggested that the crack propagations were arrested at the low-angle grain boundary plane segments and at the grain boundary triple junctions surrounded by relatively large martensite variants. Moreover, the larger martensite variants around grain boundaries contributed to the plastic accommodation of stress concentration and promoted crack-tip blunting. We propose that increasing the fraction of low-angle grain boundary plane segments as well as that of large martensite variants existing around grain boundaries can enhance local crack-arrestability and retard intergranular fracture of high-strength martensitic steels.