Ductile Crack Research Articles

Micromechanical Fracture Models of Q345 Steel and Its Weld

To better predict the ductile fracture of welded joints in steel bridges and buildings under seismic load, the fracture behavior of Q345 steel that has been widely used in China was studied under both monotonic and cyclic loadings based on the microscopic mechanism of material fracture. Based on the results of notched round bar tensile tests and finite-element analyses for Q345 steel base metal, weld metal (transverse and longitudinal), and heat-affected zone specimens, the parameters of the void growth model, stress-modified critical strain model, and cyclic void growth model were calibrated. The parameters of characteristic length in the previous models were determined by scanning electron microscope tests. The results indicated that the toughness parameters of the four materials are significantly different, and the cyclic growth capacity of the voids in weld metal and heat-affected zones deteriorates severely under cyclic loading. The toughness parameters of the two oriented weld metals are similar; therefore, the fracture toughness of the weld metal is less affected by the material orientation. The coefficients of variation of the toughness parameters of the four materials were small, and the distributions of the fracture indexes of the specimens with different notch radii were the same, which proved the effectiveness of the microscopic mechanism fracture model in predicting the initiation of ductile cracks of the members with different geometric shapes and stress states. The results of this paper provide the required material data and model parameters to predict the ductile fracture of welded joints made of Q345 steel using existing micromechanical fracture models.

Effect of the Lüders plateau on ductile fracture with MBL model

In this study, the effect of Lüders plateau on the ductile crack growth resistance has been investigated with the Gurson damage model and the modified boundary layer (MBL) model, under mode I plane strain condition. The Lüders plateau is modeled as horizontal by keeping the plateau stress equaling to the yield stress. A family of Lüders elongations ranging from 0 to 5% has been considered. The remote boundary condition of the MBL model is governed by the elastic K-field and T-stress. Numerical results show that the existence of the Lüders plateau on the stress-strain curve reduces the ductile crack growth resistance. The degree of reduction depends on the scale of the Lüders elongation. The crack tip stress field analysis indicates that the existence of the Lüders plateau varies the crack tip stress striaxiality distribution and the magnitude. It is also found that the size of plastic zone ahead of the crack tip is reduced, compared with the reference case for material without Lüders plateau. It is demonstrated that the effect of Lüders plateau on ductile crack growth is more significant at lower T-stress or for materials with higher toughness. The dependence of the initial void volume fraction and the T-stress on the ductile crack growth resistance are alleviated when the Lüders elongation is large.

Experimental study on flexural behavior of ECC/RC composite beams with U-shaped ECC permanent formwork

To enhance the durability of a reinforced concrete structure, engineered cementitious composite (ECC), which exhibits high tensile ductility and good crack control ability, is considered a promising alternative to conventional concrete. However, broad application of ECC is hindered by its high cost. This paper presents a new means to address this issue by introducing a composite beam with a U-shaped ECC permanent formwork and infill concrete. The flexural performance of the ECC/RC composite beam has been investigated experimentally with eight specimens. According to the test results, the failure of a composite beam with a U-shaped ECC formwork is initiated by the crushing of compressive concrete rather than debonding, even if the surface between the ECC and the concrete is smooth as-finished. Under the same reinforcement configurations, ECC/RC composite beams exhibit increases in flexural performance in terms of ductility, load-carrying capacity, and damage tolerance compared with the counterpart ordinary RC beam. Furthermore, a theoretical model based on the strip method is proposed to predict the moment-curvature responses of ECC/RC composite beams, and a simplified method based on the equivalent rectangular stress distribution approach has also evolved. The theoretical results are found to be in good agreement with the test data.

Fluid-structure-interaction modeling of dynamic fracture propagation in pipelines transporting natural gases and CO2-mixtures

As part of current design standards, the Battelle Two-Curve Model (BTCM) is still widely used to predict and secure ductile crack arrest in gas transmission pipelines. For modern linepipe steels and rich natural gases or CO2 mixtures, the BTCM might lead to incorrect predictions. On the one hand, it suffers from the insufficient description of the individual physical processes in the pipe material and fluid itself. Furthermore, the model does not account for fluid-structure-interaction (FSI) effects during simultaneous running-ductile fracture (RDF) and mixture decompression. Numerical FSI models allow for a more sophisticated, coupled analysis of the driving forces for the failure of pipelines. This paper deals with the development of an FSI model for the coupled prediction of 3D pressure profiles acting on the inner pipe wall during crack propagation. The coupled Euler-Lagrange (CEL) method is used to link the fluid and structure models. In a Lagrange formulation, the modified Bai-Wierzbicki (MBW) model describes the plastic deformation and ductile fracture as a function of the underlying stress/strain conditions. The fluid behavior is calculated in a 3D model space by Euler equations and the GERG-2008 reference equation of state (EOS). The coupled CEL model is used to predict the RDF in small-diameter pipe sections for different fluid mixtures. The calculated 3D pressure distributions ahead and behind the running crack tip (CT) significantly differ in axial and circumferential directions depending on the mixture composition. The predicted FSI between the pipe wall and fluid decompression in 3D CEL/FSI model provides reliable knowledge about the pressure loading of the pipeline during RDF.

Effect of Tempering Temperature on Impact Wear Behavior of 30Cr3Mo2WNi Hot-Working Die Steel

Effect of tempering temperature on impact wear morphology and properties of 30Cr3Mo2WNi hot working die steel were investigated by means of SEM, TEM, hardness and impact wear tests. In the tempering temperatures from 300℃ to 680℃, the hardness of the steel decreases and the impact toughness increases with increasing tempering temperature, while a secondary hardening with maximum hardness value of 48.6 HRC was achieved at 550℃. Fatigue delamination wear is the main mechanism during the impact wear of the experimental steel, and three damage features can be identified with different tempering temperatures. Brittle fatigue cracks are easy to occur at low tempered temperature of 300℃. Ductile fatigue cracks occurs at medium temperatures. Surfaces of steel tempered are extruded at high temperature of 680℃.

The Use of the Acoustic Emission Method to Identify Crack Growth in 40CrMo Steel.

The article presents the application of the acoustic emission (AE) technique for detecting crack initiation and examining the crack growth process in steel used in engineering structures. The tests were carried out on 40CrMo steel specimens with a single edge notch in bending (SENB). In the tests crack opening displacement, force parameter, and potential drop signal were measured. The fracture mechanism under loading was classified as brittle. Accurate AE investigations of the cracking process and SEM observations of the fracture surfaces helped to determine that the cracking process is a more complex phenomenon than the commonly understood brittle fracture. The AE signals showed that the frequency range in the initial stage of crack development and in the further crack growth stages vary. Based on the analysis of parameters and frequencies of AE signals, it was found that the process of apparently brittle fracture begins and ends according to the mechanisms characteristic of ductile crack growth. The work focuses on the comparison of selected parameters of AE signals recorded in the pre-initiation phase and during the growth of brittle fracture cracking.

Next-generation fracture prediction models for pipes with localized corrosion defects

This study presents a novel concept and a guide for developing the next generation of fracture prediction models for corroded metallic pipelines. Based on this new concept, the first 3D burst capacity models are developed herein. These models combine simplicity and accuracy through the use of a new parameter, volume of the defect, to measure the burst pressure of corroded pipes with localized defects of complex morphology. The past 50 years has seen the development of numerous 2D burst capacity models, which have focused only on the longitudinal-section area of the corrosion defect and have become increasingly more complicated and less practical in an attempt to increase their relatively low accuracy and stability. Unlike these earlier models, the 3D models presented herein do not require definition of the exact defect profile; despite this, they are significantly more accurate and more stable than the well-known conventional models. Thus, they can significantly reduce the life-cycle costs of pipelines through eliminating or substantially reducing the unnecessary repair or replacement of corroded pipelines. The 3D models have been developed from two sets of computational models that were validated against full-scale burst capacity tests on API-5L pipes with localized complex-shaped defects. In addition, high-speed imaging of the fracture was used to determine the location of ductile crack initiation and the corresponding internal pressure.

Fatigue strength evaluation of under-matched welded joints made of 800-MPa class steel based on the local strain approach

It is clear that usages of high-strength steels for steel bridges are effective to reduce weight of the bridges, and therefore, it may lead to the cost reduction of the bridges. However, adoptions of high-strength steels might result in under-matched joints, and significance of under-matched welded joints made of over 700-MPa class steels on fatigue strength and estimation of a local strain approach on the joints are not clear yet. This paper presents fatigue test results of under-matching welded joints made of 800-MPa class steels containing incomplete penetration under higher stress range region. Test results indicate that the fatigue strength in higher stress range region is less than that made of mild-strength steels, and showed that the cause of reduction of fatigue strength is that under-matched welded joints caused strain concentration and ductile crack propagation at around weld roots. This paper also presents applicability of a simple estimation method of the effective notch strain from nominal strain proposed by a previous study, and the estimation method can evaluate the effect of under-matching welded joints under higher stress range region.

Comparison between thermal, rheological and failure properties for the performance grading of asphalt cements

Thermal, rheological and failure properties of nine extracted and recovered asphalt cements were investigated to predict cracking susceptibility. Glass transition temperatures, non-isothermal Ozawa crystallization constants and binder fragilities were determined but found to lack a strong correlation with established performance indicators. All binders exhibited thermorheologically simple behavior and master curves for complex modulus and phase angle were readily constructed by applying the time-temperature superposition (TTS) principle. Failure properties of the binders were determined using ductility and double-edge-notched tension (DENT) tests. It was found that the ductility and approximate critical crack tip opening displacement (CTOD) are correlated, although the latter provides more reasonable (lower) values of strain tolerance. The Williams-Landel-Ferry (WLF) equation described the shift factors in dynamic shear, and the WLF constants C1 and C2 correlate reasonably well with ductility and CTOD. It was found that the phase angle was sensitive to asphalt cement quality, with poor performing binders showing a limiting phase angle temperature, T(δ = 45°), nearly 25 °C higher than those of superior performing materials. This investigation demonstrates that recovered binders can be prematurely hardened, likely due to overheating during production or the incorrect use of reclaimed asphalt. As this problem is not recognized when testing material from the asphalt cement supply tank, it is recommended to test extracted and recovered binder from the loose hot mix asphalt. This will assure that the material as placed actually meets performance requirements for the local climate.

Brittle and ductile creep behavior of Jinping marble under true triaxial stress

With excavation during deep underground engineering, the rock mass near the sidewall changes from ductile behavior to brittle behavior. However, the ductility of the rock mass at a certain distance from the sidewall may increase due to the change in stress. Over time, the failure mechanism of the rock mass is not necessarily consistent. The aim of this study is to obtain a better understanding of creep mechanisms under 3-D stress states. To achieve this goal, six brittle creep and two ductile creep experiments on Jinping marble were conducted under true triaxial stress conditions (σ1 > σ2 ≥ σ3). The test results show that creep deformation of rock decreases with increasing intermediate principal stress. Many micro-cracks were observed by scanning electron microscopy in the samples failed under the ductile creep. However, brittle creep and ductile creep present different crack expansion modes in microscopic terms, resulting in significant differences in macroscopic deformation behavior. The steady creep rate increases slowly with increasing σ1-σ3 when brittle creep occurs. Meanwhile, when the stress reaches a certain value during ductile creep, the steady creep rate suddenly increases exponentially from 10−6 h−1 to 10−5 h−1. From the test results, we can deduce that the true triaxial stress and time effects play key roles in the creep behavior of rock masses in deep engineering projects.

On the micromechanism of inclusion driven ductile fracture and its implications on fracture toughness

The objective is to identify the micromechanisms of ductile crack advance, and isolate the key microstructural and material parameters that affect these micromechanisms and fracture toughness of ductile structural materials. Three dimensional, finite element, finite deformation, small scale yielding calculations of mode I crack growth are carried out for ductile material matrix containing two populations of void nucleating particles using an elasto-viscoplastic constitutive framework for progressively cavitating solid. The larger particles or inclusions that result in void nucleation at an early stage are modeled discretely while smaller particles that require large strains to nucleate voids are homogeneously distributed. The size, spacing and volume fraction of inclusions introduce microstructure-based length-scales. In the calculations, ductile crack growth is computed and fracture toughness is characterized. Several features of crack growth behavior and dependence of fracture toughness on microstructural and material parameters observed in experiments, naturally emerge in our calculations. The extent to which the microstructural and material parameters affect the micromechanisms of ductile crack advance and, hence, the macroscopic fracture toughness of the material is discussed. The results presented provide guidelines for microstructural engineering to increase ductile fracture toughness, for example, the results show that for a material with small inclusions, increasing the mean inclusion spacing has a greater effect on fracture toughness than for a material with large inclusions.

Prediction of ductile crack growth in a narrow gap Inconel dissimilar weld

Predicting the behavior of large ductile crack propagation in a pipe is a major concern for many industries, especially in the nuclear industry. Leaning on the STYLE project, a European experimental program, tests are predicted using the local approach model of Gurson.The mock-up presents a narrow gap INCONEL (nickel base alloy) dissimilar metal weld joining a low alloy ferritic steel pipe and a 316L type stainless austenitic steel pipe. It was performed in the frame of the STYLE project funded by the European Commission (EC) (2010–2013). During the four points bending test on the mockup, the behavior of the crack growth was highly unexpected, and could not be deeply analyzed. The initial crack, located at the interface between the weld and the ferritic Heat-Affected Zone (HAZ), grew in the weld and deviated from it to finally reach the austenitic HAZ. A local approach modeling in order to explain the crack behavior has thus been performed, using the Gurson-Tvergaard-Needlemann (GTN) model.The characterization of the materials composing the weld was made through the STYLE project. Then, GTN parameters are identified using experimental results from the laboratory tests. The behavior of Compact Tension (CT) specimens and the pipe are then numerically predicted using finite element simulations. It is shown that the GTN model is able to catch the mechanical differences between the CT specimens and the pipe, providing a good prediction of the crack propagation during the four points bending test performed for the STYLE project.

Experimental and theoretical study on energy convergence characteristics of adiabatic shear fracture process in high-speed machining of hardened stainless steel

The formation of isolated segment chip due to the occurrence of adiabatic shear fracture (ASF) in adiabatic shear band (ASB) is a significant phenomenon under the high cutting speed. In the present work, the experimental and theoretical methods were adopted to further investigate the energy convergence characteristics in ASB during ASF process in high-speed machining. A hardened stainless steel used in turbine blade was selected as the workpiece. The chip morphology transformation from serrated chip to isolated segment chip was obtained through the high-speed machining experiment. The ductile crack propagation in ASB was observed microscopically. The relations of serrated segment geometry with the cutting conditions were revealed experimentally. According to the continuum governing equations and the deformation and energy analytical models, the distributions of shear velocity, shear strain rate, shear strain, and shear energy in ASB under various cutting speeds and feeds were analyzed combining with the constitutive and stress relations. The energy convergence characteristics in ASB during ASF process with the change of cutting conditions were analyzed and discussed. The results showed that the austenite blocks in the hardened stainless steel influenced the crack propagation in ASB. The larger shear strain induced thinner ASB would accelerate the thermal softening and strain localizing effects, resulting in severer energy convergence in ASB. The energy convergence was uniformly distributed and always kept a constant limit value in the whole ASB when ASF occurred.

Parameter window for assisted crack tip flipping: Studied by a shear extended Gurson model

Assisted flipping of a slant mode I dominated tearing crack, where the crack tip flipping mechanism is made to engage by imposing a slight mode III, has recently been studied in Felter and Nielsen (2017) [Assisted crack tip flipping under Mode I thin sheet tearing, Europ. J. Mech. A/Solids, 64;2017:58–68]. In the previous study, the Gurson–Tvergaard–Needleman model was used in its original form to limit the model parameter space and facilitate a search for a set of parameter that allows flipping of the slant crack face to occur. It is well known that the adopted version of the Gurson model can predict the shear bands that travel in front of a mode I tearing crack and these are essential features to slant crack propagation — let alone the experimentally observed crack tip flipping phenomenon. In fact, assisted crack tip flipping was achieved with this numerical model set-up, but only within a very narrow parameter window. The present work adopts a phenomenological shear extended Gurson model that allows for a study of the J3 dependency in ductile fracture at engineering scale in an attempt to widen this parameter window. By running series of large-scale computations, where a ductile tearing crack propagates multiple plate thicknesses, the authors can demonstrate tearing modes for various combinations of strain hardening, initial void volume fraction, and shear parameter. The shear damage contribution is found to shift the engagement of the crack tip flipping mechanism toward lower values of the initial void volume fraction for all levels of strain hardening considered.

Seismic performance of CHS X-connections under out-of-plane bending

This paper presents a comprehensive investigation of the hysteretic behavior of unstiffened circular hollow section (CHS) X-connections subjected to out-of-plane bending moment (OPBM). The study commences with tests on three full-scale CHS X-connection specimens with varying geometric parameters. The test results showed that the specimens mainly failed in tearing of the chord wall near the connection zone. Evident energy dissipation was provided when the connections experienced yielding, and this was followed by ductile crack propagation of the chord wall. The strength and ductility of the connections depended significantly on the brace-to-chord diameter ratio and the in-plane brace-to-chord angle. In particular, a decrease in the brace-to-chord angle led to increases in both the strength and connection efficiency (i.e., the ratio of the connection ultimate flexural capacity to nominal brace flexural yielding capacity) of the connection specimens. The test results also suggested that the current design code tends to make conservative predictions of the OPBM capacity of the connections, especially when the in-plane brace-to-chord angle is small. Moreover, either decreasing the in-plane brace-to-chord angle or increasing the brace-to-chord diameter ratio is found to benefit the ductility ratio and energy dissipation capability. This observation is further confirmed by a ring analytical model proposed in this study. FE models are then built and calibrated through comparisons against the test results, and subsequently, a parametric study was carried out to consider the influences of an extended range of geometric parameters.

Experimental and numerical study on ductile fracture of structural steels under different stress states

In addition to the stress triaxiality, the Lode angle has recently been recognized as an important parameter influencing the ductile crack initiation, propagation and final failure of structural steels. In this paper, a modified ductile fracture model is proposed to consider the effects of different stress states on the ductile fracture behavior of steels by introducing the deviatoric state parameter. Different stress states, including the tension, compression, shear, tension-shear, and compression-shear states, can be expressed by the stress triaxiality and deviatoric state parameter relating to the Lode angle. Three flat bar tension specimens and eight flat bar shear specimens were designed to model the different stress states, and ductile fracture tests were carried out on these 11 specimens. The parameters of the modified ductile fracture model in this study were determined based on these ductile fracture tests, and the corresponding finite element analytical results regarding these specimens exhibited strong agreement with the tested results. The proposed model was verified as being suitable for simulating the ductile fracture behavior of steels under different stress states.

Fracture behaviour of laser-welded 2219-T6 aluminium alloy under pulsed Lorentz force

Electromagnetic forming is a high-speed forming technology, using pulsed Lorentz force to make sheet metal deformed. The fracture behaviour of material during electromagnetic forming is a significant scientific issue in the development of this technology. In the current work, experimental methods are used to investigate the properties and microstructure of laser-welded 2219-T6 aluminium alloy and the fracture behaviour of the weld joint under pulsed Lorentz force. The results indicate that the hardness, strength, ductility, impact toughness of the laser-welded joint are all worse than that of the base metal because of the inhomogeneity of the microstructure and the existence of coarse brittle eutectic phases. This leads to the ductile fracture of laser-welded joint at the initial stage, followed by the sparking and partially melting breaking phenomena, under the pulsed Lorentz force. The molten structure is also observed on the fracture surface of the weld joint after electromagnetic tensile test rather than mechanical high-speed tensile test. During the fracture process under pulsed Lorentz force, local short circuits occur at the ductile crack tip of the weld joint due to the eddy current effect, resulting in the melting of the weld metal. The percentage reduction of area and the dimple sizes in the fracture morphology of the specimen under the conditions of high-speed deformation are larger than that under the condition of quasi-static tensile test, indicating the slight improvement in ductility of the laser-welded joint under high-speed deformation due to the inertial effect and the adiabatic temperature rise from the plastic work. Particularly, the above indicators are even larger under electromagnetic tensile test, revealing extra very slight increase in ductility compared with mechanical high-speed tensile test because of the body force and the additional temperature rise resulting from the Joule heat.

Ductile crack growth behaviors at different locations of a weld joint for an X80 pipeline steel: A numerical investigation using GTN models

The finite element method (FEM) based on the Gurson–Tvergaard–Needleman (GTN) model was used to investigate the ductile crack growth behaviors at different weld joint locations of X80 pipeline steel. As the result of the welding process, the X80 weld joints are inhomogeneous and can be divided into different zones (base metal, weld metal and heat affected zones), with different material properties and ductile crack growth resistance characteristics. By fitting the experimental results of uniaxial tension and single-edge notched bending (SENB) tests for specific zones, the GTN model parameters were calibrated in consideration of the inhomogeneity of the weld joint. The calibrated GTN models were then used to analyze the ductile crack growth behaviors in single-edge notched tension (SENT) tests at different locations of the weld joint. In addition, the effect of T-stress on the crack propagation at different locations was also studied using the small-scale yield model. It is demonstrated the calibrated GTN models are capable of predicting crack growth resistance curves at different weld joint locations, under different constraint conditions.

Fracture Assessment of Weld Joints of High-Strength Steel in Pre-Strained Condition

Unstable fractures tend to occur after ductile crack initiation or propagation. In most collapsed steel structures, a maximum 15% pre-strain was recorded, at the steel structural connections, during the great earthquake of 1995, in Japan. Almost-unstable fractures were observed in the beam-to-column connections, where geometrical discontinuities existed. Structural collapse and unstable failure occurred after large-scale plastic deformations. Ship structures can also suffer from unstable fractures in the welded joints. The fracture resistance of butt-welded joints subjected to tension in the pre-strained condition was estimated by considering the toughness deterioration, due to pre-strain and toughness correction for constraint loss in a tension specimen. The target specimen for this fracture assessment was a double-edged, through-thickness crack panel, with a crack in the weld joint (heat-affected zone (HAZ)). The critical fracture toughness value (crack tip opening displacement (CTOD)) of a large structure with pre-strain, which was applied to the HAZ region, was estimated from a small-scale, pre-stained, three-point bend specimen. Fracture toughness values, evaluated by a CTOD test, were recently mandated for shipbuilding steel plates. The critical fracture toughness value is a very useful parameter to evaluate the safety of huge ship structures.

Evolution of damage and failure in an additively manufactured 316L SS structure: experimental reinvestigation of the third Sandia fracture challenge

The third Sandia Fracture Challenge (SFC3) was a benchmark problem for comparing experimental and simulated ductile deformation and failure in an additively manufactured (AM) 316L stainless steel structure. One surprising observation from the SFC3 was the Challenge-geometry specimens had low variability in global load versus displacement behavior, attributed to the large stress-concentrating geometric features dominating the global behavior, rather than the AM voids that tend to significantly influence geometries with uniform cross-sections. This current study reinvestigates the damage and failure evolution of the Challenge-geometry specimens, utilizing interrupted tensile testing with micro-computed tomography (micro-CT) scans to monitor AM void and crack growth from a virgin state through complete failure. This study did not find a correlation between global load versus displacement behavior and AM void attributes, such as void volume, location, quantity, and relative size, which incidentally corroborates the observation from the SFC3. However, this study does show that the voids affect the local behavior of damage and failure. Surface defects (i.e. large voids located on the surface, far exceeding the nominal surface roughness) that were near the primary stress concentration affected the location of crack initiation in some cases, but they did not noticeably affect the global response. The fracture surfaces were a combination of classic ductile dimples and crack deviation from a more direct path favoring intersection with AM voids. Even though the AM voids promoted crack deviation, pre-test micro-CT scan statistics of the voids did not allow for conclusive predictions of preferred crack paths. This study is a first step towards investigating the importance of voids on the ductile failure of AM structures with stress concentrations.