Brittle Crack Propagation Research Articles

Effect of dynamic embrittlement on high temperature fatigue crack propagation in IN718 – experimental characterisation and mechanism-based modelling

At elevated temperatures, the nickel-base superalloy IN718 is prone to the failure type ‘dynamic embrittlement’. Dynamic embrittlement is assumed to be driven by tensile stress-controlled oxygen grain boundary diffusion. Oxygen embrittles the grain boundaries in front of a crack and results in a fast brittle intercrystalline crack propagation. In order to reveal the mechanism of dynamic embrittlement, high temperature fatigue crack propagation tests were carried out at 650 °C in vacuum and air applying various dwell times and testing frequencies. The crack growth was monitored by the ACPD technique and a far-field microscope. The observations show that at low stress intensity factor ranges, crack propagation mainly occurs in the unloading and loading parts of the cycle and only minor crack propagation takes place during the dwell time. With increasing dwell time, the contribution of the crack propagation at constant stress increases. A mechanism-based model was developed on the basis of these findings which allows for a quantitative description of the effect of dynamic embrittlement on fatigue crack propagation rate. The results of respective simulations correspond very satisfactorily to the experimental data. Hence, the model is suitable for lifetime assessment.

A comparative molecular dynamics-phase-field modeling approach to brittle fracture

In this work, a novel comparative method for highly brittle materials such as aragonite crystals is proposed, which provides an efficient and accurate in-sight understanding for multi-scale fracture modeling. In particular, physically-motivated molecular dynamics (MD) simulations are performed to model quasi-static brittle crack propagation on the nano-scale and followingly compared to macroscopic modeling of fracture using the phase-field modeling (PFM) technique. A link between the two modeling schemes is later proposed by deriving PFM parameters from the MD atomistic simulations. Thus, in this combined approach, MD simulations provide a more realistic meaning and physical estimation of the PFM parameters. The proposed computational approach, that encompasses mechanics on discrete and continuum levels, can assist multi-scale modeling and easing, for instance, the simulation of biological materials and the design of new materials.

Measurement of local brittle fracture stress for dynamic crack propagation in steel

The authors measured the local fracture stress of brittle crack propagation in steel plates. Many studies on brittle crack propagation in steel plates have been implemented based on energy balance, but they have not been able to provide full theoretical explanation to brittle crack propagation and arrest behavior in steel plates. On the other hand, although the local fracture stress concept has been emerging as a factor governing the behavior recently, the local fracture stress has not been evaluated sufficiently because the crack front shape and forming of shear lip have prevent us from implementing accurate FEM analysis of brittle crack propagation. So, we implemented brittle crack propagation tests using specimen with side groove which is designed to increase stress near the plate surface and make the crack front straight. Inputting crack velocities obtained from experiments, FEM analyses were implemented. Our analyses showed the local fracture stress is higher in low temperature than in high temperature. This trend can be explained from the aspect of forming of tear ridge, which is main mechanism to absorb energy during brittle crack propagation in steel plates.

An Observation of Brittle Crack Propagation in Coarse Grained 3% Silicon Steel

Brittle fracture in carbon steel affects serious impact for the safety of fracture of in steel. Especially, the arresting technology of crack propagation is the last stand for the structure. It is so important issue that the conditions which can be stopped reliably crack propagation should be clarified thoroughly. On the background of such social importance, lots of experimental and theoretical researches have been conducted from both mechanical and microstructural viewpoints.Though it has been reported that the limit speed of brittle crack propagation is the Rayleigh wave speed and the speed is about 2,900 m/s in steels, the real speed of brittle crack propagation in steels is about 1,000 m/s and lower. The reason for that is considered braking effects with crack propagation, for example unevenness in the facet, tear ridge, microcrack, twin deformation and side ligament, namely the elements that dominate the arresting toughness.In this study, the 3% silicon steel where microstructure is ferrite single phase and its grain size is 4-5mm of very large is used. It is assumed that the brittle crack propagation speed in single crystal grain is very fast because there is no need to consider tear ridge between crystal grains. In this experiment, it is succeeded in shooting of brittle fracture of steel by high speed camera. As a result of that, it was revealed that the brittle crack propagation rate of 3% silicon steel is much lower than ordinary carbon steel. Also, twin deformation and unevenness in the facet are observed by scanning electron microscope (SEM).

Multiscale modeling to clarify the relationship between microstructures of steel and macroscopic brittle crack propagation/arrest behavior

Prevention of brittle crack propagation as well as crack initiation is essential as “double integrity”. The application of steels with high arrestability to the structures is directly effective to ensure the integrity. Although it is empirically known microstructures have a possibility to macroscopically enhance the arrestability of steel, there are not any theories which have quantitatively explained the relationship between brittle fracture and microstructures in the past investigations. In the present study, we propose a multiscale fracture mechanics model by a “model synthesis” approach, which integrates the multiple models and analyses to systematically evaluate complicated macroscopic and microscopic phenomena, based on the information of microstructures of the steel. The model is composed of (i) microscopic model to simulate cleavage fracture in grain scale (10-6~10-3m), and (ii) macroscopic model to simulate brittle fracture in steel plate scale (10-3~10°m). As validation, the proposed model is applied to temperature gradient crack arrest test of steel plate having nonhomogeneous distributions of microstructures in thickness direction. The prediction results show good agreement with experimental results in both crack arrest length and shape of crack front. That is, the proposed model has a potential basis of the framework to establish the theory to clarify the relationship between microstructures and brittle crack propagation and arrest behavior.

Experimental and numerical investigation on relationship between grain size and arrest toughness in steels

Today, steel plates of container carriers become thicker because larger ships are needed for carrying more baggage and reducing the transportation cost. Thus, even though brittle fracture occurs, arresting the crack is essential as “double integrity” for structures. It is directly effective to improve the arrest toughness, which is the material resistance against the brittle crack propagation. However, there was not established theory to explain the dependence of the microstructure on arrest toughness. In particular, although it is empirically known that of the finer grain size makes higher crack arrest toughness, the quantitative relationship was not clarified.Recently, Shibanuma et al. proposed a multiscale model to simulate the brittle crack propagation/arrest behaviour qualitatively. However, the microscopic energy absorbing mechanism in the model is too simple to reveal quantitative prediction of arrest toughness. According to the above background, we conduct an experiment to quantitative clarify the relationship between arrest toughness and grain size, and then modify the above multiscale model based on the experimental results. Consequently, the results obviously shows the relationship between arrest toughness and grain size of steels. The experimental result was contrary to the result of microscopic model. Thus, it is necessary to correct a numerical expression of the microscopic energy absorbing mechanism in the model to accurately simulate the actual brittle crack propagation and arrest behavior in steel of arbitrary grain size.

Micro-crack propagation on a biomimetic bone like composite material studied with the extended finite element method

Cortical bone contributes to about 80% of the weight of the human skeleton. Along its other properties, cortical bone presents a high resistance to fracture propagation. With this paper the authors aim to model this material using the Extended Finite Element Method (X-FEM) and to understand the mechanism that allow this material to have such a property. A numerical model was developed, considering a biomimetic bone like composite material, modelling the primary anatomical and functional unit of cortical bone, the osteon, as a fiber, the interstitial lamellae as the matrix, and the cement line between them. Different properties were considered for all the above mention materials, and their influence on the micro-crack propagation was studied. The cracks introduced and their geometry allowed the authors to understand why the cracks are arresting their propagation, and why is this material so resistant to crack propagation. The results are presented using the calculated stress intensity factors, for different material and geometries, and also using several brittle fracture crack propagation examples calculated using X-FEM.

On the mechanism of dynamic embrittlement and its effect on fatigue crack propagation in IN718 at 650°C

IN718 is a commonly used nickel-base alloy for high temperature applications, e.g., in gas and steam turbines. At elevated temperatures, this and other superalloys are prone to the failure mechanism "dynamic embrittlement". Dynamic embrittlement can be considered as a kind of stress corrosion cracking, driven by tensile-stress-controlled oxygen grain boundary diffusion. Oxygen embrittles the grain boundaries by weakening the grain boundary cohesion resulting in fast and brittle intercrystalline crack propagation. In order to reveal the mechanism of dynamic embrittlement, high-temperature fatigue crack propagation tests were carried out at 650°C applying various dwell times and testing frequencies. Most of the tests were performed in laboratory air, but some experiments were run in vacuum as well, in order to eliminate environmental effects and, hence, to define the reference fatigue crack propagation behaviour. The observations show that at low stress intensity factor ranges ΔKI, continuous crack growth occurs. At intermediate values of ΔKI, no crack propagation takes place during the dwell part of the cycle. Rather, the crack extends during unloading and reloading between subsequent hold times. The time necessary to grow the crack under sustained load during the dwell time was found to decrease with increasing stress intensity factor. Therefore, at high values of ΔKI, there is a contribution of the crack propagation at constant stress, since the incubation time is shorter than the dwell time. A mechanism-based model was developed for the range of test parameters, where intergranular and transgranular areas exist side by side in the fracture surface. The total crack growth per cycle is calculated by a linear combination of the intergranular and the transgranular contribution using the corresponding area fractions as weighting factors. It is shown that simulation calculations based on this model approach correspond very reasonably to the experimental observations. Hence, the model provides a quantitative mechanismen-related description of the effect of dynamic embrittlement on fatigue crack propagation rate.

低炭素低合金ベイナイト鋼の脆性破壊における亀裂伝播単位と組織の関係

低炭素低合金ベイナイト鋼の脆性破壊における亀裂伝播単位と組織の関係

Finite element model to simulate crack propagation based on local fracture stress criterion

It is important to arrest brittle crack propagation from the perspective of double integrity concept. To arrest brittle crack, structural arrest based on structural factor is attracting attention. However, the effect of structural factor is verified only by experiments and the mechanism of arrest is still unknown. The model based on a local fracture stress criterion has proposed by Shibanuma et al.. This model can reproduce the arrest behavior quantitatively only for flat plate of homogenous materials. In order to establish a general model which can evaluate structural arrest based on local fracture stress criterion, the model by FEM and nodal force release method is effective. As a preliminary investigation, we evaluated the accuracy of local stress obtained by nodal force release method. Next we made 2D model based on local fracture stress and conducted verification by comparing with exact solution. Finally, we made 3D model considering a curve of crack front. The result obtained by the model represents the behavior of crack front considering 3D effects qualitatively. It can be the base of the model to evaluate effects of structural arrest quantitatively.

Numerical Simulation of Brittle Crack Propagation and Arrest in Steels Considering Shear-Lip Formation

A model for simulating brittle crack propagation and arrest in steel plates is presented. The model considers the influence of shear lips formed on a fracture surface acting as a crack closure force and the influence of the loss of the plane-strain condition at a propagating crack tip with an increasing dynamic stress intensity factor. The shear lips are assumed not to fracture during the test, so they will yield and provide a crack closure force. The crack closure effect depends on not only the stress intensity factor but also the crack velocity. The model was applied to temperature-gradient crack-arrest tests and ultrawide duplex crack-arrest tests. The model successfully explained anomalous crack-arrest toughness under excessively high applied stress in the temperature-gradient tests and crack arrest at a stress intensity factor far above the cleavage crack-arrest toughness in the ultrawide duplex tests.

Generic features of brittle crack propagation engendered by the dynamics of the process zone

We demonstrate that internal dynamics within the transformational process zone at the tip of a propagating crack gives rise to energy dissipation. It manifests itself as a velocity-dependent resistance force exerted on the crack tip. It has the effect of limiting the terminal crack speed to below the Rayleigh velocity as well as giving rise to a discontinuous dependence of the crack velocity upon the driving force.

Fracture morphology and its evolution A review on crack path stability and brittle fracture along butt-weld

Considerations are made for the fracture morphology and its evolution in relation to the brittle fracture along butt-weld, where crack path stability may play important roles in various aspects. Analyses of a kinked and curved crack are first reviewed in the present paper. Having learned the solutions of a kinked and curved crack, several crack path criteria are compared for the crack path prediction. Then, some aspects of stability of a crack path are examined as to whether it may propagate, keeping its original direction or not in a homogeneous material, and whether crack kinking in a material having inhomogeneous fracture toughness may be predicted by seeking the state which gives rise to an energetically stable one. Having reviewed the morphology of brittle crack propagation along butt-weld, brittle fracture of extremely thick plates of high tensile steel is discussed for the fracture control of recently emerging large container ships.

Modelling the crack propagation behavior in 9Cr/CrMoV welds

Crack propagation behavior in different zones of advanced 9Cr/CrMoV dissimilar steel welded joint was simulated by ABAQUS software. The Gurson–Tvergaard–Needleman (GTN) damage model and brittle cracking model were introduced to analyze the crack propagation behavior for ductile and brittle region on the dissimilar steel welded joint, respectively. It is found that the numerically calculated J–△a resistance curves for the ductile regions i.e., the CrMoV base metal (CrMoV-BM), the heat affected zone of CrMoV (CrMoV-HAZ) and weld metal (WM) matched the experimental results, implying the reliability of the model developed in the present study. Three cases with different initial crack length (a0) were simulated to probe into the effect of a0 on the crack propagation resistance. The simulated results revealed that higher equivalent plastic strain and stress triaxiality occurred on the right ahead of the crack tip for longer initial crack length, which induces the lower crack propagation resistance. The brittle crack propagation behavior in 9Cr base metal (9Cr-BM) and heat affected zone of 9Cr (9Cr-HAZ) was well simulated by the brittle cracking model. The local stress distribution around the crack tip before crack destabilization could be obtained from the established model for BM and HAZ of 9Cr steel.

Brittle crack propagation/arrest behavior in T-joint structure of heavy gauge steel plate

Brittle crack arrestability is extremely important in welded joints of heavy gauge steel plates used in large container ships. Recently, much attention has been focused on potential crack propagation along welds fabricated by large heat input welding. This paper examines the application of a T-joint to the strength deck structure of container ships to enhance crack arrestability. The unwelded face and the crack arrest toughness, K ca, for crack arrest were varied. In the case of a T-joint with an unwelded face, the ESSO test of the T-joint components showed that a brittle crack arrested at the T-joint embedding the unwelded face. In the case of a full penetration T-joint, the brittle crack was arrested if the steel plate used for the flange had a high K ca value in the range from 4900 to 7300 N/mm3/2. FEM analysis of the stress intensity factor K indicated that brittle crack propagation was arrested under the condition that the K value at the running crack tip was less than the K ca of the material. In the T-joint, it was noted that the K value around the deepest point of the crack decreased and was finally less than the K ca of the flange plate when the brittle crack penetrated into the flange plate. This phenomenon shows the advantage of using a T-joint for brittle crack arrest in the flange plates of strength deck structures. In order to verify the long brittle crack arrestability of the T-joint structure, ultra-large-scale tests were carried out under the recommended conditions obtained from a dynamic FEM analysis. The test results showed that long brittle cracks could arrest at the aforementioned T-joint structures.

FINITE ELEMENT MODELLING OF CRACKABLE CONNECTING RODS AT FRACTURE SPLITTING PROCESS

In this study crack propagation analysis of impul-sively loaded connecting rods with different microstuctures namely perlitic, bainitic, tempered martensitic were studied numerically for evaluation of fracture parameters. Fracture splitting parameters in experiments for uniform impact force distribution was concerned in numerical analysis. Brittle crack propagation at starting notchs was obtained by impulsive load. Dynamic crack propagation was studied by using two dimensional finite element model and crack tip positions were determined by remeshing algoritm. Dynamic stress intensity factors were calculated by J integral technique and celevage failure was detected according to dynamic fracture toughness parameters by instrumented Charpy impact test. Crack propagation, crack arrest times, cleavege times were found and compared numerically for three different microstructures. Predicted shear effective stress intensity calculations, crack tip velocity, crack extesion results are found supplementary data for experimental fracture parameter optimisation. DOI: http://dx.doi.org/10.5755/j01.mech.21.2.7748

Structural Integrity Assessments by Measuring of Residual Stresses and Brittle Crack Toughness: A Thick Large-Scale Welding Study

Residual stresses inherently occur in welds and through-thickness distributions in an extra thick weld is unclear to date, though it is important in various steel, plant, and shipbuilding industries. Here introduce a recent complementary technique to determine residual stress measurement methods. Spatial variations of residual stresses were determined through the thickness of 70-mm thick ferritic steel welds created using low heat-input (1.7 kJ/mm) one-pass electro-gas welding (EGW) and high heat-input (56 kJ/mm) multi-pass flux-cored arc welding (FCAW). The results show that significant tensile stresses (about 90% of yield strength) occur along the weld centerline near the top surface (within 10% of the depth) in the FCAW specimen. Meanwhile, in the EGW weld, the peak stress moved towards the heat-affected zone at a depth of about 40% of the thickness due to the high heat-input. Secondly, large-scale (1 x 1 m) brittle fracture experiments have been performed to examine the brittle crack-arrest toughness and crack propagation path in each case. The fracture behavior compares to the observations of the grain size, hardness, and Charpy impact energy of the localized crack region in each weld. This study provides an insight in terms of the relevance between residual stresses and the brittle fracture behavior.

Phase field modeling of fracture in multi-physics problems. Part I. Balance of crack surface and failure criteria for brittle crack propagation in thermo-elastic solids

Abstract This work presents a generalization of recently developed continuum phase field models for brittle fracture towards fully coupled thermo-mechanical and multi-physics problems at large strains. It outlines a rigorous geometric approach to the diffusive crack modeling based on the introduction of a balance of regularized crack surface, governed by a crack phase field. The regularized crack surface functional is based on a crack surface density function, that describes the macroscopic crack surface in the bulk material per unit of the reference volume. The approach overcomes difficulties associated with the computational realization of sharp crack discontinuities, in particular when it comes to complex crack topologies. The formulation proposed is essentially a gradient damage theory, however, equipped with critical ingredients rooted in fracture mechanics. A modular concept is outlined for the linking of the diffusive crack modeling with complex multi-field response of the bulk material, where focus is put on the model problem of finite thermo-elasticity. This concerns a generalization of crack driving forces from the energetic definitions towards stress-based criteria, the constitutive modeling of heat conduction across cracks and convective heat exchanges at crack faces based on additional constitutive functions. This is achieved by approximating surface load integrals of the sharp crack approach by distinct volume integrals. We demonstrate the performance of the phase field formulation of fracture at large strains by means of representative numerical examples.

Investigating the behavior of laser-sintered Nylon 12 parts subject to dynamic loading

Abstract

完全溶込みＴ継手における脆性き裂伝播停止挙動

Arrestability of brittle cracks is extremely important in the structural integrity of welded joints of heavy gauge steel plates used in large container ships. Recently, a great attention has been focused on potential crack propagation along welds made with large heat input.In this study, a T-joint is applied to the strength deck structure of container ships to enhance crack arrestability. Material crack arrest toughnesses, Kca, of some steel plates are varied. The ESSO tests of T-joint components showed that a brittle crack is arrested at the T-joint if the steel plate used in the flange has a high Kca value in the range from 4900 to 7300 N/mm3/2.FE-analyses of the stress intensity factor K indicated that brittle crack propagation is arrested under the condition that the K-value at the running crack tip is less than the Kca of the material. It is noted in the T-joint that the K-value at the deepest point of the crack decreases and is finally below the Kca of the flange plate when a brittle crack rapidly penetrates the flange plate to a depth of 30-40mm. These phenomena show the advantage of the T-joint in brittle crack arrestability in the flange plate of the strength deck structure.