Running Crack Research Articles

Analysis of the failure mechanism of a blast furnace tuyere sleeve with protective coating

The tuyere sleeve plays a crucial role in blast furnaces, enduring a harsh working environment that frequently leads to substantial damage. One approach to extend the service life of tuyere sleeve is to prepare a Ni-based alloy protective layer on its surface. However, there is a lack of experimental information concerning the failure of these protective coatings after real-world use. Therefore, in this study, the failure of a blast furnace tuyere sleeve with protective layer was investigated. Characterizations were performed on both the “original” and failed protective layers, as well as the scale layer. The “original” protective layer exhibits two main issues: excessive copper dilution (＞40%) and significant Ni-Cr enrichment at the interface, which may be attributed to inadequate preparation techniques. The failed protective layer surface is covered with molten slag and iron, and there is a crack running almost completely through the coating. Both oxidation reaction and sulfur corrosion are important factors contributing to the accelerated failure of the coating. Moreover, it is suggested that the compositional inhomogeneity of the protective layer of the tuyere sleeve leads to local stress concentration within the coating, which is an internal cause for thermal fatigue crack formation. Additionally, it can be inferred that the molten slag and iron falling onto the protective layer surface under abnormal furnace conditions not only accelerates crack initiation and propagation but also directly melts the coating material or causes interface reactions, leading to the failure of the protective layer.

A data driven computational microstructure analysis on the influence of martensite banding on damage in DP-steels

Material design and optimisation for specific applications are key factors to reduce material usage. Thus, they are an important component towards more sustainable materials and process engineering. Additionally to optimising the chemical composition and the processing chain, the microstructure is another important factor. Especially for the damage properties, which are especially influential for forming processes. To achieve the best possible component performance it is important to understand the influences of the material and its microstructure on the performance. While damage evolution during forming is not equivalent to failure, understanding the characteristics and implications is extremely important for optimal service behaviour. However, a comprehensive way to quantify the influence of the microstructure on the damage properties is still not fully available. Thus, it is unclear how individual parameters of the microstructure influence the damage properties and, in extension, the mechanical properties. For dual phase (DP) steels the martensite phase plays an important role for strength, ductility, formability and damage resistance. While the influence of martensite phase fraction and morphology was often investigated experimentally, it is not possible to draw comprehensive, quantitative conclusions from such tests, as there are many factors at play in the microstructure, that cannot be changed individually. This study applies a simulative approach utilising statistically representative volume elements (sRVE) to quantify the influence of martensite banding on damage properties. Bending tests are taken as a forming process application, and investigations on the testing material show that the martensite bands play a major role during the cracking of the sample. In front of the crack tip, damage forms inside the banded structures leading to the crack running along or through the bands when enough voids have formed. To quantify the influence the bands have on the damage initiation, two microstructure states are compared: banded and without bands. For both states 29 sRVE are simulated to ensure a statistical analysis of the influence. The simulative comparison shows that the banded structures lead to an earlier damage initiation by margins of 2 to 20% of PEEQ. The methods applied in this study enable the investigation of individual influences of microstructure parameters on different mechanical or damage properties.

Effects of a circular hole on the propagation behavior of double running cracks under impact loading

In order to study effects of a circular hole on the propagation behavior of double running cracks under impact loading, the three-point-bending impact experiments of polymethyl methacrylate (PMMA) specimens with a circular hole were carried out by using the optical caustics method, and the numerical simulation was carried out by using the extended finite element method (XFEM) in ABAQUS software. The results show that under the influence of the stress field around the circular hole, the propagation trajectory of double running cracks can be divided into three stages: "exclusion" stage; first "attraction" and then "exclusion" stage; neither "attraction" nor "exclusion" stage. The circular hole defect affects the propagation velocity of double running cracks and the dynamic stress intensity factors at the crack tip. The maximum principal stress between the running cracks and the circular hole decreases with crack spacing a. When the double running cracks’ tips are on both sides of the circular hole, the maximum principal stress between the running cracks and the circular hole reaches the minimum.

Experimental study on the effect of defect curvature on the impact fracture behavior of structures using the real-virtual caustics method.

Defects have significant influence on the impact fracture behavior of structures. In this paper, the real-virtual caustics method is used to study the impact fracture behavior of structures with elliptical arc defects under impact loading of the drop hammer, and the impact loading process is simultaneously analyzed. The research results show that impact loading of the drop hammer in this experiment is a multi-period dynamic loading process, and the fracture of specimens under impact loading of the drop hammer is an energy-controlled process. The running crack initiates under impact loading and propagates toward the elliptical arc defect. After reaching the end of the elliptical arc defect, the running crack arrests and accumulates energy, and then it initiates again and propagates toward the loading position. The greater the end curvature of the elliptical arc defect, the shorter the time for the running crack to stagnate and accumulate energy at the defect end, and the earlier the time for the running crack to initiate again at the defect end, the smaller the impact loading stress of the drop hammer and the dynamic stress intensity factor of running crack tip when the running crack initiates again at the defect end.

Experimental Investigation of Obliquely Incident Blast Wave Effect on Deflection of Running Cracks

Abstract Dynamic caustics in conjunction with high-speed photography were used in experiments to investigate the interaction of obliquely incident blast stress waves with running cracks. The dynamic photoelastic experiment method was used to supplement and improve the effects of different blast stress wave properties on the running crack. This indicated that the crack-tip mode II stress field affected the crack deflection direction. When the moving crack-tip mode II stress intensity factor KII was positive, the crack deflected clockwise. When the stress intensity factor KII was negative, the crack deflected in the counterclockwise direction. The incident direction of the blast stress wave affected the deflection of the crack. When the direction of the blast stress wave was the same as the direction of the moving crack propagation, the moving crack deflected away from the blast wave source. When the blast stress wave propagation direction was opposite to the crack propagation direction, the crack deflected toward the blast wave source. During the interaction of an obliquely incident blast stress wave with a moving crack, the P wave caused the deflection of the crack. The tails of the P wave and S wave interacted with the kinked crack to increase the mode I stress intensity factor KI and the crack propagation velocity.

Modified mixed-mode caustics interpretation to study a running crack subjected to obliquely incident blast stress waves

Crack-wave interaction is common in blast, producing mixed-mode cracks when stress waves are oblique to cracks. How to measure mixed-mode crack-tip stress under blast stress waves is difficult. To this end, optical caustics method was employed to study a running crack in a PMMA plate subjected to obliquely incident blast stress waves, by which stress problem was transformed into optical problem. Incident P and S waves, as well as reflected waves, were visualized as fringe patterns, and mixed-mode crack-tip stress was represented by a caustics pattern (shadow spot). However, mixed-mode caustics patterns under P waves were not consistent with the classical interpretation, hence a modified mixed-mode caustics interpretation was proposed and then verified by P-wave compression and tension loading cases. In modified interpretation, an iteration procedure was proposed to obtain modified stress intensity factors, i.e. KI and KII, and crack-tip positions which were more precise than those by the classical interpretation. P-wave compression phase decreased KI and crack velocity sharply but produced the largest KII, while the following P-wave tension phase had an opposite effect. S waves and reflected waves produced higher KI and crack velocity with lower KII. KII and crack velocity histories explained curved crack path and microscopic fracture surface. Finally, the reason for the success of the modified interpretation was discussed, and it is indicated that transient stress superposition and delayed redistribution in the crack tip are the mechanism underlying the modified interpretation.

Experimental and Numerical Study on Crack-Arrest Behaviours of Rock-Like SCDC Specimens

Studying rock crack-arrest toughness, which determines the arrest behaviours in rock materials with cracks, is important to prevent further damage to rock mass. To investigate the crack-arrest behaviours both qualitatively and quantitatively, dynamic impact experiments (using caustics and photoelastic methods) were performed on rock-like (epoxy resin) single cleavage drilled compression (SCDC) specimens to study the crack-arrest behaviours and the microscopic fracture characteristics. The crack-arrest toughness of the SCDC specimen was obtained and the extended finite element method (XFEM) in ABAQUS software was used to validate the experimental results. The results show that under the interaction between reflected compression and tensile waves, the running crack experiences two crack-arrest periods before the final arrest, and the crack-arrest toughness stabilizes at a value of 0.669 MPa m1/2. Moreover, the peak propagation velocity in each crack initiation process gradually decreases with the development of the crack path. Additionally, the observations by scanning electron microscopy (SEM) and photoelastic methods show that the roughness of the fracture surface is influenced by the crack-arrest behaviours, causing the spalling fragments as well as wing micro-cracks in the fracture surface.

Study of crack arrest mechanism and dynamic behaviour using arc‐bottom specimen under impacts

AbstractIn this paper, a large size trapezoidal open crack with arc‐bottom (TOCAB) configuration specimen was proposed. Experiments were carried out using 0°, 60°, 90° and 120° TOCAB specimens under drop weight impacts with loading rate from 150 to 350 GPa/s. The crack length and crack speed were calculated by using fractal method. It shows that the crack speed corrected by fractal method is more than 12% the original speed calculated using the straight length. The AUTODYN code was employed to simulate crack propagation path and crack speed. The dynamic stress intensity factors (DSIFs) were calculated using ABAQUS code. The experimental and numerical results show that all the three TOCAB specimens have arresting function on running cracks; crack propagation velocity increases with loading rates; the critical DSIFs at initiation are higher than those at propagation. The study presents how to arrest a running crack and how to protect significant structures from being completely damaged.

Experimental study on dynamic caustics of interaction between void and running crack

In order to study the interaction between running cracks and voids in the offset distance of different prefabricated cracks, setting the offset distance of the pre-crack as the unique variables, do impact three-point bending test on PMMA specimens with voids based on dynamic caustics experiment system. Researches show that: There are two critical distances, 6 mm (2R), 9 mm (3R). At these distances, the extended trajectory and dynamic fracture characteristics of the crack change significantly. (1) When the pre-crack offset distance is no greater than 3 mm, the specimen cracks for the second time. The rate and stress intensity factor of the second crack initiation are significantly greater than the first. The fractal dimension of the crack trajectory is the minimum when the pre-crack does not offset. (2) When the offset distance is increased to 6 mm, the void is attracted to the crack and then repelled, but the crack never penetrates the void. Crack velocity and stress intensity factor decrease firstly and then increase. The fractal dimension of the crack trajectory is up to the maximum. (3)When the offset distance is greater than 6 mm, the attraction of voids to cracks is gradually reduced. When the offset distance is greater than 9 mm, the attraction of void to cracks is no longer significant. Crack expands to the falling hammer after crack initiation and finally penetrates the test piece.

Experimental Study on the Dynamic Behavior of Running Crack Affected by Defect Shape and Filling Material

Using the caustics method and the experimental system of digital laser dynamic caustics, the model experiment of drop hammer impact loading was carried out, and the effect of the defect shape (circular and rectangular) and the filling material (air, epoxy, and silicone rubber) on the propagation behavior of the running crack was investigated. The experimental results show that, under the impact loading, the running crack initiates at the end of precrack and propagates toward the defect. After the running crack connects to the defect, it accumulates energy within a certain period before initiating again at the upper edge of the defect. Subsequently, only one running crack is formed at the upper edge of the circular defect, but two running cracks are formed at the upper edge of the rectangular defect. The defect shape and the filling material have a significant effect not only on the energy accumulation time of the running crack at the defect but also on the stress intensity factor when initiating at the defect. The effect degree of the defect shape on the running crack propagation behavior is in the following order: circular defect > rectangular defect, whereas the effect degree of the filling material on the running crack propagation behavior follows this order: air > silicone rubber > epoxy.

Study on the arresting mechanism of two arrest-holes on moving crack in brittle material under impacts

To develop the technique of arresting running cracks is essential because it can be used to protect cracked structures being further damaged. In this paper, a specimen of large semicircular edge crack with two arrest-holes (LSECTH) was proposed. The specimens were made of cement, river sand, fly ash and water. Impact experiments were carried out in the LSECTH specimens with different two-hole spacing (35 mm, 40 mm, 45 mm, 50 mm, 55 mm, 60 mm and 70 mm) under drop-weight impacts. Crack propagation gauges (CPGs) were used to monitor the fracturing time and crack speed. The interaction characteristic between the crack and two arrest-holes was investigated numerically by the AUTODYN code. In the numerical models, the failure criterion of maximum tensile stress and shear stress were employed for the brittle material. The crack propagation path, the circumferential stress of crack tip and the stress state between the two arrest-holes were analyzed. The calculation results indicate that compressive stresses between the two arrest-holes induced by the shape change from circle to ellipse play a key role in confining the vertical crack propagation. Both experimental and numerical results indicate that the two arrest-holes have suppressing action on moving crack; as the two arrest-hole spacing decreases, the suppressing action intensifies.

An experimental study on Kca value to arrest a running brittle crack in structural model specimens with steel plate of 100 mm thickness for container ships

With demand for ultra-large container ships, high-strength steel plates with extremely large thicknesses are applied to the structural elements around hatch side structures. The two longitudinal plates of the upper deck and hatch side coaming are the main structural elements that support bending stress during hogging of the ship structure, and brittle crack arrest toughness as well as crack initiation toughness are required in these steel plates to avoid the catastrophic ship damage. Although the International Association of Classification Societies prescribes a unified requirement for brittle crack arrest steel plates providing the value of brittle crack arrest toughness Kca at − 10 °C for plate thicknesses of 80 mm or less, ultra-large container ships using steel plates with thicknesses exceeding 80 mm are continuously required and built. In the present work, brittle crack arrest tests with large scale structural model specimens which simulate the structural element of the hatch side structure were performed to investigate the Kca value required to arrest a running brittle crack for steel plates with the thickness of 100 mm. The present investigation suggested that the test plate simulating upper deck could arrest a running brittle crack at the plate Kca of 6000 N/mm3/2, nevertheless the Kca value of 8000 N/mm3/2 was needed in the test plate simulating hatch side coaming. The different Kca values required to arrest a running crack between the test plates simulating the upper deck and the hatch side coaming are also discussed from the viewpoint of the arrested crack size and shape.

Experimental study on a Mach cone and trailing Rayleigh waves in a stress wave chasing running crack problem

This paper focuses on a blast-induced Mach cone followed by trailing Rayleigh waves during blast stress waves chasing a running crack in an epoxy plate, using the photoelasticity method and high-speed photography. A Mach cone and trailing Rayleigh waves were visualized and interpreted based on isochromatic fringes, agreeing well with theoretical wave paths. The Mach cone was the coalescence of shear wavelets generated by P waves on crack surfaces, which was verified by the half-cone angle, equal to arcsin (CS/CP). Effects of the Mach cone and trailing Rayleigh waves on the running crack were evaluated by measuring crack velocity and dynamic stress intensity factors (SIFs) of the crack tip. It was found that the Mach cone decreased both crack velocity and SIFs, while trailing Rayleigh waves had the opposite effect. Finally, because of similarities, the findings in our experiments were discussed with those in the supershear rupture. In particular, effects of the Mach cone and trailing Rayleigh waves on the running crack were consistent with particle motion near the fault in the supershear rupture.

A Yoffe-type moving crack in one-dimensional hexagonal piezoelectric quasicrystals

• Moving crack in 1D hexagonal piezoelectric quasicrystals is studied. • Full electroelastic field induced by a running crack is obtained. • Field intensity factor is independent of the crack speed. • Energy release rate is dependent on the crack speed. • Crack bifurcation is judged by the maximum energy release rate. A Yoffe-type moving crack in one-dimensional hexagonal piezoelectric quasicrystals is considered. The Fourier transform technique is used to solve a moving crack problem under the action of antiplane shear and inplane electric field. Full elastic stresses of phonon and phason fields and electric fields are derived for a crack running with constant speed in the periodic plane. Obtained results show that the coupled elastic fields inside piezoelectric quasicrystals depend on the speed of crack propagation , and exhibit the usual square-root singularity at the moving crack tip . Electric field and phason stresses do not have singularity and electric displacement and phonon stresses have the inverse square-root singularity at the crack tip for a permeable crack. The field intensity factors and energy release rates are obtained in closed form. The crack velocity does not affect the field intensity factors, but alters the dynamic energy release rate. Bifurcation angle of a moving crack in a 1D hexagonal piezoelectric quasicrystal is evaluated from the viewpoint of energy balance. Obtained results are helpful to better understanding crack advance in piezoelectric quasicrystals.

On impact by a hard cone on elasto-viscoplastic material, leading to the generation of a conical crack

The destruction of solid physical objects is a complex process in which mechanical, chemical, thermobaric and other matter transformations take place. Under mechanical destruction is understood the violation of the integrity of the object due to the occurrence of cracks. High-speed impact of a solid body on deformable materials is accompanied by the spread of cracks and is of a wave nature. This article presents an analysis of the dynamic stress-strain state in an elastoviscoplastic (EVP) material near the leading edge of a moving crack, approximated by a zone of continuous deformation. An analysis of the distribution of the intensity of tangential stresses and plastic deformations that occur behind the front of the longitudinal and shear head waves of a spherical shape generated by the impact of the vertex of the solid cone is carried out on the model EVP of the medium by the ray method. It is shown that the presence of a maximum of the jump of the tangential velocity component on the shear wave leads to a development with time of a jump in the displacements of the tangents to the front of the shear wave. This can be interpreted as the moment of initiation of the head part of a crack running along with the front of the elastic wave with the velocity of shear waves.

Atomic-scale mutual integrals for mixed-mode fracture: Abnormal fracture toughness of grain boundaries in graphene

In this study, we developed a computational scheme based on the atomic-level J-based mutual integral (or two-state conservation integral) to analyze the mixed-mode fracture along grain boundaries (GB) in polycrystalline solids. Discrete atomic information, obtained from molecular dynamics simulation of crack propagation along GBs in polycrystalline solids, is incorporated with asymptotic singular fields near an interfacial crack tip between dissimilar materials in the atomic-level J-based mutual integral, to extract the individual stress intensity factors of modes I and II. As a model problem, crack propagation along GBs in polycrystalline graphene, an ordered array of non-hexagonal defects, is analyzed. In the model, GBs are considered to be the most favorable paths for crack propagation, as the carbon atoms along the GBs experience less-ordered interatomic interactions than those in pristine graphene. When a mixed-mode loading is applied to a crack running along a graphene GB, as the mode mixity increases, the fracture toughness of GBs in graphene gradually increases. However, if the mode mixity is greater than 8°, the fracture toughness gradually decreases, which can be considered a unique characteristic of GBs in graphene. This abnormally-low fracture toughness of GBs in graphene for high mode mixity, may be interpreted as the competition between bond-rotation and bond-breaking mechanisms of carbon atoms, in conjunction with the Stone–Wales transformation and nonagon structures at the crack tip along GBs in graphene.

Hole Defects Affect the Dynamic Fracture Behavior of Nearby Running Cracks

Effects of defects on the dynamic fracture behavior of engineering materials cannot be neglected. Using the experimental system of digital laser dynamic caustics, the effects of defects on the dynamic fracture behavior of nearby running cracks are studied. When running cracks propagate near to defects, the crack path deflects toward the defect; the degree of deflection is greater for larger defect diameters. When the running crack propagates away from the defect, the degree of deflection gradually reduces and the original crack path is restored. The intersection between the caustic spot and the defect is the direct cause of the running crack deflection; the intersection area determines the degree of deflection. In addition, the defect locally inhibits the dynamic stress intensity factor of running cracks when they propagate toward the defect and locally promotes the dynamic stress intensity factor of running cracks when they propagate away from the defect.

Experimental characterization of interlaminar fracture toughness of composite laminates assembled with three different carbon fiber lamina

In the present work, the fracture resistance of a carbon fiber composite under mode I and mode II loading have been experimentally determined. For the mode I and II, the energy release rate G has been determinate for each material. In some cases, only a single estimation of G was possible due to problems in the propagation such as extensive fiber bridging and loss of planarity of the running crack. The experimental results relative to DCB tests have been analyzed in order to derive statistical trends. Only the samples for which more than three crack advance data points have been collected are considered in the analysis. The G values are those obtained with the compliance calibration method (CC). For ENF test, determination of critical GII, in addition to the value calculated with the relationship given in the prescription EN6034, other two values, the non linear and visual non linear, are also given. The crack propagation resulted to be unstable for all specimens tested and only a single value of GII could be determined.

Brittle crack propagation resistance inside grain and at high angle grain boundary in 3% Si-Fe alloy

Brittle fracture in carbon steel has a strong impact on the safety of the structures. Especially, the arresting technology of the running crack is the measure of last resort for ensuring structural integrity. Due to such high importance, many experimental and theoretical studies of brittle crack propagation have been conducted from both mechanical and microstructural viewpoints.It is thought that the elementary step of the brittle fracture of polycrystalline steel is the cleavage in each crystal grain and their connection process. However, the detailed mechanisms of brittle fracture have not been fully understood; for example, it is still unclear why the propagation rate under a large driving force is not increased up to the Rayleigh wave speed. Several difficulties hinder the achievement of a detailed understanding so far: 1) crack propagation is quite rapid, 2) the crystal grain is usually too small (10–100 μm) to collect sufficient information, 3) the microstructure is very complicated in most case, i.e., containing different phases, microstructures, precipitates and inclusions.In this study, to eliminate such difficulties, 3% silicon steel in which microstructure is the ferrite single phase and the grain size is increased to 4–5 mm is used. This steel can be fractured in a brittle manner, even under ambient temperature. For this steel, by using a high speed camera and strain gauge data with a high sampling rate, the elementary process of brittle crack propagation is elucidated.As a result, it is revealed that the brittle crack propagation rate even in a single-crystal grain is much slower than the Rayleigh wave speed. This seems to be due to the presence of twin deformations and twin boundary cracks in crystal grain as observed on the fracture surface. Using the analysis of electron back scatter diffraction (EBSD) data, the mechanism of twin deformation and twin boundary crack is revealed. Additionally, it is shown that the brittle crack propagation rate where the path includes crystal grain boundaries is much slower. This delay seems to be related to the misorientation angle at the GB. By applying our simplified model, the delay effect at the grain boundary can be successfully explained.

Stress analysis of the interaction of a running crack and blasting waves by caustics method

The purpose of this work is to investigate the mechanism of crack-wave interaction in blasting engineering. Transient effects of blasting waves on a running crack are evaluated by estimating crack-tip stress field from measurements of distorted shadow spots by caustics method. An optically geometrical superposition of light deflections from the running crack and blasting waves is proposed, considering concave and convex lens effects of waviness of initially flat plane under blasting waves. The stress field near the running crack tip in crack-wave interaction is obtained. Simulations of caustic curves derived from this superposition agree with those from instant records by a high-speed camera, highlighting that this superposition is a powerful way to evaluate the stress field near the crack tip and to extend classical caustics method to the application of unknown transient loadings by blasting waves. At last, similar photoelasticity experiments are made and results from two optical methods are discussed.