Longitudinal Surface Cracks Research Articles

Experimental study on mechanical damage characteristics of water-bearing tar-rich coal under microwave radiation

As a recognized special resource, tar-rich coal can extract the country's scarce oil and gas resources and generate semi-coke that can replace anthracite and coking coal. The tar-rich coal in northern Shaanxi is prominent, but due to the dense structure and high strength of tar-rich coal, it is easy to cause frequent dynamic disasters in coal mining. Therefore, the realization of pressure relief and disaster reduction has become the primary problem in mining tar-rich coal. There are many shortcomings in conventional pressure relief methods, so a new method of microwave-weakening coal is proposed. Through different water saturation treatments of tar-rich coal samples, the longitudinal wave velocity degradation trend and surface crack expansion law of water-bearing coal after microwave irradiation were analyzed, and the strength softening characterization and energy evolution relationship under the combined action of microwave and water were studied. Fractal dimension and its internal correlation based on the equivalent side length-mass of coal sample fragments. The experimental results show that: (1) Under the same microwave radiation condition, with the increase of water saturation, the deterioration trend of physical and mechanical parameters such as longitudinal wave velocity and peak strength is obvious. (2) After microwave radiation, the uniaxial compression results show that the coal sample is damaged by load, there is still a high residual strength, the ratio of elastic energy to dissipation energy decreases, and the possibility of rockburst of the coal sample decreases. The strength softening degree of coal specimen under the degradation of microwave and water is the highest, followed by microwave and water. (3) The fractal dimension is inversely proportional to the moisture content and microwave radiation intensity, and the fractal dimension has a significant positive correlation with the peak intensity and longitudinal wave velocity. The mechanical damage law of water-bearing tar-rich coal under microwave action is revealed, which aims to solve the problem of weakening and reducing the impact of hard coal on-site to a certain extent, ensure the safety of working face, and improve the mining efficiency of tar-rich coal. It provides basic theoretical support for microwave-assisted hydraulic fracturing technology and effective weakening measures for hard roof treatment.

Improvement of burst capacity model for pipelines containing surface cracks and its implication for reliability analysis

This paper presents the improvement of a widely used burst capacity model for steel oil and gas pipelines that contain longitudinal external surface cracks, namely the CorLAS model, through the addition of a correction factor that is quantified by the Gaussian process regression (GPR). The correction factor is assumed to depend on four non-dimensional input features that characterize both the crack geometry and pipe material properties. A database consisting of 212 full-scale burst tests of pipe specimens that contain longitudinal surface cracks is established based on the open literature, which is employed to train the GPR model and evaluate its performance. It is shown that GPR is highly effective in improving the accuracy of the CorLAS model predictions. The improvement is further shown to have a marked effect on the time-dependent probability of burst of pipelines containing growing surface cracks through two hypothetical pipeline examples: when employing the CorLAS model, the probabilities of burst are significantly higher, exceeding those obtained using the improved CorLAS model by more than one order of magnitude.

Classification of failure modes of pipelines containing longitudinal surface cracks using mechanics-based and machine learning models

This paper applies the mechanics-based approach and five machine learning algorithms to classify the failure mode (leak or rupture) of steel oil and gas pipelines containing longitudinally oriented surface cracks. The mechanics-based approach compares the nominal hoop stress remote from the surface crack at failure and the remote nominal hoop stress to cause unstable longitudinal propagation of the through-wall crack to predict the failure mode. The employed machine learning algorithms consist of three single learning algorithms, namely naïve Bayes, support vector machine and decision tree; and two ensemble learning algorithms, namely random forest and gradient boosting. The classification accuracy of the mechanics-based approach and machine learning algorithms is evaluated based on 250 full-scale burst tests of pipe specimens collected from the open literature. The analysis results reveal that the mechanics-based approach leads to highly biased classifications: many leaks erroneously classified as ruptures. The machine learning algorithms lead to markedly improved accuracy. The random forest and gradient boosting models result in the classification accuracy of over 95% for ruptures and leaks, with the accuracy of the decision tree and support vector machine models somewhat lower. This study demonstrates the value of employing machine learning models to improve the integrity management practice of oil and gas pipelines.

Effect of initial solidification characteristics on longitudinal crack formation in thin slab caster

ABSTRACT Longitudinal surface crack has been one of the serious problems of thin slab casting (TSC). The present study was undertaken to comprehend the characteristics of longitudinal facial crack (LFC) and minimise the same in hot rolled low carbon steel sheets produced through thin slab casting and rolling (TSCR). Macro-structural characteristics of solidified shell has been investigated and correlated with plant observations. The centre and edges of funnel region showed maximum propensity of LFCs. TSC solidification is initiated with fine columnar dendrites which gradually coarsened towards the slab centre. This has been attributed to the characteristics of liquid steel flow instability in the upper mould region. Meniscus flow instability may lead to non-uniform heat transfer besides mould flux entrapment in the solidifying shell. Examination revealed that the mould hot faces ware covered with brass containing voids and cracks. Brass formation was found to be the most prominent around the meniscus of the wide hot face, which gradually faded out towards the mould exit. Tramp elements (zinc and lead) in liquid steel were found to be responsible for harmful brass formation during casting. The present findings were implemented in plant operation to minimise its concentration.

A new method to determine elasto-plastic J-integral for steel pipes with longitudinal semi-elliptical surface cracks

Steel pipes have been widely used in the water and petroleum industries where crack-like flaws have long been recognised as a major cause of their failures. This paper intends to develop a new method for determining the elasto-plastic J-integral of cracked pipes made of all types of steel. A solution to elasto-plastic J-integrals is derived for external longitudinal semi-elliptical surface cracks in pressurised steel pipes. A meshing technique is developed in the detailed three-dimensional finite element analysis that incorporates the mixed types of quadratic tetrahedron and hexahedron elements. The developed method is verified with detailed finite element results. It is found in the paper that the general trend of normalised fully plastic J-integral changes with the change of crack depths and aspect ratios, and the magnitude of normalised fully plastic J-integrals can be significantly affected by material properties and pipe dimensions. The developed method can provide a useful tool for the failure assessment of pressurized steel pipes, allowing for a wide range of pipe dimensions, crack geometries and material properties.

Simulation of longitudinal surface defect in steel continuous casting

Cracks and depressions in steel continuous casting often start during initial solidification in the mold and evolve later in the process. To understand the mechanism(s) of formation of these defects, a finite-element model of transient thermal-mechanical behavior during steel solidification has been developed and applied to simulate longitudinal depressions. The thermal model features a detailed analysis of heat transfer across the interfacial gap, which is fully coupled with a thermal-mechanical elastic-viscoplastic finite-element model that calculates the evolving size and shape of surface depressions. The results are compared with analysis of a sample with a longitudinal surface depression and crack from a commercial continuously cast slab. In this scenario, a thermal event caused a local drop in heat transfer, followed by frictional tension on the domain due to slab shrinkage, led to shell necking and the formation of a localized U-shaped depression. Finally, a new visualization methodology has been developed to better reveal defect formation in real time, such as longitudinal depressions, which persist as they move down the mold at the casting speed.

A review of chamfer technology in continuous casting process

Chamfer technology, with funnel-shaped curved surface on the narrow side of the mold, is a novel technology that can effectively improve the quality of continuously cast products. This study reviews the available literature on the theoretical and applied research in chamfer technology to provide an in depth analysis of the employed approaches and the obtained results. According to theoretical research results, combined with the working conditions of slab caster, patented technologies and related equipment were developed. The research has broken the technology bottlenecks of the industrial application, while ensuring a long-life operation of the chamfer mold. In recent years, chamfer technology, which is used in slab casting processes, can prevent transverse corner cracks to form at the slab surface of micro-alloyed steel completely. Chamfer technology, which is used in thin slab casting processes, can reduce the occurrence of longitudinal surface crack of the slabs and straight edge seam defects of the rolled strip. In addition, chamfer technology, which is used in a continuous casting bloom, can reduce the risk of internal cracks and avoid the off-corner cracks occurring in as-cast bloom.

Hot Workability of a Spark Plasma Sintered Intermetallic Iron Aluminide Alloy Above and Below the Order-disorder Transition Temperature

The Fe-25Al-1.5Ta alloy has proved to be qualified for structural applications at and above 600°C due to a superior creep resistance to its binary counterpart. The creep resistance of the Fe-25Al-1.5Ta alloy at 650°C surpasses that of the P92 martensitic-ferritic steel, which is one of the most developed creep resistant alloys for steam turbine applications. From the viewpoint of cost-effectiveness in real-scale forgings, it is important to safely deform the material at as low as possible temperatures. Based on Thermo-Calc computations, the Fe-25Al-1.5Ta alloy shows a B2-to-A2 order-disorder transition at around 860ºC. This paper investigates the hot compression behavior and microstructural evolution of a Fe-25Al-1.5Ta alloy deformed in the disordered A2 (900-1100ºC) and ordered B2 (800-850ºC) regimes. Effects of ordering on plastic deformation, energy dissipation efficiency and instability parameters are identified using the concept of processing maps, and the underlying deformation mechanisms are characterized using scanning electron microscopy and electron back-scattered diffraction. The samples deformed in the A2 disorder region showed no flow instability for the deformation conditions tested, while the specimens deformed in the ordered B2 region revealed a region of flow instability located at 900ºC/1s-1. The observed flow instability region manifests itself in a longitudinal surface crack formed in the samples deformed at 900ºC/1s-1. The change of activation energy of hot deformation and efficiency of energy dissipation are discussed based on the ordering effect and movement of super-dislocations in the B2 regime. The current study identifies processing parameters to safely deform the Fe-25Al-1.5Ta alloy at lower temperatures of 800ºC and at strain rates below 1s-1.

Optimization of machine parameters to improve the quality of continuously cast slab

Industries in general target to produce high quality tonnage steel at a lowest possible cost. On this count the technology of continuous casting (con-cast) is very promising. However, one of the serious quality problems that may arise in the con-cast slabs is the formation of cracks, on the surface and/or in the interior of the slabs. The cracks appearing on the surface get oxidized in air and therefore cannot be re-welded during rolling of the slab resulting into defective rolled plates. In this work, carried out at Bhilai Steel plant Bhilai (MP), machine parameters responsible for the formation of longitudinal surface cracks are studied. The objective of doing this exercise is to optimize these parameters to reduce the frequency of crack formation. Mould taper, age of the mould, pinch roll conditions and rigidity in fixing the pinch rolls are some of the parameters studied. The study concludes that, the existing level of 1.3% of mould taper is excessive and the optimum value is found to be 1.13%. From this study, the history of the machine after maintenance is optimized to 140 heats and the rigidity maintenance period for pinch rolls to 80 heats. It is proposed that the roll gap of the pinch rolls should be checked and rectified for every 65 heats.

Application of chamfered narrow face mold technology to reduce longitudinal surface crack defects of hyperperitectic steel generated during flexible thin slab casting process

Mold wear can occur at the interface between slab and mold when slab moves downward through the mold during casting. Longitudinal surface cracks in continuous casting process of slab are closely related with inadequate amount of narrow face taper. Optimizing inner structure and adjusting taper of narrow face mold were helpful in improving contact situation between mold and solidified shell to achieve uniform solidifying shell and to avoid forming longitudinal surface cracks. A chamfered narrow face mold for flexible thin slab casting process was developed and implemented successfully in industrial production process. The relationship between wear profiles of narrow face mold and occurrence of longitudinal surface cracks has been researched. The results show that a chamfered narrow face mold can effectively compensate the narrow side shell shrinkage along slab width direction to reduce the occurrence of longitudinal surface cracks.

Stress intensity factor of semi-elliptical surface crack in a thermo-mechanically loaded cylinder with hoop wrapped FGM layer

This article focused on three dimensional fracture analyses of a pressurized cylinder with hoop wrapped functionally graded material layer, which contains a longitudinal semielliptical internal surface crack and subjected to temperature gradient. The effect of non-uniform coefficient of thermal expansion and the hoop wrapped layer thickness on the distribution of the stress intensity factor along the crack front for different crack profiles is also studied. The results, which are normalized for the advantage of non-dimensional analysis show that the hoop wrapped functionally graded material cylinder, can significantly influence on the amount and distribution of stress intensity factors along the crack front.

Comparison of human cell engraftment and differentiation abilities among strains of immunodeficient mice with different genetic backgrounds

In a 3-part series of papers, of which this paper is Part II, we investigate the applicability of the fully quadratic hexa-27 element (see Part I) to four problems of interest to the pressure vessels and piping community: (1) The solid-element-based analysis of a welded pipe elbow with a longitudinal surface crack in one of its weldments. (2) The solid-element-based analysis of the elastic bending of a simple cantilever beam, of which the exact solution is known. (3) The tetra-04 element-based analysis of the deformation of a wrench. (4) The shell-element-based analysis of a barrel vault. In this paper, we develop a two-step method first to estimate the uncertainty of a converging series of finite-element-mesh-density-parametric solutions using a 4-parameter logistic function, and then to extrapolate the results of a specific quantity (e.g., a stress component) to an extremely fine mesh density approaching the infinite degrees of freedom. The estimated parameter of the upper bound of the logistic function serves as the “best” estimate of the chosen quantity such as a specific stress component. Using a super-parametric approach, as described in Part III of this series, we show that the hexa-27 element is superior to tetra-04, hexa-08, and hexa-20.

FEM Analyses of the Radiation in Heating Forging Furnace

The purpose of this study was to identify possible causes of longitudinal surface cracks found during early stages of ingot breakdown. However, these cracks need not necessarily form during forging or as a result of poor quality of the surface in metallurgical terms. Under certain conditions, they may occur even as the ingot is being heated in the furnace to the forging temperature. The cracks probably form within a few minutes after placing the ingot in the furnace as a result of the temperature gradient, which is most severe on the ingot surface. A numerical model was created to represent the case of three ingots in a furnace. Upon casting, the ingots are cooled down to no more than 600°C and then placed in a furnace at 1,100 - 1,200°C. Numerical simulations were used to analyse their internal stresses and temperatures.

Instability analysis of pressurized pipes with longitudinal surface cracks

Recently three of the authors of this paper presented analytical solutions for reference loads of plate geometries with semi-elliptical surface cracks subjected to tension, bending, combined tension-bending and biaxial tension. These solutions were shown to provide more accurate crack driving force estimates than the conventional limit load solutions available in the literature, and the method behind them allowed for a wider application range. Within the present paper a methodology for the fracture analysis of thick-wall pressurized pipes using the R6 assessment method and considering both, biaxial and combined tension-bending loading is developed and validated. The analyses are carried out analytically, and the comparison between the predicted critical loads and experimental burst test failure loads shows satisfying agreement, this way demonstrating the potential of the proposed method.

Research of edge defect formation in plate rolling by finite element method

For hot-rolled flat products major losses of steel are due to surface defects, which are caused by the quality of an incoming continuous cast slab. Each typical surface defect of a continuous cast slab if not detected and eliminated transforms into a surface defect of a rolled plate. One of the most critical problems of hot rolled plates is connected with the longitudinal surface cracks located at the distance up to 80mm from the edges. This study covers the research and simulation of transformation of surface cracks into the edge defects of the hot-rolled plate by finite element method. It is shown that the mechanism of crack movement during plate rolling comes from inclination of front and tail faces of slab due to temperature gradient between top and bottom surfaces. To reduce the possibility of corner crack formation it is proposed to increase the temperature of the edge zone of the slab due to the rounding of edges or by induction heating. The results of the study will be useful for material saving during plate rolling.

Uncertainty Quantification of Stresses in a Cracked Pipe Elbow Weldment Using a Logistic Function Fit, a Nonlinear Least Square Algorithm, and a Super-parametric Method

In a 3-part series of papers, of which this paper is Part II, we investigate the applicability of the fully quadratic hexa-27 element (see Part I) to four problems of interest to the pressure vessels and piping community: (1) The solid-element-based analysis of a welded pipe elbow with a longitudinal surface crack in one of its weldments. (2) The solid-element-based analysis of the elastic bending of a simple cantilever beam, of which the exact solution is known. (3) The tetra-04 element-based analysis of the deformation of a wrench. (4) The shell-element-based analysis of a barrel vault. In this paper, we develop a two-step method first to estimate the uncertainty of a converging series of finite-element-mesh-density-parametric solutions using a 4-parameter logistic function, and then to extrapolate the results of a specific quantity (e.g., a stress component) to an extremely fine mesh density approaching the infinite degrees of freedom. The estimated parameter of the upper bound of the logistic function serves as the “best” estimate of the chosen quantity such as a specific stress component. Using a super-parametric approach, as described in Part III of this series, we show that the hexa-27 element is superior to tetra-04, hexa-08, and hexa-20.

Finite Element Analysis of a Pipe Elbow Weldment Creep-Fracture Problem Using an Extremely Accurate 27-Node Tri-Quadratic Shell and Solid Element Formulation

In most finite-element-analysis codes, accuracy is achieved through the use of the hexahedron hexa-20 elements (a node at each of the 8 corners and 12 edges of a brick element). Unfortunately, without an additional node in the center of each of the element's 6 faces, nor in the center of the hexa, the hexa-20 elements are not fully quadratic such that its truncation error remains at h(0)^2, the same as the error of a hexa-8 element formulation. The symbol (^) denotes raising the preceding quantity to a power of the subsequent quantity. To achieve an accuracy with a truncation error of h(0)^3, we need the fully-quadratic hexa-27 formulation. A competitor of the hexa-27 element in the early days was the so-called serendipity cubic hexa-32 solid elements (see Ahmad, Irons, and Zienkiewicz, Int. J. Numer. Methods in Eng., 2:419-451 (1970)). The hexa-32 elements, unfortunately, also suffer from the same lack of accuracy syndrome as the hexa20's. In this paper, we investigate the applicability of the fully quadratic hexa-27 elements to three problems of interest to the pressure vessels and piping community: (1) The shell-element-based analysis of a barrel vault. (2) The solid-element-based analysis of a welded pipe elbow with a longitudinal surface crack in one of its weldments. (3) The solid-element-based analysis of the elastic bending of a simple cantilever beam. Significance of the highly accurate hexa-27 formulation and a comparison of its results with similar solutions using ABAQUS hexa-8, and hexa-20 elements, are presented and discussed.

Stress Intensity Factors of External Surface Cracks in a Vertical Cylindrical Steel Tank

The present research deals with external surface cracks in vertical steel tanks. Such defects occur very often. The analytical expression of stress intensity factor (SIF) is need, for prediction residual life of tanks with cracks. There are solutions for the SIF of surface cracks in hollow cylinders. However, all of them can not be used for steel tank. The main purpose of this research is to present analytical expression SIF for longitudinal external surface cracks in vertical cylindrical steel tanks. The finite element method is used for the SIF calculation of the crack in the tank. All SIF values are determined based on the submodel technique. Firstly, the whole shell model of tank without defects is generated. Real service load and tank size take into account. After that the solid submodel with crack is generated. A wide range of cracks geometries and filling level of oil are researched. The finite element models have a good agreement with wellknown analytical decisions. All SIF results are expressed with polynomial functions, that the fracture criteria can be used easily in the estimation of the residual life of tanks. SIF polynomial function should used in Paris-Erdogan equation. Results of this work could be useful for engineers in oil storage field.

Gas dynamics and heat transfer inside a solid propellant crack during ignition transient

To study the gas dynamic and heat transfer phenomena inside a single isolated longitudinal solid propellant surface crack, two 3-D geometric models with different crack shapes were constructed. Concerning the influence of propagation of jet from the igniter on the flame spreading phenomena in the crack, flow region around the opening of the crack was also included in the above geometric models. A theoretical framework was then adopted to model the conjugate heat transfer in the combustion channel and the crack cavity. Numerical simulation results indicate that the ignition shock wave can spread into the crack cavity. Extremely high overpressure and pressurization rate were observed along the crack front. It is possible that the crack may propagate before the flame front reaches it. An ignited region located at the crack front near to the channel surface in downstream direction was generated long before the flame front reached the crack opening in both models.

Stress Intensity Factors for High Aspect Ratio Semi-Elliptical External Surface Cracks in Cylindrical Vessels

External surface cracks can occur in cylindrical vessels due to damage and propagate in the manufacturing process and during service life. Most of research focuses on stress intensity factors for surface cracks with low aspect ratios, i.e., a/c ≤1.0. Situation may well arise where the aspect ratio of cracks is larger than one. An external longitudinal surface crack is assumed to be subjected to different types of hoop stress distributions acting perpendicular to the crack faces. The stress intensity factors (SIFs) along the crack front were determined through the three-dimensional finite element method. Then these results are used to compute approximate values of SIFs in the case of complex loadings by employing both the superposition principle and the power series expansions of the actual hoop stresses. It is found that the maximum stress intensity factor for external surface cracks with high aspect ratio occurs at different point to that with low aspect ratio.