External Crack Research Articles

External Stress Corrosion Cracking Risk Factors of High Grade Pipeline Steel

The influence weights of various sensitive factors, such as steel grade, applied potential, temperature and soil environment, on stress corrosion cracking behavior of high-grade pipeline steel were studied by means of orthogonal test at mixed level and slow strain rate test. The stress corrosion behavior of X80 pipeline steel in near neutral and high pH simulated soil solution under overprotection potential was observed by SEM. The results showed that the applied potential has the greatest weight on stress corrosion index of ISSRT, followed by temperature, steel grade and soil environment. X80 pipeline steel exhibits high stress corrosion sensitivity in near-neutral and high pH simulated soil solutions at applied potential of -1500mV, and its SCC mechanism is hydrogen embrittlement (HE). This study can provide guidance for practical engineering application of pipeline steel.

Weight function for an external circumferential semielliptical crack in a cylinder

AbstractIn this paper, the weight function was extracted at the deepest point of a semielliptical circumferential crack. The crack is assumed to exist on the outer surface of the cylinder. For this purpose, the three‐dimensional finite element method was accomplished to specify two reference loads, which are indispensable for determining the weight function. The verification study confirms the accuracy of the derived weight function under prescribed mechanical loading on the crack surfaces. There is consistency among the solution results compared with those in the literature. The second part describes the application of the weight function for the thermal boundary conditions. Steady‐state thermal stress intensity factors are demonstrated using the weight function and presented as a closed‐form solution. The results were compared with the finite element data on the special case of thermal loading, and good agreement is obtained.

Influence of non-ferrous metals on the structural quality of the high-alloy mild steel 20X2H4A billets received by centrifugal electroslag remelting

The problem of the formation of cracks and the conditions of their origin in the cast of metal products have been widely studied in world practice, but the real mechanism of this process is not yet clear enough that it could be applied in production. Hot cracks – one of the most common and the most detrimental defects of steel castings, leading to a significant increase in the cost of products. The formation of hot cracks in steel castings depends on many factors, including the chemical composition of steel, the presence of non-metallic inclusions and gases, conditions of deoxidation and solidification of steel, etc. Therefore, special attention in the real conditions of production should be given to the metallurgical side of the issue. There is no quantitative measure of steel purity in the books of steel, which is divided into chemical, defined by the chemical analysis, and metallographic, defined by the content of non-metallic inclusions. The effect of the general purity of steel on the characteristics of destruction is studied sufficiently, but the data on the individual impact of impurities is very incomplete.In many respects, pure steel has a clear advantage over steel with harmful impurities. This applies to all mechanical properties associated with destruction or processes occurring on the edge of grains. The cardinal way of obtaining cleaner steel, including the reduction of impurities of non-ferrous metals, is the use of pure original charge mixture.The article discusses the topical problem of cracking formation after chemical-thermal treatment in the material of sun gears of the second row of quarry dump trucks BELAZ, obtained by the method of CESR rolled steel 20X2H4А, the possible impact of small concentrations of non-ferrous metals (Pb, Zn, Sn, Bi, As, Se) on the development of external and internal cracks in castings and on structural heterogeneity of steel after CESR.The obtained results allowed to draw conclusions about the impact of non-ferrous metals on the formation of cracks in castings and to establish their limited quantitative characteristics. Based on the results, measures have been proposed to minimize the impact of small amounts of non-ferrous metals on the formation of cracks in castings.

Creep fracture parameter C* solutions for axial internal and external surface cracks in pressurized cylinders

Accurate calculations of creep fracture mechanics parameter C* is very important for life prediction and assessment of high-temperature cracked components. The current two engineering estimation approaches (EPRI and reference stress approaches) for C* can seriously overestimate or sometimes underestimate C* of cracked components, thus it is desirable to develop accurate C* solutions. In this work, based on extensive three-dimensional (3D) finite element analyses, the C* solutions for axial internal and external surface cracks with a wide range of sizes in pressurized cylinders with different geometry sizes are investigated and developed. The creep influence function Hc for calculating C* along the crack fronts has been obtained and fitted into equations, and the accuracy of the equations has been verified. The correlation equations between C* and creep hardening exponent n are firstly developed which can be conveniently used to estimate C* for materials with different n values. The effects of crack sizes and cylinder geometries on the C* distributions along the crack fronts also are analyzed.

Anchorage of naturally corroded, plain reinforcement bars in flexural members

Reinforced concrete structures are often damaged by corrosion, which affects the interaction between reinforcement bars and concrete. Earlier studies mostly applied artificial corrosion to test the bond between deformed bars and concrete. However, there is a lack of knowledge on the effects of natural corrosion on plain bars. In this paper, 20 beams with naturally corroded plain bars and varying amount of damage were taken from an 80-year-old bridge and tested in three-point bending. All but three of the specimens anchored the yield force of the bars after the opening of one or two major bending cracks. At large deflections, the load-carrying mechanism changed from beam to arch action. Eventually, end-slip of the reinforcement bars was observed. The bars were extracted, cleaned, three-dimensionally scanned, and tested in tension. The average bond strength in the unyielded zone was found to be equal to 7.39 MPa, with a standard deviation of 3.33 MPa. The casting position was identified as an important factor: when uncorroded, bottom-cast bars had a higher bond strength than that of top-cast bars. However, they were more prone to splitting cracks and, consequently, loss of bond strength for small corrosion levels. Top-cast bars had increasing bond strength with increasing corrosion levels, owing to the absence of external cracks. These differences were likely related to a denser concrete surrounding the bottom-cast bars. The remaining bond capacity in the yielded zones was evaluated to be approximately 1.0 MPa.

Application Scope of Limit Load Criterion for Ductile Material Pipes With Circumferentially External Cracks

Abstract Bending stress at plastic collapse for a circumferentially cracked pipe is predicted by limit load criterion provided by the Appendix C of the ASME Code Section XI. The equation of the Appendix C is applicable for pipes with both external and internal surface cracks. On the other hand, the authors have developed a more precise equation taking into account the pipe mean radii at noncracked area and at cracked ligament area. From the comparison of Appendix C equation and the new equation, the plastic collapse stress estimated by the Appendix C equation gives about 20% less conservative bending capacity prediction for external cracked pipes with large crack angle and small Rm/t, where Rm is the pipe mean radius and t is the pipe wall thickness. This paper discusses the limitation scope to use the limit load criterion of the Appendix C equation.

The Characteristics of Selected Triaxiality Measures of the Stresses for a C(T) Specimen Dominated by the Plane Strain State

The paper presents a comprehensive analysis of the stress field and selected triaxiality parameters near the crack tip for C(T) specimen dominated by the plane strain state using the finite element method. It includes some theoretical information about elastic-plastic fracture mechanics, the basics of the FEM modeling and presentation of the numerical results. The FEM analysis includes calculations with large strain assumptions. The influence of the external load and crack length is discussed. Additional elements of the paper are a qualitative assessment of the size of plastic zones and the crack tip opening displacement.

Multi-material topology optimization for crack problems based on eXtended isogeometric analysis

This paper proposes a novel topology optimization method generating multiple materials for external linear plane crack structures based on the combination of IsoGeometric Analysis (IGA) and eXtended Finite Element Method (X-FEM). A so-called eXtended IsoGeometric Analysis (X-IGA) is derived for a mechanical description of a strong discontinuity state's continuous boundaries through the inherited special properties of X-FEM. In X-IGA, control points and patches play the same role with nodes and sub-domains in the finite element method. While being similar to X-FEM, enrichment functions are added to finite element approximation without any mesh generation. The geometry of structures based on basic functions of Non-Uniform Rational B-Splines (NURBS) provides accurate and reliable results. Moreover, the basis function to define the geometry becomes a systematic p-refinement to control the field approximation order without altering the geometry or its parameterization. The accuracy of analytical solutions of X-IGA for the crack problem, which is superior to a conventional X-FEM, guarantees the reliability of the optimal multi-material retrofitting against external cracks through using topology optimization. Topology optimization is applied to the minimal compliance design of two-dimensional plane linear cracked structures retrofitted by multiple distinct materials to prevent the propagation of the present crack pattern. The alternating active-phase algorithm with optimality criteria-based algorithms is employed to update design variables of element densities. Numerical results under different lengths, positions, and angles of given cracks verify the proposed method's efficiency and feasibility in using X-IGA compared to a conventional X-FEM.

A Review of Grain Kernel Damage: Mechanisms, Modeling, and Testing Procedures

HighlightsPublished literature on grain kernel damage during handling is reviewed.Types and sources of grain kernel damage are discussed.Factors affecting the level of grain kernel damage are outlined.Models to predict grain kernel damage and corresponding test devices are summarized.Abstract. Grain kernel damage during harvest and handling continues to be a challenge in grain postharvest operations. This damage causes physical and physiological changes to grain, which reduces the grain quality and leads to significant yield loss. During harvesting and handling, grain kernels are subject to complex loading conditions consisting of a combination of impact, shear, and compression forces. The main damage mechanisms include impact, which causes external and internal cracks or even fragmentation of the kernel; attrition, which generates fine material; jamming, which deforms and breaks kernels due to high compressive forces; and fatigue, which produces broken kernels and fine material via repeatedly applied loads. Grain kernel damage accumulates as the grain moves through harvesting and handling operations. Harvesting is the major cause of cracks and breakage, while conveying after drying produces fine material. This article provides a comprehensive review of the types of grain kernel damage, sources of grain kernel damage, factors affecting damage, predictive damage models, and the experimental methods used to assess the damage. This review shows that although there is considerable empirical data focused on kernel damage, there is a lack of generalizable mechanics-based predictive models. Mechanics-based models are desirable because they would be useful for providing guidance on designing and operating grain handling processes to minimize kernel damage and thus improve grain quality. In addition, several damage models developed for non-grain particulate materials based on fracture mechanics are reviewed. With some modifications and detailed property analysis, there is potential for adapting the models developed for inorganic materials to predict grain kernel damage. Keywords: Grain kernel damage, Grain harvesting and handling, Breakage susceptibility, Grain damage prediction.

Longitudinal fracture analysis of inhomogeneous non-linear elastic rod with internal crack in torsion

The present paper deals with longitudinal fracture analysis of an inhomogeneous non-linear elastic rod with an internal longitudinal circular cylindrical crack. The rod is loaded in torsion. The cross-section of the rod is a circle. The rod exhibits continuous material inhomogeneity in radial direction. The complementary strain energy is considered in order to derive the strain energy release rate. Since the crack is internal, the torsion moments in the crack arms which are needed to obtain the complementary strain energy can not be determined explicitly. Therefore, the rod is treated as a statically undermined structure with one internal redundant (the torsion moment in the external crack arm is taken as a redundant). The solution to the strain energy release rate, derived in the present paper, is verified by considering the balance of the energy. The longitudinal fracture behavior of the rod with internal crack is compared with that of a rod with circular notch in the external crack arm. It is found that the strain energy release rate in the rod without notch is lower than that in the rod with notch in the external crack arm.

Symptom changes and crack progression in untreated cracked teeth: One-year findings from the National Dental Practice-Based Research Network

ObjectivesThe study objective was to: (1) quantify symptom (pain) and crack changes during one year of follow-up, among teeth that had at least one visible crack at baseline but which did not receive treatment for those cracks; (2) identify any patient traits/behaviors and external tooth/crack characteristics correlated with these changes. MethodsIn this observational study, 209 National Dental Practice-Based Research Network dentists enrolled a convenience sample of 2858 subjects, each with a single, vital posterior tooth with at least one observed external crack; 1850 teeth remained untreated after one year of follow-up and were the cohort for analyses. Data were collected at the patient-, tooth-, and crack-level at baseline, one-year follow up (Y1), and interim visits. Associations between changes in symptoms and cracks were identified, as were changes in symptoms associated with baseline treatment recommendations. ResultsChanges in pain symptoms were observed in 32% of patients; decreases were twice as common as increases (23% vs. 10%). More changes were observed in cold pain than in biting pain and spontaneous pain combined; 2% had increases in biting pain and 2% in spontaneous pain. Only 6% had an increase in the number of cracks. Changes in pain symptoms were not associated with an increase in the number of cracks, but were associated with baseline treatment recommendations. Specifically, pain symptom changes (especially decreases) were more common when the tooth was recommended for treatment at baseline. ConclusionsCracked teeth that have not received treatment one year after baseline do not show meaningful progression as measured by increased symptoms or number of cracks during follow-up. Clinical SignificanceUntreated cracked teeth, most of which were recommended for monitoring at baseline and some of which were recommended for treatment but did not receive treatment, remained relatively stable for one year with little progression of cracks or symptoms.

An investigation on the circumferential surface crack growth in steel pipes subjected to fatigue bending

In the present paper, we propose an analytical method to calculate the Stress Intensity Factor (SIF) of circumferential surface cracks in steel pipes subjected to bending. In light of pipe geometry and bending load case, the analytical formula is raised by introducing new bending correction factors and new geometry correction factors on the basis of the Newman-Raju’s method. The bending correction factors are deduced based on the bending stress gradient, while the geometry correction factors are determined by parametric studies for internal surface cracks and external surface cracks respectively. Owing to a large data set requirement by the parametric studies, three-dimensional finite element (FE) models of evaluating SIFs of circumferential surface cracks are developed. The FE method is validated to ensure that it could provide accurate SIF estimations. Analytical verification is conducted which shows that the SIF evaluated by the proposed analytical method match well with the results evaluated by the recommended analytical method. Then experimental investigations of external surface crack growth in offshore steel pipe subjected to fatigue bending are implemented to further validate the analytical method of predicting surface crack growth rate. The analytical results match well with the test results and the available experimental data from literature, indicating that the analytical method can be used for practical purposes and facilitate the crack growth evaluation and residual fatigue life prediction of cracked steel pipes.

Numerical Analysis of the Effect of External Circumferential Elliptical Cracks in Transition Thickness Zone of Pressurized Pipes Using XFEM

The present work investigates the effect of the elliptical three-dimensional (3D) cracks on a pipe with thickness transition, considering internal pressure. Level sets were defined using the extended finite element method (XFEM), the stress intensity factors (SIFs) of 3D cracks were investigated and compared between straight pipes and pipes with thickness transition. The results show that the XFEM is an effective tool for modeling crack in pipes. A pressurized pipe with thickness transition is more sensitive to the feature compared to the straight pipe. Parameters of the transition zone have an influence on stress intensity factors. Quantification of the SIFs associated with cracks in the transition zone of pipes with thicknesses is performed.

Mapeamento do potencial álcali reativo das areias no Rio Grande do Sul

Existem vários processos de deterioração que interferem no desempenho e durabilidade de um concreto, dentre estes processos encontra-se a reação álcali-agregado (RAA). A RAA é um termo geral utilizado para descrever a reação química que ocorre internamente em estruturas de concreto, entre os hidróxidos alcalinos presentes no cimento e fases minerais reativas presentes nos agregados. Como resultado da reação, forma-se um gel sílico-alcalino que, na presença de umidade, absorve água, sendo capaz de expandir no interior da massa de concreto, podendo causar expansão e, consequentemente, fissuração interna e externa nas estruturas de concreto. As medidas preventivas à ocorrência da RAA são conhecidas há muito tempo e aplicadas rotineiramente apenas em estruturas de usinas hidrelétricas. Sabendo-se que qualquer estrutura sujeita a intensa umidade pode desenvolver a RAA, a prevenção é necessária também nas edificações residenciais, por exemplo, em locais onde o concreto esteja exposto à umidade, a fim de garantir estruturas mais duráveis e, portanto, mais sustentáveis, uma vez que a geração de resíduos pela necessidade de manutenção constante, ou demolição, vai na contramão dos pilares da sustentabilidade na construção. Nesse contexto, a primeira etapa do processo de prevenção da ocorrência da RAA é conhecer o potencial reativo dos agregados, pois conhecendo esse parâmetro é possível empregar materiais cimentícios adequados à inibição do desencadeamento da reação. Esta pesquisa, tem como objetivo avaliar o potencial álcali-reativo de oito amostras de areia, provenientes das jazidas fornecedoras do insumo para produção de concreto no Rio Grande do Sul. O estudo foi realizado baseando-se no método acelerado de barras de argamassa, de acordo com a ABNT NBR 15577-4 (2018) e na avaliação das características mineralógicas dos agregados, através da análise petrográfica. Os resultados apontaram um alto potencial deletério das amostras avaliadas. Verificou-se que 7 de um total de 8 amostras analisadas foram classificadas como potencialmente reativas. A variação percentual entre a amostra de maior para a de menor expansão foi de 93,44% (0,61%-0,04% respectivamente), sendo potencialmente inócua apenas a amostra de areia da cidade de Osório/RS.

Reliability assessment of cracked pipes subjected to creep-fatigue loading

The reliability analysis of defective pipes, which operate under creep-fatigue loading conditions has been studied. An austenitic stainless steel pipe containing an axially external semi-elliptical surface crack, subjected to variable internal pressure at elevated temperature has been considered as the benchmark problem. Different parameters in material behavior, geometry parameters, and operational condition have been assumed as random variables in the probabilistic assessments. The reliability of the cracked pipes have been evaluated at various operation times, using different reliability methods. Sensitivity analyses have been conducted to investigate the importance measure of the random variables involved in the problem. Moreover, the effects of reducing the scattering of the random variables on the variations of the reliability index, β, has been examined.

Distribution of Mode I Stress Intensity Factors for Single Circumferential Semi-Elliptical Crackin Thick Cylinder

In this paper, mode I stress intensity factors (SIFs) are calculated numerically by finite element software ANSYS, for a single semi-elliptical circumferential crack on a thick cylinder. The examined cracks were located either on the external or internal surface of the cylinder and subjected to two different types of loadings, tension and internal pressure, applied separately. To present results in a more comprehensive form, dimensionless analysis is used, and a wide variation limit of parameters that define the crack geometry is considered. The ratio of crack depth to crack length ranging between 0.4 to 1.2, the ratio of crack depth to cylinder wall thickness vary between 0.2, 0.5 and 0.8, and the ratio of the cylinder wall thickness to the cylinder internal radius 0.25. Based on the obtained results, distributions of SIFs found to be symmetric along the crack front. The location where the maximum SIFs on the crack front attained is strongly affected by the change of aspect ratio, and external cracks generally exhibit a higher SIFs than those of internal cracks. It is also found a significant effect for the relative depth of the crack on SIFs value, which could accelerate the fracture process.

Stress intensity factors analyses for external semi-elliptical crack for repaired gas-pipeline by composite overwrap under pressure

The purpose of this article is to present the stress intensity factors (SIF) solutions for semi-elliptic crack in pipelines under internal pressures, the stress intensity factors are calculated by the three-dimensional finite element method (FEM) for cracked pipelines and repaired pipe by composite patch. The distribution of normalized stress intensity factors (KI, KII and KIII) along the crack front for different crack lengths, crack depth, crack geometry, lap length and composite thickness was obtained by nodal calculations. Our FE results show that the crack frond is non-regular with respect to the rupture mode and presents three zones where the mixed mode (KII) is dominant from the deepest point to the point close to the surface of the pipe not the mode I cited in several studies. When the edge of the crack is close to the outer diameter, the mode III (KIII ) is dominant and this regardless of the state of the pipelines. it can also be noted that the composite repair reduces the SIF KI by 46% and the KII by 55% and the KIII by 72% near the outside diameter of the pipe in each zone of domination. However composite repair is very effective for a rectangular crack as for the semi-elliptical crack. this confirms that the elliptic crack is dominated by the mixed mode II not by mode I.

Recommended treatment of cracked teeth: Results from the National Dental Practice-Based Research Network

Statement of problemDespite the high prevalence of posterior cracked teeth, questions remain regarding the best course of action for managing these teeth. PurposeThe purpose of this clinical study was to identify and quantify the characteristics of visible cracks in posterior teeth and their association with treatment recommendations among patients in the National Dental Practice-Based Research Network. Material and methodsNetwork dentists enrolled patients with a single, vital posterior tooth with at least 1 observable external crack. Data were collected at the patient, tooth, and crack levels, including the presence and type of pain and treatment recommendations for subject teeth. Frequencies according to treatment recommendation were obtained, and odds ratios (ORs) comparing recommendations for the tooth to be restored versus monitored were calculated. Stepwise regressions were performed using generalized models to adjust for clustering; characteristics with P<.05 were retained. ResultsA total of 209 dentists enrolled 2858 patients with a posterior tooth with at least 1 crack. Mean ±standard deviation patient age was 54 ±12 years; 1813 (63%) were female, 2394 (85%) were non-Hispanic white, 2213 (77%) had some dental insurance, and 2432 (86%) had some college education. Overall, 1297 (46%) teeth caused 1 or more of the following types of pain: 1055 sensitivity to cold, 459 biting, and 367 spontaneous. A total of 1040 teeth were recommended for 1 or more treatments: restoration (n=1018; 98%), endodontics (n=29; 3%), endodontic treatment and restoration (n=20; 2%), extraction (n=2; 0.2%), and noninvasive treatment, for example, occlusal device, desensitizing (n=11; 1%). The presence of caries (OR=67.3), biting pain (OR=7.3), and evidence of a crack on radiographs (OR=5.0) were associated with over 5-fold odds of recommending restoration. Spontaneous pain was associated with nearly 3-fold odds; pain to cold, having dental insurance, a crack that was detectable with an explorer or blocked transilluminated light, or connected with a restoration were each weakly associated with increased odds of recommending a restoration (OR<2.0). ConclusionsApproximately one-third of cracked teeth were recommended for restoration. The presence of caries, biting pain, and evidence of a crack on a radiograph were strong predictors of recommending a restoration, although the evidence of a crack on a radiograph only accounted for a 3% absolute difference (4% recommended treatment versus 1% recommended monitoring).

Structural failure test of a 52.5 m wind turbine blade under combined loading

Blade is one of the basic and key components in wind turbines. To get closer to the actual loading condition of blade under complex and extreme wind conditions to more accurately analyse the structural failure characteristics of large wind turbine blades. The structural collapse test of a 52.5 m wind turbine blade under combined loading was carried out. The entire process of damage under ultimate load was recorded and analysed in detail. The joint analysis of the failure region after the blade collapse and the failure mode during the test process were performed. The research shows that the geometrical nonlinear buckling of spar cap and shear web, the delamination result from non-uniform stress distribution and further fracture of Aft panel cause catastrophic failure of the blade. The torsional moment cause oblique cracks and oblique bulges, aggravates the expansion of the internal and external composite cracks of blade, affects the final failure mode after blade collapses (critical failure mode), and make the spar cap at suction side to clockwise twist from blade tip to root. The research results lay a theoretical foundation for subsequent model establishment and simulation analysis of large wind turbine blades under combined loading.

Damage characterization of belt conveyor made of the 330Nb alloy after service in a carburizing atmosphere in a continuous heat treatment furnace

In this paper, the failure of the belt conveyor made of 330 Nb heat-resistant steel, working in a furnace for continuous heat treatment in a carburizing and oxidizing atmosphere is investigated. On the basis of microscopic tests and X-ray phase analysis, changes in the microstructure in the cross-section of the conveyor wire after 731, 1642 and 2138 cycles carburizing and oxidizing in the temperature range 30–850 °C in 160 min were determined. Moreover, tensile tests were carried out to investigate the effect of the microstructure on the mechanical properties of heat-resistant steel. The failure was due to a cross crack of wires of the conveyor belt mesh. The results show that the on the surface of the alloy, Cr2O3 chromium oxides and FeCr2O4, Mn1.5Cr1.5O4 oxides are formed. On the border of the alloy and chromium oxides, the airtight SiO2 layer is created. As a result of the actions of external forces, transverse and longitudinal cracks are formed in the oxide layer. These cracks are the ways of carbon and oxygen diffusion. In addition, in the oxidation and carburization process, degradation of the SiO2 layer is promoted by the presence of Na, which is applied in washing agents used for cleaning products before placing them on the conveyor. Microstructure of the unused wire of the conveyor belt consisted of G and H phases and NbC in austenitic matrix. Under the operating conditions of the conveyor belt, phase H dissolves in the matrix with the simultaneous increase of the G phase, and also the G phase is formed around the NbC carbides. High temperature carburizing leads to the precipitation of massive carbides M23C6 with simultaneous significant depletion of chromium into an austenitic matrix to about 13%at. Quantitative microstructural analysis showed that the there was an increase in the size and participation, of hard phases M23C6, G, NbC in the alloy microstructure. The volume fraction of precipitation increased from 3 to 24%vol. The resulting structure is characterized by low strength. The ductility of the alloy has decreased significantly.