Semi-elliptical Surface Crack Research Articles

Surface fatigue crack growth under variable amplitude loading

Plasticity-induced fatigue crack closure plays an important role in fatigue crack growth. Under fatigue loading with variable amplitude, however, the influence of fatigue crack closure along the crack front in three-dimensional cracked bodies is difficult to analyze. Here we develop a three-dimensional strip yield model to calculate crack opening stresses of surface fatigue cracks under variable amplitude loading. An equivalent thickness concept is incorporated to quantify the thickness effect for points on the surface crack front with a series of straight-through cracks. Using this equivalent thickness based strip yield model and fatigue crack growth rate curves obtained with standard through the thickness cracked specimen under constant amplitude loading, fatigue growth of surface cracks under variable amplitude loading are able to be predicted. We used this method to calculate fatigue crack growth of five semi-elliptical surface cracks and found that the ratios of the predicted life to the experimental life ranged from 0.86 to 1.01. The average value of the results was 0.93 and the standard deviation was 0.055. Good agreements are found between the predicted and experimental life for semi-elliptical surface cracks under variable amplitude loading. This method paves an avenue for fatigue crack growth life prediction of practical structures using materials properties obtained in laboratory according to material test standards.

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.

Weight Function Method for Stress Intensity Factors of Semi-Elliptical Surface Cracks on Functionally Graded Plates Subjected to Non-Uniform Stresses.

In this paper, a weight function method based on the first four terms of a Taylor’s series expansion is proposed to determine the stress intensity factors of functionally graded plates with semi-elliptical surface cracks. Cracked surfaces that are subjected to constant, linear, parabolic and cubic stress fields are considered. The weight functions for the surface, deepest and general points on the crack faces of long and deep cracked functionally graded plates are derived, which has never been done before in the literature. The accuracy of the method in this study is then validated by comparing the results with those of finite element modeling. The numerical results indicate that the derived weight functions are highly accurate and robust enough to predict the stress intensity factors for cracked functionally graded plates subjected to non-uniform stress distributions. The weight function method is therefore a time-saving technique and suitable for handling non-uniform stress fields.

Fatigue crack growth and interaction of bridge wire with multiple surface cracks

Multiple flaws are widely observed on the surface of corroded steel bridge wires, and the interaction between flaws can increase the stress intensity factor (SIF) to drive cracking. But the flaw combination criterion provided for plates in codes is not straight-ford to evaluate interacting surface cracks in steel wires. A model containing twin co-planar semi-elliptical surface cracks throughout a circular cross-section is thus established to investigate crack propagation of steel wires under axially tension in terms of initial crack shape, size and relative spacing. A new combination rule using a virtually effective crack is then proposed for interacting co-planar surface cracks based on the SIF amplification effect, which is quantified as a function of the initial crack aspect ratio, crack depth and relative spacing. The shape development of twin surface cracks is also simulated by numerical analysis based on the Paris law. The results show that the multiple-crack growth is sensitive to both the initial crack size and relative spacing, and the interaction should be considered for the neighbouring cracks when their relative spacing is within the crack depth range (s ≤ a). The comparison of the predicted fatigue lives between twin cracks and corresponding effective crack confirms the feasibility of the presented combination criteria.

Estimation of the resource of pipes of the main circulation pipeline of a NPP by the criterion of crack resistance with account of operational degradation of properties

The results of an experimental determination of the characteristics of cyclic crack growth resistance of specimens made of stainless steel 08Kh18N12T are given. Compact samples 20 mm thick cut from Du 550–30 pipes of the primary circuit of the main circulation pipeline (MCР) were studied under eccentric tension in the initial state and after 100 thousand hours of operation in the conditions of the Novo-Voronezh NPP. Using a new engineering approach based on measuring the specimen tightening and constructing a cyclic elastoplastic fracture diagram for a specimen with a crack in the coordinates “tightening – the stress intensity factor, calculated taking into account the plasticity correction”, a calculated life estimation is given for the pipe of Du 550–30 MCР NPP, made of 08Kh18N12T steel, according to the criterion of cyclic crack growth resistance, taking into account operational degradation of properties caused by the running time of 100 thousand hours. The life (survivability), expressed in a number of cycles of change in internal pressure, for a pipe with the most dangerous defects (surface semi-elliptical crack and combined defect “pitting + crack”) is determined. An analysis of the influence of the level of initial damage and the type of hazardous defect on the survivability of the pipes is made. The results can be used to analyze damage and predict the life of pipes made of ductile metals and alloys, in cooling systems of reactors, as well as main pipelines of hydrocarbon transport.

Reference stress based J estimation equations applicable to cracked cylinders with a Wide range of radius-to thickness ratios: Part I- axial surface cracks

This paper compares systematic finite element results with reference stress based J estimation equations for pressurised axial surface and through-wall cracked cylinders with the radius-to-thickness ratio ranging from five up to seventy. It is found that, for the reference stress, the use of the Tresca limit pressure given in R6 provides satisfactory results for all cases considered; for both inner and external axial surface cracks; for both semi-elliptical and constant-depth axial surface cracks. These results are obtained for high strain hardening materials such as Type 304 stainless steel.

In-plane and out-of-plane constraint characterization of different constraint parameters for semi-elliptical surface cracks in pipes

Six constraint parameters (namely T-stress, Q, h, Tz, Ad and Ap) have been numerically calculated for semi-elliptical surface cracks in pipes so as to comparatively investigate their capability, applicability and ease of use for characterizing in-plane and out-of-plane constraints. The results show that the parameters T-stress and Tz can separately characterize in-plane and out-of-plane constraints. The crack depth a/t and aspect ratio a/c of semi-elliptical cracks can influence both in-plane and out-of-plane constraints. With decreasing a/c (increasing crack length) and increasing crack depth a/t, both in-plane and out-of-plane constraints increase. The parameters Q and stress triaxiality factor h are equivalent, and they can capture both in-plane and out-of-plane constraints for the cracks in pipes. But they may overestimate constraint for longer cracks with lower a/c. The parameters Ad and Ap based on crack-tip plastic deformation (or strain) can equivalently characterize both in-plane and out-of-plane constraints for the cracks in pipes. The advantages and limitations of different constraint parameters in terms of their engineering applications are analyzed and appropriate parameters are recommended.

Fracture mechanics testing and crack propagation modelling in polypropylene pipes

PurposeThe main aim of this study is to determine the fracture toughness and accordingly to predict the fracture initiation, crack propagation and mode of crack extension accurately in polypropylene subsea pipes subjected to internal pressure.Design/methodology/approachTensile test was performed following the ISO 527–1 standard. An elastic-plastic constitutive model was developed based on the tensile test results, and it is implemented in the FEA model to describe the constitutive behaviour of the polypropylene material. Three-point bend tests with linear-elastic fracture mechanics (LEFM) approach were conducted following ISO-13586 standard, from which the average fracture toughness of the polypropylene pipe material in crack-opening mode was found as KIc = 3.3 MPa√m. A numerical model of the experiments is developed based on the extended finite element method (XFEM), which showed markedly good agreement with the experimental results.FindingsThe validated XFEM modelling approach is utilised to illustrate its capabilities in predicting fracture initiation and crack propagation in a polypropylene subsea pipe subjected to an internal pressure containing a semi-elliptical surface crack, which agrees well with existing analytical solutions. The XFEM model is capable of predicting the crack initiation and propagation in the polypropylene pipe up to the event of leakage.Originality/valueThe methodology proposed herein can be utilised to assess the structural integrity and resistance to fracture of subsea plastic pipes subjected to operational loads (e.g. internal pressure).

Stress Intensity Factors for Semi-elliptic Surface Cracks in Compression Coil Springs with Orthotropic Anisotropy and Those with Cylindrical Anisotropy

Stress Intensity Factors for Semi-elliptic Surface Cracks in Compression Coil Springs with Orthotropic Anisotropy and Those with Cylindrical Anisotropy

Modelling the criticality of silicon nitride surface imperfections under rolling and sliding contact

Ceramic rolling elements of hybrid bearings may initially include surface imperfections. In order to provide reliable operation of a bearing, the criticality of such imperfections under rolling contact fatigue is examined by defining them as Star features: intersecting semi-elliptical surface cracks. Parametric study is conducted using Finite Element Method and discussed with help of previously published experimental observations. The effects of the Star feature morphology and configuration, contact pressure and crack face friction are investigated in terms of stress intensity factors. Possible crack propagation scenarios are explained in the present study.

Calculation of SIFs for semi-elliptical surface cracks in U-shaped bellows expansion joints of Es-Salam research reactor vessel

Expansion joint (EJ) of Es-Salam research reactor (RR) vessel as an element for deformation compensating is subjected to the axial load due to thermal expansion and often work under hostile environment (corrosion, radioactivity, etc.). As some local regions with high elastoplastic strain of bellows type EJ undergo cyclic loadings caused by startup and shutdown and changeable operating conditions of RR, they can be ruptured by crack growth. Therefore, it is necessary to evaluate the influence of cracks to structural integrity of EJ of Es-Salam RR vessel. The fracture criterions as stress intensity factor (SIF) or J-integral (JI) are used to evaluate the resistance to fracture of the constructions with cracks. The SIF is a parameter, which is used to estimate a material resistance to brittle fracture.In the present study the problem of calculation of the SIF of semi-elliptical crack (SEC) located in the stress concentration areas of EJ installed between the lower barrel and the upper barrel of a RR pressure vessel is numerically solved by advanced global-local finite element (FE) analysis. The crack locations are considered at the intersection of the various parts that construct the EJ. The modeling details and methodology, employed in the analysis, are extensively discussed and the numerical approach is proven to be efficient for the SIFs calculation of EJ semi-elliptical cracks. The proposed methodology using software ANSYS was validated by applying the analytical methods (according ASME requirement and R6 method) and also a finite element method but using software ABAQUS. It was received that SIF data are in close agreement. It was demonstrated that applied methodology for modeling of the SIF can be used.

Calculation of the Stress Intensity Factor for a Semi-Elliptical Surface Crack on a Rotating Compressor Blade

Propagation of a semi-elliptical crack on the suction surface of a rotating blade of a gas turbine compressor is modelled and analysed. The analytical solution for the stress intensity factor (SIF) at the deepest point of the crack shows that the SIF increases with an increase in the crack depth and rotation speed, whereas the SIF decreases with an increase in the semi-elliptical aspect ratio and crack-rotation axis distance. The SIF increase rate with increasing crack depth is found to depend on the aspect ratio. Comparisons of theoretical and numerical calculation results reveal a relative error within 4%. The SIF distribution along the semi-elliptical surface crack front at different depths, positions, and aspect ratios are obtained by a numerical method.

Object-Oriented Software for Fitness-For-Service Assessment of Cracked Cylinder Based on API RP 579

Fitness-for-service assessment of a cracked component intends to evaluate its remaining strength and remaining life to support the implementation of a maintenance plan. This research aims to develop a fitness-for-service assessment software to facilitate the task. The software development process included identifying the software specifications, designing the software structure, manipulating the information from the standard for programming, the graphical user interface design, and finally verification of the software. The assessment procedure employed in this study was the third edition of the standard API RP 579. The software structure was designed using an object-oriented concept. The software can perform integrity assessment levels 1, 2, and 3 option B, leak-before-break assessment, and remaining life assessment for a cracked cylinder that has a through-thickness crack or semi-elliptical surface crack oriented in an axial or circumferential direction. The applied loads could be a nominal axial force, bending moment, and internal pressure, as well as stress profiles perpendicular to the crack plane due to mechanical, thermal, and residual stresses. Accuracy of the software was demonstrated by applying it to example problems. Some aspects of the software extensibility were conceptually discussed.

Engineering estimation method of unified constraint parameters for semi-elliptical surface cracks in plates

To develop fracture assessment methodology based on in-plane and out-of-plane unified constraint parameters for various cracked components, the engineering estimation methods and solutions of unified constraint parameters should be investigated and developed. In this work, the relationship between J-integral and crack tip opening displacement (CTOD) for semi-elliptical surface cracks with a wide range of sizes in plates under bending and tension loads have been calculated by extensive three-dimensional finite element analyses. The results show that the relationship between J-integral and CTOD along the crack fronts is approximately linear. The plastic constraint factor m relating CTOD to J-integral can be determined from the J-CTOD relation lines. The factor m solutions for the cracks in plates are obtained. Based on the m solutions, the solutions of two new unified constraint parameters Ad and Ad* can be estimated and obtained. The estimation methods of the unified constraint parameter solutions in this work may be used for other cracked components. The overall constraint levels characterized by the unified constraint parameters for the cracks in plates also are analyzed. The results show that lower constraint occurs at free surface part of a crack, and higher constraint appears at deeper part. The constraint levels increase with increasing crack depth and length. The constraint of a crack under bending load is higher than that under tension load.

Location Size and Orientation Effect of Semi elliptical Surface Crack on the Fracture of a Type 3 Composite Pressure Vessel using J integral Method

In this study, structural design and analysis of a type-3 composite over wrapped pressure vessel used in a military satellite propulsion system is presented. The aim is to design a composite tank lighter than all metal fuel tanks having the same volume. Moreover, necessary design stages have been revealed for similar composite over wrapped pressure vessels. ANSYS® is used to perform the stress-strain analysis of both metal and composite parts, to determine the optimum winding angle, tank Autofrettage and fracture characteristic for the metal liner considering the crack morphology. Tsai-Hill, Tsai-Wu and Hashin theories have been implemented to investigate the various failure modes of the composite vessel. Location, size and orientation angle of semi-elliptical surface crack has a pronounced effect on fracture characteristic of the liner. In fracture investigation J-integral method is used. It is foreseen that even in the most critical crack, the crack will not propagate and there will be no burst in the tank for proposed loading conditions. Numerical results are good agreement with the experimental results.

Kinetics of the multidirectionality of elastic-plastic fracture with allowance for anisotropy of the material properties

Survivability, service life and operational safety of the engineering structures are determined by their damage rate which is mainly regulated by the presence and development of the crack-like defects in the material. Kinetic dependences describing the development of multidirectional semi-elliptic surface cracks with allowance for the anisotropy of the material properties are proposed proceeding from experimental data and numerical solutions. The obtained results are required in studying kinematic problems in nonlinear mechanics of a continuous anisotropic medium. Refining parametric equations for elastoplastic deformation anisotropy are proposed. Functional dependences of the parameters of the kinetic diagrams of low-cycle fracture on the mechanical properties of the material are presented for a wide class of welded joints of austenite stainless cyclically stable steels (12Kh18N10T). The processes of developing inclined semi-elliptic surface cracks in the continuums of welded joints under non-linear boundary loading conditions are studied. We have carried out combined computational, experimental and numerical studies of the stress-strain state in the vicinity of the contour of stationary and growing surface semi-elliptic cracks randomly oriented in space under elastoplastic nominal cyclic loading taking into account the anisotropy of the material properties. The functional distribution of the inhomogeneity parameter of the mechanical properties of the material, which affects accumulation of the local plastic strains and direction of developing the elastoplastic fracture is obtained and presented in the form of the kinetic equation of nonlinear fracture mechanics. Comparison of the experimental results and numerical calculations of the stress-strain state along the contour of the cracks under study in nonlinear boundary loading conditions revealed a good agreement between the intensities of relative elastoplastic deformations at their surface points with allowance for the deformation anisotropy. Calculations of the elastoplastic fracture resistance of the critical elements of the equipment with allowance for considered factors of nonlinear fracture mechanics and heterogeneity of the properties can improve the accuracy of evaluation of their strength, service life and survivability.

The effect of weld residual stress on fracture toughness at the intersection of two welding lines of offshore tubular structure

Due to the spatial complexity and fabrication characteristics of offshore platforms, it is inevitable to encounter overlaps or proximity of weld lines in tubular joints. Several international standards such as American Petroleum Institute (API), American Welding Society (AWS), and American Institute of Steel Construction (AISC) regulate the minimum distance between primary weld beads; however, any logical and detailed background of this limitation has not been presented. For a non-compliant weld joint where the regulation is not met, fracture toughness calculation is a typical index to verify the structural integrity.This research consists of two parts. First, weld residual stress distributions are calculated by a 3D thermo-mechanical nonlinear Finite Element Analysis. Two crossing welds, a T-weld crossing on a butt weld, are simulated in one model. A separate tee and a butt welding simulations are also performed for a comparative purpose. Second, SIFs and J-integral values are calculated at the surface and deepest crack tip locations for four different types of semi-elliptical surface cracks. Four cracks are embedded into the weld model and the residual stress distribution from the 3D thermo-mechanical FEA are mapped to a 3D FE crack model as initial conditions. An additional axial tensile load is also imposed. SIF values are compared with those using the weighting function method for the butt weld model subject to three load cases, i.e., tensile stress only, weld residual stress only, and both of them. From the simulation, a tubular joint containing a chord girth weld intersected with weld beads of brace is found to show lower the SIF values than that having only a girth weld on chord.

Calculation of the Stress Intensity Factor for a Semi-Elliptical Surface Crack on a Rotating Compressor Blade

Calculation of the Stress Intensity Factor for a Semi-Elliptical Surface Crack on a Rotating Compressor Blade

Structural fatigue investigation of transverse surface crack growth in rail steels and thermite welds subjected to in-plane and out-of-plane loading

This study investigates the structural fatigue crack growth behavior of observed transverse surface cracks in rail steel and thermite weld subjected to in-plane and out-of-plane loading. Stress Intensity Factor (SIFs) solutions were derived from finite element model analysis for transverse surface cracks located at five different crack initiation locations in the cross-section of rail steel and thermite weld. The SIFs were presented by the conventional geometry factor Y and weld magnification factor Mk solutions for rail steel and thermite weld. Fracture toughness and fatigue crack growth rate tests were conducted to determine the mechanical properties of thermite weld materials and are compared with reported data for rail steels (R260 and R350HT). Fatigue crack growth prediction algorithm was developed using the coupled root mean square integration approach and was compared with the conventional uncoupled two-degree of freedom approach for semi-elliptical surface crack growth in the depth and surface directions respectively. The fatigue crack growth algorithm and propagation life prediction provide valuable observations on the crack shape evolution. It proves that out-of-plane bending loading is a more significant failure mode as compared to in-plane bending loading for thermite weld joints at curve tracks. Fatigue crack propagation results provide fatigue stress range-life curves for weld-toe surface cracks initiated at the head-to-web region of thermite welded joint structure.

Parametric equation to predict the SIF of cracked tubular T/Y-joints

Accurate prediction of the Stress Intensity Factors (SIFs) along the crack front plays an important role in the fatigue life estimation of steel structures. As far as the authors know, all of the SIF parametric equations presented in literature ignore the influence of the crack propagation angle in their equations. Therefore, this paper focuses on deducing a new parametric SIF equation for circular hollow section T/Y-joint with semi-elliptical surface crack, with the consideration of the effect of the crack propagation angle (θcr) on the stresses along the crack front for the first time. The location of the hot spot stress along the weld toe of T/Y-joints under axial loading is firstly determined. Then, a robust finite element (FE) mesh generator is created, calibrated and employed in analysis procedures of 21,600 tubular T/Y-joints. Meanwhile, nonlinear regression analyses are carried out to develop SIF parametric equation of welded tubular T/Y-joints. The proposed parametric equation is found to agree well with the FE results and the experimental results presented in literatures. Additionally, it gives more accurate prediction of the SIF than the existing parametric equations. The error analysis of the proposed equation proves that 90% of the results have errors less than15% and 99.7% of the results have errors <30%.