Semi-elliptical Surface Crack Research Articles

J-Integral Evaluation of Surface Cracks in Round Bar under Mode III Loadings

Fracture mechanics approach has successfully used to characterize the existing cracks in engineering materials and structures. Tremendous number of publications on surface cracks can be found for crack behaviour which was assumed to behave linear elastic. However, lack of information on elastic-plastic crack behaviour or <em>J</em>-integral was demanded especially for 3D surface cracks. In this present study, semi-elliptical surface cracks embedded in a solid round bar subjected to mode III loadings are considered. Then, <em>J</em>-integral or <em>h</em>-function and limit load were determined and analyzed. In order to predict <em>J</em>-integral along the crack front, a mathematical model was then developed. It is found that the developed model capable to predict <em>J</em>-integral well. However, the predictions breakdown occurred when the elastic dominated region of cracks.

Reference load solutions for plates with semi-elliptical surface cracks subjected to biaxial tensile loading

The yield or limit load is a key parameter with respect to the accuracy of flaw assessment based on R6 type procedures such as the R6 routine, the SINTAP and FITNET method, the standard BS 7910 and others. In a number of previous papers two of the present authors proposed the use of a reference load instead of the common limit load, which not only provided more exact fracture mechanics predictions, but showed also a wider and more general application range than the conventional parameter. Here the method has been extended to biaxial tensile loading and it has been successfully validated by a thorough comparison with finite element results and alternative solutions available in the literature.

Fatigue Crack Growth and Coalescence Algorithm Starting from Multiple Surface Cracks

Fatigue crack growth and propagation analysis in welded joints have to deal with the complexity of modeling multiple weld toe surface cracks originating from weld toes. Fitness-For-Service (FFS) assessments for weld toe surface cracks employ a fracture mechanics and Paris Law approach to predict the fatigue crack propagation life of a semi-elliptical surface crack (SESC) to failure. A fatigue crack growth algorithm for assessing multiple surface crack growth, coalescence and propagation life was initially validated with previuously report crack growth data for a fillet shoulder specimen. Next a parametric study for single, double, and triple SESCs located along the weld toe line of a fillet weld was investigated with three starting crack depth sizes (0.1mm, 0.5mm, 1.0mm) coupled with three different crack aspect ratios (a/c = 1.0, a/c = 0.5 and 0.25) giving a total of 27 cases studied.

Fracture toughness testing of small ceramic discs and plates

Abstract A new fracture toughness test for discs and plates is presented, which can be applied to small specimens (>5 mm diameter). A semi-elliptical surface crack is made into the centre of the top plane using a Knoop intender. Then the layer containing the plastically deformed zone is ground off and the crack is loaded in tension using the Ball-on-3-Balls test. Applied to five different ceramic materials the results gained with the new method agree well with those of standardised methods.

Fatigue Resistant Design of Round Bars Weakened by a V-Shaped Circumferential Notch

In the present paper, fatigue crack propagation in V-notched round bars under cyclic eccentric axial loading is numerically evaluated using both a finite element analysis and an automated numerical model proposed by the first author. The stress- intensity factors are computed at a set of points on the crack front by the finite element method. Then, a suitable fatigue crack growth law is applied to the above set of points. By varying the loading eccentricity value and the notch root geometry, several initial semi-elliptical surface crack configurations are analysed.

Fatigue Threshold R‐curves Predict Fatigue Endurance Strength for Self‐Reinforced Silicon Nitride

There is a need for methods that can help predict and avoid fatigue failures of silicon nitride ceramic components. The fatigue threshold R‐curve has been proposed as potential solution to this problem. In this study, the fatigue threshold R‐curve for small, semielliptical surface cracks was calculated for a silicon nitride ceramic using the published bridging stress distribution developed from fatigue threshold tests on macroscopic crack specimens. To test the accuracy of the endurance strengths predicted using the fatigue threshold R‐curve, fatigue tests were conducted using four‐point bend beams of silicon nitride containing semielliptical surface cracks introduced by Knoop indentation. The effectiveness of the methodology was verified; indeed, 77% of the beams tested at stress levels above the predicted endurance strength failed within 107 cycles and 0% of the beams tested below the predicted endurance strength failed within 107 cycles. Furthermore, using the bridging stress distribution, which is thought to be a material property, the need for prohibitively difficult fatigue threshold experiments on small surface cracks is avoided. Accordingly, this methodology is potentially quite practical for use in the engineering design of ceramic mechanical components.

The Interaction between a Piezoelectric Screw Dislocation and a Semi-Elliptical Surface Crack in Piezoelectric Bi-Material

By using the complex variable method, the electro-elastic interaction between a piezoelectric screw dislocation in the infinite half-plane and a semi-elliptical interface crack under remote anti-plane shear stresses and in-plane electric loads in piezoelectric bi-material is investigated, and the general solutions to the problem are obtained. The complex potentials, the generalized stress fields, the generalized stress intensity factors and energy release rate of crack tip, and the image force acting on the dislocation are also calculated. The influences of the electro-elastic constants and the location of the screw dislocation upon the stress intensity factor and the image force are discussed, while the influence of remote loading upon the energy release rate is also analyzed. The results indicate that the crack and the lower half plane may attract or repel dislocation with parameters changes when dislocation lies on the upper half plane and the shape of the crack is fixed. The shielding effect is stronger when the piezoelectric coefficient of the upper half plane is larger than that of the lower half plane, and the influence of relative shear modulus upon the shielding effect is relatively smaller when the relative piezoelectric coefficients take fixed values. The energy release rate may be positive or negative under a combined electric load and mechanical load. Both increasing electric field and decreasing mechanical load can decrease the energy release rate thereby enhancing the restraint of the crack expansion.

Fatigue Crack Growth Analysis of Semielliptical Surface Crack

The aim of this paper is to present the simulation technique and experimental work for surface crack in a structure. For the sake of test apparatuss feasibility, a semielliptical surface crack in a structure was converted into small specimen size. The specimens represented semielliptical surface crack problem in real application of engineering structure. The specimens with semielliptical surface crack were tested by experiment and simulation. The S-version Finite Element Model (S-FEM) was implemented in simulation, and verification was performed through experimental works. The S-FEM consists of virtual crack closure method and Paris law as a crack growth criterion for 3-dimensional problem with hexahedron elements was used. Auto-mesh generation technique with a fully automatic crack growth simulation system was employed in S-FEM simulation. Specimens with 0oof crack surface angle were prepared and analysed for the modeIloading. A four-point bending fatigue test was conducted and simulated in order to gain crack path, fatigue crack growth and stress intensity factors value. Results from simulation and experiment showed good agreement with a certain condition. The implications from the results were discussed through the analysis of fatigue crack growth.

Crack-healing behavior of hot-pressed TZ3Y20A-SiC ceramics

Crack healing behavior of hot-pressed TZ3Y20A-SiC ceramics has been investigated by high-temperature oxidation. Semi-elliptical surface cracks with a length of 100μm have been set on the tensile side of each specimen using a Vickers hardness indenter. Based on flexural strength and observations of crack appearance, the effect of crack healing is finally judged. Cracks on ZrO2(Y2O3)–Al2O3 (TZ3Y20A) ceramics cannot be healed by heat treatment. However, for TZ3Y20A-SiC ceramics, complete crack-healing has been realized by heat treatment at 800°C, 1000°C and 1100°C for 30h, 10h and 5h, respectively. The crack-healing mechanism is attributed to the formation of SiO2 caused by high temperature oxidation during heat treatment.

Development Of Semi-Elliptical Surface Cracks In Lightly Sensitized Al-Mg Alloys

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Simulation of Cracking of Electrode Assembly in Planar Solid Oxide Fuel Cell

Cracking characteristics of the positive electrode-electrolyte-negative electrode (PEN) assembly under thermal stresses in a planar solid oxide fuel cell (SOFC) stack are studied, using finite element analysis (FEA). A 3-D FEA model is constructed for a multiple-cell stack to perform thermal stress analysis at both steady operation and shutdown stages. The stress intensity factors for various combinations of surface crack shape (semicircular and semi-elliptical) and size (1 µm, 10 µm, and 100 µm) at the highly stressed regions in the PEN are calculated at both room temperature and steady stage. Simulation results indicate the stress intensity factor is increased with crack size for semicircular surface cracks. A deep, semi-elliptical surface crack tends to grow wider, while a shallow one tends to grow deeper.

Strengthening of Misaligned Welded Pipes with Outer Circumferentially Crack Using FRP Bandage Finite Element Analysis

Finite element analysis is used to investigate the effectiveness of Fiber Reinforced Polymers (FRP) bandage in rehabilitating misaligned welded pipes with outer circumferential surface crack. It is assumed that the pipes are subjected to an axial tensile stress. Linear Elastic Fracture Mechanics (LEFM) regime is adopted to estimate the stress intensity factor (SIF). ANSYS software is used in the study through 3D-axisymmetric finite element analysis. The results demonstrated that stiffening with FRP bandage has a great influence on the reduction of the stress intensity factor at the crack tip improving the fracture integrity of the misaligned welded pipes. The most common solution of the rehabilitation process for damaged pipes fabricated from metals is very important mater. The rehabilitation technique for pipes is needed because of external erosion and corrosion for pipes or due to external damages .Usually such process requires using heavy machines and skilled workers in addition to the time needed. By using Fiber Reinforced Polymers (FRP) techniques to increase the strength for existing pipes, it is very important for the corroded and damaged pipes to increase the resistance to the surrounding environment. The conventional techniques for repairing damaged pipes need operation of cutting, welding and inspecting in addition. This may also require stopping the system during the repair period. Furthermore, for many cases the repair process is undesirable because of the residual stress induced in the pipe during the welding process which tends to decrease the fatigue performance of the pipe material in addition to the weakness of the pipe wall due to any of corrosion types such as pitting. Therefore, the final rehabilitation cost using FRP is low compared with the traditionally usual trends using welding techniques. Pipeline welding contractors are very often used in mechanized and semi-automatic welding processes. It is generally accepted that the most welded structures enter service containing tiny cracks or flaws (1). Moreover, these flaws may extend to a size where the safety of the structure becomes dangerous. It is therefore essential that engineers and designers have methods of assessing the significance of flaws detected during manufacture or after the structure has entered its service life, especially the structures that are connected with the human safety, such as pipes. Generally, cracks in piping system often occur at welding joint and around it (2). Most of these cracks are located in circumferential direction. For this reason circumferential cracks play an important role in the safety analysis of pipes (3). Strengthening techniques have drawn attention by many researchers to investigate the effectiveness of the stiffeners in the rehabilitation processes through using experimental as well as finite element methods. Ahmed et al. (4-6) investigated the effect of using FRP strengthening techniques to rehabilitate structural members under numerous loading conditions through FEA. Mourad et al. (7-18) have studied the effect of stiffening on crack growth through three-point bending TPB, compact tension CT and compact shear CS specimens. Chang-Young Oh et al. (19) investigated existence methods to estimate stress intensity factors due to welding residual stresses and compared with finite element (FE) solution for welding residual stress profiles, generated by simulating repair welds and compared with analytical solution. On the other hand, finite element analysis (FEA) was carried out to obtain the elastic stress distribution at cylinder to cylinder junction in pressurized shell structures that have applications in space vehicle design (20). The peak stress value is found to reduce due to filleted butt joint as expected and also confirmed through test results. Pipes with a flawed girth weld for two common girth weld tension tests: the curved wide plate test and the full scale pressurized pipe tension test, and subjected to global plastic deformation were studied by finite element models (21). Xiaohui Chen et. al. (22) presented an overview of recent progresses in experimental investigation and finite element analysis (FEA) of ratcheting behaviour of pressurized piping. The stress-intensity factor for the circumferential semi-elliptical surface cracks in a hollow cylinder's cross

Interaction Relationship for High Aspect Ratio Semi-Elliptical Internal Surface Cracks in CO&lt;sub&gt;2&lt;/sub&gt; Pipelines

Surface cracks with aspect ratio can occur in pipelines subjected to corrosion attacks. There are few studies on interaction relationship for double semi-elliptical surface cracks with such high aspect ratio. This paper attempts to develop a finite element method to determine on interaction of two semi-elliptical internal surface cracks with high aspect ratio by analyzing two key fracture mechanics parameters, i.e., the stress intensity factor and the-stress. The numerical results prove that the existing flaw-interaction criteria are too general to lose the worthy of application especially in pipeline engineering.

Transient thermal cracking of a brittle half space medium with a semi-elliptical surface crack

The paper conducts the cracking study of a brittle half space medium with a semi-elliptical surface crack. The transient thermal stress intensity factors are obtained and are graphically presented. Effects of thermal shock time, crack size, and crack shape on the crack growth and thermal shock resistance behavior of the medium are investigated in details. A thermal shock resistance analysis methodology is established, allowing us to study the fracture strength of brittle media subjected to thermal shock.

Finite Element Investigation of Stress Intensity Factors of Multiple Semi-Elliptical Surface Cracks in Solid Shaft

Fatigue cracks nucleate at the surface of the shaft operating under cyclic load. These cracks then grow into semi-elliptical shapes while extending into the shaft. The paper presents numerical estimation of stress intensity factor (SIF) of such cracks in circular solid shaft when subjected to bending and twisting loads in elastic conditions. The finite element model is first validated with the help of results reported elsewhere for a single surface crack. Later on, multiple coplanar surface cracks are modeled and the effect of the number of cracks and the distance between them on SIF of the main crack is investigated. The results are in line with the expectations.Nomenclature

Customization of the CEFM for predicting stress corrosion cracking in lightly sensitized Al–Mg alloys in marine applications

The Coupled Environment Fracture Model (CEFM) has been modified and calibrated to predict crack growth rate (CGR) in aluminum marine alloys. The customized CEFM provided quantitative predictions of the effects of O2, electrochemical potential, stress intensity factor, and conductivity on CGR in lightly sensitized AA5083-H321 in 3.5 wt.% NaCl solution, as well as explaining the development of a semi-elliptical surface cracks. The importance of the properties of the external environment, such as conductivity, oxidant/reductant concentration, and the kinetics of the cathodic reactions on the surfaces external to the crack has been confirmed. Crack growth is attributed to a sequence of microfracture events at the crack front, the frequency of which is determined by the mechanical conditions that exist at the crack tip, as governed by the stress intensity while the microfracture dimension is determined by hydrogen-induced fracture, with the CGR being the product of these two quantities. The success in explaining the intergranular stress corrosion cracking (IGSCC) of aluminum alloys, argues that the basic concept of the CEFM, that the internal and external environments are strongly coupled, is sound and that the CEFM, which was originally developed to describe IGSCC in sensitized stainless steels is equally applicable for describing IGSCC in lightly sensitized aluminum alloys.

Stress intensity factor of semi-elliptical surface crack in a cylinder with hoop wrapped composite layer

In this paper the fracture behavior of the compressed natural gas (CNG) cylinder with hoop wrapped composite layer is investigated for the axial crack at the inner surface. By the aid of the three-dimensional finite element method, the stress intensity factors along the crack front are obtained for different crack profiles. The effects of the cylinder geometry, hoop wrapped layer thickness and the property distributions of the composite layer on the stress intensity factor are discussed in detail. The numerical results show that the hoop wrapped composite cylinder can lower the stress intensity factor value greatly and ensure the safe use of pressure vessels containing defects in service. The composite property distribution owns a distinct effect on the crack behavior in different directions that provides a clear guide to the maintenance of CNG cylinder. Finally an approximate formula with high precision is proposed to evaluate the stress intensity factor along the axial crack front in the hoop wrapped cylinder with composite layer.

Stress intensity factors for circumferential semi-elliptical surface cracks in a hollow cylinder subjected to pure torsion

The stress-intensity factor for the circumferential semi-elliptical surface cracks in a hollow cylinder’s cross section under torsion is calculated using a finite-element technique. The configuration of standard semi-elliptical surface cracks is described using the outer surface crack-lengths and the crack depths. The compendia for the stress-intensity factor solutions for Mode II, Mode III and both mixed modes are given for the broader aspect ratios and relative crack-depths than previously available. The magnitude of the stress-intensity factor of Mode II near the free surface becomes more significant for stable crack-initiation than that of Mode III at any point along semi-elliptical crack.

On the Shape of Stress Corrosion Cracks in Water-Cooled Nuclear Power Reactor Piping

The evolution of the shape of surface cracks in sensitized Type 304 SS in Boiling Water Reactor primary coolant at 288 oC was explored as a function of the corrosion potential and stress intensity using the Coupled Environment Fracture Model (CEFM). The revised CEFM that incorporates Shoji’s model for calculating the crack tip strain rate and more advanced expressions for estimating the stress intensity factor for semi-elliptical surface cracks provides more accurate prediction of the dependence of the crack growth rate on stress intensity factor and offers an alternative explanation for the development of semi-elliptical cracks than that provided by fracture mechanics. The evolution of the shape of surface cracks depends strongly upon environmental variables, such as the corrosion potential, predicting that the minor axis of the ellipse should be oriented perpendicular to the surface due to the dependence of the crack growth rate on the electrochemical crack length.

Three-Dimensional Study of Fatigue Crack Growth in a Rotating Disc

The problem of fatigue-crack-growth in a rotating disc at different crack orientation angles is studied by using an automated numerical technique, which calculates the stress intensity factors on the crack front through the three-dimensional finite element method. Paris law is used to develop the fatigue shape of initially semi-elliptical surface crack. Because of needs for the higher mesh density and accuracy near the crack, the sub-modeling technique is used in the analysis. The distribution of SIF’s along the crack front at each step of growth is studied and the effect of crack orientation on the rate of crack-growth is investigated. The calculated SIF’s are reasonable and could be used to predict the probable crack growth rates in fracture mechanics analysis and can help engineers to consider in their designing and to prevent any unwanted failure of such components.