Semi-elliptical Surface Crack Research Articles

Prediction of the residual lifetime of gas pipelines considering the effect of soil corrosion and material degradation

The calculation model is developed to evaluate the residual lifetime of a pipe with a crack considering in-service degradation of the pipeline steel and the effect of soil corrosion. The model is based on the first law of thermodynamics. The problem of determination of the period of subcritical crack growth in a pipe is reduced to the differential equation subjected to definite initial and final conditions. Obtained equation together with the boundary conditions are used for prediction of the residual lifetime of the pipe made of the X52 pipe steel. The pipe contains external surface semi-elliptical crack. The task consists in the determination of the time of crack propagation through the pipe wall till its decompression. According to the experimental data, cracks in the studied steel under long-term loading and soil corrosion propagate mainly at constant rate, which is different for the as-received pipeline steel and the steel after 30 years in service. Based on these data, the equation for approximate determination of the crack growth rate for arbitrary service time of the pipe made of the X52 steel is proposed. To evaluate the crack propagation time, the energy approach is employed. For a crack that remains semi-elliptical during propagation, the problem is reduced to the differential equation system that determines the variations of the crack semi-axes. The system solution under definite initial and final conditions renders the expression for the pipe residual lifetime. The dependences of pipe residual lifetime on initial crack depth and pipe operation time are derived. The strong influence of the time of steel previous service on the pipe residual lifetime is manifested.

Prediction of transport vehicles’ durability with consideration of corrosive surface cracks propagation in structural elements

Support frameworks of transport vehicles operate under varying terrain conditions under the influence of extreme climate and corrosive environments. When transporting cargo, dust is deposited on the surface of metal structures. The combination of dust and moisture creates an aggressive environment resulting in intense corrosion damage. The damage is caused by the defects of corrosion pitting, which occur on the surface and transform into corrosion cracks. Based on energy approaches, with the application of wellknown results for the mathematical description of electrochemical reactions and the principles of fragile fracture mechanics, an analytical model of durability is proposed for the first time. The model determines the residual life of maximum loaded elements of undercarriages with surface cracks under the action of dynamic loads and corrosive environments. For this case, a set of mathematical relations in the form of a non-linear differential equation was developed, as well as the initial and final conditions for determining the life of vehicles’ structural elements with corrosive surface cracks. The analytical model implementation is proven by solving the problem of determining the residual life of a vehicle’s element, i.e., a steel plate, weakened by a semi-elliptical surface crack, which is under the action of dynamic loads in a 3% sodium chloride solution. The insignificant increase in the crack’s initial size is proven to greatly reduce the period of its subcritical growth. The developed model was applied to define the residual life of thin-walled elements of structures with surface cracks.

Prediction of Stress Intensity Magnification Factors of a Weld Toe Crack in Load-Carrying Cruciform Fillet Welded Joints Under Axial Loading

Load-carrying cruciform fillet welded joint is commonly encountered in on-shore and offshore structures. In this study, the stress intensity magnification factors (Mkm) of such configuration containing a 3-D semi-elliptical surface crack at the weld toe are analysed using 3-D finite element analyses. Due to the non-penetrating fillet welds at the intersection of the main plate and transverse stiffener, the Mkm values are found to be much larger than the corresponding results of non-load-carrying cruciform welded joints. Therefore, a new set Mkm equations for load-carrying joints are built up using multiple regression analyses, and statistical analyses are carried out to validate the goodness of fit of these equations. It is noted that the existing British Standard 7910 (BS 7910) assessment equation produces an over-conservative prediction on Mkm values as compared with the newly proposed Mkm equations, and the largest over-estimation percentage difference is as high as 257.8%.

Analysis of stress intensity factor along semi-elliptical surface crack reinforced by composite patching

Analysis of stress intensity factor along semi-elliptical surface crack reinforced by composite patching

Effect of interface region on fatigue crack growth in diffusion-bonded laminate of Ti-6Al-4V

This paper aims to illustrate the effect of the bonding interface on fatigue crack growth (FCG) in the diffusion-bonded (DB) laminate of Ti-6Al-4V (Ti64). To achieve this end, we performed FCG rate measurements for the DB laminate of Ti64 (2 mm + 2 mm + 2 mm + 2 mm) using a semi-elliptical surface crack, the size of which was determined by marker bands. The FCG behavior and laws along the surface direction and the thickness direction were investigated through fractographic analyses. It is found that there are interface regions in the DB laminate of Ti64, significantly influencing the FCG rate along the thickness direction (da/dN). When the crack front approaches the interface region from the base material, da/dN decreases with crack growth and reaches a local minimum when the crack front enters the interface region. With crack propagation in the interface region, da/dN increases regularly until the crack front approaches the base material, after which crack growth accelerates until the crack extends to a specific point in the base material. To illustrate the mechanism of the interface region influencing da/dN, we obtained the microtexture and the elastic/plastic properties of the base material and the interface region. The results show that although there is no apparent difference between the base material and the interface region in terms of elasticity modulus and yield strength, their differences in microtexture are significant, resulting in nonlinear variation in log(da/dN) with the logarithmic stress intensity factor range near the interface region.

Nb2AlC-particle induced accelerated crack healing in ZrO2–matrix composites

The oxidation induced crack healing behavior of pre-cracked Nb2AlC particle loaded ZrO2-matrix composites was explored by annealing in air at 1200 °C for short periods of 10 and 20 min. Composites loaded with 0, 6.5, 13, and 19.5 vol% Nb2AlC powder dispersed in 3Y-TZP matrix powder were manufactured by spark plasma sintering (SPS) at 1300 °C. Semi-elliptical artificial surface cracks with a length exceeding 220 µm were produced by Vickers indentation. The modulus of rupture of virgin, indented and annealed samples was measured in three-point bending mode. Compared to single phase 3Y-TZP strength recovery of the Nb2AlC loaded composite upon annealing at 1200 °C in air is accelerated and reaches > 60% of the initial strength after a short healing period of 10 min only. A semi-empirical oxidation cohesive zone healing model was derived which describes the crack microstructure evolution as a combined effect of 3Y-TZP-matrix healing superimposed by Nb2AlC particle oxidation induced healing.

Stress distribution and fatigue crack propagation analyses in welded joints

AbstractAn approach is presented, based on the weight function method to calculate the stress intensity factors of semielliptical surface cracks originating from the notch root of welded joints. The stress distribution along the potential crack plane required in the weight function method is constructed on the basis of the notch stress intensity factor approach in the highly stressed zone and of the equivalent linearized stress distribution and is compared with those determined by the finite element method and existing predictions. The stress intensity factors determined by the proposed approach are compared with available solutions. These comparisons show that the results determined by the proposed approach generally agree well with the existing solutions. For the cases where the agreement is poor, the reasons are identified. One important feature of the proposed approach is that the stress singularity at sharp notch tip can be considered, which cannot be appropriately simulated by the finite element method. Finally, to demonstrate the applicability of the proposed approach, the fatigue life and the fatigue crack shape evolution of welded joints are predicted and they are compared with experimental results.

Engineering Critical Assessment for Offshore Pipeline with Semi Elliptical Surface Cracks in Girth Weld – Comparison of FEM and BS7910 Guideline

Engineering Critical Assessment for Offshore Pipeline with Semi Elliptical Surface Cracks in Girth Weld – Comparison of FEM and BS7910 Guideline

Fatigue crack propagation prediction for marine structures based on a spectral method

A method based on spectrum analysis is proposed for predicting fatigue crack propagation (FCP) of a crack in marine structures. The stress intensity factors (SIF) for a crack can be different under different load cases, such as loading conditions, heading angles and wave frequencies, even at the same level of nominal or hotspot stress. Therefore, the load spectrum obtained from the stress transfer function and wave spectrum may be failing to calculate the FCP accurately. In current study, SIF transfer functions were evaluated through detailed structural analysis for different crack sizes, and the short term SIF distribution obtained from the SIF transfer function. FCP life could then be calculated from numerous short term SIF distribution and the improved Paris formula. A sub-model technique was integrated, facilitating SIF re-analysis due to crack growth, and an Improved Euler method was adopted to reduce the computational steps. FCP of a semi elliptical surface crack at the weld toe between hatch coaming and forward bulkhead of a container ship superstructure was used to demonstrate the application of the proposed spectral method. This paper offers a way of combining the spectral analysis and fracture mechanics in fatigue crack growth calculation for marine structures.

Analysis of stress intensity factors obtained with the fem for surface semielliptical cracks in the zones of structural stress concentrators

Проблемними питаннями при оцінюванні живучості конструкції з поверхневими тріщинами є складності обчислення коефіцієнтів інтенсивності напружень (КІН) для тріщин, які розвиваються у зонах концентраторів напружень. Ці труднощі пов’язані з особливостями зміни форми контуру поверхневої тріщини в процесі циклічного навантаження, який залежить від рівня неоднорідності напружено-деформованого стану в області концентраторів. Проведено аналіз існуючих методів визначення коефіцієнтів інтенсивності напружень у зонах концентраторів: методу вагових функцій, чисельного і гібридного методів. Моделювання зварних з’єднань різних типів з тріщинами проводили у рамках представленої моделі як геометричного фактора – підсилення зварного шва, що дозволило методом скінчених елементів оцінити особливості впливу концентратора напружень на зміну КІН по контуру поверхневої тріщини. При цьому розглянуто особливості побудови сітки скінчених елементів, вплив розмірів сітки на отримані розрахункові значення КІН, порівняння отриманих результатів з відомими рішеннями. При циклічних випробуваннях розтягом отримано кінетику форми втомних поверхневих півеліптичних тріщин у зонах досліджуваних концентраторів напружень та, для порівняння, в однорідному полі напружень. Проаналізовано отримані форми тріщин з результатами модельних досліджень.

Prediction of Bending Fatigue Life of Cracked Out-of-Plane Gusset Joint Repaired by CFRP Plates

Carbon fiber reinforced polymer (CFRP), plates bonding repair method is one of the simple repair methods for cracked steel structures. In this study, the influence of width of CFRP plates on bending fatigue life of out-of-plane gusset joint strengthened with CFRP plates was investigated from the experimental and numerical point of view. In the bending fatigue test of cracked out-of-plane gusset joint strengthened with CFRP plates, the effect of width of CFRP plates on crack growth life was clarified experimentally. Namely, it was revealed that the crack growth life becomes larger with increasing the width of CFRP plates. In the numerical approach, the stress intensity factor (SIF) at the surface point of a semi-elliptical surface crack was estimated based on the linear fracture mechanics. Furthermore, the extended fatigue life of cracked out-of-plane gusset joint strengthened with CFRP plates was evaluated by using the estimated SIF at the surface point and the empirical formula of the aspect ratio of semi-elliptical crack. As the results of numerical analysis, the estimated fatigue life of the specimen strengthened with CFRP plates showed the good agreement with the test results.

Creep constraint and fracture parameter C∗ for axial semi-elliptical surface cracks with high aspect ratio in pressurized pipes

The unified creep constraint parameter Ac and fracture parameter C∗ for axial semi-elliptical surface cracks with high aspect ratio in pressurized pipes have been investigated by three-dimensional finite element analyses. The results show that the constraint level near surface part is lower than that at the deeper part of crack front. With increasing crack aspect ratio a/c and decreasing crack depth a/t, the constraint levels decrease. The parameter Ac solution for high aspect ratio cracks has been obtained. The crack aspect ratio a/c has significant effect on the C∗ distributions along the crack fronts. For high aspect ratio cracks, the maximum C∗ along the crack fronts occurs near surface part. With increasing crack aspect ratio a/c from 0.2 to 2, the creep crack initiation location will change from the deepest part of the crack front to the near surface part. For high aspect ratio cracks, if the calculations of C∗ and Ac at the deepest point is used to predict creep crack initiation life, non-conservative results may be produced. More benefits from incorporating constraint effects may be obtained for creep life assessments of pipes with high aspect ratio cracks due to their lower constraint.

The Effect of Indentation Load, Tilted Angle, and the Amount of Surface Removal to the Fracture Toughness of Silicon Nitride Measured by the Surface Crack in Flexure (SCF) Test Method

The surface crack in flexure (SCF) is a method for the evaluation of the fracture toughness of advanced ceramics. Conventionally is practiced by using a Knoop indenter to make a very small precrack. Removal on indent and the plastically deformed zone is required before the fracture test. The purpose of this removal is to eliminate residual stresses under the Knoop impression and to obtain a semi elliptical precrack shape. In this work the influence of the variation of several experimental parameters is investigated. Fracture toughness values by the SCF method are compared with those measured using the SEVNB (single edge V-notched beam) method. The material chosen for this purpose was gas pressured sintered silicon nitride (Si3N4) containing 3wt.%Al2O3 and 3 wt.%Y2O3 (SL200B, Ceram Tec, Plochingen, Germany). The varied parameters were indentation load, orientation of the indentation crack with respect to the bending axis and the amount of material removed from the surface. Additional investigations were performed to determine the crack geometry for various indentation loads. A procedure was developed to facilitate the location and measurement of the precrack size on the fracture surfaces. The effect of measurement accuracy of the results is evaluated. The fracture toughness of specimens with a surface removal in the range suggested from ASTM C 1421 were found to agree with the results obtained from SEVNB. A surface removal below the recommendation resulted in low values of fracture toughness. Increasing the amount of surface removal moderately was found to still fit with results obtained from SEVNB. Surface removal of much more from the recommended amount leads to failure from natural flaws. Observations from serial sectioning revealed that precracks obtained from HK20 and HK30 have an irregular precrack shape with large lateral cracks. It is suggested to use HK5 and HK10 to produce semi-elliptical surface cracks.

Analysis of surface cracks in round bars using dual boundary element method

A closed-form solution for stress intensity factors (SIFs) of a semi-elliptical surface crack in round bars subjected to torsional loading using the dual boundary element method are presented in this paper. The SIFs of surface cracks having different crack aspect ratios and inclination angles were calculated using the numerical software BEASY. In general, the results revealed that the Mode III SIF was strongly affected by both the crack size and crack shape whereas that crack aspect ratio had the greatest influence on the SIFs regardless of the fracture modes.

The Influence of Crack Propagation Angle on the Stress Intensity Factors (SIFs) of Cracked Tubular T-Joints

This paper focuses on the crack propagation angle effects on the stress intensity factors of a 3-D semi-elliptical surface crack in tubular T-joints. A robust mesh generator is created and used in modeling and analyzing a wide range of tubular T-joints. The mesh elements are generated by using MATLAB programming language then analyzed using ABAQUS software. The numerical model results are firstly compared with existing experimental results for verification which shows a good agreement with a maximum deviation of 3.44%. Then, the influence of the crack propagation angle on the stress intensity factors is determined, along the crack front, on 1200 CHS T-joint with semi-elliptical surface crack. It has been found that the crack propagation angle influenced the stress intensity factor at the crack front and subsequently the fatigue life of tubular T-joints. The stress intensity factor specially at the deepest point of the crack increases as crack propagation angle increases. The effect of the crack propagation angle on the SIF is directly proportional with the crack depth and crack length.

Non-Fourier thermal shock fracture of solids with shallow semi-elliptical surface crack

In this study, a thick plate with a shallow semi-elliptical surface crack subjected to a transient thermal load was investigated based on the non-Fourier heat conduction law. Using a Laplace transform and its numerical inversion transform methods, the non-Fourier temperature fields and associated thermal stress formulas in the solid uncracking were obtained. Then, using the weight function and analogy methods, the thermal stress intensity factor formula at the crack tip was determined. The effects of the thermal shock time, crack depth, and crack shape on the thermal stress intensity factor and the crack growth behavior were explored numerically by comparing the Fourier model with a non-Fourier model. The results demonstrate that the non-Fourier model is much safer for studying the thermal shock cracking and crack growth behavior of solids under high-temperature applications.

An experimental-finite element method based on beach marks to determine fatigue crack growth rate in thick plates with varying stress states

The relation between the fatigue crack growth rate and stress state is significant for fatigue life estimation, especially for thick-welded structures with complex stress states. This paper proposes a hybrid Experimental-Finite Element Method aimed to obtain the fatigue crack growth rate corresponding to plane stress and plane strain states. A static load marking method is proposed to mark a series of crack fronts in a single edge notched bending specimen at known numbers of cycles, which is analysed by nonlinear finite element method to evaluate the stress state by a local constraint factor. Stress intensity factor at the center of the thickness and the surface position where plane strain and plane stress states approximately exist and remain unchanged with crack propagation, is calculated by the 1/4-node displacement method with consideration of non-orthogonal mesh influence. The J-integral method and the virtual crack closure technique are also applied to check the performance of the methods in dealing with the actual cracked body problem. Crack length increments are determined based on the crack front profiles. The da/dN-ΔK curves of plane strain and plane stress states are determined and compared with the results from normal fatigue crack growth rate tests according to Chinese standard: GB/T 6398-2000. The fatigue crack growth behavior of a semi-elliptical surface crack located at the weld toe of a T-plate welded joint subjected to cyclic tension loading is assessed based on the curves and the analytical results are compared with the experimental results. The asymptotic crack shape observed in experiments is successfully predicted by using the curves determined by the hybrid method.

Empirical solutions for stress intensity factors of a surface crack in a solid cylinder under pure torsion

No empirical solution for the stress intensity factors (SIFs) of a surface crack in a solid cylinder under torsion was reported in literature. In this paper, empirical solutions for Modes I, II, and III stress intensity factors of a semi-elliptical surface crack in a solid cylinder under pure torsion are proposed. The dual boundary element method (DBEM) based software was used to generate the SIFs for different crack size, crack aspect ratio, and different inclination angles. Next, the solutions for the SIFs were produced through a systematic curve fitting approach.

Theoretical aspects of stress corrosion cracking of Alloy 22

Theoretical aspects of the stress corrosion cracking of Alloy 22 in contact with saturated NaCl solution are explored in terms of the Coupled Environment Fracture Model (CEFM), which was calibrated upon available experimental crack growth rate data. Crack growth rate (CGR) was then predicted as a function of stress intensity, electrochemical potential, solution conductivity, temperature, and electrochemical crack length (ECL). From the dependence of the CGR on the ECL and the evolution of a semi-elliptical surface crack in a planar surface under constant loading conditions it is predicted that penetration through the 2.5-cm thick Alloy 22 corrosion resistant layer of the waste package (WP) could occur 32,000 years after nucleation. Accordingly, the crack must nucleate within the first 968,000 years of storage. However, we predict that the Alloy 22 corrosion resistant layer will not be penetrated by SCC within the 10,000-year Intermediate Performance Period, even if a crack nucleates immediately upon placement of the WP in the repository.

Characterization of the creep interaction effect for twin semi-elliptical surface cracks under combined tension and bending loading

The interaction of twin semi-elliptical surface cracks subjected to combined tension and bending in the creep regime is studied. Moreover, the crack growth behavior in the presence of multiple cracks is simulated to reveal the effect of creep interaction factor. The tension load produces a larger creep interaction factor than those for pure bending and combined tension and bending. For the combination of cracks, a greater value of the creep interaction factor is permitted compared with the linear-elastic condition. The enhancement of creep crack growth rate in multiple cracks is diminished as the improvement of creep interaction factor is lower than 30%.