Papers by Keyword: Semi-Elliptical Surface Crack

Abstract: In this investigation, the weight function method was employed to calculate the stress intensity factors for semi-elliptical surface crack in a hollow cylinder. An uniform stress and a linear stress distributions were used as the two references to determine the weight function. The weight function was then applied to a wide range of semi-elliptical surface cracks subjected to non-linear loadings. The stress intensity factors obtained by the weight function were compared with literature results. Good agreement demonstrates the accuracy of the present approach.

Abstract: The drill pipe near the surface stands the largest tension and torsion load for the full hole during drilling operation. And fatigue crack growth is always the major cause of failure of drill string. As an example, 5ʺ drill pipe that was near the well head of an ultra-deep straight well and made of 30CrMo, whose constitutional relation was fitted by experiment, was analyzed here. Simplifying the initial crack of the drill pipe as circumferential semi-elliptical surface crack, we simulated the elastic-plastic fracture feature of the drill string with surface crack, partly through-wall crack and fully through-wall crack under combined loading of axial force and torsion. Crack front geometry evolvement is simulated for the different stages of crack propagation. This work would provide a basis for the full-range analysis of fatigue crack growth.

Abstract: In this paper, a fatigue test of a X80 pipeline steel specimen with a semi-elliptical defect was performed to study the fatigue propagation stages of the Surface cracks. Based on the obtained test data, a three dimentional finite element procedure was developed for the crack growth estimation of the surface crack in the specimen. In the numerical analysis of crack growth, both the crack depth direction and the surface direction were investigated by using two different evolution equations. The stress intensity factors along the crack front were determined by applying the virtual crack closure technique. The predictions of crack growth were compared with the test data.

Abstract: The calculation of stress intensity factor (SIF) in fracture mechanics-based fatigue life prediction is mainly based on empirical formulas, the poor applicability of which limits the wide application of fatigue assessment based on crack propagation. This paper proposed a SIF calculation method for structures under wave loads. It was proved correct by comparing with the empirical formulas. Then, the method proposed was applied for the surface crack at weld toe of a connection between the column and brace of a semi-submersible platform. The predicted results showed that SIF ranges varied with wave loads, which were subjected to the effects of wave directions and frequencies. In addition, SIF ranges calculated by empirical formulas were very different from those of the proposed method. In order to accurately predict fatigue crack propagation life for ocean engineering structures, it is necessary to take the characters of wave loads into consideration. The proposed method in this paper may provide a reference.

Abstract: A detailed elastic-plastic analysis on different semi-elliptical surface crack geometries under uniaxial and biaxial load was conducted using 3D finite element analysis for pipeline steel X100. After quantitating fracture driving force J-integral and plastic yielding load, the effects of biaxial load and crack geometry were investigated by using the Option 3 of R6 failure assessment diagram approach for the surface cracks with the different crack sizes ( aspect ratios c/a = 1, 1.5 and 3 ) and the various biaxial loading ratios λ from -1 to 2. And three option curves of the R6 assessment were compared to evaluate the applicability of R6 assessment method for the X100 steel.

Abstract: This study presents a three dimensional finite element method for mixed-mode fracture analysis of an FGM coating-bond coat-substrate structure. The FGM coating is assumed to contain an inclined semi-elliptical crack at the free surface. The trilayer structure is examined under the effect of transient thermal stresses. Strain singularity around the crack front is simulated by utilizing collapsed wedge-shaped singular elements. The modes I, II and III stress intensity factors are computed by applying the displacement correlation technique and presented as a function of time. Four different FGM coating types are examined in the parametric analyses which are metal-rich, ceramic-rich, linear variation and homogeneous coatings. The results provided illustrate the influences of the FGM coating type and crack inclination angle on the transient behavior of the mixed-mode stress intensity factors.

Abstract: For accurate failure assessment, a second parameter like T-stress describing the constraint is needed in addition to the single parameter J-integral. In this work, selecting the structures of surface-cracked plate and pipe, we perform line-spring finite element modeling, and accompanying elastic-plastic finite element analyses. We then present a framework, which includes the constraint effects in the R6 FAD approach for failure assessment of cracked-structures.

Abstract: For accurate failure assessment, a second parameter like T-stress describing the constraint is needed in addition to the single parameter J-integral. In this work, selecting the structures of surface-cracked plate and pipe, we perform line-spring finite element modeling, and accompanying elastic-plastic finite element analyses. We then present a framework, which includes the constraint effects in the R6 FAD approach for failure assessment of cracked-structures.

Abstract: Critical defects in pressure vessels and pipes are generally found in the form of a semielliptical surface crack, and the analysis of which is consequently an important issue in engineering fracture mechanics. Furthermore, in addition to the traditional single parameter K or J-integral, the second parameter like T-stress should be measured to quantify the constraint effect. In this work, the validity of the line-spring model is investigated by comparing line-spring J-T solutions to the reference 3D finite element J-T solutions. A full 3D-mesh generating program for semi-elliptical surface cracks is employed to provide such reference 3D solutions. Then some structural characteristics of the surface-cracked T and L-joints are studied by mixed mode line-spring finite element. Negative T-stresses observed in T and L-joints indicate the necessity of J-T two parameter approach for analyses of surface-cracked T and L-joints.

Abstract: Fatigue crack propagation life of weld toe crack through residual stress field was estimated using Elber's crack closure concept. Propagation of weld toe crack is heavily influenced by residual stresses caused by the welding process, so it is essential to take into account the effect of residual stresses on the propagation life of a weld toe crack. Fatigue cracks at transverse and longitudinal weld toe was studied, these two cases represent the typical weld joints in ship structures. Numerical and experimental studies are performed for both cases. Residual stresses near the welding area were estimated through a nonlinear thermo-elasto-plastic finite element method and the residual stress intensity factor with Glinka's weight function method. Effective stress intensity factor was calculated using the Newman-Forman-de Koning-Henriksen equation, which is based on the Dugdale strip yield model in estimating the crack closure level, U, at different stress ratios. Calculated crack propagation life coincided well with experimental results.