• Transactions of the Korean Society of Mechanical Engineers A
• /
• v.30 no.10 s.253
• /
• pp.1255-1260
• /
• 2006

To maintain the structural integrity of steam generator tubes, 40% of wall thickness plugging criterion has been developed. The approach is for the steam generator tube with single crack, so that the interaction effect of multiple cracks can not be considered. Although, recently, several approaches have been proposed to assess the integrity of steam generator tube with two identical cracks whilst actual multiple cracks reveal more complex shape. In this paper, the coalescence pressure of steam generator tube containing multiple cracks of different length is evaluated based on the detailed 3-dimensional (3-D) elastic-plastic finite element (FE) analyses. In terms of the crack shape, two collinear axial through-wall cracks with different length were considered. Furthermore, the resulting FE coalescence pressures are compared with FE coalescence pressures and experimental results for two identical collinear axial through-wall cracks to quantify the effect of crack length ratio on failure behavior of steam generator tube with multiple cracks. Finally, based on 3-D FE results, the coalescence evaluation diagrams were proposed.

• Transactions of the Korean Society of Mechanical Engineers A
• /
• v.20 no.4
• /
• pp.1275-1289
• /
• 1996

The plane elasticity problem of collinear cracks in a layered medium is investigated. The medium is modeled as bonded structure constituted from a surface layer and a semi-infinite substrate. Along the bond line between the two dissimilar homegeneous constituents, it is assumed that as interfacial zone having the functionally graded, nonhomogeneous elastic modulus exists. The layered medium contains three collinear cracks, one in each constituent material oriented perpendicular to the nominal interfaces. The stiffness matrix formulation is utilized and a set of homogeneous conditions relevant to the given problem is readily satisfied. The proposed mixed boundary value problem is then represented in the form of a system of integral equations with Cauchy-type singular kernels. The stress intensity factors are defined from the crack-tip stress fields possessing the standard square-root singular behavior. The resulting values of stress intensity factors mainly address the interactions among the cracks for various crack sizes and material combinations.

• Transactions of the Korean Society of Mechanical Engineers A
• /
• v.28 no.11
• /
• pp.1813-1821
• /
• 2004

Until now, the 40% of wall thickness criterion, which is generally used for the plugging of steam generator tubes, has been applied only to a single cracked geometry. In the previous study by the authors, a total number of 9 local failure prediction models were introduced to estimate the coalescence load of two collinear through-wall cracks and, then, the reaction force model and plastic zone contact model were selected as the optimum ones. The objective of this study is to estimate the coalescence load of two collinear through-wall cracks in steam generator tube by using the optimum local failure prediction models. In order to investigate the applicability of the optimum local failure prediction models, a series of plastic collapse tests and corresponding finite element analyses for two collinear through-wall cracks in steam generator tube were carried out. Thereby, the applicability of the optimum local failure prediction models was verified and, finally, a coalescence evaluation diagram which can be used to determine whether the adjacent cracks detected by NDE coalesce or not has been developed.

• Structural Engineering and Mechanics
• /
• v.29 no.1
• /
• pp.55-75
• /
• 2008

In this paper, the behavior of two collinear Mode-I cracks in piezoelectric/piezomagnetic materials subjected to a uniform tension loading was investigated by the generalized Almansi's theorem. Through the Fourier transform, the problem can be solved with the help of two pairs of triple integral equations, in which the unknown variables were the jumps of displacements across the crack surfaces. To solve the triple integral equations, the jumps of displacements across the crack surfaces were directly expanded as a series of Jacobi polynomials to obtain the relations among the electric displacement intensity factors, the magnetic flux intensity factors and the stress intensity factors at the crack tips. The interaction of two collinear cracks was also discussed in the present paper.

• Transactions of the Korean Society of Mechanical Engineers A
• /
• v.22 no.4
• /
• pp.779-789
• /
• 1998

In this paper, the thermal shock responses of collinear cracks in a layered medium are investigated based on the uncoupled, quasi-static plane thermoelasticity. The medium is modeled as a bonded structure composed of a surface layer and a semi-infinite substrate. Between these two dissimilar homogeneous constituents, a functionally graded interfacial zone exists with the nonhomogeneous features of continuously varying thermoelastic properties. Three cracks are assumed to be present in the layered medium, one in each one of the constituent materials, aligned collinearly normal to the nominal interfaces. A system of singular integral equations is solved, subjected to the forcing terms of equivalent transient thermal tractions acting on the locations of cracks via superposition. Main results presented are the transient thermal stress intensity factors to illustrate the parametric effects of various geometric and amterial combinations of the medium with the thermoelastically graded interfacial zone and the collinear cracks.

• Transactions of the Korean Society of Mechanical Engineers A
• /
• v.39 no.3
• /
• pp.269-273
• /
• 2015

The study investigated the effect of the distance between two collinear circumferential surface cracks on the primary stress corrosion crack (PWSCC) growth in alloy 600TT steam generator tubes using a finite element damage analysis based on the PWSCC initiation model and macroscopic phenomenological damage mechanics approach. The damage analysis method was verified by comparing the results to the previous study results. The verified method was applied to collinear circumferential surface PWSCCs. As a result, it was found that the collinear cracks showed earlier coalescence and penetration times than the a single crack, and the times increased with the distance. In addition, it is expected that penetration may occur before coalescence of two cracks if they are more than a specific distance apart.

• Structural Engineering and Mechanics
• /
• v.43 no.3
• /
• pp.311-320
• /
• 2012

The higher-order asymptotic C(t) - $A_2(t)$ approach was employed to investigate the crack-tip stress of two collinear cracks in a power-law creeping material under the plane strain conditions. A comprehensive calculation was made of the single crack, collinear crack model with S/a = 0.4 and 0.8, by using the C(t) - $A_2(t)$ approach, HRR-type field and the finite element analysis; the latter two methods were used to check the constraint significance and the calculation accuracy of the C(t) - $A_2(t)$ approach, respectively. With increasing the creep time, the constraint $A_2$ was exponentially increased in the small-scale creep stage, while no discernible dependency of the constraint $A_2$ on the creep time was found at the extensive creep state. In addition, the creep time and the mechanical loads have no distinct influence on accuracy of the results obtained from the higher-order asymptotic C(t) - $A_2(t)$ approach. In comparison with the HRR-type field, the higher-order asymptotic C(t) - $A_2(t)$ solution matches well with the finite element results for the collinear crack model.

• Journal of Welding and Joining
• /
• v.22 no.4
• /
• pp.73-81
• /
• 2004

Failure mechanisms of welded joints under fatigue loads are interpreted that multiple collinear surface cracks initiating randomly along the weld toes propagate under the mutual interaction and coalescence of adjacent two cracks. To estimate fatigue crack propagation life for three types of the representative welded joints, i.e. non-load carrying cruciform, cover plate and longitudinal stiffener joint, the stress intensity factors at the front of the surface cracks have to be calculated, which are influenced strongly by the geometry of attachments, weld toes and the crack shapes. For the effective calculation of the stress intensity factors the Mk-factor was introduced which can be derived by a parametric study performed by FEM considering influence of the geometrical effects. The fatigue life of the cruciform joint was estimated by using the Mk-factors and the method considering the propagation mechanisms of the multiple surface cracks. Analysis results for the fatigue life had a good agreement with that of experiment.

• Transactions of the Korean Society of Mechanical Engineers A
• /
• v.20 no.12
• /
• pp.3838-3846
• /
• 1996

A method is proposed to obtain the stress intensity factors of multiple cracks lying on many straight llnes in an infinite isotropic plate. In this mehtod, analytical solutions for collinear multiple cracks subject to surface point forces are obrained and used as Green functions. For the multiple cracks lying onmany straight lines, the equivalent crack surface tractions are obtained by using the alternating method and the stress intensity factors are calculated. By using the proposed method several useful problems are solved and discussed.

• Nuclear Engineering and Technology
• /
• v.36 no.4
• /
• pp.316-326
• /
• 2004

The $40\%$ of wall criterion, which is generally used for the plugging of steam generator tubes, is applied only to a single crack. In a previous study, a total number of 9 failure models were proposed to estimate the local failure of the ligament between cracks, and the optimum coalescence model of multiple collinear cracks was determined among these models. It is, however known that parallel axial cracks are more frequently detected than collinear axial cracks during an in-service inspection. The objective of this study is to determine the plastic collapse model that can be applied to steam generator tubes containing two parallel axial through-wall cracks. Three previously proposed local failure models were selected as the candidates. Subsequently, the interaction effects between two adjacent cracks were evaluated to screen them. Plastic collapse tests for the plate with two parallel through-wall cracks and finite element analyses were performed to determine the optimum plastic collapse model. By comparing the test results with the prediction results obtained from the candidate models, a COD base model was selected as an optimum model.