• Transactions of the Korean Society of Mechanical Engineers A
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• v.35 no.3
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• pp.251-257
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• 2011

This paper deals with the statistical properties of parameters B and q in the creep crack growth rate (CCGR) law, da/dt=B$(C^*)^q$, in Mod. 9Cr-1Mo (ASME Gr.91) steel which is considered a candidate materials for fabricating next generation nuclear reactors. The CCGR data were obtained by creep crack growth (CCG) tests performed on 1/2-inch compact tension (CT) specimens under an applied load of 5000N at a temperature of $600^{\circ}C$. The CCG behavior was analyzed statistically using the empirical equation between CCGR, da/dt and the creep fracture mechanics parameter, $C^*$. The B and q values were determined for each specimen by the least-squares fitting method. The probability distribution functions for B and q were investigated using normal, log-normal, and Weibull distributions. As far as this study is considered, it can be appeared that B and q followed the log-normal and Weibull distributions. Moreover, a strong positive linear correlation was found between B and q.

• KSCE Journal of Civil and Environmental Engineering Research
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• v.30 no.4D
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• pp.387-393
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• 2010

This paper studies static crack growth of asphalt pavement using the extended finite element method (XFEM). To consider nonlinear characteristics of asphalt concrete, a viscoelastic constitutive equation using the Maxwell chain is used. And a linear cohesive crack model is used to regularize the crack. Instead of constructing the viscoelastic constitutive law from the Prony approximation of compliance and retardation time measured experimentally, we use a smooth log-power function which optimally fits experimental data and is infinitely differentiable. The partial moduli of the Maxwell chain from the log-power function make analysis easy because they change more smoothly in a more stable way than the ordinary method such as the least square method. Using the developed method, we can simulates the static crack growth test results satisfactorily.

• Transactions of the Korean Society of Mechanical Engineers A
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• v.30 no.10 s.253
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• pp.1219-1226
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• 2006

The ASTM test standard recommends the use of the compact tension specimen for creep crack growth rates measurement. In the creep crack growth rate test, the displacement rate due to creep is obtained by subtracting the contribution of elastic and plastic components from the total load line displacement rate based on displacement partitioning method fur determining $C^*-integral$, which involves Ramberg-Osgood (R-O) fitting procedures. This paper investigates the effect of the R-O fitting procedures on plastic displacement rate estimates in creep crack growth testing, via detailed two-dimensional and three-dimensional finite element analyses of the standard compact tension specimen. Four different R-O fitting procedures are considered; (i) fitting the entire true stress-strain data up to the ultimate tensile strength, (ii) fitting the true stress-strain data from 0.1% strain to 0.8 of the true ultimate strain, (iii) fitting the true stress-strain data only up to 5% strain, and (iv) fitting the engineering stress-strain data. It is found that the last two procedures provide reasonably accurate plastic displacement rates and thus should be recommended in creep crack growth testing. Moreover, several advantages of fitting the engineering stress-strain data over fitting the true stress-strain data only up to 5% strain are discussed.

• Journal of the Korea Concrete Institute
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• v.13 no.6
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• pp.568-574
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• 2001

From measured responses of concrete three-point bend tests, the average values of the responses have been calculated. The fracture behavior of continuously propagating concrete crack has been analyzed from the average responses. The experimental parameters of this study were the initial notch sizes of 25.4㎜ and 6.4 ㎜ and the processing times of 2,000 sec. and 20 sec . The different notch sizes were used for the effects of the size of fracture process zone and specimen geometry, and the processing times for those of initial creep. However the load-point displacement rate in this study did not affect the experimental responses seriously. The average loads were calculated from the average external work of a series of tests, and average crack lengths were determined by using strain gages. Before the peak load, the resistance curve could be determined from the size of fracture process zone, but unstable crack propagation of 88㎜ occurred at the load-point displacement of 0.088∼0.154㎜ after the peak load. The average fracture energy density G$\_$F/$\^$ave/ = 115 N/m occurred during the unstable crack propagation. The fracture process zones were fully developed at the crack length of 111㎜, and the sizes of fracture process zone for initial notches of 25.4㎜ and 6.4㎜ were 86㎜ and 105㎜, respectively. Average fracture energy densities of the resistance curves after full development of fracture process zone were 229 N/m for the initial notch of 25.4㎜ and 284 N/m for 6.4㎜. The values were more than twice of G$\_$F/$\^$ave/.

• Transactions of the Korean Society of Mechanical Engineers A
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• v.24 no.5 s.176
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• pp.1331-1342
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• 2000

In power generation systems a variety of structural components typically operate at high temperature and pressure. Therefore a life assessment methodology accounting for gradual creep fracture is increasingly needed for these components. The most critical defects in such structure are generally found in the form of semi-elliptical surface cracks in the welded tubular joints. Therefore the analysis of a semi-elliptical surface crack in a plate or a shell is an important problem in engineering fracture mechanics. On this background, via shell/line-spring finite element analyses of such surface cracks in the welded T and L joints under various loadings, we investigate J-integral along the crack front We first develop T and L joints auto mesh generation program providing ABAQUS input file composed of shell/line-spring finite elements. We then further develop a T and L joints life assessment program based on the experimental creep crack growth law and auto mesh generation program in a graphical user interface format Finally the remaining life of T and L joints for various analytical parameters are assessed using the developed life assessment program.