Woo-Yeon Cho

Stress Intensity Factor and Elastic COD for Circumferential Through-Wall Cracks in the Interface Between an Elbow and a Straight Pipe Under Internal Pressure

Leak-Before-Break (LBB) is one of important approaches applied to nuclear piping design. In the LBB assessment, it is important to evaluate crack instability and to predict leak rate based on a fracture mechanics concept, in which an idealized straight pipes with through-wall cracks (TWCs) are generally considered in the typical LBB analysis. On the other hand, in nuclear piping system, elbows are often connected with straight pipes by welding, in which cracks could occur as well known. Hence, accurate assessment needs to be performed for cracks in weldments joining an elbow and attached straight pipe.In the previous study, it has been revealed that crack instability of a TWC in the interface between an elbow and a straight pipe under bending moment could be different with that of a straight pipe with a TWC depending on a change of pipe thickness, radius of curvature and crack length. Especially, elbows attached to a pipe were more severe than straight pipes for relatively shorter crack length. Thus, a need of engineering solutions for cracks in the interface between an elbow and a pipe is raised for accurate LBB analysis on nuclear piping system.In this present study, stress intensity factor (SIF) and crack opening displacement (COD) are estimated via detailed 3-dimensional finite element (FE) elastic analyses for circumferential TWCs in the interface between an elbow and a straight pipe subjected to internal pressure. The geometric variables (pipe thickness, radius of curvature and crack length) affecting SIFs and CODs were systematically considered in order to cover actual ranges of geometric variables. Also, the effect of elbow on elastic fracture parameters was investigated by comparing the present results with the results from the previous straight pipe solution. Moreover, based on the present FE results, the shape factors (F, V) used for calculating SIFs and CODs are proposed for circumferential TWCs in the interface between an elbow and a pipe. The present results can be used to perform the accurate LBB assessment for nuclear piping system including elbows welded to a straight pipe.Copyright

On Probabilistic Fracture Assessment of Defective Pipeline Based on Target Failure Probabilities

This paper investigates the priorities as well as the safety factors of each assessment variable for a defective pipeline based on a partial safety factor concept considering the target failure probabilities during operating period of components of interest.For this, firstly wall-thinned pipeline under internal pressure is considered, which is important in fitness-for-service assessment of corroded pipeline. For the analysis, scatters in the applied pressure, mechanical properties and geometries of wall-thinned pipeline are considered using normal and log-normal distributions. In addition, partial safety factors of a circumferential through-wall cracked pipeline subjected to global bending moment are also evaluated based on the elastic-plastic fracture mechanics. In this case, scatters in the applied bending moment, mechanical and fracture properties are considered based on normal and log-normal distributions.More importantly, two different deterministic integrity assessment methods are applied to wall-thinned pipeline and two different estimation methods of elastic-plastic J-integral are applied to circumferential through-wall cracked pipeline to evaluate the partial safety factors. Resulting values of partial safety factors are calculated using both the advanced first-order second moment method (AFOSM) and the second-order reliability method (SORM). Moreover, the effects of statistical distributions and variations of standard deviations of assessment variables on the partial safety factors are also demonstrated.Copyright

Tensile Strain Capacity of X80 and X100 Welds

High-strength pipelines (API 5L grade X70 and above) provide viable economic options for large-diameter and high-pressure transmission of energy products. To facilitate the understanding and potential use of high-strength pipelines, the tensile strain capacity (TSC) of X80 and X100 girth welds was evaluated through a series of mechanical tests and analytical/computational modeling. The experimental tests include tensile, Charpy, SENT, and curved-wide-plate (CWP) tests. The TSC measured from CWP tests is compared with the prediction from TSC models developed at CRES. The TSC of the girth welds is assessed by comparing experimentally measured values with the expected TSC from similar welds. The assessment confirms that this particular set of X80 and X100 girth welds provide very good tensile strain capacity.Copyright