Welding Research Council Bulletin Research Articles

Long-Term Performance of Semimetallic Gaskets

Abstract Flexible graphite-based gaskets are used extensively in high-temperature applications as a replacement for asbestos-based gaskets. The effects of aging and temperature exposure on flexible graphite sheet gaskets were the subject of a previous work (Derenne et al., 1997, “Elevated Temperature Characterization of Flexible Graphite Sheet Materials for Bolted Flanged Joints,” Welding Research Council Bulletin, Issue 419, pp. 1–87.). In this paper, the effects of aging on flexible graphite under a confined-gasket configuration will be examined as they are yet unknown. This study outlines the performance evaluation of the elevated temperature behavior of flexible graphite-based gaskets under a confined configuration and long-term exposure employing a high-temperature aged leakage relaxation (HALR) fixture. This aged relaxation leakage adhesion test (ARLA)-like fixture retains the mechanical features of the aged tensile relaxation screening (ATRS)/high-temperature aged tensile relaxation (HATR) while enabling cold leakage rate and weight loss measurements. Four distinct semimetallic gaskets with different graphite-confined configurations, namely, corrugated metal, spiral wound, kammprofile, and double jacketed, were evaluated within a temperature range of 800–1200 °F (430–650 °C) for an exposure time of 2500 h. Graphite weight loss, gasket thickness change, leakage and tightness parameters, and creep and relaxation measurements were taken at regular intervals for each gasket style. To better assess the aging process, these critical mechanical and leakage properties were scrutinized; the influence of the degradation process, related mainly to graphite oxidation, was emphasized.

Suitability of fatigue crack growth thresholds at negative stress ratios for ferritic steels and aluminum alloys in flaw evaluation procedures

Fatigue crack growth thresholds for ferritic steels and aluminum alloys in flaw evaluation documents are reviewed. The WRC (Welding Research Council) Bulletin, ASME (American Society of Mechanical Engineers) Code Section VIII, IIW (International Institute of Welding) and BS (British Standards) 7910 give constant fatigue crack growth threshold values at negative stress ratios. However, the definitions of the thresholds at negative stress ratios are different between these flaw evaluation documents. From fatigue crack growth tests and collection of fatigue crack growth threshold data, the thresholds are affected by compressive stresses. It can be said that the thresholds at negative stress ratios are not constant. Therefore, the thresholds given by the WRC Bulletin, ASME Section VIII and IIW are slightly un-conservative. The threshold given by BS 7910 is considerably conservative. A suitable definition of the threshold at negative stress ratios for application in flaw evaluation procedures is proposed as the variable threshold expressed by full range of stress intensity factors.

NUMERICAL METHODS RESEARCH AND FRACTURE FAILURE ANALYSIS FOR CRITICAL CRACK SIZE

Safety analysis of fracture mechanics is an important part of structural integrity assessment and critical crack size is one ofmain concerns for fracture failure. From the perspective of mathematics, the calculation of critical crack size is the solving and analysis for nonlinear equation f(x)=0. About the solution of nonlinear equation, there are the bisection method, the false position method, the simple iteration method, the newton method and the secant method. For the complexity of load imposed on nuclear power equipment, there may be the multi-value phenomenon of critical crack size in analyzing fracture failure problems. Considering that the corresponding mechanical states of the shallow crack and the deep crack are different when the fracture failure occurs, the crack toughness factor needs to be considered in the fracture failure analysis of shallow cracks. Connecting the bottom head of nuclear power plant main equipment, the pipeline large break transient of accident condition and the welding research council bulletin 175, the algorithm and the convergence of multi-value phenomenon of critical size are studied. Using the crack toughness factor, the fracture failure critical crack size is deeply analyzed and discussed. The algorithm and fracture failure theory in the present study can provide the technical support for the prolonging operation life projects of nuclear power plant main equipment in future.

Three-Dimensional Stress Criteria—Summary of the PVRC Project

This paper summarizes work done by the Pressure Vessel Research Council (PVRC) on three-dimensional stress criteria, using primarily elastic two and three-dimensional analytical techniques, finite element analysis. The focus of the work was to recommend guidelines on evaluation of elastic stresses relative to the ASME Boiler and Pressure Vessel Code (Code) defined failure modes as they relate to stress limits. The guidelines are developed such that they may be used to update and expand the procedures for evaluating the required stress limits in Code Section VIII, Division 2 and Code Section III, Subsection NB. This paper summarizes the recommendation using ten guidelines. The project addresses eleven example problems, nine of which include finite element analyses. These example problems validate the recommendations. The detailed information is published in the Welding Research Council Bulletin 429, “3-D Stress Criteria Guidelines for Applications.” [S0094-9930(00)01601-2]

Local stress factors of a pipe-nozzle under internal pressure

A comprehensive study of local pressure stresses at the juncture of a pipe-nozzle is presented. Currently, neither experimental nor analytical data exists that is sufficient for pressure vessel designers to analyze the local pressure stresses at the juncture of a pipe-nozzle. In order to provide a comprehensive database to calculate these pressure stresses, this paper, through a finite element technique, provides a series of local pressure stress factor plots as a function of pipe-nozzle geometrical parameter β (nozzle mean radius/pipe mean radius) with a range from 0.1 to 1.0 and γ (pipe mean radius/pipe thickness) with a range from 10 to 300. These local pressure stress components supplement the stress calculations presented by the Welding Research Council Bulletin 107 for other external loadings.

Local Flexibilities in Cylindrical Shells Due to External Loadings (Design Paper)

A shortcoming in Welding Research Council Bulletin 107, “Local Stresses in Spherical and Cylindrical Shells Due to External Loadings,” Wichman et al. (1965, 1979), has been the absence of a method for determining the flexibilities of vessels under local loads. Bulletin 107 is based in part on the work of Bijlaard (1954, 1955, 1959). These works of Bijlaard contain sufficient data such that flexibilities can be determined graphically for cylindrical shells under local (nozzle) loads. A relatively simple graphic method is presented for determining the radial, circumferential rotational and longitudinal rotational flexibilities at nozzle connections in cylindrical vessels where radial and bending loads are introduced through the nozzle.

On the estimation of stress concentration and flexibility at nozzles in ellipsoidal vessel heads

The US Welding Research Council Bulletin Number 107 (WRC-107) is currently being used by many designers in the estimation of stress concentrations at nozzle-vessel junctures. The validity of the application of this design document to the case of nozzles in ellipsoidal vessel heads is studied in this paper. A number of comparisons of the stress intensity and flexibility at the nozzles between approximate approaches and the exact situation were made and discussed.