Limits of Allowable Static and Cyclic Loads on Lens Ring Gaskets

Due to their relative simple handling and the high reliability, lens ring gaskets are widely used in high pressure connections. In addition, they are able to withstand large external forces without being damaged but at the same time they do not lose leak tightness even with very small contact forces. This ability to withstand high loads allows a lens ring gasket to be reused several times with minimal polishing after disassembling the flange connection. However, this reusability has its end when the lens ring gasket is damaged or destroyed by static or cyclic overload. Static overload is mainly caused by too high bolt tightening force and leads to large scale plastic deformation. This damage can be easily avoided by choosing the correct bolt preload and proper flange alignment. A greater difficulty is the damage prediction by cyclic overload. This is caused by pressure fluctuations, excitations in the range of natural frequencies or other external cyclic loads. The effects of these mentioned loads on the service life of the lens ring gasket additionally depend on factors such as the stiffness of the individual components of the flange connection, the initial bolt preload or the materials used. In the present work a method is presented which is able to predict the load limits of lens ring gaskets under static and cyclic loading. This method allows the consideration of different geometric influences of the individual components, of different materials and bolt preloads. From the static and cyclic forces on the flange connection, local stresses in the lens ring are calculated. This enables making a statement regarding plastic deformation due to bolt load and fatigue failure due to cyclic loads.

Retardation Effect of Cyclic Overload on Stress Corrosion Crack Growth in Stainless Steel

Stress Corrosion Cracking (SCC) has been observed in some components of austenitic stainless steels in the Boiling Water Reactors (BWRs). The structural integrity evaluation for flawed component is performed for continued service for a specified time period based on the Rules on Fitness-for-Service (FFS) for Nuclear Power Plants, such as JSME FFS Code or ASME Section XI. SCC growth evaluation is generally performed only by taking into account steady loads, such as welding residual stress. It is important to examine various factors affecting SCC growth behavior for further understanding and improvement in predicting growth behavior in the BWR environment. Cyclic overloading due to such as earthquake force is one of the important factors to be evaluated. In this study, the effect of cyclic overload on SCC growth in simulated BWR environment has been examined by using CT specimens of cold-rolled stainless steels (Type 316L). The retardation phenomenon was observed in SCC growth behavior immediately after the cyclic overloading was applied. It was considered that SCC propagation was retarded due to the compressive plastic region at the crack tip, introduced by overloads. The method of predicting the SCC growth behavior after cyclic overloading was also discussed.

Estimation of Remaining Fatigue Life for Elbow Pipe Subjected to Cyclic Overload

Low-cycle fatigue tests for STPT410 elbow pipes were conducted under displacement control with and without an internal pressure of 9 MPa. First, preliminary fatigue tests were conducted under constant displacements of ±15, ±20 and ±30 mm. Next, two-step fatigue tests were carried out in which the elbows were first subjected to cyclic displacements of ±30 or ±20 mm, which correspond to cyclic overload, and then subjected to a second displacement of ±20 or ±15 mm until a fatigue crack penetrated. The total usage factor was 0.8∼1.2. Thus, the remaining fatigue life of a given elbow pipe can be predicted by the cumulative fatigue damage rule.