Papers by Keyword: Thermal Fatigue Crack

Abstract: In this study, we performed thermal stress analyses on a ventilated disk brake with a 3D model for two cases (whether the pressure distribution on a contact surface is uniform or not). A pressure distribution analysis was performed to determine the pressure distribution on the contact surface. Then, by using the results that were obtained from the pressure distribution analyses, we performed thermal stress analyses. Finally, we have found the spots where the maximum thermal stresses occur. Also, for the life evaluation of a disk brake, we have conducted the fatigue test and obtained the S-N curve. From those results, we evaluated the life of a disk brake.

Abstract: Pipelines of nuclear power plants undergo high pressure and temperature. Thermal stratification typically occurs in the surge line and the main feed water lines by flow and this stratification will initiate and propagate thermal fatigue cracks. This may cause rupture and leakage and it is a serious problem to nuclear power plants operation. Therefore it is very important to detect and measure thermal fatigue cracks. In this study, thermal fatigue cracks were generated in austenitic stainless steel specimens by a thermal cycle in notched pipes and weld jointed pipes. Ultrasonic techniques were used to evaluate the thermal fatigue crack depth. When ultrasonic waves propagate from an angle beam probe to thermal fatigue cracks, waves are reflected and diffracted. Crack depth was evaluated by the reflected signals from back wall and diffracted signals from the crack tip, but diffracted signals were too weak to detect so the reflected signals were more useful. The TOFD and dB drop methods were used in this study. The TOFD method is uses a time delay of diffracted signal from the crack tip. The dB drop method is an application of an amplitude decreasing rate by a probe moving distance.

Abstract: Damages in nuclear facilities during the operation of the Reactor Coolant System (RCS) are caused by cyclic loadings due to mechanical or thermal fatigue. Therefore, the development of an integrated technology including fabrication of standard specimens and their practical usage is needed to enhance the reliability of nondestructive testing for surge lines or main feed water lines. In this study, thermal fatigue cracks on STS 304 plates (t = 6mm) and tubes (O.D = 89.7mm. t = 7.7mm) for performance demonstration inspection were fabricated for the Reactor Coolant System (RCS) in nuclear power plants. In case of plates, cyclic thermal changes, from 20
to 450
, were loaded on the V-notched specimens under tensile stressed conditions. The applied tensile stress was 1,700MPa at the notch portion. In the case of tubes, cyclic thermal changes, from 35
to 355
, were applied on the V-notched specimens under compressive stressed conditions. In the case of plates, the initial crack was generated at 17,000 cycles and the depth of crack was 2.54mm at 22,000 cycles. Unlike the plates, in the case of tubes, the initial crack formed at 14,000 cycles and the crack penetrated the tube at 25,000 cycles. In this paper, shapes and fractographies of fabricated thermal fatigue cracks, and the used cyclic thermal loads are presented.