T/CNS 4-2018 in English

Scope

In this section, the guidelines for axial fatigue tests were referred to from the national standards GB/T 15248-2008 "Methods of Axial Isotropic Low-Cycle Fatigue Tests on Metallic Materials," GB/T 3075-2008 "Metallic Materials—Fatigue Testing—Axial Force Control Method," and GB/T 20120.1-2006 "Corrosion of Metals and Alloys—Cyclic Fatigue Testing—Part 1: Circumferential Failure Tests." Additionally, the reference was made to ASTM standards E466 "Standard Practice for Conducting Force Controlled Constant Amplitude Axial Fatigue Tests of Metallic Materials" and E606-04 "Standard Practice for Strain-Controlled Fatigue Testing," as well as technical documents prepared by the Metal Research Institute of Chinese Academy of Sciences on high-temperature, high-pressure water corrosion fatigue testing. These were combined with the experiences from the implementation of high-temperature, high-pressure water corrosion fatigue tests at the Metal Research Institute of Chinese Academy and relevant international research institutions. The specific technical content is as follows:

(1) To ensure that the fatigue performance obtained using standard specimens can represent the parent material's fatigue performance and to guarantee the validity of the fatigue data, it is stipulated that the shape and dimensions of standard fatigue specimens should conform to certain regulations apart from the components: for rod-shaped specimens, the diameter d of the gauge length segment must be no less than 6.35 mm, with a gauge length L = 3d ± d; the radius r of the transitional round arc is 4d ± 2d. For rectangular specimens, the minimum thickness T of the gauge length segment should be 2.54 mm, with a gauge length L = 3T ± T/2 and width H = 2T; the radius r of the transitional round arc is 2T ± T/2.

(2) To ensure the validity of the fatigue data, it is stipulated that parallelism, perpendicularity, and coaxiality of specimens should generally be better than 0.08 mm.

(3) For convenience in connecting the fatigue specimen to the testing machine under high-temperature and high-pressure water conditions, a step-end connection method for rod-shaped specimens is recommended, while an ohmic jaw holding method is suggested for rectangular specimens.

(4) To achieve real-time control and monitoring of the water chemical parameters of the high-temperature and high-pressure water solution, the corrosion fatigue testing device must be equipped with a circulation water loop, which includes a storage tank, a circulation pump, a high-pressure pump, a heat exchanger, a preheater, a condenser, a back pressure valve, and ion exchange resins. The outlet of the storage tank is connected to the circulation pump via piping. The outlet from the circulation pump is split into two paths: one for monitoring the water chemistry loop and another for the high-temperature and high-pressure water testing loop. The chemical monitoring circuit includes conductivity probes, dissolved oxygen sensors, pH meters, ion exchange resins, etc., which then return to the storage tank. The high-temperature and high-pressure water testing circuit comprises a high-pressure pump, a heat exchanger, a specimen holder, a high-pressure vessel, a condenser, and a back pressure valve. The outlet of these piping connects back to the storage tank.

(5) To ensure that the test can simulate typical light-water reactor power plant service water environments, it is stipulated that the high-pressure vessel must be capable of maintaining stable sealing under 280-325°C, 8-16.5 MPa high-temperature and high-pressure water conditions for long-term operation.

(6) To ensure that fatigue tests are conducted at a constant temperature, the temperature measurement position should be near the gauge length segment of the specimen, with a fluctuation range of less than ±2°C.

(7) To guarantee key water chemical parameters in the test environment, it is necessary to monitor and control the dissolved oxygen content in the circulating water in real-time and with precision.

(8) To ensure the correct chemical parameters of the high-temperature and high-pressure water in the high-pressure vessel, the circulation flow rate should be sufficient to replace the water in the high-pressure vessel every hour. For example, for a 10L volume high-pressure vessel, the minimum circulation flow rate is 10L/h.

(9) To facilitate loading and unloading of fatigue specimens, the high-pressure vessel must have adequate size to accommodate a specimen holder with convenient operations.

(10) To achieve fatigue loading on specimens in the high-temperature and high-pressure water environment, the fatigue loading system should be sealed dynamically connected to the high-pressure vessel and maintain stable operation under high temperature and high pressure conditions.

(11) To ensure proper coaxiality during the fatigue test process, precise cooperation is required among the testing machine's loading system, the high-pressure vessel, the specimen holder, and the specimen clamps. Efforts should be made to eliminate gaps between components to provide stable and effective holding of the fatigue specimens, ensuring a coaxiality better than 0.08 mm.

(12) To ensure the effectiveness and accuracy of low-cycle fatigue tests under high-temperature and high-pressure water environments, it is stipulated that the corrosion fatigue testing device must be able to achieve strain control in such conditions or provide real-time, in-situ monitoring of the strain in the gauge length segment of the specimen under displacement control mode.

(13) To ensure a smooth, safe, and effective conduct of high-temperature and high-pressure water corrosion fatigue tests, it is stipulated that operators must strictly follow operational procedures during testing.

(14) To ensure safety, the device should automatically alarm and shut down when the outlet temperature exceeds acceptable levels; rupture valves are installed at the inlet of the high-pressure vessel to release pressure in case of fault-induced pressure increases; automatic alarms and shutdowns will occur upon excessively high temperatures or leakage from the high-pressure vessel, preheater, heat exchanger, etc.; an alarm system is also required for cooling water interruption.

(15) To ensure the stability and reliability of the testing device, accurate calibration should be conducted regularly according to regulations on force sensors, displacement sensors, high-precision linear variable differential transformer (LVDT) displacement sensors, temperature sensors in the high-pressure vessel and preheater, dissolved oxygen probes, pressure sensors, etc.

(16) To ensure the validity of test data, each experimental point typically requires three repetitions.