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CN-121977950-A - Method for determining crack growth rate and threshold value at extremely high temperature

CN121977950ACN 121977950 ACN121977950 ACN 121977950ACN-121977950-A

Abstract

The invention discloses a method for measuring crack growth rate and threshold value at extremely high temperature, which comprises the steps of firstly manufacturing a test sample and a clamp, then installing a high-temperature furnace, installing the test sample and installing a thermocouple, then connecting the test sample, a reference sample and a potential measurement system by using a high-temperature resistant wire, wherein the reference sample is arranged outside the high-temperature furnace, finally starting the test, namely, the high-temperature furnace is raised to 1000-1200 ℃ and is insulated, meanwhile, water cooling is conducted into a cooling connecting rod, a low-cycle fatigue testing machine is started to load and test the crack of the sample by the potential measurement system, adopting a K-down method to perform the test, adopting a direct-current potential method to monitor crack growth by the potential measurement system in real time, ending the test when the crack growth rate reaches 1X 10 ‑ mm/cycle, and obtaining the fatigue crack growth threshold value by data processing. The invention solves the problems of current conduction stability, load transmission accuracy and crack length measurement accuracy in the environment of 1000-1200 ℃.

Inventors

  • WU LEI
  • Ning Wenlei
  • ZHANG CHUNYU
  • MA LIFEI
  • LIU YUEJIAO

Assignees

  • 国标（北京）检验认证有限公司

Dates

Publication Date
20260505
Application Date
20251212

Claims (10)

  1. 1. A method of determining crack growth rate and threshold at very high temperatures, comprising: step 1, manufacturing a test sample and a clamp, namely preparing a high-temperature alloy material into a crack propagation rate test sample and a reference sample, and manufacturing a cooling connecting rod and a ceramic U-shaped clamp; step 2, installing a high-temperature furnace, installing a test sample and installing a thermocouple, and then connecting the test sample, a reference sample and a potential measurement system by using a high-temperature-resistant wire, wherein the reference sample is arranged outside the high-temperature furnace; And 3, starting the test, namely raising the temperature of the high-temperature furnace to 1000-1200 ℃ and preserving heat, simultaneously gradually introducing a cooling connecting rod through water cooling, starting loading the low-cycle fatigue testing machine and testing the crack of the sample through a potential measuring system, adopting a K-reduction method for the test, adopting the potential measuring system to monitor the crack growth in real time through a direct-current potential method, ending the test when the crack growth rate reaches 1X 10 -7 mm/cycle, and obtaining a fatigue crack growth threshold value delta K th through data processing.
  2. 2. The method of determining crack growth rate and threshold value at very high temperatures as claimed in claim 1, wherein the thickness of the test specimen and the reference specimen is 5mm.
  3. 3. The method for measuring crack growth rate and threshold value at extremely high temperature according to claim 2, wherein the test sample and the reference sample comprise a main body part and two stretching parts, wherein the two stretching parts are symmetrically arranged at the upper and lower parts and are integrally fixedly connected to the same side of the main body part, the two stretching parts are separated by a stretching opening, and the innermost side of the stretching opening is an included angle part of 30 degrees; the upper and lower stretching parts are uniformly provided with a stretching part mounting hole so as to be connected with respective ceramic U-shaped clamp pins, each stretching part is provided with a stretching part wire connecting hole on the side surface on the same side as the stretching opening, the upper and lower end surfaces of the main body part are symmetrically provided with a group of main body part wire connecting holes, the stretching part wire connecting holes are connected with a voltage interface, and the main body part wire connecting holes are connected with a current interface.
  4. 4. The method for determining crack growth rate and threshold value at extremely high temperature according to claim 3, wherein the high temperature furnace is a split high temperature furnace, at least two independent heating resistance wire sets are separated from each other along a plane or a plane set where a load direction of the low-cycle fatigue testing machine is located in the high temperature furnace, and each heating resistance wire set adopts an independent temperature control system to realize effective isolation of a magnetic field in a furnace chamber.
  5. 5. A method for determining crack growth rate and threshold value at very high temperature as claimed in claim 3, wherein in step 2, the process of installing the test specimen is: step 21, installing cooling connecting rods and ceramic U-shaped clamps, wherein the two cooling connecting rods 3 are firstly respectively installed on screw holes of an upper actuating cylinder and a lower actuating cylinder of a low-cycle fatigue testing machine through screw threads, and the ceramic U-shaped clamps are installed on the two cooling connecting rods 3 through the screw holes; Step 22, installing the test specimen, namely inserting the test specimen into the thin groove of the ceramic U-shaped clamp, and connecting the test specimen and the ceramic U-shaped clamp by using pins.
  6. 6. The method for determining crack growth rate and threshold value at extremely high temperature according to claim 5, wherein the ceramic U-shaped clamp is formed by custom processing of alumina ceramic, the matching precision of the ceramic U-shaped groove and the pin hole of the test sample reaches H7/g6 level, and the installation coaxiality deviation of the whole clamp system is within +/-0.8 mm.
  7. 7. A method for determining crack growth rate and threshold value at very high temperature as claimed in claim 3, wherein in step2, the process of connecting the test specimen, the reference specimen and the potential measurement system using the high temperature resistant wire is; The first interface of the current of the potential measurement system is sequentially connected with one main body part wire connecting hole of the reference sample, the other main body part wire connecting hole of the reference sample, one main body part wire connecting hole of the test sample, the other main body part wire connecting hole of the test sample and the current second interface of the potential measurement system; One tensile portion wire connection hole of the test specimen is connected with a first set of voltage first interfaces, the other tensile portion wire connection hole of the test specimen is connected with a first set of voltage second interfaces, one tensile portion wire connection hole of the reference specimen is connected with a second set of voltage first interfaces, and the other tensile portion wire connection hole of the reference specimen is connected with the second set of voltage second interfaces.
  8. 8. The method for determining crack growth rate and threshold value at extremely high temperature according to claim 3 or 7, wherein the high temperature resistant wires are platinum-rhodium alloy wires, the high temperature resistant wires are all installed by using wire installation bolts, and the high temperature resistant wires wound outside the wire installation bolts are fixed between the wire installation bolts and the alumina ceramic gaskets; the potential measurement system is a DCPD test device produced by MTS; In the test process of the step 3, the load ratio R is more than or equal to 0.1, the normalized gradient of the stress intensity factor is set to be less than or equal to-0.08 mm - in an environment with the temperature of more than-0.08 mm - 1,1000 ℃, and the test waveform adopts sine waves with the frequency of 0-30Hz.
  9. 9. A method for determining crack growth rate and threshold value at very high temperature as claimed in claim 1 or 3, wherein said step 3 comprises the steps of: Step 31, moving the high-temperature furnace to a working position through a sliding rail according to test setting, setting a test temperature, heating the high-temperature furnace to a target temperature, and preserving heat for a period of time to enable a test sample to reach a stable test temperature; step 32, applying axial fatigue load to the test sample on the low-cycle fatigue testing machine, wherein the load range is determined according to the test requirement, and the test frequency is controlled to be 0-30Hz; Step 33, utilizing a potential measurement system to monitor the change of the crack length in real time by measuring the change of the voltage value on the test sample, calculating the fatigue crack growth rate according to the relation between the voltage value and the crack length, continuously performing a fatigue test, and ending the test when the crack growth rate value reaches 10 -7 mm/time and enough data logarithm exists; And 34, selecting at least 5 da/dN-delta K data pairs which are evenly distributed between 10 -6 ~10 -7 mm/time, using da/dN as independent variable and delta K as dependent variable, fitting data points by using a linear regression method, and calculating delta K value with crack growth rate of 10 - 7 mm/time according to the fitting result, wherein the delta K value is fatigue crack growth threshold value delta K th .
  10. 10. A method of determining crack growth rate and threshold value at very high temperatures as claimed in claim 3, wherein said preparing superalloy material into crack growth rate test sample, reference sample, comprises: Step 11, rough machining of an outer contour; Step 12, machining a stretching opening and a wire connecting hole, and finely machining an external profile; step 13, tapping the wire connecting hole; And 14, removing burrs of the outer contour, and cleaning the surface by using dust-free cloth and absolute ethyl alcohol.

Description

Method for determining crack growth rate and threshold value at extremely high temperature Technical Field The invention belongs to the technical field of mechanical property testing of metal materials, and particularly relates to a method for measuring crack growth rate and threshold value at extremely high temperature. Background Superalloy is a critical material for the fabrication of hot end components (e.g., turbine disks, blades) for high-end equipment such as aircraft engines, gas turbines, and the like. With the increasing demands on efficiency and thrust weight ratio of propulsion systems, the operating temperatures of these components have generally exceeded 1000 ℃, and even progressed toward 1200 ℃. Under the extreme condition, the damage tolerance performance of the material under the action of cyclic load, namely the capability of resisting the initiation and the extension of fatigue cracks, directly determines the service safety and the service life of the component. The fatigue crack growth threshold value delta K th is a core parameter for representing the capability of the material to prevent the fatigue crack from growing, and is also a base stone for damage tolerance design and life prediction. The delta K th data of the high-temperature alloy at the real service temperature is accurately obtained, and the method has irreplaceable important significance for optimizing material components, improving manufacturing process, ensuring structural reliability and making scientific overhaul period. Currently, for fatigue crack growth tests at temperatures below 1000 ℃, there are relatively well established standard methods for measuring the crack length of test specimens, which mainly comprise: (1) Visual inspection by direct observation and recording of crack length by high temperature microscopy or industrial camera systems. The method has simple equipment, but under the ultra-high temperature, the serious oxidization of the surface of the test sample, the interference of heat radiation and the reduced definition of an environment box window lead to difficult identification of crack tips, large human error of the measurement result and basic failure under the ultra-high temperature. (2) Compliance method-the crack length is reversed by measuring Crack Opening Displacement (COD). The method performs well at higher loads and crack growth rates. However, when the threshold value Δkth is measured, the applied load level is low, resulting in a very small variation in COD. Under the high-temperature environment, the thermal expansion of the extension rod, the relaxation of the clamp and the noise of the measuring system are greatly amplified, so that the displacement measurement signal-to-noise ratio is extremely low, the data dispersibility is high, and the precision cannot be guaranteed. (3) DC potentiometry (DCPD) consists in indirectly calculating the crack length by applying a constant DC to the test specimen and by monitoring the voltage change caused by the resistance change due to crack growth. The method has high calculation precision on standard test samples such as C (T) and M (T), is easy to realize automation and closed-loop control, and is a mainstream technology of the current medium-high temperature test. However, when the temperature exceeds the critical point of 1000 ℃, conventional DCPD technology faces three major difficult surmounting obstacles: The temperature resistance limit of the wire system is that a common high-temperature-resistant wire can be rapidly oxidized and embrittled in an oxidizing atmosphere at 1200 ℃, so that the resistance is increased suddenly and even fused, and a large current (10-25A is usually required) required for measurement cannot be conducted stably. High-temperature insulation failure, namely that the insulation performance of the common high-temperature insulation material is sharply reduced at the temperature, or the common high-temperature insulation material reacts with a lead wire and a test sample to cause current leakage, so that potential measurement signals are distorted, drifted and even form short circuit. The high-temperature performance of the clamp system is insufficient, namely, the strength of the metal clamp is reduced and creep occurs at extremely high temperature, and the thermal expansion coefficient of the metal clamp is not matched with that of a test sample, so that additional assembly stress can be introduced, and the accuracy of load transmission and the coaxiality of a test are affected. In summary, the prior art lacks a complete test scheme which can stably work for a long time within the range of 1000 ℃ to 1200 ℃ and can ensure the measurement precision and reliability, and the research and the development and the safety application of a new generation of ultra-high temperature materials are seriously restricted. The invention aims to overcome the key technical bottleneck. Disclosure of Invention Aiming at the problems of