CN113551965B - Method for testing fatigue crack propagation threshold value of high-temperature alloy at high temperature - Google Patents
Method for testing fatigue crack propagation threshold value of high-temperature alloy at high temperature Download PDFInfo
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Abstract
The invention belongs to the technical field of fatigue crack propagation, and particularly relates to a method for testing a fatigue crack propagation threshold value of a high-temperature alloy at a high temperature. The method comprises the following steps: prefabricating a test sample; performing first crack prefabrication at room temperature, and releasing residual stress at the gap; heating the sample by using a high-temperature furnace, preserving heat after reaching the test temperature, and prefabricating cracks again at high temperature; measuring a threshold value delta K of a lifting pair by adopting a step-by-step force reducing method; reducing the load to a crack non-propagation state, wherein the corresponding delta KiAnd delta K under the stress of the previous stagei‑1Pairing is carried out (Δ K)i+ΔKi‑1) The/2 is a threshold value delta K of the lifting pair; and after a point which corresponds to the crack and does not expand is obtained, the load is increased to enable the crack tip to begin to expand again, the steps are repeated until the confidence coefficient requirement is met, and the average value of the threshold values of the lifting pairs is the threshold value of the test condition of the sample. The method and the device can reduce the influence of oxide-induced crack closure caused by crack surface oxidation, so that the test result is more reliable.
Description
Technical Field
The invention belongs to the technical field of fatigue crack propagation, and particularly relates to a method for testing a fatigue crack propagation threshold value of a high-temperature alloy at a high temperature.
Background
Damage tolerance is a modern fatigue fracture control method that has evolved and been applied gradually from the 70's of the 20 th century. The American civil aviation administration increased the design rule of 'damage tolerance' in civil aviation regulation FAR 25.571 as early as 1978, and the current repair and advanced aircraft design of the active aircraft structure at home and abroad adopted the design rule. In damage tolerance design, the fatigue crack propagation threshold Δ Kth (hereinafter referred to as "threshold Δ Kth") of a material is a very important performance indicator, which directly affects the aircraft overhaul interval and the critical crack length. For aluminum alloy, titanium alloy and structural steel widely applied to aeronautical structures, reference is mainly made to ASTM E647-15E1, GB/T6The test standards such as 398-. The fatigue crack propagation rate is generally assumed to be equal to 10-7The stress intensity factor range value corresponding to mm/cycle is delta Kth under the condition of the alloy. However, the current standard is suitable for room temperature environment, and no standard and systematic method exists for high temperature environment.
For aircraft engines, damage tolerance based engine life control methods are also a development trend for future engine repair, new engine designs. However, due to the fact that temperature field coupling exists during the service of the engine, and due to the fact that the service conditions of the engine are more complex compared with the service conditions of an airplane due to complex structures of an engine rotor, a mortise, a blade and the like, damage tolerance is slow to develop in the field of aeroengine engineering. Wherein, because the fracture performance of the material at high temperature is difficult to measure, the damage tolerance design of the engine lacks a large amount of reliable basic data. For a high-temperature alloy which is a key material of an aircraft engine, research shows that the high-temperature alloy has comprehensive influence on factors such as creep effect of the high-temperature alloy at high temperature, oxide-induced crack closure caused by crack surface oxidation caused by high temperature and the like, and the fatigue crack propagation rate is 10-6At mm/cycle, the crack growth rate decreases rapidly, and even cracks stop growing. Therefore, the method for measuring the threshold value delta Kth at room temperature cannot be expanded to high temperature, and a reliable and feasible method for measuring the threshold value delta Kth at high temperature is urgently needed to be established.
The fatigue crack propagation threshold value is an important index for reflecting the fracture resistance of the member containing the crack defects, and provides a reliable basis for infinite life design of a crack-containing structure and small load interception without fatigue damage under variable amplitude load. The fatigue crack propagation threshold value data deletion has great influence on the use of the high-temperature alloy in the fields of aviation, aerospace and the like which relate to high-temperature long-term service.
Disclosure of Invention
The invention aims to provide a method for testing a fatigue crack propagation threshold value of a high-temperature alloy at high temperature aiming at the defects of the conventional testing technology so as to establish a set of reliable and feasible high-temperature threshold value delta Kth measuring method.
In order to achieve the purpose, the invention mainly provides the following technical scheme:
a method for testing the fatigue crack propagation threshold value of a high-temperature alloy at high temperature comprises the following steps,
step 1, prefabricating a sample;
step 2, performing first crack prefabrication at room temperature, releasing residual stress at the gap, and forming a through crack to avoid corner cracks or crack deviation at two sides at high temperature;
step 3, heating the sample by using a high-temperature furnace, preserving heat after reaching the test temperature, and prefabricating cracks again at high temperature to enable delta a = 0.4-0.6 mm;
step 4, measuring the threshold value of the lifting pair by adopting a step-by-step force reducing method, gradually reducing the force value on the premise of keeping the stress ratio until the crack does not expand, and enabling the delta K corresponding to the crack does not expandiAnd delta K under the stress of the previous stagei-1Pairing to obtain a critical lifting pair of crack propagation and crack non-propagation (delta K)i+ΔKi-1) The/2 is a threshold value delta K of the lifting pair;
and 5, after a point corresponding to the crack which does not expand is obtained, increasing the load to a force value of 3-4 levels, so that the tip of the crack can break through the obstruction of the front edge, restarting expansion, repeating the step 4 until the confidence requirement is met, and taking the average value of the threshold values of all the lifting pairs as the threshold value of the test condition of the sample.
In the test method, in the step 1, the sample is a standard compact tensile sample, the sample is a C (T) sample, the B/W of the sample is less than 0.15, the width W of the sample is more than 40mm, and the thickness B of the sample is about 4-6 mm under the condition that the load precision is ensured.
In the testing method, in the step 2, crack prefabrication is carried out at room temperature, the prefabrication length is 1.8-2.5 mm, the crack is observed on two sides at room temperature by adopting a reading microscope, and the difference between the measured values of the front side crack and the rear side crack is not more than 0.25B;
in the test method, in step 3, the temperature is kept for 30min after the test temperature is reached, and the single-side observation is carried out at high temperature by adopting a long-focus microscope through a window.
In the above test method, in step 4, the step-by-step force reduction method includes a first stage and a second stage, in the first stage, the force value is reduced by no more than 10%, so that the crack propagation rate reaches (5-10) × 10-6mm/cycle, when the crack propagation rate is reduced to (5-10) × 10-6And entering the second stage after mm/cycle, and reducing the descending amplitude of the force value in the second stage to enable the crack propagation length delta a under each stage of stress to be between 0.2mm and 0.5 mm.
In the test method described above, the magnitude of the drop in the force value is reduced to not more than 5% or 0.05kN in the second phase.
In the above test method, in step 4, the crack non-propagation state is: Δ N =106The increment delta a of crack propagation is less than 0.1 mm.
In the above test method, in step 4,. DELTA.Ki-1Corresponding crack propagation rates da/dN of not more than 5X 10- 6mm/cycle。
In the test method described above, the obstacle is the closure of the crack tip of the material at high temperature in step 5.
In the above test method, in step 5, the calculation process that satisfies the confidence requirement is: and obtaining 3-5 lifting pairs, and then calculating whether the data points of the lifting pairs meet the corresponding confidence requirement.
By the technical scheme, the invention at least has the following advantages:
1) when the crack propagation rate is reduced to (5-10) x 10-6After mm/cycle, the descending amplitude of the force value is reduced to be not more than 5% or 0.05kN, the crack expansion length delta a under each level of stress is 0.2 mm-0.5 mm, the phenomenon that the crack expansion rate is rapidly reduced and even stops expanding due to oxidation or creep and the like is avoided, and therefore the method for testing the fatigue crack expansion threshold value of the high-temperature alloy at the high temperature is more reliable.
2) According to the invention, by prefabricating the cracks at room temperature, the generation of asymmetric cracks at two sides and angular cracks is avoided, and meanwhile, the prefabrication is carried out again at the test temperature, so that the influence of temperature effects such as oxidation and thermal expansion on the elastic field of the crack tip line is avoided.
3) The invention defines the descending range of the force value and reduces the fatigue crack propagation rate to 5 multiplied by 10-6 The mm/cycle party can participate in threshold value pairing; the number of the paired data of the lifting pairs cannot exceed 3-5, the confidence degrees of all pairs are calculated, and the test is stopped if the requirement of the confidence degrees is met, so that the experimental data can be more real and effective.
4) The invention defines the invalid condition of the sample when the fatigue crack threshold value is tested by the lifting pair, and if the number of the lifting pair exceeds 5, the data is invalid, thereby avoiding the influence of long-time heat preservation on the alloy performance.
The foregoing description is only an overview of the technical solutions of the present invention, and in order to make the technical solutions of the present invention more clearly understood and to implement them in accordance with the contents of the description, the following detailed description is given with reference to the preferred embodiments of the present invention and the accompanying drawings.
Drawings
FIG. 1 shows the 650 ℃ stress ratio 0.05 da/dN- Δ K data for certain aging type nickel-base superalloys of the present invention.
Detailed Description
To further explain the technical means and effects of the present invention adopted to achieve the predetermined objects, the following detailed description of the embodiments, structures, features and effects according to the present invention will be made with reference to the accompanying drawings and preferred embodiments.
The test method needs test equipment comprising a medium-frequency fatigue tester, a high-temperature furnace (comprising a window), a reading microscope, a long-focus microscope and other related instrument accessories.
The fatigue crack propagation rate of the high-temperature alloy is 10-6At mm/cycle, the crack growth rate is rapidly reduced, and even the crack stops growing. The method for testing the threshold value of the high-temperature alloy at high temperature is as follows.
A method for testing the fatigue crack propagation threshold value of a high-temperature alloy at high temperature comprises the following steps,
step 1, prefabricating a sample;
step 2, performing first crack prefabrication at room temperature, releasing residual stress at the gap, and forming a through crack to avoid corner cracks or crack deviation at two sides at high temperature;
step 3, heating the sample by using a high-temperature furnace, preserving heat after reaching the test temperature, and prefabricating cracks again at high temperature to enable delta a =0.5 mm;
step 4, adopting a step-by-step force reducing method, and gradually reducing the force value under the premise of keeping the stress ratio, wherein the reduction range is not more than 10 percent, so that the force value reaches (5-10) x 10-6About mm/cycle, when the crack propagation rate is reduced to (5-10) × 10-6After mm/cycle, reducing the descending amplitude of the force value to be not more than 5% or 0.05kN, and enabling the crack expansion length delta a under each level of stress to be between 0.2mm and 0.5 mm; reducing the load step by step until the crack is in a non-expansion state, and enabling the crack to be in a corresponding delta K in the non-expansion stateiAnd delta K under the stress of the previous stagei-1Pairing to obtain a critical lifting pair of crack propagation and crack non-propagation (delta K)i+ΔKi-1) The/2 is a threshold value delta K of the lifting pair;
and 5, after the point where the corresponding crack does not expand is obtained, increasing the load to a force value of 3-4 levels, so that the tip of the crack can break through the obstruction of the front edge, restarting expansion, repeating the operation steps of the step 4 to the step 5 until the requirement of confidence is met, and taking the average value of the corresponding delta K of each lifting pair as the threshold value of the test condition of the sample.
Material at high temperature 10-6In the mm/cycle rate range, the crack growth rate is rapidly reduced and even stops growing due to oxidation or creep, and in the embodiment, a more reliable threshold value can be obtained by reducing the force value to obtain a smaller crack growth rate.
Specifically, in order to enable the sample to meet the requirements of experimental standards, in the step 1, the sample is a standard compact tensile sample, the sample is a C (T) sample, the B/W of the sample is less than 0.15, the width W of the sample is more than 40mm, and the thickness B of the sample is about 4-6 mm under the condition that the load precision is guaranteed.
Step 2, performing crack prefabrication at room temperature, wherein the prefabrication length is about 2mm, performing double-sided observation on the crack at room temperature by adopting a reading microscope, wherein the difference between the measured values of the front and rear side cracks is not more than 0.25B, and the stress application process of the crack prefabrication is the same as GB/T6398;
and 3, preserving the temperature for 30min after reaching the test temperature to eliminate the influence of temperature effects such as oxidation, thermal expansion and the like on the tip of the initial crack, and performing single-side observation at high temperature by adopting a long-focus microscope through a window.
In step 4, the crack non-propagation state is: Δ N =106The increment delta a of crack propagation is less than 0.1 mm.
To ensure the accuracy of the test, Δ Ki-1Corresponding crack propagation rates da/dN of not more than 5X 10-6mm/cycle is effective.
In step 5, the barrier is a plastic region caused equivalently by creep.
The number of the lifting pairs is not more than 5, so that the influence on the result caused by long-term tissue evolution and sample oxidation at high temperature is avoided. If the confidence requirement is not met even if the lifting pairs exceed 5, the sample data is considered invalid, and a real and effective threshold value is not obtained.
The calculation process meeting the confidence requirement is as follows: and obtaining 3-5 lifting pairs, and then calculating whether the data points of the lifting pairs meet the corresponding confidence requirement.
That is, in this embodiment, 3 to 5 lifting pairs can be obtained, so there should be 3 to 5 Δ K, and the average value of these Δ K is the threshold value of the experimental condition.
The method for testing the fatigue crack propagation threshold value of the high-temperature alloy at high temperature is further described by specific examples.
By adopting a conventional threshold value test method, the threshold value under the condition can not be obtained under the condition that the stress ratio of the time-effect nickel-based superalloy at 650 ℃ is 0.05.
The fatigue crack propagation door obtained after the test is carried out by adopting the method for testing the fatigue crack propagation threshold value of the high-temperature alloy at the high temperatureThe threshold, corresponding to da/dN- Δ K data points, is shown in FIG. 1 at fatigue crack propagation rates of less than 10-6And in mm/cycle, the crack propagation rate is rapidly reduced, and the threshold value cannot be measured at high temperature by adopting the traditional crack propagation threshold value test method.
The corresponding threshold value is measured by adopting the method, and the result is as follows:
TABLE 1 stress ratio of 650 deg.C 0.05 threshold value data for certain aging type nickel-base superalloy
The three lifting pairs are respectively 13.31, 13.39 and 13.55, the average value is 13.42, the standard deviation is 0.256, and the coefficient of variation is 0.019. The requirements of 95% confidence coefficient and 5% relative error are met. Therefore, the result 13.42 of the above experiment is an effective threshold value, and the experiment is stopped.
The above description is only a preferred embodiment of the present invention, and is not intended to limit the present invention in any way, and any simple modification, equivalent change and modification made to the above embodiment according to the technical spirit of the present invention are still within the scope of the technical solution of the present invention.
Claims (10)
1. A method for testing the fatigue crack propagation threshold value of a high-temperature alloy at high temperature is characterized by comprising the following steps,
step 1, prefabricating a sample;
step 2, performing first crack prefabrication at room temperature, releasing residual stress at the gap, and forming a through crack to avoid corner cracks or crack deviation at two sides at high temperature;
step 3, heating the sample by using a high-temperature furnace, preserving heat after reaching the test temperature, and prefabricating cracks again at high temperature to enable delta a = 0.4-0.6 mm;
step 4, measuring the threshold value of the lifting pair by adopting a step-by-step force reducing method, gradually reducing the force value on the premise of keeping the stress ratio until the crack does not expand, and enabling the delta K corresponding to the crack does not expandiAnd delta K under the stress of the previous stagei-1Pairing to obtain a critical lifting pair of crack propagation and crack non-propagation (delta K)i+ΔKi-1) The/2 is a threshold value delta K of the lifting pair;
and 5, after a point corresponding to the crack which does not expand is obtained, increasing the load to a force value of 3-4 levels, so that the tip of the crack can break through the obstruction of the front edge, restarting expansion, repeating the step 4 until the confidence requirement is met, and taking the average value of the threshold values of all the lifting pairs as the threshold value of the test condition of the sample.
2. The test method according to claim 1,
in the step 1, the sample is a standard compact tensile sample, the sample is a C (T) sample, B/W of the sample is less than 0.15, width W of the sample is more than 40mm, and thickness B of the sample is 4-6 mm under the condition that load precision is guaranteed.
3. The test method according to claim 1,
and 2, performing crack prefabrication at room temperature, wherein the prefabrication length is 1.8-2.5 mm, performing double-sided observation on the cracks at room temperature by adopting a reading microscope, and the difference between the measured values of the front side cracks and the rear side cracks is not more than 0.25B.
4. The test method according to claim 1,
and 3, preserving the heat for 30min after the test temperature is reached, and performing single-side observation at high temperature by adopting a long-focus microscope through a window.
5. The test method according to claim 1,
in the step 4, the step-by-step force reducing method comprises a first stage and a second stage, wherein the force value in the first stage is reduced by no more than 10%, so that the crack propagation rate reaches (5-10) x 10-6mm/cycle, when the crack propagation rate is reduced to (5-10) × 10-6mm/cycle followed by the second stage inAnd in the second stage, the descending amplitude of the force value is reduced, so that the crack propagation length delta a under each stage of stress is between 0.2mm and 0.5 mm.
6. The test method according to claim 5,
in the second phase, the magnitude of the drop in the force value is reduced to not more than 5% or 0.05 kN.
7. The test method according to claim 1,
in step 4, the crack non-propagation state is: Δ N =106The increment delta a of crack propagation is less than 0.1 mm.
8. The test method according to claim 1,
in step 4,. DELTA.Ki-1Corresponding crack propagation rates da/dN of not more than 5X 10-6mm/cycle。
9. The test method according to claim 1,
in step 5, the obstruction is the closure of the crack tip of the material at high temperature.
10. The test method according to claim 1,
in step 5, the calculation process meeting the confidence requirement is as follows: and obtaining 3-5 lifting pairs, and then calculating whether the data points of the lifting pairs meet the corresponding confidence requirement.
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