CN109520856B - A small sample creep crack propagation test method - Google Patents
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- PXHVJJICTQNCMI-UHFFFAOYSA-N Nickel Chemical compound [Ni] PXHVJJICTQNCMI-UHFFFAOYSA-N 0.000 description 2
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Abstract
本发明提供一种所需材料的体积小、试验取材对设备的损伤少且能够得到与标准试样相近的蠕变裂纹扩展数据的小试样蠕变裂纹扩展试验方法。该试验方法包括：从设备或试料采取并制备用于蠕变裂纹扩展试验的小试样；利用疲劳试验机在小试样上预制初始裂纹；在小试样的表面焊接用于电流输入和电压测量的导线；将小试样安装在蠕变裂纹扩展试验装置上，对小试样加温，充分保温后施加载荷；利用位移引伸计采集小试样的载荷线位移量；根据测得的电压，通过直流或交流电位法获得小试样的裂纹长度；对试验数据进行处理，获得蠕变裂纹起裂时间、蠕变裂纹扩展速率。该试验方法适合于难以在在役构件、薄壁件、焊缝、使用功能性梯度材料的设备上取样的情况等。
The invention provides a small sample creep crack propagation test method, which requires small volume of materials, less damage to equipment due to test materials, and can obtain creep crack propagation data similar to standard samples. The test method includes: taking and preparing small samples from equipment or samples for creep crack propagation test; using a fatigue testing machine to prefabricate initial cracks on the small samples; welding on the surface of the small samples for current input and Conductor for voltage measurement; install the small sample on the creep crack propagation test device, heat the small sample, and apply a load after sufficient insulation; use the displacement extensometer to collect the load line displacement of the small sample; according to the measured Voltage, the crack length of the small sample is obtained by the DC or AC potential method; the test data is processed to obtain the creep crack initiation time and creep crack growth rate. This test method is suitable for situations where it is difficult to sample in-service components, thin-walled parts, welds, equipment using functionally graded materials, etc.
Description
Technical Field
The invention belongs to the technical field of material performance measurement, and particularly relates to a creep crack growth test method suitable for a small sample.
Background
A compact tensile specimen 62.5mm long, 60mm wide and 25mm thick is generally required for creep crack growth tests, and the blank volume required for the tests is large. The material selection on some active equipment can affect the service life of the equipment and even cause the damage of the equipment. In some thin-wall equipment such as heat exchange tubes, engine blades and the like, welding seams and equipment using functionally graded materials are formed, and the standard samples cannot be manufactured due to small material thickness. Under these circumstances, a small sample test method is urgently needed in order to obtain creep crack growth performance of the material.
Previous research results show that the shape and size of the sample affect the restraint state of the creep crack tip (i.e., crack tip restraint), and further affect the creep crack propagation rate.
The higher the level of crack tip restraint, the faster the creep crack growth rate achieved by the test specimen. When the restraint level reaches a certain degree, the creep crack propagation rate tends to be stable. The deep crack compact tensile specimen specified in the standard for creep crack growth rate test methods for metallic materials (HB7623-1998) is a higher level of restraint and more conservative test data can be obtained with the standard specimen.
Generally, the crack tip restraint level of a small sample is lower, the crack propagation rate obtained by using the small sample is slower than that of a standard sample, and the creep crack propagation rate between a large sample and a small sample can be accurately correlated by some technical treatment, but the existing method is not mature.
Therefore, there is a need for a small sample creep crack growth test method that can obtain creep crack growth data similar to that of a standard sample.
Disclosure of Invention
The invention is completed for solving the problems in the prior art, and aims to provide a small sample creep crack growth test method which has the advantages of small volume of required materials, less damage of test materials to equipment and capability of obtaining creep crack growth data similar to that of a standard sample.
In order to achieve the above object, the inventors have intensively studied and provided the following means.
The small sample creep crack growth test method comprises the following steps:
s1: taking and preparing small samples for creep crack growth tests from the equipment or the samples;
s2: prefabricating an initial crack on the small sample by using a fatigue testing machine;
s3: welding a lead for current input and voltage measurement on the surface of the small sample;
s4: mounting the small sample on a creep crack growth test device, heating the small sample, and applying a load after sufficient heat preservation;
s5: collecting the load line displacement of the small sample by using a displacement extensometer;
s6: obtaining the crack length of the small sample by a direct current or alternating current potential method according to the measured voltage;
s7: and processing the test data to obtain creep crack initiation time and creep crack propagation rate.
In the creep crack growth test method for a small sample, the small sample is preferably a round compact tensile small sample having a diameter of 20mm, a thickness B of 1 to 5mm, and an initial crack length a0The ratio of the width W of the sample to the width W of the sample is 0.3-0.8.
In the small sample creep crack growth test method, the small sample is preferably a compact tensile small sample having a width W of 12.5mm, a thickness B of 1 to 5mm, and a length a of an initial crack0The ratio of the width W of the sample to the width W of the sample is 0.3-0.8.
In step S1 of the creep crack growth test method for small samples, the prepared small samples are preferably subjected to surface grinding, surface polishing and/or electropolishing to remove the surface residual stress of the small samples.
In step S2 of the small sample creep crack growth test method, the maximum load at the time of pre-cracking is preferably lower than the load applied to the small sample at the time of testing.
In step S4 of the small sample creep crack growth test method, the creep crack growth test apparatus is preferably a test apparatus for measuring creep crack growth of the small sample, the small sample is attached to a sample-loading plate by a jig, one end of the small sample is fixed to the sample-loading plate, and the other end of the small sample is connected to a sample-loading lever.
In step S7 of the small sample creep crack growth test method, it is preferable that the time corresponding to the creep crack initiation time of 0.2mm of crack growth be the creep crack initiation time; the creep crack growth rate described above is expressed in da/dt and is derived from time t by plotting crack length a against time t using origin software.
In step S6 of the small specimen creep crack growth test method, the crack length of the round compact elongated small specimen is preferably calculated using the following formula (1),
where a is the crack length after propagation, W is the specimen width, V0Is the initial voltage and V is the measured voltage.
In step S7 of the small sample creep crack growth test method, the creep crack growth rate is preferably characterized by a parameter C calculated by the following formula (2):
where P is the applied load, B is the thickness of the specimen, a is the crack length after propagation, and W is the specimen width; n is a power law creep index and is obtained by testing and fitting the relation between the steady-state creep strain rate and the stress under the log-log coordinate through a creep experiment;
is a creep load linear displacement rate, and is calculated by the following formula (3),
wherein P, B and a are as defined above;
is the linear displacement rate of the load, and is formed by the relation curve of the linear displacement of the load and the timeObtaining the time derivative; e ═ E/(1-v)2) E is the modulus of elasticity and v is the Poisson's ratio; m is the power law hardening equation εp＝α(σ/σys)mIn (1), alpha is the coefficient of the power-law hardening equation, ∈pIs plastic strain, σ is stress, σysObtaining alpha and m values by fitting the relation between the plastic strain and the stress of the material for yield strength; j. the design is a squarepIs a plastic component, and is calculated by the following formula (4),
wherein P, B, a, W, alpha, m and sigmaysIs as defined above, h of small sample1(a/W,m)Obtained by finite element software calculation.
Effects of the invention
According to the small sample creep crack growth test method, creep crack growth data similar to that of a standard sample can be obtained, the volume of required materials is greatly reduced, and the damage of test materials to equipment is reduced.
In addition, the test method is suitable for testing by using the test device which can simultaneously complete creep and creep crack growth tests of various types of small samples in a vacuum environment, can prevent oxidation, performs the test at high flux and obtains more stable creep crack growth test data.
Other advantageous effects of the present invention are explained in the following further disclosure.
Drawings
FIG. 1 is a schematic view of an example of a round compact tensile small sample used in the small sample creep crack growth test method of the present invention.
FIG. 2 is a schematic view of an example of a compact tensile small sample used in the small sample creep crack growth test method of the present invention.
FIG. 3 is a schematic diagram of a crack length measurement method in the small specimen creep crack growth test method of the present invention.
Figure 4 shows a schematic comparison of a standard compact tensile specimen with a round compact tensile small specimen.
Fig. 5 is a graph showing the change in linear displacement of load with time of a round compact tensile small sample in the test example of the present invention.
FIG. 6 is a graph showing the voltage change with time of a round compact tensile small sample in the test example of the present invention.
FIG. 7 is a graph showing the change of crack length with time of a round compact tensile small sample in the test example of the present invention.
FIG. 8 is a graph of creep crack rate da/dt versus C for round compact tensile coupons in test examples of the present invention.
FIG. 9 is a comparison of crack propagation rates obtained from the round compact tensile coupon test in the test examples of the present invention with the results of the standard compact tensile coupon test.
Description of the symbols
1 Power supply
2 Voltage acquisition instrument
3 conducting wire
4 Small sample
Detailed Description
The technical features of the present invention will be described below with reference to preferred embodiments and drawings, which are intended to illustrate the present invention and not to limit the present invention. The drawings are greatly simplified for illustration purposes and are not necessarily drawn to scale.
It is to be understood that the preferred embodiments of the present invention are shown in the drawings only, and are not to be considered limiting of the scope of the invention. Various obvious modifications, variations and equivalents may be made to the present invention by those skilled in the art on the basis of the examples shown in the drawings, and the technical features in the different embodiments described below may be arbitrarily combined without contradiction, and these are within the scope of protection of the present invention.
[ Small sample for creep crack growth test ]
The present invention is a method for performing a creep crack growth test using a small sample, and the small sample used in the present invention is described below with reference to fig. 1 and 2.
The small sample used in the present invention may be a round compact tensile small sample or a compact tensile small sample, and a round compact tensile small sample is preferable.
In preparing small specimens, specimens are taken from equipment or specimens and prepared into a prescribed size and shape as needed, and initial cracks are usually prepared on the small specimens using a fatigue tester.
As shown in FIG. 1, a round compact tensile small sample has a diameter of 20mm, a thickness B of 1 to 5mm, and an initial crack length a0The ratio of the width W of the sample to the width W of the sample is 0.3-0.8. Among them, the thickness is more preferably 1 to 3mm, still more preferably 1 to 2mm, and particularly preferably 1 mm. Length of initial crack a0The ratio of the width W to the width W of the sample is more preferably 0.3 to 0.8, still more preferably 0.3 to 0.6, and particularly preferably 0.4 to 0.5.
As shown in FIG. 2, the compact tensile test piece had a width W of 12.5mm, a thickness B of 1 to 5mm, and a length a of an initial crack0The ratio of the width W of the sample to the width W of the sample is 0.3-0.8. Among them, the thickness is more preferably 1 to 3mm, still more preferably 1 to 2mm, and particularly preferably 1 mm. Length of initial crack a0The ratio of the width W to the width W of the sample is more preferably 0.3 to 0.8, still more preferably 0.3 to 0.6, and particularly preferably 0.4 to 0.5.
The round compact tensile small sample and the compact tensile small sample are provided with two loading holes, and the centers of the two holes are connected with each other (O)1And O2The connecting line) is a loading line, and a plane containing the loading line and orthogonal to the side face is taken as a reference plane. The sample width W is the distance from the reference plane to the back surface of the sample.
[ creep crack growth test method for Small sample ]
The small sample creep crack growth test method of the present invention includes the following steps S1 to S6.
Step S1:
small specimens for creep crack growth tests were taken and prepared from the equipment or specimens. The small specimens used in the present invention were prepared as described in [ creep crack growth test small specimen ]. The small size of the small sample used greatly reduces the consumption of test materials.
In a preferred embodiment, the prepared small sample is subjected to surface grinding, surface polishing and/or electropolishing to remove the surface residual stress of the small sample.
Step S2:
an initial crack was prepared on the above small sample using a fatigue tester. Initial cracks were prepared on the coupons as described in [ creep crack propagation test coupons ] above.
When an initial crack is preformed on a small sample by using a fatigue testing machine, the maximum load when the crack is preformed is not larger than the load applied to the small sample in the test, and the stress ratio is 0.1. Furthermore, the prepared fatigue crack length of small specimens is usually not less than 1 mm.
Step S3:
before proceeding to step S3, the sample may be washed with an ultrasonic washer and dried.
In step S3, wires for current input and voltage measurement are welded on the surface of the small sample. As shown in fig. 3, the current input leads are connected at the farthest ends from the crack surface, respectively; the wires for voltage measurement are respectively connected to the side surfaces close to the wire cutting gaps.
Step S4:
next, the small sample is mounted on a creep crack growth test apparatus for measuring creep crack growth of the small sample, the small sample is mounted on a sample loading plate by a jig, one end of the small sample is fixed to the sample loading plate through a loading hole, and the other end is connected to a sample loading lever. Then the small sample is heated and fully insulated, and then a load is applied.
In a preferred embodiment, the creep crack growth test apparatus is a high-throughput creep and creep crack growth test apparatus for small samples, which can provide a vacuum environment, reduce the influence of oxidation on the measurement result, and improve the measurement accuracy, and which can simultaneously perform creep and creep crack growth tests for a plurality of small samples to perform a high-throughput test.
When the small sample is heated, a mica sheet can be adopted to be padded between the thermocouple and the sample so as to avoid the voltage on the sample from influencing the temperature measurement and control.
Two lead wires for current input welded on the small sample are connected with a direct current power supply or an alternating current power supply, and two lead wires for voltage measurement welded on the small sample are connected with a voltage acquisition instrument. When the crack length is measured by a direct current potential method, the input current is 1-3A, and 1A is particularly preferred.
The target temperature of the heating furnace is set, and heating is started. And after the target temperature is reached, carrying out heat preservation for at least 0.5h, and then starting a loading test.
Step S5:
and collecting the load line displacement of the small sample by using a displacement extensometer. The displacement extensometer is fixed on the sample clamp, and the clamp is connected with the small sample by adopting a pin. And (4) with the continuous expansion of the crack, the distance between the upper clamp and the lower clamp is continuously increased, and the displacement is determined as the displacement of the crack load line.
Step S6:
and obtaining the crack length of the small sample by a direct current or alternating current potential method according to the voltage measured by the voltage acquisition instrument.
In a preferred embodiment, in the case of using a round compact tensile small sample, the crack length of the round small sample can be calculated by the following formula (1),
where a is the crack length after propagation, W is the specimen width, V0Is the initial voltage and V is the measured voltage.
The above equation (1) is applicable to the case where the half distance between the voltage leads is measured to be 1.5 mm.
Step S7:
and processing the test data to obtain creep crack initiation time and creep crack propagation rate.
In a preferred embodiment, the creep crack initiation time is defined as the time corresponding to 0.2mm of crack propagation. The creep crack growth rate described above is expressed in da/dt and is derived from time t by plotting crack length a against time t using origin software.
In a preferred embodiment, the above-mentioned creep crack growth rate da/dt is characterized by a parameter C. The parameter C is suitable for cracks that propagate in a brittle or ductile mode, and is used for data correlation, characterized by the fact that the creep crack propagation data under different loads will tend to be identical in log-log coordinates. The above parameter C is calculated by the following formula (2):
where P is the applied load, B is the thickness of the specimen, a is the crack length after propagation, and W is the specimen width; n is a power law creep index and is obtained by testing and fitting the relation between the steady-state creep strain rate and the stress under the log-log coordinate through a creep experiment;
is a creep load linear displacement rate, and is calculated by the following formula (3),
wherein P, B and a are as defined above;
the linear load displacement rate is obtained by time derivation according to a relation curve of the linear load displacement and time; e ═ E/(1-v)2) E is the modulus of elasticity and v is the Poisson's ratio; m is the power law hardening equation εp＝α(σ/σys)mIn (1), alpha is the coefficient of the power-law hardening equation, ∈pIs plastic strain, σ is stress, σysObtaining alpha and m values by fitting the relation between the plastic strain and the stress of the material for yield strength; j. the design is a squarepIs a plastic component, and is calculated by the following formula (4),
wherein P, B, a, W, alpha, m and sigmaysIs as defined above, h of small sample1(a/W,m)Obtained by finite element software calculation.
According to the small sample creep crack growth test method, the small sample is used for creep crack growth test, creep crack growth data similar to that of a standard sample can be obtained, the volume of the required material is greatly reduced, and the damage of the test material to equipment is reduced.
In addition, the test method is suitable for testing by using the test device which can simultaneously complete creep and creep crack growth tests of various types of small samples in a vacuum environment, can prevent oxidation, performs the test at high flux and obtains more stable creep crack growth test data.
Test examples
In order to further explain the small sample creep crack growth test method and its effects of the present invention, the technical means of the present invention will be explained by the following test examples.
As a material tested in the test examples of the present invention, Inconel625 nickel-based alloy for aircraft engine regenerator manufacturing (supplied by hualong special steel ltd, yang, jiang su) was used, and the thickness of the steel plate was 20 mm. Prior to testing, the material was solution treated at 1050 ℃ for 4 hours and then cooled to room temperature. The basic mechanical properties of the Inconel625 nickel-base alloy at 650 ℃ are shown in Table 1.
[ Table 1]
A circular blank having a diameter of 20mm and a thickness of 1.6mm was cut out of the steel sheet by electric discharge machining. Adopt linear cutting and drilling machine processing initial breach and loading hole, through processing such as surface abrasive paper is polished, electrolysis, process thickness B to 1mm, sample width W is 15mm in addition, and loading hole diameter is 3.75 mm.
Then, the processed sample was subjected to initial fatigue crack preparation. In the test in this test example, the applied load was 420N, so that the maximum load of the prepared fatigue crack was 420N, the stress ratio was 0.1, and the prepared fatigue crack length was 1mm, thereby obtaining a round compact tensile small sample (# 1) shown in FIG. 4.
Next, the small sample was cleaned using an ultrasonic cleaning machine. Platinum wire leads for current input and voltage measurement were spot welded to the surface of the small sample. The current input wires are respectively connected with the farthest ends away from the crack surface; the wires for voltage measurement are respectively connected to the side surfaces close to the linear cutting gap, and the half distance between the voltage measurement lead wires is 1.5 mm.
A high-flux small sample creep and creep crack growth test device is adopted, one end of a small sample is fixed on a sample loading disc through a loading hole, and the other end of the small sample is connected with a loading rod. The test device provides a vacuum environment, can prevent a small sample from being oxidized at high temperature, and adopts the mica sheet to be padded between the thermocouple and the sample so as to avoid the influence of voltage on the sample on temperature measurement and control. And four wires are respectively connected with a direct current source and a voltage collector. The target temperature of the furnace was set at 650 ℃ and heating was started. And after the target temperature is reached, the temperature is kept for 0.5h, and then the loading test is started, wherein the load is 420N.
And acquiring the crack load line displacement of the small sample by using a displacement extensometer. In the test process, the displacement extensometer automatically collects and records load linear displacement data and records the data in real time. The load line displacement versus time for the round compact tensile coupon is shown in figure 5.
In the test process, the crack length is measured by adopting a direct current potential method, and the input current is 1A. And automatically acquiring and recording voltage data in the whole process. The voltage versus time for the round compact tensile small coupon is shown in figure 6. By using the function relation between the crack length and the voltage and combining the voltage data in fig. 6, the change curve of the crack length of the round compact tensile small sample along with the time is calculated and obtained, as shown in fig. 7.
The obtained test data are then processed. From fig. 7, the time corresponding to the crack growth amount of 0.2mm, i.e., the creep crack initiation time was found, and the creep crack initiation time in this test example was 330 hours. The creep crack growth rate da/dt is obtained by deriving the time t from the crack length versus time curve in FIG. 7. The creep crack growth rate da/dt is related by C, the calculation equation of C is shown in the invention content step S7, and the linear displacement rate of the load in the calculation process
Derived from the load line displacement versus time curve in fig. 5. The creep crack growth rate da/dt is plotted against C in fig. 8. It can be seen that the creep crack growth rate da/dt is linear with C on a log-log scale. Using the power law equation da/dt ═ D0C*φThe creep crack growth rate of the Inconel625 nickel-base alloy at 650 ℃ was determined by fitting the curves of fig. 8. Further, the creep crack growth rate at any C x was calculated.
Creep crack growth tests were conducted in parallel on three sets of round compact tensile small specimens (1#, 2#, 3#) according to the method described above in the test examples, and the creep crack growth rates da/dt and C obtained by data processing are shown in fig. 9.
In addition, a creep crack growth test was conducted using a standard compact tensile specimen having a length of 62.5mm, a width of 60mm and a thickness of 25mm, and as shown in FIG. 4, W was 50mm and a025mm and the diameter of the loading hole is 12.5 mm. Creep crack growth tests were performed in parallel on standard compact tensile specimens (1#, 2#, 3#), and the creep crack growth rates da/dt versus C were obtained according to ASTM E1457, and are shown in fig. 9.
As can be seen from fig. 9, the creep crack growth rate data obtained by the creep crack growth test method using the round compact tensile small sample of the present invention is within the same dispersion band as the creep crack growth rate data obtained by using the standard compact tensile sample, indicating that the creep crack growth rate data similar to that of the standard sample can be obtained by the small sample creep crack growth test method of the present invention.
In the above test examples, the case of performing the creep crack growth test using the round compact tensile small sample was exemplified, but the creep crack growth rate data similar to that of the standard sample can be similarly obtained even when the compact tensile small sample is used.
In the above test examples, the crack length of the small sample was obtained by the dc potential method, but the same test results were obtained even when the ac potential method was used, and finally creep crack growth rate data similar to that of the standard sample was obtained.
Finally, it should be understood that the above description of the embodiment and the test example is illustrative in all aspects, and not restrictive, and various modifications may be made within the scope not departing from the spirit of the present invention. The scope of the present invention is shown by the claims, not by the above embodiments or test examples. The scope of the present invention includes all modifications within the meaning and range equivalent to the claims.
Industrial applicability of the invention
The small sample creep crack growth test method can obtain the creep crack growth rate data similar to that of a standard sample by using the round compact tensile small sample or the compact tensile small sample, greatly reduces the volume of the required material, reduces the damage of the test material to equipment, and is particularly suitable for the conditions that the sample material is less and the sampling is difficult to be carried out on certain sampling limited occasions such as in-service components, thin-wall parts, welding seams and equipment using functional gradient materials.
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