Fatigue crack prefabrication method suitable for TiA1 alloy CT sample

Abstract

The invention relates to a fatigue crack prefabricating method suitable for a TiA1 alloy CT sample, which comprises the following steps of: sequentially generating non-penetrable fatigue cracks with certain lengths on two side surfaces of the CT sample through asymmetric loading under sine cyclic load by utilizing the crack propagation resistance characteristic under plane stress, and then generating a middle crack through symmetric loading, so as to prefabricate the fatigue cracks on the two side surfaces of the CT sample. And connecting the non-penetrable cracks on the two sides to form penetrable fatigue cracks, and expanding the cracks to the required length. Compared with a traditional method for directly prefabricating the penetrable fatigue crack, the method is easy to operate, high in success rate and high in efficiency, the fatigue crack with the needed length can be rapidly and accurately prefabricated, and the defect that when a TiA1 alloy CT sample uses a traditional fatigue crack prefabricating method, due to the fact that force is too large, the TiA1 alloy CT sample is suddenly unstable and fractured is overcome; or cracks cannot be generated for a long time due to too small force.

Description

Technical Field

[0001] The invention relates to a material testing technology, in particular to a method for prefabricating fatigue cracks in TiΑ1 intermetallic compound materials. Background Art

[0002] TiΑl intermetallic compounds have both heat resistance and light weight, and their density is less than 1 / 2 of that of nickel-based high-temperature alloys (TiΑl density is 3.9-4.2, while nickel-based high-temperature alloys are 7.8-9.2). They are internationally recognized as ideal materials for manufacturing hot-end components such as rotor blades of aircraft engines above 600°C. The high-temperature and high-performance TiΑl single crystal new material invented by the team of Academician Chen Guang of Nanjing University of Science and Technology has a better match between strength and plasticity, increasing the temperature bearing capacity of TiΑl alloy from 650-750°C to above 900°C, showing broad application prospects in the field of aerospace. The fracture toughness and fatigue crack growth rate of TiΑl alloys are important performance indicators that reflect their engineering application capabilities.

[0003] According to GB / T 21143-2014, GB / T 6398-2017 and other standards, the test of fracture toughness and fatigue crack growth rate requires a natural crack to be induced in advance on the specimen. However, for TiΑl alloy intermetallic compounds, due to their low room temperature plasticity and high brittleness, it is often difficult to crack when prefabricating fatigue cracks, or once cracked, the crack expands rapidly, making it difficult to control the crack length. Directly using a specimen with a notch for testing will result in inaccurate data and low test accuracy.

[0004] At present, there is no good pre-crack method for TiΑl alloy materials, especially TiΑl single crystals. Therefore, exploring a simple, convenient and efficient fatigue crack pre-fabrication method is of great significance for the study of damage tolerance performance of new high-temperature resistant materials such as TiΑl alloys. Summary of the invention

[0005] The invention aims to provide a method for prefabrication of fatigue cracks of TiAl alloy CT specimens which is simple to operate and has a high success rate.

[0006] To achieve the above object, the technical solution of the present invention is:

[0007] A fatigue crack prefabrication method for a TiΑ1 alloy CT specimen comprises the following steps:

[0008] (1) Prepare CT specimens, and grind and polish the specimen surfaces;

[0009] (2) Then, a conical notch with a notch radius of ≤0.12 mm is pre-processed at the middle position of the line connecting the two pin holes of the specimen as a crack source;

[0010] (3) Then, two pins of the same specification and unequal diameters are used to pass through the pin holes from the first end to fix the sample in the U-shaped hook groove, and a sine wave spectrum with a tensile peak of 2000N and a stress ratio of 0.1 is applied. Under the action of cyclic load, a semi-penetrating crack is preferentially generated on one side of the sample, and the length of the crack is controlled to be 0.8-1 mm;

[0011] (4) Then, use the two unequal diameter pins mentioned above to pass through the pin holes from the second end to fix the sample in the U-shaped hook groove, and repeat step (3) to generate a semi-penetrating crack of the same length on the other side of the sample;

[0012] (5) Finally, two pins of the same diameter and the same specifications are used to pass through the pin holes to fix the specimen in the U-shaped hook groove, and the peak tensile force of the cyclic load is reduced to 500N. As the middle crack expands, the two semi-penetrating cracks will connect to form a penetrating crack. The cyclic load is maintained until the penetrating crack continues to expand by 0.4-0.6mm, and then the specimen is unloaded.

[0013] Preferably, the CT sample is prepared according to GB / T 21143-2014, with W=15.2 mm and W / B=2.

[0014] Preferably, the diameter of the first end of the unequal diameter pin is 3.7-3.8 mm, the diameter of the second end is 3.5-3.6 mm, the length is 23-25 ​​mm, and the longitudinal section of the unequal diameter pin is trapezoidal.

[0015] Preferably, the diameter of the equal-diameter pins is 3.7-3.8 mm and the length is 23-25 ​​mm.

[0016] Preferably, a high-definition camera is used to observe and measure the crack length.

[0017] Compared with the prior art, the present invention has the following advantages:

[0018] 1. The present invention is not only applicable to TiAl alloy materials, but also applicable to fatigue crack prefabrication problems of other quasi-brittle materials.

[0019] 2. The test method of the present invention is simple, highly operable, and does not require complicated equipment or instruments.

[0020] 3. The present invention utilizes the crack propagation resistance characteristics under the plane stress state to cause cracks to form on both sides of the sample first. Since the cracks are non-penetrating and the crack size is small, the applied load is much smaller than the load required for directly prefabricating a whole penetrating crack in the traditional way. In addition, since the surface cracks are closer to the plane stress state, the cracks are less likely to expand unstably than in the plane strain state. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] Figure 1 Schematic diagram of sample dimensions.

[0022] Figure 2 Prefabricate tooling for fatigue cracks.

[0023] Figure 3 It is a schematic diagram and a physical diagram of the process of the method described in the present invention. DETAILED DESCRIPTION

[0024] The general idea of ​​the present invention is: a conical notch is pre-processed at the place where cracks need to be induced, and a non-penetrating fatigue crack of a certain length is generated on the two sides of the CT sample in turn by using a fixture under a sinusoidal cyclic load. Since the crack is non-penetrating, the lateral depth of the crack is shallow, so it is more inclined to a plane stress state. From the crack extension resistance characteristics under plane stress, it can be seen that if the crack needs to continue to expand under plane stress, it is necessary to increase the crack extension driving force and increase the stress intensity factor at the crack tip. At this time, since the unilateral crack is shorter and the load is smaller, the stress intensity factor is also smaller, thus ensuring that the crack is not easy to expand instability. Afterwards, symmetrical loading is performed again. At this time, only a small load is required to generate the middle crack, and the non-penetrating cracks on both sides are connected to form a whole penetrating fatigue crack. Since the load is small at this time, the stress intensity factor at the crack tip is small, so the crack is not easy to expand instability. Finally, the small load is maintained to expand the crack to the required length.

[0025] Example

[0026] The fatigue crack prefabrication method of the TiΑ1 alloy CT specimen of the present invention is to prepare the CT specimen according to GB / T 21143-2014, the specimen W=15.2mm, W / B=2, and the specimen size is shown in Figure 1, the surface of the sample is ground and polished. Then, a conical notch with a radius of ≤0.12mm is pre-processed in the middle of the line connecting the two pin holes as a crack source. Then, two unequal diameter pins with a trapezoidal cross-section and a diameter of 3.7mm at the thickest end, 3.5mm at the thinnest end, and a length of 23mm are used (unequal diameter pins are in the shape of a truncated cone, and their diameters gradually decrease from the thickest end to the thinnest end). The pin is passed through the pin hole from the thinnest end until the sample is located at the center of the two pins. The sample is then fixed in the U-shaped hook groove and a sinusoidal wave spectrum with a peak tensile force of 2000N and a stress ratio of 0.1 is applied. Due to the asymmetry of the two ends of the pin, the tensile stress on the two sides of the sample is different. Therefore, under the action of cyclic load, one side of the sample is superior to the other. First, a semi-penetrating crack is generated, and the length of the crack is controlled to be 0.8mm. Then, the pin is passed through the pin hole from the thickest end until the sample is located at the center of the two pins. The sample is fixed in the U-shaped hook groove and a sine wave spectrum with a peak tensile force of 2000N and a stress ratio of 0.1 is applied to generate a semi-penetrating crack of the same length on the other side of the sample. Then, two equal-diameter pins with a diameter of 3.7mm are used, and the sample is located at the center of the two pins. The peak tensile force of the cyclic load is reduced to 500N. As the middle crack expands, the two semi-penetrating cracks will connect to form a penetrating crack. The load spectrum is maintained to continue to expand the penetrating crack by 0.4mm, and then unloaded. See the prefabrication tooling for cracks. Figure 2 .

[0027] Figure 3 The following is a schematic diagram and a physical diagram of the process of forming a crack using the method of the present invention. It can be seen from the cross section of the sample that the pre-made crack using the method is symmetrical on both sides and the front contour of the pre-crack is clear.

Claims

1. A fatigue crack prefabrication method suitable for TiAl alloy CT specimens, characterized in that: The steps include: (1) Prepare CT specimens, and grind and polish the specimen surfaces; (2) Then, a conical notch with a notch radius of ≤0.12 mm is pre-processed in the middle of the line connecting the two pin holes of the specimen as a crack source; (3) Then, two pins of the same specifications and unequal diameters are used to pass through the pin holes from the first end to fix the sample in the U-shaped hook groove, and a sine wave spectrum with a tensile peak of 2000N and a stress ratio of 0.1 is applied. Under the action of cyclic load, a semi-penetrating crack is preferentially generated on one side of the sample, and the length of the crack is controlled to be 0.8~1mm; (4) Then, use the two unequal diameter pins mentioned above to pass through the pin holes from the second end to fix the sample in the U-shaped hook groove, and repeat step (3) to generate a semi-penetrating crack of the same length on the other side of the sample; (5) Finally, two pins of the same diameter and the same specifications are used to pass through the pin holes to fix the specimen in the U-shaped hook groove, and the peak tensile force of the cyclic load is reduced to 500N. As the middle crack expands, the two semi-penetrating cracks will connect to form a penetrating crack. The cyclic load is maintained until the penetrating crack continues to expand by 0.4-0.6mm, and then the specimen is unloaded.

2. The method according to claim 1, characterized in that The CT specimens were prepared according to GB / T 21143-2014, with W=15.2mm and W / B=2.

3. The method according to claim 1, characterized in that The diameter of the first end of the unequal diameter pin is 3.7-3.8 mm, the diameter of the second end is 3.5-3.6 mm, and the length is 23-25 ​​mm. The longitudinal section of the unequal diameter pin is trapezoidal.

4. The method according to claim 1, characterized in that The diameter of the equal-diameter pin is 3.7~3.8mm and the length is 23~25mm.

5. The method according to claim 1, characterized in that A high-definition camera is used to observe and measure the crack length.

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