High-temperature acoustic fatigue S-N curve test piece and clamp design method

Abstract

The invention belongs to the technical field of aviation acoustic fatigue, and particularly relates to a high-temperature acoustic fatigue S-N curve test piece and a clamp design method. The method comprises the following steps: step 1, determining the configuration of a test piece, determining the size of the test piece according to a preset index, and completing test piece design; 2, determining a maximum stress position on the test piece, and pasting a strain gauge on the maximum stress position; 3, determining the number of test pieces which are simultaneously installed according to the dispersity requirement; 4, determining a mounting gap between the test pieces according to the test temperature condition; 5, designing a test fixture according to the sizes of the test pieces and the number of the test pieces installed at the same time; and 6, completing the combined installation of the test piece and the test fixture. According to the invention, the test efficiency can be obviously improved, and efficient and accurate material thermo-acoustic fatigue S-N curve test can be realized.

Description

Technical Field

[0001] The present application belongs to the field of aviation acoustic fatigue technology, and in particular relates to a high-temperature acoustic fatigue SN curve test piece and a fixture design method. Background Art

[0002] During flight, aircraft will be subjected to various noise loads such as power plant noise and aerodynamic noise. When the noise load exceeds a certain level, it will produce a large excitation on the structure. In particular, when the spectral characteristics of the noise are coupled with the dynamic characteristics of the structure, the structure will produce a significant vibration stress response. This stress acts on the structure repeatedly for a long time, and cracks will be generated in stress concentration areas or other defective areas, which will then cause crack propagation until damage or destruction occurs, resulting in acoustic fatigue problems. In the study of acoustic fatigue in the fields of aerospace and aviation, it is often necessary to understand the acoustic fatigue life of certain components in high temperature environments, and to know the relationship between the stress response level and acoustic fatigue life of the materials used in these parts under high temperature environments and noise loads, that is, to obtain the thermal acoustic fatigue S-N curve of the material through experimental means.

[0003] For material fatigue special effects testing, using a vibration table to conduct vibration fatigue S-N curve testing is a mainstream method. However, there are differences between acoustic fatigue and vibration fatigue, and in many cases, special testing of acoustic fatigue / high-temperature acoustic fatigue SN curves is required. The high-temperature acoustic fatigue S-N curve test requires the test piece to be installed on the side wall of the traveling wave tube test device. This installation method determines that the high-temperature acoustic fatigue S-N curve test piece must be able to achieve a closed traveling wave tube without sound leakage. Therefore, the structural form of the vibration fatigue S-N curve test piece cannot be applied to acoustic fatigue / high-temperature acoustic fatigue SN curve tests. Special test pieces and fixtures must be designed.

[0004] In the past, square flat plate test pieces were mostly used in high-temperature acoustic fatigue S-N curve tests. The square plate was connected to the fixture on all sides and installed on the side wall of the traveling wave tube. During the test, cracks may appear at the junction of the test piece and the fixture. During the test, the acoustic fatigue S-N curve of the material is measured and calculated by the strain gauge attached to the test piece. This test piece and fixture design method has many disadvantages, mainly manifested as follows: (1) The stress gradient of the square flat plate acoustic fatigue test piece is small, the repeatability of the test piece failure position is poor, and the failure stress cannot be accurately measured by the strain gauge. (2) The test piece undergoes thermal expansion after being heated. The installation form of the four-side clamps restricts the thermal expansion of the test piece, resulting in high thermal stress inside, introducing thermal stress loads that are not required for the test, and causing serious system errors. (3) In order to ensure a reasonable response frequency and response magnitude, the square flat plate test piece is large in size. Due to the size limitation of the test piece, only one test piece can be loaded in each test, which is inefficient.

[0005] Therefore, it is desired to have a technical solution to overcome or at least alleviate at least one of the above-mentioned defects of the prior art. Summary of the invention

[0006] The purpose of the present application is to provide a high-temperature acoustic fatigue SN curve test piece and a fixture design method to solve at least one problem existing in the prior art.

[0007] The technical solution of this application is:

[0008] A high temperature acoustic fatigue SN curve test piece and fixture design method, comprising:

[0009] Step 1: Determine the test piece configuration, determine the test piece size according to the preset indicators, and complete the test piece design;

[0010] Step 2: determine the maximum stress position on the test piece and paste the strain gauge at the maximum stress position;

[0011] Step 3: Determine the number of test pieces to be installed simultaneously according to the dispersion requirements;

[0012] Step 4: Determine the installation gap between the test pieces according to the test temperature conditions;

[0013] Step 5: Design the test fixture according to the size of the test piece and the number of test pieces to be installed simultaneously;

[0014] Step 6: Complete the combined installation of the test piece and the test fixture.

[0015] In at least one embodiment of the present application, the test piece is a beam with constant thickness and variable cross-section, and the test piece configuration includes:

[0016] A fixing portion, wherein a mounting hole is formed on the fixing portion;

[0017] The movable part includes a first movable section, a second movable section, and a third movable section connected in sequence, wherein:

[0018] The first movable section is connected to the fixed portion, and outer edges of both sides of the first movable section have symmetrical arcs;

[0019] The outer edges of both sides of the second movable section are parallel to each other;

[0020] The third movable segment has a pointed head, and outer edges on both sides of the third movable segment have symmetrical arcs, and the arcs on the same side of the third movable segment and the first movable segment are in opposite directions.

[0021] In at least one embodiment of the present application, in step 1, determining the size of the test piece according to a preset indicator includes:

[0022] The calculation formula for obtaining the first-order resonance frequency of the test piece is:

[0023]

[0024] Where, f1 is the first-order resonance frequency of the test piece, L is the length of the test piece, E is the elastic modulus of the material, I is the moment of inertia, ρ is the material density, and A is the cross-sectional area of ​​the test piece;

[0025] Substituting the moment of inertia of the beam into the calculation formula of the first-order resonance frequency of the test piece, we get:

[0026]

[0027] Wherein, x is the length of the active part, r is the chamfer curvature of the arc of the first active segment, a is the width of the fixed part, b is the width of the active part, t is the thickness of the test piece, and R is the chamfer curvature of the arc of the third active segment;

[0028] Obtain the first-order resonance frequency of the test piece;

[0029] The dimensions of each test piece are determined according to the first-order resonance frequency of the test piece.

[0030] In at least one embodiment of the present application, the first-order resonance frequency of the test piece is set to 50 Hz to 100 Hz.

[0031] In at least one embodiment of the present application, in step 2, determining the maximum stress position on the test piece and attaching a strain gauge at the maximum stress position includes:

[0032] The pasting position of the strain gauge on the test piece is located on the left-right symmetrical center line of the first movable section, and the distance between the strain gauge and the end of the fixed part is:

[0033]

[0034] Among them, e is the distance between the strain gauge and the end of the fixed part, c is the length of the fixed part, x is the length of the movable part, r is the chamfer curvature of the arc of the first movable segment, a is the width of the fixed part, and b is the width of the movable part.

[0035] In at least one embodiment of the present application, in step 4, determining the installation gap between the test pieces according to the test temperature conditions includes:

[0036] The installation clearance is:

[0037]

[0038] Where d is the installation gap, α is the thermal expansion coefficient of the material, is the difference between the preload temperature and the ambient temperature, b is the width of the active part, and m is the anti-collision dimension.

[0039] In at least one embodiment of the present application, in step 5, designing a test fixture according to the size of the test piece and the number of test pieces installed simultaneously includes:

[0040] The test fixture is designed as a mouth frame structure;

[0041] The inner opening length of the mouth frame is:

[0042] g = n × (a + d)

[0043] The inner opening width of the mouth frame is:

[0044] h=x+2d

[0045] Wherein, g is the inner opening length, n is the number of test pieces installed simultaneously, a is the fixed portion width, d is the installation gap, h is the inner opening width, and x is the movable portion length.

[0046] In at least one embodiment of the present application, in step six, completing the combined installation of the test piece and the test fixture includes:

[0047] Multiple test pieces are arranged alternately on the test fixture, the test pieces are clamped by the pressure block and the test fixture, and the test pieces are fixedly mounted on the test fixture with bolts to form a whole.

[0048] The invention has at least the following beneficial technical effects:

[0049] The high-temperature acoustic fatigue SN curve test piece and fixture design method of the present application fixes a part of the test piece, can release the internal thermal stress of the test piece, can install multiple test pieces at one time, significantly improves the test efficiency, and can realize efficient and accurate material thermal acoustic fatigue S~N curve testing. BRIEF DESCRIPTION OF THE DRAWINGS

[0050] Figure 1 It is a front view of the test piece configuration of one embodiment of the present application;

[0051] Figure 2 This is a front view of the combined installation of a test piece and a test fixture according to one embodiment of the present application;

[0052] Figure 3 is a schematic diagram of the dimensions of a test piece according to an embodiment of the present application;

[0053] Figure 4 It is a schematic diagram of the combined installation dimensions of a test piece and a test fixture according to one embodiment of the present application;

[0054] Figure 5 is a cross-sectional view of a test piece and a test fixture combined installation according to one embodiment of the present application;

[0055] Figure 6It is a high temperature acoustic fatigue SN curve of one embodiment of the present application.

[0056] in:

[0057] 1-test piece; 2-test fixture; 3-clamp; 4-bolt. DETAILED DESCRIPTION

[0058] In order to make the purpose, technical scheme and advantages of the implementation of this application clearer, the technical scheme in the embodiment of this application will be described in more detail below in conjunction with the drawings in the embodiment of this application. In the drawings, the same or similar reference numerals throughout represent the same or similar elements or elements with the same or similar functions. The described embodiments are part of the embodiments of this application, not all of them. The embodiments described below with reference to the drawings are exemplary and are intended to be used to explain this application, and should not be construed as limitations on this application. Based on the embodiments in this application, all other embodiments obtained by ordinary technicians in this field without making creative work are within the scope of protection of this application. The embodiments of this application are described in detail below in conjunction with the drawings.

[0059] In the description of the present application, it should be understood that the terms "center", "longitudinal", "lateral", "front", "back", "left", "right", "vertical", "horizontal", "top", "bottom", "inside", "outside", etc., indicating orientations or positional relationships, are based on the orientations or positional relationships shown in the accompanying drawings, and are only for the convenience of describing the present application and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, be constructed and operated in a specific orientation, and therefore should not be understood as limiting the scope of protection of the present application.

[0060] The following is combined with Figures 1 to 6 This application is described in further detail.

[0061] The present application provides a high temperature acoustic fatigue SN curve test piece and a fixture design method, comprising the following steps:

[0062] Step 1: Determine the test piece configuration, determine the test piece size according to the preset indicators, and complete the test piece design;

[0063] Step 2: determine the maximum stress position on the test piece and paste the strain gauge at the maximum stress position;

[0064] Step 3: Determine the number of test pieces to be installed simultaneously according to the dispersion requirements;

[0065] Step 4: Determine the installation gap between the test pieces according to the test temperature conditions;

[0066] Step 5: Design the test fixture according to the size of the test piece and the number of test pieces to be installed simultaneously;

[0067] Step 6: Complete the combined installation of the test piece and the test fixture.

[0068] The high temperature acoustic fatigue SN curve test piece and fixture design method of the present application firstly determines the test piece configuration. Figure 1 As shown, in this embodiment, the test piece is a beam of variable cross-section of uniform thickness, which is a pointed flat-bottomed thin plate. The test piece configuration includes a fixed portion and a movable portion. The fixed portion is provided with a mounting hole for fixed installation of the test piece; the movable portion includes a first movable segment, a second movable segment, and a third movable segment connected in sequence, wherein the first movable segment is connected to the fixed portion, and the outer edges of both sides of the first movable segment have symmetrical arcs; the outer edges of both sides of the second movable segment are parallel to each other; the third movable segment has a pointed head, and the outer edges of both sides of the third movable segment have symmetrical arcs, and the arcs on the same side of the third movable segment and the first movable segment are in opposite directions.

[0069] Secondly, the size of the test piece is determined according to a preset index. In this embodiment, the selected index is the first-order resonance frequency of the test piece.

[0070] In the case of root clamped boundary, the calculation formula for the first-order resonance frequency of the single-side clamped beam is:

[0071]

[0072] Where, f1 is the first-order resonant frequency of the test piece, L is the length of the test piece (mm), E is the elastic modulus of the material (MPa), I is the moment of inertia, ρ is the material density, and A is the cross-sectional area of ​​the test piece (mm 2 );

[0073] Considering that the test piece is a beam with constant thickness and variable cross-section, the above formula needs to be changed according to the structure of the test piece. The moment of inertia expression of the beam is substituted into the calculation formula of the first-order resonance frequency of the test piece to obtain:

[0074]

[0075] Wherein, x is the length of the active part, r is the chamfer curvature of the arc of the first active segment, a is the width of the fixed part, b is the width of the active part, t is the thickness of the test piece, and R is the chamfer curvature of the arc of the third active segment;

[0076] Obtain the first-order resonance frequency of the test piece;

[0077] The dimensions of each test piece are determined according to the first-order resonance frequency of the test piece.

[0078] The design of the size of the test piece is achieved by obtaining a preset value of the first-order resonance frequency of the test piece. Similarly, the first-order resonance frequency of the test piece can also be controlled by the size of the test piece in the design as needed.

[0079] In a preferred embodiment of the present application, when the configuration of the test piece remains unchanged, the specific size of the test piece can be adjusted according to the different materials of the test piece. The first-order resonance frequency of the test piece is generally set to 50Hz~100Hz. First, preset the width a of the fixed part, and then determine the arc chamfer curvature R. The chamfer curvatures of the two arcs at the tip and root of the test piece are consistent, that is, R=r, and the arc curvature is generally set to 1.25 times the width a of the fixed part. The width b of the active part of the test piece is generally set to 1 / 2 of the width a of the fixed part of the test piece. The thickness t of the test piece is generally set to 1 / 20 of the width a of the fixed part. According to the formula, the length x of the active part of the test piece can be calculated from the preset frequency value and the elastic modulus of the material of the test piece. Four mounting holes are also required in the fixed part of the test piece to fix the test piece on the test fixture. There is no strict limitation on the position and size of the test piece mounting hole. Holes between φ6 and φ10 can be selected, but they need to be slightly larger than the size of the positioning bolt to ensure that the bolt rod can pass through the hole. At this point, the size of the test piece can be fully determined.

[0080] In step 2, the maximum stress position on the test piece is determined, and a strain gauge is attached to the maximum stress position, including:

[0081] The pasting position of the strain gauge on the test piece is located on the left-right symmetrical center line of the first movable section, and the distance between the strain gauge and the end of the fixed part is:

[0082]

[0083] Among them, e is the distance between the strain gauge and the end of the fixed part, c is the length of the fixed part, x is the length of the movable part, r is the chamfer curvature of the arc of the first movable segment, a is the width of the fixed part, and b is the width of the movable part.

[0084] In this embodiment, through the test piece configuration design, the acoustic fatigue failure site of the test piece is clear and controllable, so that the acoustic fatigue failure site of the test piece must be located near the boundary of the fixed part of the test piece, which is very convenient for observation and easy to find fatigue cracks at the first time. The stress at the acoustic fatigue failure site can be effectively measured by a resistance strain gauge. The stress concentration position of the test piece is relatively fixed (the failure position of the test piece), and a resistance strain gauge can be pasted at the failure position of the test piece in advance to effectively measure the strain at the failure position of the test piece.

[0085] In step 3, the number of test pieces to be installed simultaneously is determined based on the dispersion requirements. SN curve fitting requires the support of sufficient data points. The specific number of test pieces is related to the dispersion of the test data. For metal materials, 5 load levels are generally selected, and multiple test pieces, such as 8 pieces, are installed simultaneously at each load level.

[0086] In the high-temperature acoustic fatigue SN curve test piece and fixture design method of this application, a gap needs to be reserved between each test piece to ensure that the test piece can expand freely after being heated without generating thermal stress, and the internal thermal stress is effectively released. It is also necessary to control the vibration of the test piece after being acoustically excited without collision. In step 4, the installation gap between the test pieces is determined according to the test temperature conditions, including:

[0087] The installation clearance is:

[0088]

[0089] Where, d is the installation gap (mm), α is the thermal expansion coefficient of the material (1 / ℃), is the difference between the preload temperature and the ambient temperature (℃), b is the width of the active part (mm), and m is the anti-collision dimension (mm). The anti-collision dimension is the gap between the test piece after thermal expansion. The reserved value is generally determined based on the gap between the test piece installation hole and the positioning bolt, and should not be less than this value.

[0090] Furthermore, in step 5, the test fixture is designed according to the size of the test piece and the number of test pieces to be installed simultaneously, including:

[0091] The test fixture is designed as a mouth frame structure, and the size of the mouth frame structure is calculated according to the size and number of test pieces;

[0092] The inner opening length of the mouth frame is:

[0093] g = n × (a + d)

[0094] The inner opening width of the mouth frame is:

[0095] h=x+2d

[0096] Wherein, g is the inner opening length, n is the number of test pieces installed simultaneously, a is the fixed portion width, d is the installation gap, h is the inner opening width, and x is the movable portion length.

[0097] Finally, in step six, the test piece and the test fixture are assembled and installed, including: Figure 2 As shown, multiple test pieces 1 are arranged in a staggered manner on the test fixture 2, the test pieces 1 are clamped by the pressure block 3 and the test fixture 2, and the test pieces 1 are fixedly installed on the test fixture 2 with the bolts 4 to form a whole, and finally installed on the traveling wave tube for testing. The staggered installation and complementary shape control the acoustic leakage of the traveling wave tube to a minimum range, and multiple test pieces 1 can be installed on the test fixture 2 at one time, which significantly improves the test efficiency.

[0098] In one embodiment of the present application, the thermoacoustic fatigue S-N curve of the high temperature alloy GH188 under the thermal noise load is measured. The physical parameters of GH188 at 20°C and 450°C are shown in Table 1.

[0099] Table 1

[0100] T(℃) 20 450 E(GPa) 213 170.37 μ 0.301 0.320 <![CDATA[α(10 -6 ℃ -1 )]]> 13.4 13.7 <![CDATA[σ(10 3 kg / m 3 )]]> 9.09 9.09 <![CDATA[K(W / m C° -1 )]]> 15.4 20.1

[0101] The design steps of the test piece and fixture are as follows:

[0102] The number of test pieces 1 installed at the same time is set to 8, and the fixed part width a=60mm is preset according to the opening size of the traveling wave tube. Then determine the arc chamfer curvature R and r. The arc curvature is generally set to 1.25 times the fixed part width a, so the arc chamfer curvature can be set to 75mm. The active part width b of the test piece 1 is generally set to 1 / 2 of the fixed part width a, which is 30mm. The test piece thickness t is generally set to 1 / 20 of the fixed part width a, which is 1.5mm. The first-order resonance frequency of the test piece is preset to 75Hz, and the material elastic modulus is 213MPa. According to the calculation formula of the first-order resonance frequency of the test piece, the active part length x of the test piece 1 is calculated to be 135mm from the preset frequency value and the elastic modulus of the test piece material. Set 4 mounting holes in the fixed part of the test piece 1, with a hole diameter of 6.2mm, and install M6 bolts. The installation clearance of the test piece 1 is calculated according to the formula, d=5mm. According to the formula, the inner opening size of the test fixture 2 is g = 315mm, h = 145mm. The designed test piece size and test fixture are as follows: Figure 3-5 shown.

[0103] The acoustic fatigue SN curve test of GH188 deformed high-temperature alloy material at 450℃ was carried out using the above test pieces and fixtures. The acoustic fatigue life S curve of GH188 deformed high-temperature alloy material at 450℃ was obtained by fitting with the least squares method. rms -N r Curve like Figure 6 As shown, the curve equation is lgS rms =-0.263lgN r +4.019, Adj. R-Square is 0.647.

[0104] The high-temperature acoustic fatigue SN curve test piece and fixture design method of the present application are as follows: the first-order resonance frequency of the test piece can be controlled in the design as needed; the acoustic fatigue failure site of the test piece is clear and controllable; the stress at the acoustic fatigue failure site of the test piece can be effectively measured by a resistance strain gauge; the test piece can expand freely after being heated, and the internal thermal stress is effectively released; a part of the test piece is fixed to release the internal thermal stress of the test piece, and multiple test pieces can be installed at one time, which significantly improves the test efficiency and can achieve efficient and accurate material thermal acoustic fatigue S~N curve testing.

[0105] The above is only a specific implementation of the present application, but the protection scope of the present application is not limited thereto. Any changes or substitutions that can be easily thought of by a person skilled in the art within the technical scope disclosed in the present application should be included in the protection scope of the present application. Therefore, the protection scope of the present application shall be based on the protection scope of the claims.

Claims

1. A high temperature acoustic fatigue SN curve test piece and fixture design method, characterized in that: include: Step 1: Determine the test piece configuration, determine the test piece size according to the preset indicators, and complete the test piece design; Step 2: determine the maximum stress position on the test piece and paste the strain gauge at the maximum stress position; Step 3: Determine the number of test pieces to be installed simultaneously according to the dispersion requirements; Step 4: Determine the installation gap between the test pieces according to the test temperature conditions; Step 5: Design the test fixture according to the size of the test piece and the number of test pieces to be installed simultaneously; Step 6: Complete the combined installation of the test piece and the test fixture.

2. The high temperature acoustic fatigue SN curve test piece and fixture design method according to claim 1 is characterized in that: In step 1, the test piece is a beam of constant thickness and variable cross-section, and the test piece configuration includes: A fixing portion, wherein a mounting hole is formed on the fixing portion; The movable part includes a first movable section, a second movable section, and a third movable section connected in sequence, wherein: The first movable section is connected to the fixed portion, and outer edges of both sides of the first movable section have symmetrical arcs; The outer edges of both sides of the second movable section are parallel to each other; The third movable segment has a pointed head, and outer edges on both sides of the third movable segment have symmetrical arcs, and the arcs on the same side of the third movable segment and the first movable segment are in opposite directions.

3. The high temperature acoustic fatigue SN curve test piece and fixture design method according to claim 2 is characterized in that: In step 1, the size of the test piece is determined according to the preset indicators, including: The calculation formula for obtaining the first-order resonance frequency of the test piece is: Where, f1 is the first-order resonance frequency of the test piece, L is the length of the test piece, E is the elastic modulus of the material, I is the moment of inertia, ρ is the material density, and A is the cross-sectional area of ​​the test piece; Substituting the moment of inertia of the beam into the calculation formula of the first-order resonance frequency of the test piece, we get: Wherein, x is the length of the active part, r is the chamfer curvature of the arc of the first active segment, a is the width of the fixed part, b is the width of the active part, t is the thickness of the test piece, and R is the chamfer curvature of the arc of the third active segment; Obtain the first-order resonance frequency of the test piece; The dimensions of each test piece are determined according to the first-order resonance frequency of the test piece.

4. The high temperature acoustic fatigue SN curve test piece and fixture design method according to claim 3 is characterized in that: The first-order resonance frequency of the test piece is set to 50 Hz to 100 Hz.

5. The high temperature acoustic fatigue SN curve test piece and fixture design method according to claim 4, characterized in that: In step 2, the maximum stress position on the test piece is determined, and a strain gauge is attached to the maximum stress position, including: The pasting position of the strain gauge on the test piece is located on the left-right symmetrical center line of the first movable section, and the distance between the strain gauge and the end of the fixed part is: Among them, e is the distance between the strain gauge and the end of the fixed part, c is the length of the fixed part, x is the length of the movable part, r is the chamfer curvature of the arc of the first movable segment, a is the width of the fixed part, and b is the width of the movable part.

6. The high temperature acoustic fatigue SN curve test piece and fixture design method according to claim 5, characterized in that: In step 4, the installation gap between the test pieces is determined according to the test temperature conditions, including: The installation clearance is: Where d is the installation gap, α is the thermal expansion coefficient of the material, is the difference between the preload temperature and the ambient temperature, b is the width of the active part, and m is the anti-collision dimension.

7. The high temperature acoustic fatigue SN curve test piece and fixture design method according to claim 6, characterized in that: In step 5, the test fixture is designed according to the size of the test piece and the number of test pieces to be installed simultaneously, including: The test fixture is designed as a mouth frame structure; The inner opening length of the mouth frame is: g = n × (a + d) The inner opening width of the mouth frame is: h=x+2d Wherein, g is the inner opening length, n is the number of test pieces installed simultaneously, a is the fixed portion width, d is the installation gap, h is the inner opening width, and x is the movable portion length.

8. The high temperature acoustic fatigue SN curve test piece and fixture design method according to claim 7, characterized in that: In step six, the test piece and the test fixture are assembled and installed, including: Multiple test pieces are arranged alternately on the test fixture, the test pieces are clamped by the pressure block and the test fixture, and the test pieces are fixedly mounted on the test fixture with bolts to form a whole.

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