Method for converting low cycle test stresses to pulsating stresses

Abstract

The application provides a method for converting low cycle test stress to pulsating stress, calculates the root mean square value of a concerned position according to the root mean square method, obtains a stress scaling coefficient by dividing the maximum elastic stress value of the concerned position by the root mean square value, divides the stress value difference of the concerned position by the scaling coefficient, makes the maximum elastic stress value brought into a Goodman straight line model not exceed the tensile strength value of the material, iteratively obtains the scaled pulsating stress value through the Goodman straight line model, and finally multiplies the scaled pulsating stress value by the scaling coefficient to obtain the actual pulsating stress value. The application solves the problem that when the concerned position has stress concentration and the maximum elastic stress value σmax exceeds the tensile strength σb value of the material, the Goodman straight line model is no longer applicable to converting the test stress cycle to the pulsating stress cycle, and if the Goodman straight line model is used to solve the pulsating stress value, the converted pulsating stress value will be greater than the maximum elastic stress value σmax, which is extremely unreasonable.

Description

Technical Field

[0001] This invention relates to a method for converting low-cycle test stress into pulsating stress, belonging to the field of finite element simulation calculation of aero-engine structural strength, specifically involving a method for converting low-cycle test stress into pulsating stress when the strength limit is exceeded. Background Technology

[0002] When designing the load spectrum for low-cycle fatigue tests of aero-engine rotor disks, the ideal speed cycle should be 0 r / min - peak speed - 0 r / min. However, due to limitations in the test apparatus's control capabilities, the valley speed cannot be unloaded to zero. In actual tests, the valley speed is generally taken as 5% of the peak speed cycle, resulting in a speed cycle spectrum of 5% peak speed - peak speed - 5% peak speed, causing the stress ratio of the test cycle to be non-zero. When subsequently calculating the test load coefficient, to ensure that the test conditions are comparable to the engine's load level, the test stress cycle must be a pulsating cycle (stress ratio of zero). Therefore, converting the test stress cycle into a standard pulsating stress cycle based on the Goodman linear model is a necessary and crucial step.

[0003] Currently, by performing finite element simulation calculations on the disk structure, the maximum elastic stress value σ at the test speed corresponding to the test speed of the part of interest on the disk at the peak speed can be obtained. max However, when there is stress concentration in the area of ​​interest and the maximum elastic stress value σ max Exceeding the tensile strength σ of the material b When the value is set, the calculated pulsating stress value will be greater than the maximum elastic stress value σ. max This is inconsistent with reality, and in this case, the Goodman linear model will not be applicable to the calculation of pulsating stress. Summary of the Invention

[0004] To address the aforementioned technical problems, this invention provides a method for converting low-cycle test stress into pulsating stress.

[0005] The present invention is achieved through the following technical solutions.

[0006] This invention provides a method for converting low-cycle test stress into pulsating stress. Based on the mesh size, multiple mesh regions are selected within the stress decrease trend range, and the average stress values ​​of these regions are read. Then, the root mean square (RMS) value of the region of interest is calculated using the RMS method. The maximum elastic stress value of the region of interest is divided by the RMS value to obtain a stress scaling factor. The stress difference between the regions of interest is then divided by the scaling factor to ensure that the maximum elastic stress value input into the Goodman linear model does not exceed the tensile strength of the material. The scaled pulsating stress value is then iteratively calculated using the Goodman linear model. Finally, the scaled pulsating stress value is multiplied by the scaling factor to obtain the actual pulsating stress value.

[0007] Specifically, the following steps are included:

[0008] ① The structural strength at valley and peak rotational speeds is calculated using the mesh size to obtain the stress distribution at the location of interest;

[0009] ② Select the maximum elastic stress value σ at the same node under the peak rotational speed of the part of interest based on the structural strength calculation results. max Minimum elastic stress σ at valley speed min and the tensile strength value σ at the operating temperature b ;

[0010] ③ Using the maximum elastic stress value σ max The node is the initial node. With a radius of 2 times the mesh size a, select the mesh within this range and read the average stress value X1.

[0011] ④ Based on the structural characteristics of the area of ​​interest, select the line on the trend of stress variation from large to small in the characteristic region and the line corresponding to the maximum elastic stress value σ. max The next center is located at a distance of 4 times the mesh size 2a from the node. Using 2 times the mesh size a as the radius, select the mesh within this range and read the average stress value X2.

[0012] ⑤ Repeat step ④ to continue reading the average stress values ​​X3, X4…X of (n-2) structural locations of interest. n , ;

[0013] ⑥ The average stress values ​​X1, X2...X of the areas of interest n The root mean square value is obtained by combining the root mean square method.

[0014] ⑦ The maximum elastic stress value σ of the area of ​​interest max The root mean square value X on the downward trend line RMS The ratio is defined as the scaling factor K;

[0015] ⑧ Based on the maximum elastic stress value σ max Minimum elastic stress σ at valley speed min The stress amplitude σ is obtained by using the scaling factor K. a and average stress value σ m ;

[0016] ⑨ The fatigue limit value σ when the stress ratio R = -1 is obtained according to the Goodman linear model. -1 Then, the scaled pulsating stress value σ0 when the stress ratio is 0 is calculated again based on the Goodman linear model.

[0017] ⑩ Multiply the scaled pulsating stress value by the scaling factor K to amplify it to the original stress level and obtain the actual pulsating stress value σ'.

[0018] In step ①, the structural strength is calculated using ANSYS Workbench software.

[0019] In step ⑤, n is selected based on the stress reduction trend of the structural parts of interest.

[0020] In step ⑤, X n Greater than

[0021] In step ⑥, the formula for calculating the root mean square value is:

[0022]

[0023] Among them, X RMS Let X be the root mean square value, N be the number of observations, i be the number of the i-th observation, and X be the root mean square value. n The Nth value should be the average force.

[0024] In step ⑦, the scaling factor K is calculated using the following formula:

[0025]

[0026] Among them, X RMS σ is the root mean square value. max This represents the maximum elastic stress value at the region of interest.

[0027] In step ⑧, the formula for calculating the stress amplitude is:

[0028]

[0029] The formula for calculating the average stress value is:

[0030]

[0031] In step ⑨, the formula for calculating the fatigue limit value when the stress ratio R = -1 is:

[0032]

[0033] The formula for calculating the scaling pulsating stress value is:

[0034]

[0035] In step ⑩, the formula for calculating the actual pulsating stress value is:

[0036]

[0037] The beneficial effects of this invention are as follows: In the process of calculating the experimental pulsating stress value, a calculation method is proposed that involves obtaining a stress scaling factor K at the location of interest, scaling the elastic stress value according to the scaling factor K to obtain the scaled pulsating stress, and then proportionally amplifying the pulsating stress. This method solves the problem when there is stress concentration at the location of interest and the maximum elastic stress value σ is high. max Exceeding the tensile strength value σ of the material b When converting experimental stress cycles into pulsating stress cycles, the Goodman linear model is no longer suitable. Using the Goodman linear model to solve for pulsating stress values ​​will result in calculated pulsating stress values ​​that are greater than the maximum elastic stress value σ. max This is an extremely unreasonable problem. Attached Figure Description

[0038] Figure 1 This is the stress distribution at the tenon joint R at a valley speed of 1000 r / min;

[0039] Figure 2 This is the stress distribution at the tenon joint R at a peak speed of 19080 r / min;

[0040] Figure 3 It represents the stress distribution in the mean stress region X1;

[0041] Figure 4 It represents the stress distribution in the mean stress area X2.

[0042] Figure 5 It represents the stress distribution in the mean stress area X3.

[0043] Figure 6 It represents the stress distribution in the mean stress area X4.

[0044] Figure 7 It represents the stress distribution in the mean stress area X5.

[0045] Figure 8 This is a map showing the overall location of the center of the area of ​​interest. Detailed Implementation

[0046] The technical solution of the present invention is further described below, but the scope of protection is not limited to what is described.

[0047] This invention provides a method for converting low-cycle test stress into pulsating stress, taking a compressor disc as an example, which specifically includes the following steps:

[0048] 1. The strength of the wheel was calculated using ANSYS Workbench software. With appropriate mesh sizes (1mm overall, 0.2mm for areas of interest), the structural strength was calculated at valley speeds of 1000 r / min and peak speeds of 19080 r / min. The stress distribution at the tenon-groove transition R-section of the area of ​​interest is presented. Figure 1 and Figure 2 ;

[0049] 2. Select the maximum elastic stress value σ at the peak rotational speed of the injection point based on the strength calculation results. max The minimum elastic stress σ at the same node location under the valley rotational speed of 1075 MPa. min The stress is 7 MPa, and the stress cycle spectrum is 7 MPa~1075 MPa~7 MPa. The working temperature is 296℃. The corresponding tensile strength value σ at this working temperature is... b The pressure is 836 MPa.

[0050] 3. Using the node with the maximum elastic stress value as the initial node, and a radius of 0.2 mm (twice the mesh size), select the mesh within this range. Read the average stress X1 of the first region as 987 MPa. See details... Figure 3 ;

[0051] 4. Based on the structural characteristics of the area of ​​interest, select the next center point on the stress-to-decrease trend line of the characteristic region, at a distance of 4 times the grid size (0.4 mm) from the node with the maximum elastic stress value. Using a radius of 2 times the grid size (0.2 mm), select the grid within this range and read the average stress value x2 as 887 MPa. See details... Figure 4 ;

[0052] 5. Based on the stress decrease trend of the structurally important parts, appropriately select n as 5, repeat step 4, and continue to read the average stress values ​​X3, X4, and X5 of the three structurally important parts. See details below. Figures 5-7 The pressures are 749 MPa, 656 MPa, and 548 MPa, respectively. The overall location map of the regional centers of the five structurally significant areas is shown below. Figure 8 ;

[0053] 6. Equation for calculating the root mean square value on the stress decrease trend line at structural features: X was calculated RMS =781 MPa;

[0054] 7. The maximum stress value σ of the area of ​​interest max (1075 MPa) and the root mean square value X on the downward trend line RMS The ratio (781 MPa) is defined as the scaling factor at the tenon joint, and is calculated as follows:

[0055]

[0056] 8. Obtain σ based on the Goodman linear model. -1 The value is:

[0057]

[0058] 9. Again, using the Goodman linear model, calculate the scaled pulsating stress value when the stress ratio is 0:

[0059] 10. Multiplying the pulsating stress by a scaling factor to amplify it to the original stress level yields the formula for calculating the actual pulsating stress value: At this time, the pulsating stress cycle spectrum is 0 MPa ~ 1074 MPa ~ 0 MPa.

[0060] In summary, the present invention achieves the maximum elastic stress value σ max Exceeding the tensile strength value σ of the material b At that time, the maximum elastic stress value σ max After scaling by the scaling factor K, the actual pulsating stress value σ' is obtained by proportionally enlarging it.

Claims

1. A method for converting low-cycle test stress into pulsating stress, characterized in that: Based on the mesh size, multiple mesh regions are selected within the stress decrease trend range, and the average stress values ​​of these regions are read. Then, the root mean square (RMS) value of the area of ​​interest is calculated using the RMS method. The maximum elastic stress value of the area of ​​interest is divided by the RMS value to obtain the stress scaling factor. The stress difference of the area of ​​interest is then divided by the scaling factor to ensure that the maximum elastic stress value input into the Goodman linear model does not exceed the tensile strength of the material. The scaled pulsating stress value is then iteratively obtained using the Goodman linear model. Finally, the scaled pulsating stress value is multiplied by the scaling factor to obtain the actual pulsating stress value. The specific steps include: ① The structural strength at valley and peak rotational speeds is calculated using the mesh size to obtain the stress distribution at the location of interest; ② Select the maximum elastic stress value σ at the same node under the peak rotational speed of the part of interest based on the structural strength calculation results. max Minimum elastic stress σ at valley speed min and the tensile strength value σ at the operating temperature b ; ③ Using the maximum elastic stress value σ max The node is the initial node. With a radius of 2 times the mesh size a, select the mesh within this range and read the average stress value X1. ④ Based on the structural characteristics of the area of ​​interest, select the line on the trend of stress variation from large to small in the characteristic region and the line corresponding to the maximum elastic stress value σ. max The next center is located at a node distance of 4 times the grid size 2a. Using 2 times the grid size a as the radius, select the grid within this range and read the average stress value X2. ⑤ Repeat step ④ to continue reading the average stress values ​​X3, X4…X of (n-2) structural locations of interest. n , ; ⑥ The average stress values ​​X1, X2...X of the areas of interest n The root mean square value is obtained by combining the root mean square method. ⑦ The maximum elastic stress value σ of the area of ​​interest max The root mean square value on the downward trend line The ratio is defined as the scaling factor K; ⑧ Based on the maximum elastic stress value σ max Minimum elastic stress value at valley speed The stress amplitude σ is obtained by using the scaling factor K. a and average stress value σ m ; ⑨ The fatigue limit value σ when the stress ratio R = -1 is obtained according to the Goodman linear model. -1 Then, the scaled pulsating stress value σ0 when the stress ratio is 0 is calculated again based on the Goodman linear model. ⑩ Multiply the scaled pulsating stress value by the scaling factor K to amplify it to the original stress level, and obtain the actual pulsating stress value σ'; In step ⑦, the scaling factor K is calculated using the following formula: in, The root mean square value, The maximum elastic stress value for the area of ​​interest; In step ⑨, the formula for calculating the fatigue limit value when the stress ratio R = -1 is: The formula for calculating the scaling pulsating stress value is: ； In step ⑩, the formula for calculating the actual pulsating stress value is: 。 2. The method for converting low-cycle test stress to pulsating stress as described in claim 1, characterized in that: In step ①, the structural strength is calculated using ANSYS Workbench software.

3. The method for converting low-cycle test stress to pulsating stress as described in claim 1, characterized in that: In step ⑤, n is selected based on the stress reduction trend of the structural parts of interest.

4. The method for converting low-cycle test stress to pulsating stress as described in claim 1, characterized in that: In step ⑤, X n Greater than .

5. The method for converting low-cycle test stress to pulsating stress as described in claim 1, characterized in that: In step ⑥, the formula for calculating the root mean square value is: ； in, Let X be the root mean square value, N be the number of observations, i be the number of the i-th observation, and X be the root mean square value. n The Nth value should be the average force.

6. The method for converting low-cycle test stress to pulsating stress as described in claim 1, characterized in that: In step ⑧, the formula for calculating the stress amplitude is: ； The formula for calculating the average stress value is: 。

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