Some standard content:
National Standard of the People's Republic of China
Method of axial force controlled fatiguetesting of metals
669:620
.178.3
GB 3075—82
This standard applies to the constant load amplitude axial fatigue test of metal specimens subjected to any type of cyclic stress shown in Figure 1 in air at room temperature. This test is used to determine the axial fatigue strength of metals with a number of cycles not less than 5×104. 5
Asymmetric compression
Pulsating compression
Symmetric tension and compression
Asymmetric tension and compression
Asymmetric tension
Pulsating tension
Asymmetric tension
Figure 1 Types of cyclic stress
Note: If there is a special need, other waveforms of cyclic stress can be used. Symbols, names, definitions and units
The symbols, names, definitions and units related to stress cycle (see Figure 2) and fatigue test are shown in Table 1. Table 1
Symbol
Maximum stress
Minimum stress
Average stress
The stress with the largest algebraic value in the stress cycle. Tensile stress is positive and compressive stress is negative
The stress with the smallest algebraic value in the stress cycle. With tensile stress as positive and compressive stress as negative
Algebraic mean of maximum stress and minimum stress Issued by the State Administration of Standards on May 10, 1982
(kgf imm2)
（kgf/mm2)
(kgf.mm2)
Implemented on March 1, 1983
Stress amplitude
Stress range
Stress ratio
Theoretical stress concentration factor
Cyclic rate
Conditional fatigue limit
Fatigue limit
Survival rate
Note: 1kgf/mm\= 9.8MPa.
2 Test specimen
2. Symbols and names of test specimens
GB3075-82
Continued table!
The average of the maximum and minimum stresses in the stress harmonic cycleThe algebraic difference between the maximum and minimum stresses in the stress harmonic cycleThe algebraic ratio of the minimum stress to the maximum stressThe ratio of the local stress to the nominal stress
Minimum number of stress cycles per unit time
The number of stress cycles until the specimen fails (fatigue cracks of a specified length or visible to the naked eye, complete cracks, etc. appear)
The median fatigue strength corresponding to the specified number of cycles N
The percentage of fatigue life above the specified value
Fatigue cycles
Figure 2 Fatigue stress cycle
The symbols and names related to the specimen are shown in Table 2. 120
(kgf mn)
(kgf mn*)
(kgi.mm2)
2.2 Shape and size
GB 3075—82
Diameter of the holding part of a circular cross-section specimen or the outer diameter of the threaded partDiameter of the specimen at the maximum stress point
Parallel length of the working part of the specimen
Test section thickness of a rectangular cross-section specimenWidth at the maximum stress point of a rectangular cross-section specimenWidth of the holding part of a rectangular cross-section specimenThe minimum radius of curvature of the transition from d to D or from b to B, or the arc between the holding parts of the specimenmm
The shape and size of the specimen depend on the purpose of the test, the model and capacity of the testing machine, and the shape of the test material. Its holding part should be coaxial or symmetrical with the axis of the specimen or the axis of the reduced test section (see Figures 3 to 6). The selected test section of the specimen should be such that the maximum load expressed in absolute value is not less than 25% of the full scale of the load gear used by the testing machine. The same batch of specimens used for fatigue strength determination shall have the same shape, size and surface condition. 2.2.1 The recommended shapes and sizes of smooth specimens are shown in Figures 3, 4 and Table 3. 0.0-
Circular cross-section specimen
GB3075—82
0.024 B
Figure 4 Rectangular cross-section specimen
Note: ① When the test section is reduced in the thickness direction of the specimen due to the limited capacity of the testing machine, the surface finish of the added surface shall not be lower than 9. ②The connection between the working part of the specimen and the arc transition part shall be smooth and free of dents. ③The edges of the working part of the rectangular specimen shall be smooth and have appropriate small fillets. Table 3
Nominal size
Nominal size
(2 ~6)
Note: ① When conducting a cyclic compressive stress test, Lc4d or Lc4b should be used. mm
5d or 5b
D/d or Bb
3d or 3b
② Under the condition of taking special measures, the test of torch-shaped cross-section specimens with ab<30mm2 can be negotiated. 31.5
2.2.2 In view of the particularity of the purpose and requirements of the notch fatigue test, there is no restriction on the design of the notch specimen. However, its shape, size and K should be indicated in the test report.
GB3075-82
Recommended V-notch circular cross-section specimens and U-notch rectangular cross-section specimens are shown in Figures 5 and 6. po.o1.4
R0.34± 0.02
R0.43 ± 0.02
Figure 5 V-notch circular cross-section specimen (K, = 3) 123
GB 3075--82
= 0. 02 {-- 8
Figure 6 U-notch rectangular cross-section specimen
(K = 3 R/B = 0.05 6/B =0.7)
2.2.3 The measurement error of the actual minimum diameter of the circular specimen shall not exceed ±0.01mm; the measurement error of the actual minimum cross-sectional dimension of the rectangular specimen shall not exceed ±0.5%.
When measuring the specimen dimensions, the surface of the specimen shall be prevented from being damaged. 2.2.4 The shape and size of the specimen clamping part shall be reasonably designed according to the clamping tool and the specimen of the testing machine. The ratio of its cross-sectional area to the maximum stress cross-sectional area of the specimen depends on the clamping method, but should not be less than 1.5. If the specimen is clamped by thread, the above ratio should be as large as possible, and fine pitch threads should be used.
2.3 Preparation and storage
2.3.1 The selected specimen roughness should be representative of the organizational properties of the raw material. The sampling location, orientation and method shall be carried out in accordance with relevant standards. 2.3.2 The processing technology used should minimize the residual stress and hardening generated on the surface of the specimen. , during the processing, overheating or other factors should be prevented from changing the fatigue properties of the material, and the surface quality of the specimen should be uniform. During milling, turning and grinding, the cutting depth and feed amount should be appropriately reduced step by step, and sufficient cooling should be provided. 2.3.3 When the specimen is heat treated, deformation and surface layer deterioration should be prevented. 2.3.4 It is recommended to use longitudinal polishing method to perform the final finishing of the working part surface after longitudinal milling, fine turning and fine grinding. 2.3.5 After finishing, the specimen should be carefully cleaned and well preserved to prevent specimen deformation, surface damage and corrosion. Note: See Appendix A for specimen processing technology. If necessary, the parties can negotiate and agree on this technology. 3 Test conditions
3.1 Load
Different types of axial fatigue testing machines can be used. During the test, the following requirements should be met: 3.1.1 Static load indication accuracy:
a. The load indication error is not greater than ±1%.
b. The load indication variation is not greater than 1%.
3.1.2 Within 10 hours of continuous testing, the fluctuation of the dynamic load indication is: a. The average load indication fluctuation is not more than 1% of the full load range. b. The load amplitude indication fluctuation is not more than 2% of the full load range. 3.1.3 The load needs to be applied axially
l:, the lower clamp should firmly clamp the end of the specimen. The center line of the clamp should coincide with the force axis of the testing machine as much as possible to ensure that the cyclic load is accurately transmitted along the axis of the specimen 124
without gap. GB 3075-82
recommends using a resistance strain gauge to measure the bending percentage of the specimen on the testing machine. The measurement method is shown in Appendix B. 3.2 Frequency
The stress cycle frequency depends on the type of testing machine used, the stiffness of the specimen and the test requirements. The selected frequency must not cause heating of the test part of the specimen. It is recommended that the test frequency be in the range of 10 to 200 Hz. The tests of the same batch of specimens should be carried out at approximately the same frequency. Note: In general, the testing machine should be calibrated at least once a year according to relevant national standards or regulations. 4 Test procedures
4.1 Installing the specimen
When installing the specimen, the specimen must be carefully operated to keep the specimen coaxial with the upper and lower clamps of the testing machine, and minimize the stress other than the specified axial stress on the specimen.
4.2 Applying the load
The load should be applied smoothly and accurately, and no overload should be allowed. During the entire test process, the fluctuation of the dynamic load indication value should comply with the provisions of 3.1.2.
4.3 Termination of the test
The specimen is usually tested continuously under the specified stress until the specimen fails or the specified number of cycles. The specimen failure should occur within the Lc of the (a)-shaped specimen or at the maximum stress section of the (b)-shaped specimen, otherwise the test results are invalid. If the test process is interrupted, the number of cycles and the rest time at the time of interruption must be noted in the test report. 4.4 Determination of conditional fatigue limit and S-N curve 4.4.1 Determination of conditional fatigue limit
The conditional fatigue limit of the material is determined by the lifting method. The number of specimens usually requires more than 13. The stress increment 4α is generally within 5% of the expected fatigue limit, and the test can be carried out at 3-5 stress levels. The test stress level of the first specimen should be slightly away from the expected fatigue limit. According to the test results of the previous specimen (failure or pass), the test stress level of the next specimen is determined (reduced or increased) until all tests are completed. For the test data before the first opposite result (failure and pass; pass and failure), if it is outside the fluctuation range of the subsequent test data, it will be discarded. If it is within the above fluctuation range, it will be used as valid data. During the test process, they will be gradually translated to after the first pair of opposite results as the first valid data at the stress level of the specimen. The calculation formula for conditional fatigue limit:
ON=m
Where: m-total number of valid tests (failure and pass data points are counted); n-test stress level level
Oi--the first stress level;
V; the number of tests at the i-th stress level. The survival rate of the conditional fatigue limit obtained by the above formula is 50%. If necessary, the test results can be processed by mathematical statistics to obtain the conditional fatigue limit under any survival rate. Note: According to the material technical conditions or agreement, other methods can be used to determine the conditional fatigue limit. 4.4.2 Determination of S-N curve
Usually, at least 5 stress levels are taken. The number of specimens at each stress level should be gradually increased as the stress level decreases. The conditional fatigue limit obtained by the lifting and lowering method is used as the lowest stress water half point on the S-N curve. With . as the ordinate and N as the abscissa, a curve is drawn using the best fitting method, as shown in Figure 7. The number of specimens used at each stress level shall be negotiated and agreed upon by both parties if necessary. 12
5 Result Expression and Test Report
5.1 Result Expression
GB 3075—82
·Lifting Method Test
In Lifting Method Test,
0 Grouping Method Test Data
Fatigue Life, Times
Figure 7 SN Curve
Because the fatigue test data have a large dispersion, in order to obtain more reliable test results, in addition to designing a reasonable fatigue test plan, the fatigue test data should be processed by statistical methods. The test results are generally expressed by graphical methods, and the following graphical representation methods are recommended. 5.1.1 S-N Curve
This is the most commonly used method for expressing fatigue test results. When drawing the SN curve, the stress amplitude or other stress values depending on the type of cyclic stress (under asymmetric tension and compression, it is usually the maximum stress amplitude) is used as the ordinate, and the number of cycles V (fatigue life) is used as the coordinate.
N all use logarithmic coordinates, stress can use linear coordinates or logarithmic coordinates according to specific circumstances, see Figure?. In addition, according to different requirements, various parameters (such as average stress om, stress ratio R, survival rate p, etc.) S.N curves can be drawn. 5.1.2 Endurance diagram
For a specified endurance time N (fatigue life), a diagram showing the relationship between limit cycle stress and average stress. Stress (a) and average fatigue (Um) relationship diagram, see Figure 8. a.
Maximum stress (Oma), minimum stress (min) and average stress (Om) relationship diagram (GoodmanSmith diagram), see Figure 9.
Maximum stress (Omax) and minimum stress (Omn) relationship diagram, see Figure 10. Equal life diagram, see Figure 11.
5.2 Test report
Report the test results as required. The following contents should be clearly stated in the report: 5.2.1 Material brand, furnace number, specification, chemical composition, heat treatment process and conventional mechanical properties. 1 Preparation process of the sample and its shape, size and surface condition. 5.2.2
Testing machine model.
Should be cycle form, ①m, 2, R.
Test frequency.
Exceeding the test environment temperature of 10~35℃ and the relative coagulation degree of 50~70%, 5.2.6
The test process does not meet the requirements. 126
Corresponding to the foot line of GB 3075—82
Suitability
Tensile strength
Figure 8 Experimental (theoretical) results
Haigh diagram
|m curve corresponding to Um.
-0m compression
|min curve corresponding to m
|asymmetric compression
||||max curve corresponding to +m
||Tmin curve corresponding to +m
|asymmetric tension
Figure 9 Relationship between Umax and Omin and 0m (theoretical results) Goodman - Smith Figure
+Exact tensile
—560
Maximum stress limit
GB3075-82
Small stress extreme
Figure 10?max-min relationship diagram (theoretical results) ROS diagram
Minimum stress
Minimum stress
kgf/mm2
Equi-life diagram
Yield point
pmu/ysy
A.1 Turning
A.1.1 Rough turning
GB 3075—82
Appendix A
Machining technology of fatigue specimens
(reference)
When turning the diameter of the specimen from α+5mm (c is the nominal diameter d of the specimen plus an appropriate surface polishing allowance) to a+0.5mm, the cutting depth should be reduced gradually. The recommended cutting depth is: 1.25mm
Note: The surface polishing allowance for high-strength material specimens is 0.025mm. A.1.2 Turning finishing
When turning the diameter of the specimen from α+0.5mm to r, the cutting depth should be further reduced gradually. The recommended cutting depth is: 0.125 mm
Small feed rate should be used, such as no more than 0.06mm per revolution. A.2 Milling
This method can be used to cut the rough specimen from the material and machine the rough specimen of rectangular cross section to the nominal size of the specimen. The cutting speed and feed rate should be determined according to the specimen material. When finishing milling, the required surface finishing quality should be taken into account. Grinding
For materials that are not easy to turn due to increased strength due to heat treatment, the rough specimen diameter can be turned to +0.5mm and then heat treated. Then grinding is used to finish the diameter. The following grinding depth is recommended:
The grinding depth is 0.030mm before it is 0.1mm larger than the nominal diameter, and the grinding depth is 0.005mm before it is 0.025mm larger than the nominal diameter. With a grinding depth of 0.0025mm, the specimen diameter is ground to. It should be fully cooled during grinding. bzxz.net
A.4 Surface polishing
After the test part is machined to the diameter, use a sandpaper or sandpaper that is gradually finer to perform mechanical or manual polishing along the axis approximately parallel to the specimen. Make its surface finish reach V9 (R0.16～0.32μm). 6000# water-abrasive silicon carbide sandpaper can be used for the final polishing of the test part surface. Note: Although this process is suitable for processing a variety of metal materials, it is not the only one. Therefore, a reasonable processing technology should be selected according to the material properties of the specimen. A.5 Processing of notched specimens
The processing technology of notched specimens is basically the same as that of smooth specimens. A.5.1 Rough turn the notch, leaving a margin of 0.3～0.5mm. A5.2 According to the strength level of the material, turn or grind the notch for fine processing. The fine processing technology refers to Article A.1.2, Chapter A.2 and Chapter A.3.
A.5.3 If the notch finish requirement is still not met after the above process, polishing is required. 129
B.1 Principle
GB 3075—82
Appendix B
Introduction to the method for determining the bending percentage of the specimen on the axial fatigue testing machine using resistance strain gauges
(reference)
Install the calibration rod with strain gauges on the testing machine, measure the deformation of each strain gauge under stress, and calculate the bending percentage of the calibration rod caused by the different axes under stress by the following formula: e
Formula: Smax
B.2 Apparatus
The maximum deformation measured on the calibration rod;
The average deformation measured on the calibration rod.
B.2.1 Calibration rod:
B.2.1.1 The material, shape and size of the calibration rod should be similar to the test specimen, and its diameter can be 10mm or 20mm. B2.1.2 The strain gauges should be evenly distributed on the calibration rod, and the maximum bending or close to the maximum bending can be measured. B.2.2 Measurement system
The measurement can be carried out by a measurement system composed of a voltage-stabilized power supply, an amplifier, a digital voltmeter, etc., or by other deformation measurement systems. The measurement error shall not be less than 3%.
B.3 Procedure
B.3.1 Clamp one end of the calibration rod on the testing machine, and leave the other end unclamped and in a free state. B.3.2 Zero the loading system and deformation measurement system of the testing machine. B.3.3 Clamp the free end (non-clamped end) of the calibration rod. B.3.4 Apply loads at 10%, 20%, 30%, 40%, and 50% of the maximum load range of the testing machine, and measure the deformation of each strain gauge on the calibration rod at each load level.
B.3.5 Calculate the bending percentage of the calibration rod according to the formula in B, 11. Additional remarks:
This standard was proposed by the Ministry of Metallurgical Industry of the People's Republic of China. This standard was drafted by the Iron and Steel Research Institute of the Ministry of Metallurgy. The drafters of this standard are Gao Shunzhi and He Rongnian.
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