GB/T 15248-1994 Axial constant amplitude low cycle fatigue test method for metallic materials
Some standard content:
National Standard of the People's Republic of China
Axial constant-amplitude low-cycle fatigue of metallic materials
Test method
The test method for axial loading constant-amplitude low-cycle fatigue of metallic materials1Subject content and scope of application
GB/T15248—94
Replaces GB 6399-86
This standard specifies the terminology, specimen preparation, equipment, test procedures, result processing and test report for axial constant-amplitude low-cycle fatigue test of metallic materials.
This standard is applicable to low-cycle fatigue tests of metallic materials with constant cross-section and funnel-shaped specimens subjected to axial constant-amplitude tensile-compressive stress or continuous strain change, excluding tests of full-size components and structural parts. This standard is applicable to tests at temperatures and strain rates where the time-dependent inelastic strain is negligibly small compared to the time-independent inelastic strain. The inelastic strain referred to here refers to all inelastic strains. This standard allows tests to be conducted under environmental factors such as temperature, pressure, saturation, and medium, but these factors should remain constant throughout the test.
This standard can be used as a guide for low cycle fatigue testing in material development, mechanical design, process and quality control, product performance determination and failure analysis.
2. Reference standards
JJG556 Verification procedures for axial loading fatigue testing machines 3 Symbols, terms, definitions and function expressions The symbols, terms, definitions and units related to stress-strain cycle and low-cycle fatigue tests are shown in Table 1. 3.1
True strain
The instantaneous gauge length of the specimen after axial deformation
The original gauge length of the specimen
Engineering strain
Total strain range
Approved by the State Administration of Technical Supervision on October 7, 1994 982
The natural logarithm of the ratio of the instantaneous gauge length to the original gauge length or e=ln(I+e)
The ratio of the deformation increment of the specimen to its original gauge length LL
In one cycle, the algebraic difference between the maximum and minimum strains: A et
1995-06-01 implementation
maximum strain
minimum strain
elastic strain range
plastic strain range
true stress
engineering stress
cyclic stress range
maximum stress
minimum stress
stress ratio
strain ratio
strain amplitude
stress amplitude
failure cycles
failure reversal number
fatigue ductility index
fatigue strength index
fatigue ductility coefficient
fatigue strength coefficient
cyclic strain hardening index
cyclic strength coefficient
elastic modulus
GB/T 15248-—94
Continued Table 1
The maximum algebraic value of strain in a cycle. Tension is positive, compression is negative
The minimum algebraic value of strain in a cycle is equal to the stress range divided by the elastic modulus:
Ae. Aa/E
The difference between the total strain range and the elastic strain range Ae, -- Aet - de.
The ratio of instantaneous load to instantaneous cross-sectional area, = P/A or
a=S(1+e)
The ratio of load to original cross-sectional area,
S=P/Ao
The algebraic difference between the maximum and minimum stresses in a cycle: Aa= dmax Dmia
The maximum algebraic value of stress in a cycle The minimum algebraic value of stress in a cycle R, = Omin /omx
Re Emin / max
Half of the strain range
Half of the stress range
Number of cycles to failure
Number of reverse cycles to failure
Slope of the Ig(Ae,/2) -Ig2N
curve
lg(Ag/2)—Ig2Nr or
Ig(A/2)Ig2Nr curve
Take the ordinate cutoff at
2N,=1 on the Ig(Aep/2)lg2Nr curve
Take Ig(Ag/2)—Ig2N The ordinate intercept at
2Ni—1 on the curve
The slope of the Ig(Ao/2)-Ig(Ae,/2) curve is taken as the ordinate intercept at
Ae./2=1 on the lg(4a/2)-lg(4e/2) curve
In the elastic range, the ratio of stress to strain is single
GB/T15248—94
3.2 Axial constant amplitude low cycle fatigue strength refers to the ability of the specimen to withstand high axial tension-compression stress or strain. Generally speaking, the total number of failure cycles is less than 5×10*, and there are two evaluation methods. 3.2.1 The specimen is tested under the specified stress or strain value, and the number of cycles to failure is determined. This test is called a comparative test. 3.2.2 Use a group of specimens, select several stress or strain values, and measure the number of cycles to failure respectively, and then draw the △g/2-2N or Ae/2-2N curve, as shown in Figures 1 and 2. According to the relationship:
As = Ae+ Aep
The above Ae/2-2N, curve can also be processed into the form of Figure 3. 10
Figure 1Aa/2-2N. Curve
Figure 2Ae/2-2N, Curve
GB/T15248-94
Figure 3Ae/2, Ae/2, Aep/2-2N, Curve 3.2.3 The cyclic stress-strain relationship on the specimen is shown in Figure 4. Figure 4 Stress-strain loop during cycling
3.3 Functional relationship
3.3.1 Appendix A provides empirical relationships that are commonly used to represent low cycle fatigue data. 3.3.2 Appendix B provides the relationship between radial strain and axial strain. 4 Specimens
4.1 Specimen design
Specimens should be designed to be short and thick to ensure normal operation under load and not to cause instability. Several specimens shown in Figure 5 are recommended. 98.
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R=2dtda
R= 4d±2d
2d± d
Id±0. 02
R= 6d ± 2d
Figure 5 Recommended low cycle fatigue specimen
4.1.1 Figures 5(a) and 5(b) are uniform cross-section specimens. The specimen in Figure 5(b) is usually selected, but due to the limitation of the strain extensometer type, the specimen in Figure 5b with a gauge length of 3d±d and the specimen in Figure 5(a) can also be used. Since the specimen in Figure 5(a) has two transitional rounded parts within the gauge length, its strain range should be corrected according to the formula provided in Appendix C. 4.1.2 Figure 5(c) is a single-section funnel-shaped specimen. The choice of Figures 5(a) and 5(b) or 5(c) should be determined based on the anisotropy and bending resistance of the material. Uniform cross-section specimens are usually used for a total strain range of about 2%. For tests with a total strain range greater than 2%, a single-section funnel-shaped specimen is recommended. The ratio of the radius of curvature of this specimen to the minimum radius of the specimen is generally 12:1. If there is a special need, various ratios within the range of 8:1 and 16:1 can be used. Lower ratios will increase stress concentration and may affect fatigue life. Higher ratios will reduce the bending resistance of the specimen. When the material is anisotropic, uniform cross-section specimens should be used. 4.1.3 The three recommended specimens have a solid circular cross-section with a minimum working portion diameter of 6 mm. Within the scope of this standard, these specimens can be designed as specimens with other diameters or tubular cross-sections. It is recommended that the ratio of the gauge length to the working part diameter of the axial strain-controlled equal-section specimen be less than 4, and the ratio of the cross-sectional area of the specimen clamping part to the cross-sectional area of the working part be greater than 4. The coaxiality of the working part and the clamping part of the specimen is within 0.01mm.
4.1.4 The head of the specimen is selected according to the fixture and material used. When bonding occurs due to local oxidation of the thread at high temperature, it is recommended to use a specimen head with a shoulder or add an anti-bonding agent.
4.1.5 For plate specimens, generally speaking, when the plate thickness is less than 6mm, the specimen shown in Figure 6 can be used, but a special clamping device must be provided. 986
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(a) Rectangular section
(b) Circular section
L=3T±T/2
W=4T+T
R,=2T± T/2
Figure 6 Plate low cycle fatigue specimen
T(plate thickness)
T(plate thickness)
The rectangular cross-section specimen shown in Figure 6(a) is suitable for a plate thickness of 2.5 mm and a total strain amplitude of 1%. For higher strain amplitudes, a circular cross-section funnel-shaped specimen as shown in Figure 6(b) is recommended. 4.1.6 Other shapes of specimens may be used for testing according to specific circumstances, but this must be stated in the test report. 4.2 Preparation and storage of specimens
4.2.1 The specimens shall be cut from homogeneous raw materials or rough pieces in order to statistically characterize the properties of the material. When material conditions permit, specimens may be cut from components and the required rolling direction according to the test purpose. 4.2.2 When the specimen is tested after heat treatment, it shall be heat treated first and then processed into the specimen. If the hardness after heat treatment is too high and difficult to machine, rough machining may be performed first and then fine machining may be performed after heat treatment. However, the roughness should include the final machining allowance and the dimensions that may cause deflection during heat treatment. 4.2.3 During the entire production process of the specimen, the metal should not be subjected to cold work hardening or overheating, except for the purpose of testing to determine the effect of specific surface conditions on fatigue. It is recommended to use consistent machining to make the surface finish high and uniform, and to use machining or polishing processes that minimize surface metal distortion as the final process. Appendix D provides an example of machining methods. 4.2.4 After the specimen is finely machined, it should be carefully cleaned, immediately protected, and properly stored to prevent specimen deformation, surface damage and corrosion. 5 Equipment
5.1 Testing machine
The test can be carried out on any tension-compression low-cycle fatigue testing machine that can control load and deformation. 5.1.1 The static load of the testing machine is calibrated according to the JJG556 axial loading fatigue testing machine verification procedure, and its systematic error is not more than ±1%, and the deviation is not more than 1%. If the error reaches ±2%, it can still be used, but a calibration curve must be made and corrected. If the error exceeds 2%, it is not allowed to be used.
5.1.2 Stability of stress or strain control. The repeatability of two consecutive cycles should be within 1% of the tested stress or strain range, or within 0.5% of the average range, and the entire test process should be stable within 2%. 5.2 Fixtures
5.2.1 The chuck connecting the specimen can be in any way, such as threaded or with shoulders. However, during the test, the connection between the specimen, the chuck and the testing machine must be tightened to prevent the specimen and the chuck from loosening or causing a gap when the load is reversed. 987
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5.2.2 It should have good coaxiality. Use a standard specimen, evenly distribute 4 strain gauges with the same resistance on its central circumference, and measure its bending deformation rate within the elastic range of the specimen. Repeat the measurement three times, then rotate 90° and measure again. The deformation rate is within 5%. The deformation rate is calculated by the following formula:
Where: 8. (g1+g2+g3+g)/4
V(g1.s)*+(g2.)*
×100%
g1.g=[(g1—go) —(g3 go))/2-(g1—g3)/2g2.4-[(g2—go)—(g1go)]/2(g2-g)/2g1,g2, where g1+g are the average strains of the four strain gauges measured three times. 5.3 Strain extensometer
5.3.1 A strain extensometer suitable for long-term dynamic measurement and control should be equipped to measure the deformation within the gauge length of the specimen. The measurement accuracy should not be less than ±1%.
5.3.2 The type of extensometer can be electromechanical and photoelectric (such as photoelectric or laser). Axial or radial strain extensometers should be selected according to the specimen used, as shown in Figures 7 and 8. Be extremely careful when installing the extensometer to prevent damage to the specimen surface and premature fracture. 5.3.3 The extensometer should be calibrated after each test. Specimen
Upper extension rod
Lower extension rod
Lower clamp
Differential transformer
Figure 7 Schematic diagram of axial extensometer measurement
5.4 Sensor
A load sensor designed for fatigue testing should be equipped and connected in series with the specimen to measure the axial load. The sensor should have high bending resistance, low axial flexibility, good linearity, accuracy and sensitivity, and small hysteresis. Its measurement accuracy should not be less than ±1% of the maximum load value measured.
5.5 Recording device
Extensometer
GB/T 15248-94
Elastic element
Figure 8 Schematic diagram of radial extensometer measurement
5.5.1 The accuracy of the recording device should be kept within 1% of the full scale. Differential transformer "adjustable screw
5.5.2 To record the load-deformation or stress-strain hysteresis loop, an XY recorder or an oscilloscope with camera and other recording instruments of corresponding accuracy should be equipped.
5.5.3 To record the change of stress or strain over time, a long chart recorder should be equipped. The stress or strain cycle rate should be such that the speed of the recording pen does not exceed half of the conversion speed of the recorder. It is recommended to calibrate the recorder at the frequency of use. 5.6 Cycle counter
Record the total number of cycles, and it is desirable to have a timer attached to verify the cycle counter and frequency. 5.7 Strain computer
For radial stress of funnel-shaped specimens A variable-controlled low-cycle fatigue test that can convert radial strain into axial strain signals. The principles and relationships are shown in Appendix B.
5.8 Calibration of equipment
All electrical recording and sensing devices should be calibrated regularly according to the manufacturer's instructions. Unless otherwise specified, the calibration period shall not exceed one year. If necessary, calibration should be performed at any time. 6 Test step set
6.1 Test environment
6.1.1 During room temperature testing, the temperature change of the specimen shall not exceed ±2°C during the test period. During high temperature testing, the temperature fluctuation of the working part of the specimen shall not exceed ±2°C, and the temperature gradient within the gauge length shall be within ±2°C, otherwise it shall be reported. Instructions. 6.1.2 High-temperature tests can be heated by high-frequency induction furnaces, radiation furnaces or electric furnaces. In order to make the temperature of the specimens uniform, there should be sufficient insulation time. When using the first two methods, it is recommended to install a heat shield between the furnace and the specimen. 6.1.3 When testing in air, unless it has been determined that humidity has little or no effect on the fatigue properties of the test material, the humidity should be controlled. If it cannot be controlled, it should be carefully monitored. 6.2 Measurement of specimen size
In order to accurately calculate the cross-sectional area of the specimen, it must be detected with a measuring instrument with a reading accuracy of not less than 0.01mm. For specimens of equal cross-section, the diameter should be detected at two different positions within the gauge length. 6.3 Testing machine 6.3.1 Depending on the purpose of the test, one or several variables can be controlled during the test, and the changes of other variables with the cycle can be monitored at the same time. 6.3.2 In low-cycle fatigue tests, the most commonly used controlled variable is the total strain range. The plastic strain range can also be controlled according to the test requirements. For low-ductility materials and long-life low-cycle fatigue tests, the plastic strain range is very small. If the required strain range can be maintained and the load range can be adjusted regularly, load control is also allowed. 6.3.3 In order to achieve continuous control of the specified test variables, closed-loop control fatigue testing machines are often used. If the closed-loop testing machine used is not a continuously controllable 989
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, the limits of the variables used must be strictly controlled. 6.3.4 For anisotropic materials, such as directional solidification, single crystal materials, etc., axial strain control must be used. 6.3.5 Except for the purpose of the test to study the initial loading effect, all tests should start with the same half cycle (tension or compression). 6.4 Waveform
6.4.1 Except when the purpose of the test is to determine the effect of the waveform, the strain (or stress) waveform over time shall remain consistent throughout the test. In the absence of specific requirements or when the equipment is limited, a triangular wave is generally used. 6.4.2 Low cycle fatigue tests with holding time shall be conducted in accordance with Appendix E. 6.5 Strain rate or cycle frequency
6.5.1 Except when the purpose of the test is to determine the effect of the strain rate or cycle frequency, the strain rate or cycle frequency shall remain constant for each test. Generally speaking, a constant strain rate can shorten the test cycle, while a constant cycle frequency test may be more practical for fatigue analysis of certain mechanical parts.
6.5.2 If a constant strain rate test cannot be performed due to non-triangular waves or equipment limitations, or a constant frequency test cannot be performed due to time limitations, other rate control methods may be used. A constant average strain rate is usually used, which is one times the product of the strain range and the frequency. Other control methods may also be used. When conducting a plastic strain limit control test, the most appropriate method is to keep the average plastic strain rate constant.
6.5.3 The selected strain rate or frequency should be low enough to prevent the specimen from heating up to more than 2°C and to accommodate the frequency response characteristics of the strain extensometer. The actual values used should be noted in the test report. 6.6 Recording
If the electronic computer data acquisition system is not used continuously, the total strain range-axial stress hysteresis loop should be recorded. For tests with more than 100 cycles, intermittent recording or sampling is allowed, but it is expected that no less than 10 hysteresis loops should be recorded, and other relevant variables that change with the cycle should be recorded.
6.7 Determination of failure
6.7.1 Determine the failure criterion based on the test purpose and the characteristics of the material being tested. 6.7.2 Break into two pieces at a certain position within the gauge length of a uniform cross-section specimen, or near the small diameter of a funnel-shaped specimen. If the fracture is outside the above-specified position or impurities, holes, machining defects, etc. are found on the fracture surface, the results are invalid. 6.7.3 For strain-controlled tests, an inflection point appears in the compression portion of the hysteresis loop. When the value of the inflection point, i.e., the peak compressive stress minus the stress at the inflection point of the compression loading curve, reaches a certain specified percentage of the peak compressive stress, failure is considered, as shown in Figure 9. Inflection point
Figure 9 Definition of inflection point for determining failure
Value of inflection point
GB/T 15248-94
6.7.4 Failure is considered when the rate of change of cyclic stress or strain, i.e., the ratio of the cyclic stress or strain range to the stable stress or strain range, exceeds a certain predetermined percentage.
6.7.5 For surface crack tests, when the crack grows to a certain predetermined size that meets the test purpose requirements. 6.7.6 For fully repeated strain or stress tests, the decrease or increase in the peak tensile stress or strain is greater than a certain predetermined percentage of the decrease or increase in the peak compressive stress or strain.
6.7.7 When the test requirements or conditions permit, in addition to failure according to the predetermined failure standard, it is best to continue the test until the specimen breaks.
6.8 Test sequence
6.8.1 When testing at room temperature, load the specimen into the chuck and tighten it, install the extensometer, then connect the chuck to the testing machine, adjust the concentricity and tighten it. Place the test variables such as the predetermined load or strain, frequency, waveform, etc. at the given position, and then the test can be carried out. 6.8.2 When testing at high temperature, first load the specimen into the chuck and tighten it, tie the thermocouple, install the extensometer, and then install the heater. Heat up to the specified temperature and keep it warm. The other sequence is the same as 6.8.1. 6.9 Number of specimens
For the comparative test of metals and alloys, there should be no less than 3 specimens. To measure the △e/2 (△/2)-2N curve, generally 12 to 15 specimens are required, and several stress or strain points are selected to measure their failure cycles respectively. If statistical analysis is required, the number of specimens should be determined according to the purpose of the test.
7 Processing of test results
7.1 Draw the △e/2(△g/2)-2N curve (as shown in Figures 1 and 2) and the △e/2, △e/2-2N curve (Figure 3). It is recommended to use double logarithmic coordinates.
7.2 According to the curve drawn in Figure 3 and the symbols and definitions in 2.2, calculate the fatigue ductility index c, fatigue strength index b, fatigue ductility coefficient e\ and fatigue strength coefficient a of the metal and alloy under the test conditions. When determining △a, if there is no obvious cyclic stability value, take the stress range at Nt/2.
7.3 Regarding the cyclic stress-strain curve, it is compared with the primary stress-strain curve to judge whether the metal and alloy are cyclic hardening or softening signs, and is made by half of the paired stable stress range and half of the total strain range. Generally, the multi-sample method is adopted. If the conditions do not allow, the single-sample escalation test method with a strain range from small to large can also be adopted, as shown in Figure 10. Or it can be made using a paired stable stress range and a plastic strain range. The mathematical expression is shown in Formula (A1) in Appendix A. 600
Figure 10 Cyclic stress-strain curve
7.4 Determination of cyclic strain hardening index n. According to the data of paired stable stress amplitude and plastic strain amplitude, a △a/2-△ep/2 curve is drawn on a double logarithmic coordinate, as shown in Figure 11. The slope of the curve is the cyclic strain hardening index of the metal or alloy under the test conditions.
GB/T 15248—94
7.5 Inspection after the test. According to the research purpose, the fracture morphology can be observed by scanning electron microscopy. If you want to study the organizational changes that occur during fatigue or the influence of metallurgical organization on fatigue resistance, you can use optical metallographic technology and transmission electron microscopy. 10#
1Aa/2-Aep/2 curve
8Test report
Report the test results as required. The report may include the following: 10~
8.1 Material brand and standard number, manufacturer, furnace batch, specification, chemical composition, heat treatment process and conventional mechanical properties. 8.2 Sampling location, specimen shape, size and surface condition. 8.3 Testing machine model.
8.4 Test conditions, including test temperature and control method, environmental medium, cycle frequency or cycle strain rate, waveform, stress ratio or strain ratio, control method.
8.5 Any situation that does not comply with this standard during the test. 8.6 Test results.
8.6.1 The initial value, stable value or Nt/2 value of the stress range, strain range and plastic strain range. 8.6.2 The number of cycles to failure Nt, and the criteria for determining failure. 8.6.3*Analysis results of cyclic stress-strain properties, including cyclic hardening index and cyclic strength factor. 8.6.4Analysis results of strain-life properties, including fatigue strength index, fatigue ductility index, fatigue strength factor and fatigue ductility factor.
Test date, tester and proofreader.
GB/T 15248—94
Appendix A
Functional relationship
(reference)
A1 Formulas (A1)~(A4) have been conveniently used to describe low cycle fatigue data of many metals. A1.1 Cyclic stress-strain characteristics:
Aa/2=K'(Aep/2)
A1.2 Fatigue life relationship:
A0/2-)(2N)b
(2N）
Aep/2=e'(2N)c
(2N,)h+e'(2N)e
Appendix B
Conversion from radial to axial strain for isotropic materials (reference)
When radial strain is converted into axial strain, it is first necessary to separate the elastic and plastic components from the total strain: B1
E-e+ep
Ed= Ede+edp
-elasticity,
where, e-
-plasticity;
d-radial
-total axial strain.
The axial and radial strains are related by Poisson's ratio√, that is, E,-—Ede /v and E,=— Edp/up. The above formula can be rearranged as:
Edp Ed
Ede / v,- (Ed -- Ede) /vp
With the help of the elastic modulus E, ed can be related to the axial stress: Ede=--(vo)/E
So,
E-/E—E/vp-(vo)/(vpE)
Assuming that plastic deformation occurs under constant conditions, that is, uniform 1/2, then E=(a/E)(1-2V)-2Ed
In the test, using a radial extensometer and an axial load cell, α and ea are continuously available. If E does not change with the cycle, then the slope of the elastic part of the α-Ea line can be obtained from /E. The block diagram of the radial-axial strain conversion principle is shown in Figure B1. 9933 For strain-controlled tests, an inflection point appears in the compression portion of the hysteresis loop. When the value of the inflection point, i.e., the peak compressive stress minus the stress at the inflection point of the compression loading curve, reaches a certain specified percentage of the peak compressive stress, failure is considered, as shown in Figure 9. Inflection point
Figure 9 Definition of inflection point for determining failure
Value of inflection point
GB/T 15248-94
6.7.4 Failure is considered when the rate of change of cyclic stress or strain, i.e., the ratio of the cyclic stress or strain range to the stable stress or strain range, exceeds a certain predetermined percentage.
6.7.5 For surface crack tests, when the crack grows to a predetermined size that meets the requirements of the test purpose. 6.7.6 For full repeated strain or stress tests, the decrease or increase in the peak tensile stress or strain is greater than a predetermined percentage of the decrease or increase in the peak compressive stress or strain.
6.7.7 When the test requirements or conditions permit, in addition to failure according to the predetermined failure standard, it is best to continue the test until the specimen breaks.
6.8 Test sequence
6.8.1 When testing at room temperature, load the specimen into the chuck and tighten it, install the extensometer, then connect the chuck to the testing machine, adjust the concentricity and tighten it. Place the test variables such as the predetermined load or strain, frequency, waveform, etc. at the given position, and then the test can be carried out. 6.8.2 When testing at high temperature, first load the specimen into the chuck and tighten it, tie the thermocouple, install the extensometer, and then install the heater. Heat up to the specified temperature and keep it warm. The other sequence is the same as 6.8.1. 6.9 Number of specimens
For the comparative test of metals and alloys, there should be no less than 3 specimens. To measure the △e/2 (△/2)-2N curve, generally 12 to 15 specimens are required, and several stress or strain points are selected to measure their failure cycles respectively. If statistical analysis is required, the number of specimens should be determined according to the purpose of the test.
7 Processing of test results
7.1 Draw the △e/2(△g/2)-2N curve (as shown in Figures 1 and 2) and the △e/2, △e/2-2N curve (Figure 3). It is recommended to use double logarithmic coordinates.
7.2 According to the curve drawn in Figure 3 and the symbols and definitions in 2.2, calculate the fatigue ductility index c, fatigue strength index b, fatigue ductility coefficient e\ and fatigue strength coefficient a of the metal and alloy under the test conditions. When determining △a, if there is no obvious cyclic stability value, take the stress range at Nt/2. www.bzxz.net
7.3 Regarding the cyclic stress-strain curve, it is compared with the primary stress-strain curve to judge whether the metal and alloy are cyclic hardening or softening signs, and is made by half of the paired stable stress range and half of the total strain range. Generally, the multi-sample method is adopted. If the conditions do not allow, the single-sample escalation test method with a strain range from small to large can also be adopted, as shown in Figure 10. Or it can be made using a paired stable stress range and a plastic strain range. The mathematical expression is shown in Formula (A1) in Appendix A. 600
Figure 10 Cyclic stress-strain curve
7.4 Determination of cyclic strain hardening index n. According to the data of paired stable stress amplitude and plastic strain amplitude, a △a/2-△ep/2 curve is drawn on a double logarithmic coordinate, as shown in Figure 11. The slope of the curve is the cyclic strain hardening index of the metal or alloy under the test conditions.
GB/T 15248—94
7.5 Inspection after the test. According to the research purpose, the fracture morphology can be observed by scanning electron microscopy. If you want to study the organizational changes that occur during fatigue or the influence of metallurgical organization on fatigue resistance, you can use optical metallographic technology and transmission electron microscopy. 10#
1Aa/2-Aep/2 curve
8Test report
Report the test results as required. The report may include the following: 10~
8.1 Material brand and standard number, manufacturer, furnace batch, specification, chemical composition, heat treatment process and conventional mechanical properties. 8.2 Sampling location, specimen shape, size and surface condition. 8.3 Testing machine model.
8.4 Test conditions, including test temperature and control method, environmental medium, cycle frequency or cycle strain rate, waveform, stress ratio or strain ratio, control method.
8.5 Any situation that does not comply with this standard during the test. 8.6 Test results.
8.6.1 The initial value, stable value or Nt/2 value of the stress range, strain range and plastic strain range. 8.6.2 The number of cycles to failure Nt, and the criteria for determining failure. 8.6.3*Analysis results of cyclic stress-strain properties, including cyclic hardening index and cyclic strength factor. 8.6.4Analysis results of strain-life properties, including fatigue strength index, fatigue ductility index, fatigue strength factor and fatigue ductility factor.
Test date, tester and proofreader.
GB/T 15248—94
Appendix A
Functional relationship
(reference)
A1 Formulas (A1)~(A4) have been conveniently used to describe low cycle fatigue data of many metals. A1.1 Cyclic stress-strain characteristics:
Aa/2=K'(Aep/2)
A1.2 Fatigue life relationship:
A0/2-)(2N)b
(2N）
Aep/2=e'(2N)c
(2N,)h+e'(2N)e
Appendix B
Conversion from radial to axial strain for isotropic materials (reference)
When radial strain is converted into axial strain, it is first necessary to separate the elastic and plastic components from the total strain: B1
E-e+ep
Ed= Ede+edp
-elasticity,
where, e-
-plasticity;
d-radial
-total axial strain.
The axial and radial strains are related by Poisson's ratio√, that is, E,-—Ede /v and E,=— Edp/up. The above formula can be rearranged as:
Edp Ed
Ede / v,- (Ed -- Ede) /vp
With the help of the elastic modulus E, ed can be related to the axial stress: Ede=--(vo)/E
So,
E-/E—E/vp-(vo)/(vpE)
Assuming that plastic deformation occurs under constant conditions, that is, uniform 1/2, then E=(a/E)(1-2V)-2Ed
In the test, using a radial extensometer and an axial load cell, α and ea are continuously available. If E does not change with the cycle, then the slope of the elastic part of the α-Ea line can be obtained from /E. The block diagram of the radial-axial strain conversion principle is shown in Figure B1. 9933 For strain-controlled tests, an inflection point appears in the compression portion of the hysteresis loop. When the value of the inflection point, i.e., the peak compressive stress minus the stress at the inflection point of the compression loading curve, reaches a certain specified percentage of the peak compressive stress, failure is considered, as shown in Figure 9. Inflection point
Figure 9 Definition of inflection point for determining failure
Value of inflection point
GB/T 15248-94
6.7.4 Failure is considered when the rate of change of cyclic stress or strain, i.e., the ratio of the cyclic stress or strain range to the stable stress or strain range, exceeds a certain predetermined percentage.
6.7.5 For surface crack tests, when the crack grows to a predetermined size that meets the requirements of the test purpose. 6.7.6 For full repeated strain or stress tests, the decrease or increase in the peak tensile stress or strain is greater than a predetermined percentage of the decrease or increase in the peak compressive stress or strain.
6.7.7 When the test requirements or conditions permit, in addition to failure according to the predetermined failure standard, it is best to continue the test until the specimen breaks.
6.8 Test sequence
6.8.1 When testing at room temperature, load the specimen into the chuck and tighten it, install the extensometer, then connect the chuck to the testing machine, adjust the concentricity and tighten it. Place the test variables such as the predetermined load or strain, frequency, waveform, etc. at the given position, and then the test can be carried out. 6.8.2 When testing at high temperature, first load the specimen into the chuck and tighten it, tie the thermocouple, install the extensometer, and then install the heater. Heat up to the specified temperature and keep it warm. The other sequence is the same as 6.8.1. 6.9 Number of specimens
For the comparative test of metals and alloys, there should be no less than 3 specimens. To measure the △e/2 (△/2)-2N curve, generally 12 to 15 specimens are required, and several stress or strain points are selected to measure their failure cycles respectively. If statistical analysis is required, the number of specimens should be determined according to the purpose of the test.
7 Processing of test results
7.1 Draw the △e/2(△g/2)-2N curve (as shown in Figures 1 and 2) and the △e/2, △e/2-2N curve (Figure 3). It is recommended to use double logarithmic coordinates.
7.2 According to the curve drawn in Figure 3 and the symbols and definitions in 2.2, calculate the fatigue ductility index c, fatigue strength index b, fatigue ductility coefficient e\ and fatigue strength coefficient a of the metal and alloy under the test conditions. When determining △a, if there is no obvious cyclic stability value, take the stress range at Nt/2.
7.3 Regarding the cyclic stress-strain curve, it is compared with the primary stress-strain curve to judge whether the metal and alloy are cyclic hardening or softening signs, and is made by half of the paired stable stress range and half of the total strain range. Generally, the multi-sample method is adopted. If the conditions do not allow, the single-sample escalation test method with a strain range from small to large can also be adopted, as shown in Figure 10. Or it can be made using a paired stable stress range and a plastic strain range. The mathematical expression is shown in Formula (A1) in Appendix A. 600
Figure 10 Cyclic stress-strain curve
7.4 Determination of cyclic strain hardening index n. According to the data of paired stable stress amplitude and plastic strain amplitude, a △a/2-△ep/2 curve is drawn on a double logarithmic coordinate, as shown in Figure 11. The slope of the curve is the cyclic strain hardening index of the metal or alloy under the test conditions.
GB/T 15248—94
7.5 Inspection after the test. According to the research purpose, the fracture morphology can be observed by scanning electron microscopy. If you want to study the organizational changes that occur during fatigue or the influence of metallurgical organization on fatigue resistance, you can use optical metallographic technology and transmission electron microscopy. 10#
1Aa/2-Aep/2 curve
8Test report
Report the test results as required. The report may include the following: 10~
8.1 Material brand and standard number, manufacturer, furnace batch, specification, chemical composition, heat treatment process and conventional mechanical properties. 8.2 Sampling location, specimen shape, size and surface condition. 8.3 Testing machine model.
8.4 Test conditions, including test temperature and control method, environmental medium, cycle frequency or cycle strain rate, waveform, stress ratio or strain ratio, control method.
8.5 Any situation that does not comply with this standard during the test. 8.6 Test results.
8.6.1 The initial value, stable value or Nt/2 value of the stress range, strain range and plastic strain range. 8.6.2 The number of cycles to failure Nt, and the criteria for determining failure. 8.6.3*Analysis results of cyclic stress-strain properties, including cyclic hardening index and cyclic strength factor. 8.6.4Analysis results of strain-life properties, including fatigue strength index, fatigue ductility index, fatigue strength factor and fatigue ductility factor.
Test date, tester and proofreader.
GB/T 15248—94
Appendix A
Functional relationship
(reference)
A1 Formulas (A1)~(A4) have been conveniently used to describe low cycle fatigue data of many metals. A1.1 Cyclic stress-strain characteristics:
Aa/2=K'(Aep/2)
A1.2 Fatigue life relationship:
A0/2-)(2N)b
(2N）
Aep/2=e'(2N)c
(2N,)h+e'(2N)e
Appendix B
Conversion from radial to axial strain for isotropic materials (reference)
When radial strain is converted into axial strain, it is first necessary to separate the elastic and plastic components from the total strain: B1
E-e+ep
Ed= Ede+edp
-elasticity,
where, e-
-plasticity;
d-radial
-total axial strain.
The axial and radial strains are related by Poisson's ratio√, that is, E,-—Ede /v and E,=— Edp/up. The above formula can be rearranged as:
Edp Ed
Ede / v,- (Ed -- Ede) /vp
With the help of the elastic modulus E, ed can be related to the axial stress: Ede=--(vo)/E
So,
E-/E—E/vp-(vo)/(vpE)
Assuming that plastic deformation occurs under constant conditions, that is, uniform 1/2, then E=(a/E)(1-2V)-2Ed
In the test, using a radial extensometer and an axial load cell, α and ea are continuously available. If E does not change with the cycle, then the slope of the elastic part of the α-Ea line can be obtained from /E. The block diagram of the radial-axial strain conversion principle is shown in Figure B1. 9932, calculate the fatigue ductility index c, fatigue strength index b, fatigue ductility coefficient e\ and fatigue strength coefficient a of metals and alloys under the test conditions. When determining △a, if there is no obvious cyclic stability value, take the stress range at Nt/2.
7.3 Regarding the cyclic stress-strain curve, it is compared with the primary stress-strain curve to judge whether the metal and alloy are cyclic hardening or softening. It is made by half of the paired stable stress range and half of the total strain range. Generally, the multi-sample method is used. If conditions do not allow, the single sample escalation test method with a strain range from small to large can also be used, as shown in Figure 10. Or it can be made using a paired stable stress range and a plastic strain range. Its mathematical expression is shown in Formula (A1) in Appendix A. 600
Figure 10 Cyclic stress-strain curve
7.4 Determination of cyclic strain hardening index n. According to the data of paired stable stress amplitude and plastic strain amplitude, a △a/2-△ep/2 curve is drawn on the double logarithmic coordinates, as shown in Figure 11. The slope of the curve is the cyclic strain hardening index of the metal or alloy under the test conditions.
GB/T 15248-94
7.5 Inspection after the test. According to the research purpose, the fracture morphology can be observed by scanning electron microscopy. If you want to study the organizational changes that occur during fatigue or the influence of metallurgical organization on fatigue resistance, you can use optical metallographic technology and transmission electron microscopy. 10#
1Aa/2-Aep/2 curve
8Test report
Report the test results as required. The report may include the following: 10~
8.1 Material brand and standard number, manufacturer, furnace batch, specification, chemical composition, heat treatment process and conventional mechanical properties. 8.2 Sampling location, sample shape, size and surface state. 8.3 Testing machine model.
8.4 Test conditions, including test temperature and control method, environmental medium, cycle frequency or cycle strain rate, waveform, stress ratio or strain ratio, and control method.
8.5 Any situation that does not comply with this standard during the test. 8.6 Test results.
8.6.1 The initial value, stable value or Nt/2 value of the stress range, strain range and plastic strain range. 8.6.2 The number of cycles to failure, Nt, and the criteria for determining failure. 8.6.3* The analysis results of cyclic stress-strain properties, including the cyclic hardening index and cyclic strength coefficient. 8.6.4 The analysis results of strain-life characteristics, including the fatigue strength index, fatigue ductility index, fatigue strength coefficient and fatigue ductility coefficient.
Test date, tester and proofreader.
GB/T 15248—94
Appendix A
Functional relationship
(reference)
A1 Formulas (A1)~(A4) have been conveniently used to describe the low cycle fatigue data of many metals. A1.1 Cyclic stress-strain characteristics:
Aa/2=K'(Aep/2)
A1.2 Fatigue life relationship:
A0/2-)(2N)b
(2N）
Aep/2=e'(2N)c
(2N,)h+e'(2N)e
Appendix B
Conversion from radial to axial strain for isotropic materials (reference)
When radial strain is converted into axial strain, it is first necessary to separate the elastic and plastic components from the total strain: B1
E-e+ep
Ed= Ede+edp
-elasticity,
where, e-
-plasticity;
d-radial
-total axial strain.
The axial and radial strains are related by Poisson's ratio√, that is, E,-—Ede /v and E,=— Edp/up. The above formula can be rearranged as:
Edp Ed
Ede / v,- (Ed -- Ede) /vp
With the help of the elastic modulus E, ed can be related to the axial stress: Ede=--(vo)/E
So,
E-/E—E/vp-(vo)/(vpE)
Assuming that plastic deformation occurs under constant conditions, that is, uniform 1/2, then E=(a/E)(1-2V)-2Ed
In the test, using a radial extensometer and an axial load cell, α and ea are continuously available. If E does not change with the cycle, then the slope of the elastic part of the α-Ea line can be obtained from /E. The block diagram of the radial-axial strain conversion principle is shown in Figure B1. 9932, calculate the fatigue ductility index c, fatigue strength index b, fatigue ductility coefficient e\ and fatigue strength coefficient a of metals and alloys under the test conditions. When determining △a, if there is no obvious cyclic stability value, take the stress range at Nt/2.
7.3 Regarding the cyclic stress-strain curve, it is compared with the primary stress-strain curve to judge whether the metal and alloy are cyclic hardening or softening. It is made by half of the paired stable stress range and half of the total strain range. Generally, the multi-sample method is used. If conditions do not allow, the single sample escalation test method with a strain range from small to large can also be used, as shown in Figure 10. Or it can be made using a paired stable stress range and a plastic strain range. Its mathematical expression is shown in Formula (A1) in Appendix A. 600
Figure 10 Cyclic stress-strain curve
7.4 Determination of cyclic strain hardening index n. According to the data of paired stable stress amplitude and plastic strain amplitude, a △a/2-△ep/2 curve is drawn on the double logarithmic coordinates, as shown in Figure 11. The slope of the curve is the cyclic strain hardening index of the metal or alloy under the test conditions.
GB/T 15248-94
7.5 Inspection after the test. According to the research purpose, the fracture morphology can be observed by scanning electron microscopy. If you want to study the organizational changes that occur during fatigue or the influence of metallurgical organization on fatigue resistance, you can use optical metallographic technology and transmission electron microscopy. 10#
1Aa/2-Aep/2 curve
8Test report
Report the test results as required. The report may include the following: 10~
8.1 Material brand and standard number, manufacturer, furnace batch, specification, chemical composition, heat treatment process and conventional mechanical properties. 8.2 Sampling location, sample shape, size and surface state. 8.3 Testing machine model.
8.4 Test conditions, including test temperature and control method, environmental medium, cycle frequency or cycle strain rate, waveform, stress ratio or strain ratio, and control method.
8.5 Any situation that does not comply with this standard during the test. 8.6 Test results.
8.6.1 The initial value, stable value or Nt/2 value of the stress range, strain range and plastic strain range. 8.6.2 The number of cycles to failure, Nt, and the criteria for determining failure. 8.6.3* The analysis results of cyclic stress-strain properties, including the cyclic hardening index and cyclic strength coefficient. 8.6.4 The analysis results of strain-life characteristics, including the fatigue strength index, fatigue ductility index, fatigue strength coefficient and fatigue ductility coefficient.
Test date, tester and proofreader.
GB/T 15248—94
Appendix A
Functional relationship
(reference)
A1 Formulas (A1)~(A4) have been conveniently used to describe the low cycle fatigue data of many metals. A1.1 Cyclic stress-strain characteristics:
Aa/2=K'(Aep/2)
A1.2 Fatigue life relationship:
A0/2-)(2N)b
(2N）
Aep/2=e'(2N)c
(2N,)h+e'(2N)e
Appendix B
Conversion from radial to axial strain for isotropic materials (reference)
When radial strain is converted into axial strain, it is first necessary to separate the elastic and plastic components from the total strain: B1
E-e+ep
Ed= Ede+edp
-elasticity,
where, e-
-plasticity;
d-radial
-total axial strain.
The axial and radial strains are related by Poisson's ratio√, that is, E,-—Ede /v and E,=— Edp/up. The above formula can be rearranged as:
Edp Ed
Ede / v,- (Ed -- Ede) /vp
With the help of the elastic modulus E, ed can be related to the axial stress: Ede=--(vo)/E
So,
E-/E—E/vp-(vo)/(vpE)
Assuming that plastic deformation occurs under constant conditions, that is, uniform 1/2, then E=(a/E)(1-2V)-2Ed
In the test, using a radial extensometer and an axial load cell, α and ea are continuously available. If E does not change with the cycle, then the slope of the elastic part of the α-Ea line can be obtained from /E. The block diagram of the radial-axial strain conversion principle is shown in Figure B1. 993- (Ed -- Ede) /vp
With the help of elastic modulus E, ed can be related to axial stress: Ede=--(vo)/E
So,
E-/E—E/vp-(vo)/(vpE)
Assuming that plastic deformation occurs under constant conditions, that is, uniform 1/2, then E=(a/E)(1-2V)-2Ed
In the test, radial extensometer and axial load sensor are used, α and ea are continuously available. If E does not change with the cycle, then the slope of the elastic part of the α-Ea line can be obtained from /E. The block diagram of the radial-axial strain conversion principle is shown in Figure B1. 993- (Ed -- Ede) /vp
With the help of elastic modulus E, ed can be related to axial stress: Ede=--(vo)/E
So,
E-/E—E/vp-(vo)/(vpE)
Assuming that plastic deformation occurs under constant conditions, that is, uniform 1/2, then E=(a/E)(1-2V)-2Ed
In the test, radial extensometer and axial load sensor are used, α and ea are continuously available. If E does not change with the cycle, then the slope of the elastic part of the α-Ea line can be obtained from /E. The block diagram of the radial-axial strain conversion principle is shown in Figure B1. 993
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