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GB/T 26077-2021: Metallic materials - Fatigue testing - Axial-strain-controlled method
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Basic data | Standard ID | GB/T 26077-2021 (GB/T26077-2021) | | Description (Translated English) | Metallic materials - Fatigue testing - Axial-strain-controlled method | | Sector / Industry | National Standard (Recommended) | | Classification of Chinese Standard | H22 | | Word Count Estimation | 36,373 | | Issuing agency(ies) | State Administration for Market Regulation, China National Standardization Administration | GB/T 26077-2021: Metallic materials - Fatigue testing - Axial-strain-controlled method---This is a DRAFT version for illustration, not a final translation. Full copy of true-PDF in English version (including equations, symbols, images, flow-chart, tables, and figures etc.) will be manually/carefully translated upon your order.
(Metallic materials Fatigue test Axial strain control method)
ICS 77.040.10
CCSH22
National Standards of People's Republic of China
Replace GB/T 26077-2010
Axial strain control method for fatigue test of metallic materials
(ISO 12106.2017, MOD)
Released on 2021-04-30
2021-11-01 implementation
State Administration of Market Supervision and Administration
Issued by the National Standardization Management Committee
Table of contents
Foreword Ⅲ
Introduction Ⅴ
1 Scope 1
2 Normative references 1
3 Terms and definitions 1
4 Symbol 3
5 Test equipment 5
6 Sample 8
7 Test procedure 14
8 High temperature strain control creep-fatigue test 18
9 Result expression 19
10 Test report 21
Appendix A (informative) Uncertainty of measurement 25
Appendix B (Informative) Graphical representation example of test results 26
Reference 30
Axial strain control method for fatigue test of metallic materials
1 Scope
This document specifies the test equipment, specimens, test procedures, high temperature strain control creep
Variable fatigue test, result expression and test report.
This document is applicable to uniaxially loaded specimens with strain control and strain ratio Re=-1 under constant temperature and constant amplitude conditions. This document can also be used
Instructs to conduct tests at other strain ratios Re, and to conduct tests at high temperatures where creep deformation may be active.
2 Normative references
The contents of the following documents constitute the indispensable clauses of this document through normative references in the text. Among them, dated quotations
Only the version corresponding to that date is applicable to this document; for undated reference documents, the latest version (including all amendments) is applicable to
This document.
GB/T 12160 Calibration of extensometer system for uniaxial testing of metallic materials (GB/T 12160-2019, ISO 9513.2012,
IDT)
GB/T 16825.1 Inspection of static uniaxial testing machine Part 1.Inspection and calibration of force measurement system of tensile and (or) compression testing machine
Standard (GB/T 16825.1-2008, ISO 7500-1.2004, IDT)
GB/T 25917.1 Uniaxial fatigue test system Part 1.Dynamic force calibration (GB/T 25917.1-2019, ISO 4965-1.
2012, IDT)
GB/T 34104 Metal material testing machine loading coaxiality inspection (GB/T 34104-2017, ISO 23788.2012,
MOD)
JJF1637 Low-cost metal thermocouple calibration specification
JJG141 Verification Regulation of Precious Metal Thermocouple for Work
JJG556 Verification Regulations of Axial Force Fatigue Testing Machine
JJG617 Verification Regulation of Digital Temperature Indicating Regulator
3 Terms and definitions
The following terms and definitions apply to this document.
3.1
Engineering stress
The instantaneous force divided by the initial cross-sectional area within the gauge length.
S=F/A0
Where.
S ---Engineering stress;
F --- instantaneous force;
A0-the original cross-sectional area.
3.2
True stress
The instantaneous force divided by the instantaneous cross-sectional area within the gauge length.
σ=F/A or σ=S(1 e)
Where.
S---Engineering stress F/A0;
e ---Engineering strain ΔL/L0.
Note. When the strain is about 10%, the real stress is similar to the engineering stress F/A0.It is also important that when the strain is about 10%, the engineering strain is derived from the extension
The meter actually measures and is the controlled parameter under test.
3.3
Initiallength
Gaugelength
L0
The initial length between the measuring points of the extensometer at the test temperature.
3.4
Parallel length
Lp
The length between the transition radii of the specimen.
3.5
Strain
Engineering strain e=
ΔL
L0 =
Li-L0
L0
Real total strain ε=∫
Li
L0
dL
Where.
Li --- the instantaneous length of the gauge length part;
L0---initial length or gauge length.
Note. When the true strain value is about 10%, ε is approximate to engineering strain e=ΔL/L. It is also important that with a strain of about 10%, the engineering strain
It is the quantity measured by the extensometer and the controlled parameter in the strain control fatigue test.
3.6
Cycle
The smallest unit of cyclic repetition on the strain-time function.
3.7
Maximum
The maximum algebraic value of a variable in a loop.
3.8
Minimum
The smallest algebraic value of a variable in a loop.
3.9
Mean
Half of the algebraic sum of the maximum and minimum values of the variable.
6.2 Preparation of test samples
6.2.1 General requirements
The low-cycle fatigue test is designed to characterize the inherent characteristics of the material, and it is very important to prepare the sample in accordance with the following recommendations. If try
The purpose of the test is to investigate the influence of some special factors (such as surface heat treatment, oxidation, etc.) without following the recommendations below. Under any condition,
Any deviation should be noted in the test report.
6.2.2 Machining program
If the test material needs heat treatment, it should be done before processing the sample. Machining may cause residual stress on the surface of the specimen, thereby
Affect the test results. This stress is caused by the thermal gradient generated during the processing stage, and is related to the deformation of the material or the change of the microstructure.
For high temperature tests, the effect of residual stress on the test results is not obvious, because part or all of the heat is released during the heat preservation process.
Residual Stress. The selection of appropriate finishing process can reduce the residual stress of the sample. For hard materials, grinding should be given priority.
Grinding. It is used within 0.1mm of the size of the finished product, and the feed amount does not exceed 0.005mm/time;
Polishing. Use sandpaper with a gradually decreasing particle size to complete the polishing within 0.025mm of the finished product size. It is recommended to sample the sample in the final stage.
Take longitudinal polishing.
Note 1.Changes in the microstructure of the material. This phenomenon is caused by the temperature increase and strain hardening during the processing process, and it also causes phase transitions and surface recombination.
The reason for the crystal.
Its direct effect is to invalidate the test result, because the test material is no longer in its original state, and care should be taken to prevent it.
Note 2.Pollutant intervention. certain elements or compounds may degrade the mechanical properties of a certain material, such as the presence of chlorine in steel or titanium alloys. So add
The use of these elements should be avoided in the process (especially in the cutting emulsion). Attention should also be paid to the cleaning and degreasing process during the pre-storage of the sample.
The problem.
The processing technology used should be traceable and subsequently recorded in the test report. In addition, the position of each sample should be pointed out carefully.
For placement and orientation, it is advisable to use an initial shape other than the bar or round bar as the test material. For example, forged, cast or additively manufactured products, which
The above variables will affect the fatigue response. GB/T 2975 specifies the mechanical properties of steel profiles, bars, flats and pipes specified in ISO 6929
The identification, location and preparation requirements of test specimens and test pieces. In addition, GB/T 20832 specifies samples related to product characteristics
Axis.
6.2.3 Sampling and marking
Test materials sampled from semi-finished products or components may have a significant impact on the interpretation of the results obtained during the test. Therefore record
The detailed information of the sample sampling site is necessary.
A clear sampling plan should be attached to the test report. The sampling plan should include.
---The sampling location of each sample;
--- Processing direction of semi-finished products (such as rolling direction, extrusion direction, etc.);
---Marking of each sample.
The sample shall have a unique number during the processing of the sample. Any marking method can be used on the area that will not be processed away on the sample
Marking should not affect the quality of the test.
6.2.4 Surface condition of the sample
The surface condition of the sample will affect the test results. The impact is usually related to the following factors.
---The surface roughness of the sample;
---Residual Stress;
--- Changes in the microstructure of the material;
---Involvement of pollutants.
Follow the recommendations below to minimize the impact of the above factors.
The surface quality of the sample is usually characterized by surface roughness. The importance of the surface roughness of the sample to the obtained test results is
To a large extent is related to the test conditions, the surface corrosion or plastic deformation of the sample will reduce its influence.
The average surface roughness of the sample should be less than 0.2μm (or the equivalent value of other parameters).
Another important parameter different from the average surface roughness is the presence of local mechanical scratches. For the final work of round specimen processing
The sequence is usually to remove all scratches in the circumferential direction caused by the lathe. The specimen should be polished longitudinally after grinding. At low power (approximately
20 times) check the sample, there should be no scratches along the circumferential direction and obvious processing marks.
If the heat treatment is performed after the surface of the sample is processed, it is recommended to polish the surface of the sample after the heat treatment. If it can't be polished,
It is advisable to conduct heat treatment in a vacuum environment or inert gas protection to avoid oxidation of the sample. It is recommended to remove the residual stress of the specimen.
The heat treatment should not change the microstructure of the material being studied. The details of heat treatment and machining should be indicated in the test report.
6.2.5 Dimension check
The size of the sample should be inspected after finishing finishing, and the inspection method used should not change the surface condition of the sample.
6.2.6 Storage and transportation
After the sample is prepared, it should be stored to prevent any damage (contact scratches or oxidation, etc.). It is recommended to use a separate box or a tube with a cap
Save the sample. In some cases, the sample should be stored in a vacuum bottle or a desiccator containing silica gel.
The transportation of samples should be minimized.
Special care should be taken when marking the sample. It is recommended to mark the specimen at both ends of the specimen, so that the half of the specimen can also be
To be recognized.
7 Test procedure
7.1 Laboratory environment
Low cycle fatigue and creep-fatigue test is a very complicated test. The quality of test results and selected test methods and laboratories
The environment has a lot to do with it.
The test should be carried out in the following suitable environment.
---Constant room temperature and relative humidity;
---Minimum air pollution (such as dust, chemical vapor, etc.);
---No external electrical signal interference that affects the control and data acquisition of the testing machine;
---Minimal external mechanical vibration.
Note. When testing certain materials (such as aluminum alloy), it is extremely important to observe and record the relative humidity, because humidity has a significant impact on fatigue life.
7.2 Test machine control
During the test, the peak strain of the servo control should not exceed ±1% of the set value.
In the strain control test of long-life fatigue (i.e., the nominal cyclic plasticity is negligible, for example, about 106 to 107 cycles) strain control test,
In order to shorten the test time, the test control mode can be switched, and the force control fatigue test can be carried out at a higher frequency. The fatigue that started in the strain control
The labor test can obtain a stable stress-strain response. In this case, the cyclic plasticity is small and can be ignored. In this case, the test can be controlled
The control mode is switched to the force control mode, and the test frequency should be increased cautiously, so as not to increase the sample temperature due to the hysteresis effect. Trying in conversion
When testing the control mode, it is advisable to monitor the strain at any time and make adjustments in the force control mode to ensure that the strain is within the range of the maximum strain and the minimum strain.
The limit deviation is within ±0.5%. When changing the test control mode, it is advisable to refer to GB/T 3075 for more information.
In a creep fatigue test involving constant stress (force) in the cycle, the indicated peak value of the applied force should be kept within ±1% of the set value.
7.3 Installation of the sample
When installing the specimen, try to avoid pre-strain. For the previously aligned test system, it is recommended to use displacement control to reduce the
One end is clamped on the chuck, and the actuator is moved so that the other end of the sample touches the other chuck. Then keep the specimen small on the other end of the chuck
Compression preload (force control) clamping. Next, set up the extensometer, when its output returns to zero, the strain control conversion can be carried out. Especially at high temperature
In the test, the use of anti-adhesion compounds or pre-oxidation of the end of the sample can help remove the sample from the fixture. When the force is zero, it may
The extensometer needs to be reset to zero. Set the gauge length of the extensometer and adjust it mechanically to zero. If necessary, it is recommended to use a mechanical zero adjustment device or use
Adjust the gauge length with your fingers to make it as close to zero as possible (that is, within about ±0.5% of the test range). Once the “rough” zero position is obtained, build
It is recommended to reset the electronic zero of the extensometer again. When handling samples that are susceptible to corrosive erosion, cotton or acrylic protective gloves should be worn.
7.4 Cycle Waveform-Strain Rate or Cycle Frequency
In order to determine the trend of material properties, a strain ratio other than Re=-1 is usually used for strain control experiments. Figure 8 shows the
The stress-strain hysteresis curve during the test when Re=0.As shown in Figure 8, for the average strain of this tension offset, the cycle of the average stress
The ring relaxation tends to zero average stress value. In this case, it is recommended to carefully monitor and record such stress-strain and average stress information to
Then carry out subsequent data analysis and determine material trends. When Re=∞, the hysteresis loop exhibits similar behavior, but under the opposite stress meaning
In a sense, when the hysteresis loop moves upward in the stress-strain space, the stress relaxation tends to a smaller negative value.
Note. Testing under the condition of Re=0.1 does not always prevent bifurcation buckling, because the cyclic average stress relaxation during the test may cause the hysteresis loop
The lower part is compressed, as shown in the stress-strain response in Figure 8.
Fig. 8 Stress-strain hysteresis loop from 0 to maximum strain test when Re=0
The waveform of the control parameter (strain) should remain unchanged during the test. The purpose of the test study is to study the effect of the cyclic waveform on the fatigue behavior of the material.
Except for the impact test. The test is usually carried out with a constant total strain rate, in the range of 5×10-4s-1~5×10-2s-1 (0.05%s-1~
5%s-1), which means a triangular loop shape. Generally, it can be used in the range of 0.01Hz~1Hz at room temperature
Sine wave, but it is not recommended to perform at high temperature, because this will cause the strain rate to change.
The creep-fatigue cycle usually consists of a constant total strain rate ramp, which is likely to be different in the tensile and compressive directions, and at the same time.
The force and/or strain residence time ranges from a few minutes to tens of hours or even hundreds of hours. Chapter 8 explains this further.
7.5 Test procedure
7.5.1 Forecast
At the beginning of the test, it is recommended to repeatedly apply cyclic force to the sample within the elastic range at room temperature to determine the elastic modulus of the material and confirm
The correctness of the measurement system (force and strain). The deviation of the measured value of the elastic modulus from the expected value should not exceed ±5%.
For the same test condition, it is recommended to increase the test temperature from room temperature to the test temperature by detecting the temperature (the force value is at zero in the force control mode of the testing machine)
Point), the thermal strain of the extensometer measures the average thermal expansion coefficient of the material. The deviation of the measured value of this coefficient from the expected value should not exceed
Over ±5%.
Generally, the installation of the extensometer is carried out at room temperature, and the value of the extensometer is adjusted to zero when the test temperature is reached. In this case, at high
The strain value measurement should be corrected in the temperature test, because the gauge length of the extensometer has been changed due to thermal expansion. Therefore, in the pre-test
At least the thermal expansion value of the gauge length shall be recorded during the verification and correction. For automatic systems, the corrected gauge length should be used for online control and
data collection.
For some systems, especially when the test temperature exceeds 1000°C, the extensometer may be installed when the sample is in a hot state. In this case,
Under circumstances, it is impossible to measure the coefficient of thermal expansion. At this time, the actual gauge length should be measured by a reliable method.
If the fatigue test is performed at high temperature, it is recommended.
a) Pre-compress the sample at room temperature, and the pre-compression is equal to the product of the metal linear expansion coefficient and the difference between the room temperature and the test temperature (that is, αΔT);
b) Determine the gauge length at the test temperature;
c) Set the extensometer to zero and compensate to the newly established gauge length under the test temperature in the test procedure.
7.5.2 Start test
For a clear test procedure, it is necessary to choose the direction of the first quarter cycle. Tensile stress is usually selected; for induction heating
In the low-cycle fatigue test, due to the effect of thermal induction, the compressive stress is the direction of the first quarter of the cycle.
In the experiment of strain control, the usual procedure is to change the control mode from force value to strain after heating up and checking the elastic modulus. test
The machine should complete this conversion without overshoot (that is, additional strain beyond the required strain level). Overshoot will affect subsequent test production
Health impact.
The difference between the actual half-width of the strain at the beginning of the test and the half-width of the control strain should not exceed 5% of the half-width of the control strain. In order to reach
When the strain level specified in 7.2 is reached, adjustments should be made to the corresponding variables. The entire adjustment process should be within the first 10 cycles or 1% of the number of failure cycles (taken as
Whichever is smaller).
For the suspension of the test due to negligence or other accidents, it should be confirmed before resuming the test.
---The specimen was not damaged (i.e. bent) during the stopping process;
---The extensometer does not slip;
---The stress is continuous before the unexpected stop;
---The modulus is the same as before the accidental stop;
---The strain limit is the same as before the unexpected stop.
The above situation can be confirmed by analyzing the test data. Under the above circumstances, it is allowed to recover without overshoot.
test.
7.6 Number of samples
The recommended number of samples is at least 8; the abscissa of the obtained strain amplitude-cycle number curve should cover at least three orders of magnitude.
7.7 Test record
7.7.1 Stress-strain hysteresis loop
For the XY recording system at the beginning of the test, the original stress-strain hysteresis loop fed back by the strain control should be continuously recorded. Of course
Later, this curve should be recorded regularly during the test. The choice of data recording frequency is related to the expected number of test cycles. Commonly used numbers
The data recording group includes the first 10 cycles of the test, followed by logarithmic growth (such as 20, 50, 100,.200, 500, etc.), and collected every ten times the cycle
3 hysteresis loops.
For automatic data acquisition systems, it can be through a program with a preset number of cycles or according to the changes of two parameters (stress and strain).
The quantification formulation program completes the selection of the number of acquisition cycles. No matter which recording method is used, the given data sampling rate should be clearly described
Hysteresis loop (see 5.4.1).
7.7.2 Data Collection
If the test equipment allows, record the time-varying stress, strain, and temperature, as well as the retention time in the creep-fatigue test, as shown in the section
Described in Chapter 8.If this is not possible, at least the peak stress, strain and temperature should be recorded in order to determine the failure of the specimen according to 7.8.
7.8 Test abort
The test will be terminated when the selected termination test conditions are reached and the testing machine is equipped with condition-controlled shutdown equipment. Without this device,
Other feasible methods of stopping the testing machine should be used, such as when the force value does not reach a certain force threshold (generally speaking, this is a method that depends on the amplitude
The test is terminated when the control signal reaches a certain difference between the control signal and the feedback signal.
It is worth noting that the method of preselecting the shutdown stress for some tests is not appropriate, and the test should not be automatically stopped, as shown in Figure 8.
The continuous loop softens the situation. In this case, it is recommended to monitor the response of the material before determining the conditional stress. In fact, it may be experimenting
After the end, determine the number of failed cycles.
If the test stops automatically before the sample breaks, the test data should be checked to determine whether the failure criterion is reached before the sample is removed.
according to. If it is found that the shutdown is too early, the test can be resumed (see 7.5.2). If the failure criterion is reached, the force value control mode should be selected and the force
Drop to zero and remove the sample after cooling down. If the sample has broken, usually choose the displacement control mode to cool the sample before removing it.
For the high temperature test, in order to limit the oxidation of the sample and facilitate the microscopic observation of the section in the future, the high temperature should be turned off immediately after the test is terminated.
furnace. If the test is terminated before the sample breaks, try to avoid overloading the sample during the cooling process of the heating device.
Note. The latter approach is especially suitable for electronic-mechanical testing machines.
7.9 Failure criterion
There are many definitions of failure. This may depend on the interpretation of the fatigue test results and the nature of the test material. For failure
The consideration of the criterion is usually based on the occurrence, development and enhancement of certain phenomena, and these phenomena that can be monitored indicate that the sample will be severely affected.
Heavy damage or instantaneous failure.
The fatigue life Nf can be defined by the number of cycles that meet the following failure criteria.
a) A certain percentage change of the maximum tensile stress relative to the level determined by the test;
b) The ratio of tensile and compressive modulus of elasticity on the hysteresis loop changes to a certain extent; usually ET/EC=0.9 is the evaluation failure
Effective standards (see Figure 9);
c) The specimen is completely broken into two parts.
Note. The post-test inspection of the failed specimens defined above will give a lot of useful information.
It is recommended to record the crack initiation position related to the frame of the testing machine, that is, the quadrant where the sample is cracked. This feature will help determine the possibility
Different axis of the testing machine or fixture.
The failure criterion used for a group of tests should be indicated in the report. Figure 10 shows an example of stress reduction failure, in this case
Below, Nf is defined as the number of cycles when the stress value drops sharply by x% on the tensile stress-cycle number curve. Usually the value of x is 10.
However, the typical range of x value is 2%~30%.
The number of failure cycles is usually evaluated from the range of upper boundary stress or upper boundary force or upper boundary force. For extensometers outside the gauge length
In the case of fracture, the upper boundary stress/maximum tensile stress will increase by x%.
This failure criterion is related to one (or more) visible cracks on the sample. Generally speaking, the crack area and the specimen
The ratio of the original cross-section is comparable to the ratio of the stress drop.
For specimens that are completely or partially broken, the position of the fracture surface or crack relative to the gauge length shall be determined, and the test results shall be reported.
It is indicated in the report.
In order to confirm the validity of the test, the sample should be inspected after the test. This means that on the one hand, the inspection sample fails or has a main crack
On the other hand, confirm whether there are defects or abnormalities that may cause initial failure or premature failure (such as surface defects, holes, clamps).
Miscellaneous, excessive card marks left by the extensometer or bending of the specimen due to alignment problems).
8 High temperature strain control creep-fatigue test
This document provides a strain control that maintains constant stress and/or constant strain for a certain period of time during cycling at uniform temperature
The creep fatigue test method.
In many actual structural components, creep and low-cycle fatigue are usually the result of service conditions involving thermal cycles, which include
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