This standard specifies the definition, symbols, principle, test equipment, specimens, test procedures, calculation and test report of the tensile strain hardening index (n value) test method for metal sheets and strips. This standard is applicable to the determination of the tensile strain hardening index (n value) of metal sheets and strips with a thickness of 0.1~3mm. This standard is only applicable to the part where the stress-strain curve is monotonically continuous within the uniform plastic deformation range. GB/T 5028-1999 Test method for tensile strain hardening index (n value) of metal sheets and strips GB/T5028-1999 Standard download decompression password: www.bzxz.net

GB/T5028—1999
This standard is equivalent to ISO10275:1993 "Metallic materials-sheets and strips-Determination of tensile strain hardening index". The main technical contents are the same as ISO10275:1993, but the writing method is not completely corresponding. Chapter 9 "Calculation" has been added. This revision of this standard has modified the following main technical contents of GB/T5028--1985: the chapters have been rearranged;
- Added principles and removed test significance;
- Added reference standards;
- Added specimen types;
Deleted Appendix A;
Listed the calculation of intercept, strength coefficient, standard deviation and coefficient of variation as Appendix A. This standard replaces GB/T5028--1985 "Test method for tensile strain hardening index (n value) of metal sheets" from the date of implementation. Appendix A of this standard is a prompt appendix.
This standard was proposed and coordinated by the National Technical Committee on Steel Standardization. The drafting units of this standard are: Wuhan Iron and Steel (Group) Corporation, Central Iron and Steel Research Institute. The main drafters of this standard are: Dai Ruiling, Wu Guoyun, Liang Xinbang. This standard was first issued in March 1985.
GB/T5028—1999
ISO Foreword
ISO (International Organization for Standardization) is a worldwide federation composed of national standardization groups (ISO member groups). The work of formulating international standards is usually completed by ISO's technical committees. If each member group is interested in a project established by a technical committee, it has the right to participate in the technical committee. International organizations (official or unofficial) that maintain contact with ISO also participate in the work. In terms of electrotechnical standardization, ISO maintains a close cooperative relationship with the International Electrotechnical Commission (IEC). The draft international standard adopted by the technical committee is submitted to the member groups for voting. The international standard needs to obtain the consent of at least 75% of the member groups participating in the voting before it can be officially released. International standard ISO10275 was developed by ISO/TC164 Technical Committee on Mechanical Properties of Metals, SC2 Ductility Testing Subcommittee.
Appendix A of this international standard is a prompt appendix. 106
National Standard of the People's Republic of China
Metallic materials--Sheet and strip--Determinationof tensile strain hardening exponent(n-values)GB/T 5028 - 1999
eqv IS0 10275:1993
Replaces GB/T50281985
This standard specifies the definition, symbols, principles, test equipment, specimens, test procedures, calculations and test reports of the tensile strain hardening exponent (n-value) test method for metal sheets and strips. This standard is applicable to the determination of the tensile strain hardening exponent (n-value) of metal sheets and strips with a thickness of 0.1 to 3 mm. This standard instrument is applicable to the monotonically continuous portion of the stress-strain curve within the uniform plastic deformation range. 2 Referenced standards
The clauses contained in the following standards constitute the clauses of this standard through reference in this standard. When this standard is published, the versions shown are valid. All standards will be revised, and parties using this standard should explore the possibility of using the latest versions of the following standards. GB/T228-1987 Metal tensile test method GB/T10623-1989 Metal mechanical properties test terminology GB/T16825-1997 Calibration of tensile testing machine JJG762-1992 Extensometer verification procedure
GB/T 5027-1999↑
Metal sheet and strip plastic strain ratio (r value) test method 3 Definitions
This standard adopts the following definitions.
3.1 Strain hardening index (n): Under uniaxial tensile force, the true strain index in the mathematical equation of true stress and true strain can be expressed by formula (1):
This equation can be transformed into the following logarithmic equation: Ing = Ink + nlne
The slope of the straight line on the double logarithmic coordinate plane is the strain hardening index: n tanα
3.2 Yield point elongation: The elongation value of the specimen in the entire discontinuous interval on the force-elongation curve (only refers to materials with obvious yield phenomenon). For other related definitions, see GB/T10623.
4 Symbols
The symbols used in this standard and their explanations are shown in Table 1. Approved by the State Administration of Quality and Technical Supervision on 1999-71-01 and implemented on 2000-08-01
Original thickness of specimen
Original width within the gauge length of specimen
Extensometer gauge length
Instantaneous elongation of the gauge length section
Instantaneous length of the gauge length section - L
GB/T5028-1999
Original cross-sectional area of ​​the parallel length section of the specimenSymbol and description
When force F is applied The cross-sectional area of ​​the parallel length of the specimen is S = s.)
Parallel length of the specimen
True strain of the specimen under the action of force F
True stress of the specimen under the action of force F
Instantaneous force applied to the specimen
Strain hardening index
Strength coefficient
Slope of the straight line of the relationship between Ina and Ine
Number of measurement points when determining the strain hardening index Note; 1N/mm2-1 MPa
The specimen is axially tensile deformed at a specified constant rate within the uniform plastic deformation range. The tensile strain hardening index (n value) is calculated using the stress-strain curve for the entire uniform plastic deformation range, or using a part of the stress-strain curve for the uniform plastic deformation range. 6 Testing equipment
6.1 Testing machine
6.1.1 The testing machine shall comply with the requirements of GB/T16825, and the level of the testing machine shall not be lower than level 1. 6.1.2 The clamping method of the specimen shall comply with the provisions of GB/T228. The testing machine shall be able to control the moving speed of the chuck according to the requirements of 8.4.1 to conduct uniaxial tensile tests.
6.1.3 When the chuck of the testing machine clamps the specimen, it shall ensure that the center line of the longitudinal axis of the specimen and the center lines of the upper and lower chucks coincide with the tensile axis of the testing machine.
6.1.4 The testing machine shall be calibrated regularly by the metrology department. 6.2 Measuring tools
The thickness of the specimen shall be measured with at least a grade 1 dry ruler or other measuring tools with the same accuracy, and the measurement error of the specimen width shall not exceed ±0.2%. The measuring tools shall be inspected regularly by the metrology department. 6.3 Extensometer
6.3.1 The extensometer for measuring the change in the gauge length of the specimen shall comply with the grade 1 or higher provisions of JJG762. 6.3.2 The extensometer shall be calibrated according to JJG762. During calibration, the working state of the extensometer shall be as similar as possible to that during the test. 6.3.3 The calibrated extensometer should be carefully checked before testing. When the extensometer is repaired or abnormal, it should be recalibrated. 7 Test specimens
7.1 Sampling
GB/T5028-1999
The sampling location, direction and quantity shall comply with the requirements of relevant product standards or be determined by negotiation between the two parties. 7.2 Test specimen shape
The test specimen shall generally have a clamping portion wider than the parallel length portion (referred to as a shoulder test specimen, see Figure 1) The rest
R≥20
Shoulder test specimen
By agreement, the test specimen can also be a strip with parallel edges (referred to as a non-shoulder test specimen, see Figure 2). For products with a width equal to or less than 20mm, the test specimen width can be equal to the width of the product. However, for arbitration tests, a shoulder test specimen should be used. The rest
Figure 2 Non-shoulder test specimen
7.3 Test specimen size
The parallel length should not be less than Lo+b. /2. During the arbitration test, the parallel length should be L. +2bo. For non-shoulder specimens with a width less than or equal to 20mm, the free length between the grips should not be less than L+3bg. Table 2 gives the dimensions of two non-proportional specimens. If the measurement results with the same accuracy can be given, other non-standard types of specimens may also be used. Table 2 Dimensions of two non-proportional specimens
Original width hOriginal gauge length L.
Parallel length L
Clamping part
Width BwwW.bzxz.Net
Clamping part
Length h
! Maximum width within the gauge length
Non-shoulder specimen between grips!
Free length
≥140
Difference with the minimum width
7.4 Specimen preparation
GB/T 5028— 1999
7.4.1 The rough specimens must be cut individually. All specimens must be machined to eliminate the effects of work hardening. Note: For very thin specimens, it is recommended to separate the rough specimens of equal width with oil paper one by one, clamp thicker plates of equal width on both sides and machine them together until the required size is reached.
7.4.2 Unless otherwise specified, the specimen thickness should be the full thickness of the product. 7.4.3 The specimen surface should not have defects such as scratches. 7.5 If the plastic strain ratio (r value) is measured at the same time, it should comply with the requirements of GB/T5027. 8 Test procedure
8.1 The test is generally carried out at room temperature of 10 to 35°C. For tests with strict temperature requirements, the room temperature should be (23 ± 5)°C. 8.2 Measure and record the original thickness a. and width bo within the gauge length of the specimen. The measurement error should make the error of the original cross-sectional area S. not exceed ± 2%.
8.3 The specimen is clamped on the testing machine and the axial force of the specimen should be ensured. 8.4 Test speed
8.4.1 The speed of the chuck movement of the testing machine shall not exceed 50% L./min. The chuck movement speed shall be constant throughout the strain range of the n value. For special materials, the tensile speed shall be in accordance with the provisions of the relevant product standards. 8.4.2 If the specified non-proportional elongation stress, yield point, yield point elongation and other properties are determined at the same time as the n value test, the test speed shall comply with the provisions of GB/T228.
8.5 Record the force and the corresponding strain value.
8.5.1 When the n value is determined within the entire uniform plastic deformation range, the upper limit of the measured strain shall be slightly less than the strain corresponding to the maximum force; the lower limit shall be slightly greater than the yield strain (materials that do not yield significantly) or the strain at the end of the yield point elongation (materials that yield significantly). When the elastic strain part is less than 10% of the total strain, it does not need to be deducted. 8.5.2 For manual measurement, at least 5 strain data points distributed in geometric series should be taken within the strain range to be examined (see Figures 3 and 4).
Elongation AL, mm
Figure 3 Distribution of data points (not obvious yield) +10
GB/T5028-1999
Elongation Al., mm
Figure 4 Distribution of data points (obvious yield)
8.5.3 For automatic measurement, the data processing program can directly obtain the strain hardening exponent, but the measured strain data points should not be less than 5. If the strain data points are less than 20, these data points should be distributed in geometric series; if the data points are greater than 20, they can be distributed at equal intervals. 8.5.4 When multiple n values ​​need to be determined, the data points in each strain interval for calculating n values ​​shall not be less than 5. 9 Calculation
9.1 According to the force and the corresponding deformation values, use equations (4) and (5) to calculate the true stress and true strain respectively: a
LesS.
e= ln(E
. (4)
9.2 According to the logarithmic expression of the true stress-true strain power exponent within the uniform plastic deformation range, the strain hardening exponent n is calculated using the least squares method. Therefore, formula (2) can be expressed in the following form: Y=AX+B
Where: Y=lna
B--lnK
According to this, the relationship for calculating the strain hardening exponent (n) can be derived: N
N(x,)2
9.3 The calculated strain hardening exponent (n value) should be rounded to an accuracy of 0.005. (6)
Test report
The test report shall include the following contents:
a) National Standard No.;
b) Description of the test material;
GB/T5028
c) Method adopted (manual measurement or automatic measurement);
d) Type of specimen adopted;
e) Orientation of the specimen relative to the rolling direction;
f) Uniform strain range for determination of strain hardening exponent;
g) Number of measurement points for determination of strain hardening exponent;
h) Other special conditions in the test
i) Test results.
GB/T 5028
3—1999
Appendix A
(Suggested Appendix)
Calculation of intercept, strength coefficient, standard deviation and coefficient of variation According to the definition of strain hardening index (n value), the logarithm is obtained as follows: Ing=InK+nlne
Then lnK is the intercept in the double logarithmic coordinates.
Assume B=InK
The intercept B and strength coefficient K are calculated according to formula (A1): Sy
K = exp(B)
The strength coefficient (K) is numerically equal to the extrapolated value of the true stress when the true strain is 1.00. The standard deviation S(n) is calculated according to formula (A3), which reflects the degree of dispersion of the slope n of the straight line. A2
TN(Y,)?
S(n) =
Sn）depends on the size of the strain interval for measuring n value and the number of stress-strain data pairs used in the regression calculation. A3n value test accuracy depends on the stress-strain measurement accuracy. The relative test accuracy of n value is expressed by the coefficient of variation (n): v(n)=
A4The linear correlation between the test values ​​is expressed by Q: N
([NEX?
(Zx)\inzy?
（A4）
（A5)）(6)
Test report
The test report shall include the following contents:
a) National Standard No.;
b) Description of the test material;
GB/T5028
c) Method adopted (manual measurement or automatic measurement);
d) Type of specimen adopted;
e) Orientation of the specimen relative to the rolling direction;
f) Uniform strain range for determination of strain hardening exponent;
g) Number of measurement points for determination of strain hardening exponent;
h) Other special conditions in the test
i) Test results.
GB/T 5028
3—1999
Appendix A
(Suggested Appendix)
Calculation of intercept, strength coefficient, standard deviation and coefficient of variation According to the definition of strain hardening index (n value), the logarithm is obtained as follows: Ing=InK+nlne
Then lnK is the intercept in the double logarithmic coordinates.
Assume B=InK
The intercept B and strength coefficient K are calculated according to formula (A1): Sy
K = exp(B)
The strength coefficient (K) is numerically equal to the extrapolated value of the true stress when the true strain is 1.00. The standard deviation S(n) is calculated according to formula (A3), which reflects the degree of dispersion of the slope n of the straight line. A2
TN(Y,)?
S(n) =
Sn）depends on the size of the strain interval for measuring n value and the number of stress-strain data pairs used in the regression calculation. A3n value test accuracy depends on the stress-strain measurement accuracy. The relative test accuracy of n value is expressed by the coefficient of variation (n): v(n)=
A4The linear correlation between the test values ​​is expressed by Q: N
([NEX?
(Zx)\inzy?
（A4）
（A5)）(6)
Test report
The test report shall include the following contents:
a) National Standard No.;
b) Description of the test material;
GB/T5028
c) Method adopted (manual measurement or automatic measurement);
d) Type of specimen adopted;
e) Orientation of the specimen relative to the rolling direction;
f) Uniform strain range for determination of strain hardening exponent;
g) Number of measurement points for determination of strain hardening exponent;
h) Other special conditions in the test
i) Test results.
GB/T 5028
3—1999
Appendix A
(Suggested Appendix)
Calculation of intercept, strength coefficient, standard deviation and coefficient of variation According to the definition of strain hardening index (n value), the logarithm is obtained as follows: Ing=InK+nlne
Then lnK is the intercept in the double logarithmic coordinates.
Assume B=InK
The intercept B and strength coefficient K are calculated according to formula (A1): Sy
K = exp(B)
The strength coefficient (K) is numerically equal to the extrapolated value of the true stress when the true strain is 1.00. The standard deviation S(n) is calculated according to formula (A3), which reflects the degree of dispersion of the slope n of the straight line. A2
TN(Y,)?
S(n) =
Sn）depends on the size of the strain interval for measuring n value and the number of stress-strain data pairs used in the regression calculation. A3n value test accuracy depends on the stress-strain measurement accuracy. The relative test accuracy of n value is expressed by the coefficient of variation (n): v(n)=
A4The linear correlation between the test values ​​is expressed by Q: N
([NEX?
(Zx)\inzy?
（A4）
（A5)）
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