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GB/T 38684-2020 (GBT38684-2020)

GB/T 38684-2020_English: PDF (GBT 38684-2020, GBT38684-2020)
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BASIC DATA
Standard ID GB/T 38684-2020 (GB/T38684-2020)
Description (Translated English) Metallic materials -- Sheet and strip -- Biaxial stress-strain curve by means of bulge test -- Optical measuring systems
Sector / Industry National Standard (Recommended)
Classification of Chinese Standard H22
Classification of International Standard 77.040.10
Word Count Estimation 22,278
Date of Issue 2020-03-31
Date of Implementation 2020-10-01
Adopted Standard ISO 16808-2014, MOD
Drafting Organization Baoshan Iron and Steel Co., Ltd., Daum Optical Technology (Shanghai) Co., Ltd., Dalian University of Technology, Shenzhen Wance Testing Equipment Co., Ltd., Metallurgical Industry Information Standards Institute
Administrative Organization National Steel Standardization Technical Committee (SAC/TC 183)
Proposing organization China Iron and Steel Association
Issuing agency(ies) State Administration for Market Regulation, National Standardization Administration
Summary This standard specifies the symbols and descriptions, principles, test equipment, optical measurement systems, test specimens, test procedures, evaluation of vertex curvature deformation and strain of optical measurement methods for biaxial stress-strain curve bulging tests of metal sheets and strips Methods, calculations of biaxial stress-strain curves and test reports. This standard applies to metal sheets and strips with a thickness of less than 3 mm, whose biaxial stress-strain curves are determined during pure bulging. Note: The term "biaxial stress-strain curve" in this standard is abbreviated. What the test measures is a "biaxial true stress-true strain curve".

Standards related to: GB/T 38684-2020

GB/T 38684-2020
NATIONAL STANDARD OF THE
PEOPLE’S REPUBLIC OF CHINA
ICS 77.040.10
H 22
Metallic materials - Sheet and strip - Biaxial stress-
strain curve by means of bulge test - Optical
measuring systems
(ISO 16808:2014, Metallic materials - Sheet and strip - Determination of biaxial
stress-strain curve by means of bulge test with optical measuring systems, MOD)
ISSUED ON: MARCH 31, 2020
IMPLEMENTED ON: OCTOBER 01, 2020
Issued by: State Administration for Market Regulation;
Standardization Administration of PRC.
Table of Contents
Foreword ... 3 
1 Scope ... 5 
2 Symbols and descriptions ... 5 
3 Principle ... 6 
4 Test equipment ... 7 
5 Optical measurement system ... 10 
6 Specimen ... 10 
7 Test procedure ... 11 
8 Evaluation method of apex curvature deformation and strain ... 12 
9 Calculation of biaxial stress-strain curve ... 14 
10 Test report ... 15 
Appendix A (Normative) Verification procedures for optical measurement
systems ... 17 
Appendix B (Informative) Curvature calculation based on response surface . 20 
Appendix C (Informative) Determination of equiaxial stress points of yield and
work hardening curves ... 22 
References ... 30 
Metallic materials - Sheet and strip - Biaxial stress-
strain curve by means of bulge test - Optical
measuring systems
1 Scope
This standard specifies the symbols and descriptions, principles, test
equipment, optical measurement systems, specimens, test procedures, apex
curvature deformation and strain evaluation method for the optical
measurement method for the biaxial stress-strain curve bulging test of sheets
and strips of metallic materials, as well as the calculation and test report of
biaxial stress-strain curve.
This standard applies to the determination of biaxial stress-strain curve of the
metallic sheet and strip which has a thickness of less than 3 mm during the pure
bulging process.
Note: The term "biaxial stress-strain curve" in this standard is abbreviated. The
test determines the "biaxial true stress-true strain curve".
2 Symbols and descriptions
The symbols and descriptions used in this standard are as shown in Table 1.
Table 1 -- Symbols and descriptions
Symbol Descriptions Unit
ddie Diameter of concave die (inner diameter) mm
dBH Diameter of blank holder (inner diameter) mm
R1 Fillet radius of concave die (internal) mm
h Height of drawn specimen (external surface) mm
t0 Initial thickness of specimen (unprocessed) mm
t True thickness of specimen mm
p Cavity pressure MPa
rms Standard deviation (root mean square) -
ρ Radius of curvature mm
r1 Radius of curved surface for determining curvature mm
r2 Radius of curved surface for determining strain mm
r1_100 Radius of curved surface determined by 100 mm concave die mm
coordinate of a grid point on the surface of the bulged specimen, thereby
calculating the shape change and true strain curve of the bulging center area
of the specimen.
4.3 During the test, the system shall be able to determine the XYZ coordinates
(non-contact) of the grid points on the surface of the bulged specimen through
the optical system; use these coordinates to calculate the true strain ε1 and ε2
of each grid point in the selected area, the thickness direction strain ε3, as well
as the curvature radius ρ of the dome of the bulging specimen.
4.4 The system should be equipped with a fluid pressure measurement system
or an indirect measurement system. Starting from 20% of the maximum range
of the system, the accuracy of the measurement system should meet level 1
requirements.
4.5 The concave die, blank holder and liquid cavity shall have sufficient rigidity,
to ensure the minimum deformation of these parts during the test. The blank
holding force shall be high enough to ensure the tightness of the blank holder.
The specimen shall not move between the blank holder and the concave die.
Usually during the test, the bulging pressure will weaken the blank holding force.
When determining the blank holding force required for the test, it shall consider
the effect of this effect on the blank holding force.
4.6 The fluid medium for pressurization shall be in full contact with the surface
of the specimen (no bubbles), to prevent the occurrence of high energy
pressure release or oil splash due to the energy storage effect of the
compressed air bubbles at the moment of energy release or rupture. During the
test, until the specimen ruptures, the fluid shall not leak through the blank holder,
concave die, or plate and anywhere else.
4.7 It is recommended to use calendering ribs (or devices with similar shapes
on a round surface) to prevent material flow. The use of calendering ribs shall
not cause the material to crack. The position of the calendering rib can be
located between the concave die and the blank holder. The size of the
calendering ribs should avoid blocking the material from flowing during the test,
thereby causing excessive bending and wrinkling of the material.
4.8 It is recommended to place a glass plate in front of the lens and lighting
equipment, to ensure that when the specimen is broken, the splash of test oil
will not affect the optical measurement system. The glass plate can be fixed on
the blank holder (thick glass) or placed in front of the lens and lighting system
(thin glass), as shown in Figure 3. This plug-in protection device shall not affect
the measurement quality of the optical measurement system. After each test,
the glass plate shall be wiped clean to avoid damage or scratches; Meanwhile
it shall be accurately placed back to the original position so that no recalibration
of the measurement system is required. In order to obtain better measurement
5 Optical measurement system
In order to determine the radius of curvature ρ of the specimen surface, as well
as the true strains ε1 and ε2, it is recommended that the optical measurement
system have the following characteristics:
a) Optical sensors based on 2 or more cameras.
b) The measurement range shall be greater than 1/2 of the concave die’s
diameter. The measurement area used should be a concentric circle of
the blank holder; its diameter shall be greater than half the diameter of the
blank holder. Throughout the forming process, this area can be observed
at any height of the drawn specimen.
c) Local resolution (distance between two individual grid points): The
distance gmax between two adjacent measurement points on the
undeformed specimens shall meet the following requirements:
d) The determination of the curvature requires that in the concentric circle
area of the blank holder with a diameter of 1/2 ddie, the measurement
accuracy can be verified by testing the optical measurement system, as
shown in Appendix A. The accuracy of the z-axis coordinate shall meet:
Note: Strain measurement accuracy: rms (ε1) = 0.003, rms (ε2) = 0.003.
For each of the true strain values as mentioned in the root mean square
above, the acceptable measurement range is as follows:
- εreal = 0, acceptable measurement range: -0.003 ~ 0.003;
- εreal = 0.5, acceptable measurement range: 0.479 ~ 0.503.
e) Lost measurement points: In order to avoid curvature discontinuities, in
concentric circles with a diameter half the diameter of the blank holder,
only measurement points that do not exceed 5% (not including
interpolated points) are allowed to be lost. If two adjacent points are lost,
the point shall not be fitted into the circle.
6 Specimen
6.1 General
7.2 Measure the initial thickness of the specimen, accurate to 0.01 mm.
7.3 Press the specimen through the blank holder and concave die. During the
test, air bubbles shall be avoided between the specimen and the hydraulic fluid
medium, to prevent the compressed air from splashing the hydraulic oil when
the specimen ruptures.
7.4 It is recommended to use a constant strain rate of 0.05 s-1 for the
deformation area of the specimen. If a constant strain rate cannot be achieved,
it should keep the constant speed of the punch or hydraulic fluid medium. In the
process of measuring the biaxial stress-strain curve, in order to avoid a greater
impact on materials that are sensitive to temperature or strain rate, the bulge
test should be completed within 2 min ~ 4 min.
7.5 During the test, measure the pressure of the hydraulic fluid medium.
7.6 During the test, measure the XYZ coordinates of the grid points on the
specimen surface. Among them, the origin of coordinates shall be located at
the center of the blank holder. The XY plane is parallel to the surface of the
blank holder (parallel to the metal plate pressed before the test). Moreover, the
X-axis direction corresponds to the rolling direction. The Z-axis shall be
perpendicular to the surface of the clamped metal sheet specimen and face the
direction of the optical sensor.
7.7 Pressure data and forming data shall be measured and saved
simultaneously. It is recommended to measure at least 100 sets of data during
the test. In order to show the entire strain and pressure changes, it is
recommended to record at least 100 pictures of the bulge test.
7.8 When the crack completely penetrates the thickness direction of the
specimen, the specimen shall be considered to have failed. That is, cracks in
the thickness direction of the specimen can be judged by detecting the drop in
fluid pressure.
7.9 Prepare enough specimens, to ensure that at least three valid test results
are obtained.
8 Evaluation method of apex curvature deformation
and strain
8.1 In order to better explain the following calculation methods of curvature and
strain, it is necessary to assume a spherical surface (best fit spherical surface)
near the apex. In a picture before the failure of the specimen, as defined in 7.8,
select the arc top area with the largest deformation; define it as the location
where the true stress and true thickness strain ε3. In order to obtain a stable
9.3 The ratio of the diameter of the concave die and the thickness of the
specimen shall be larger within a reasonable range, so as to ensure that the
specimen is under a thin-walled stress state, meanwhile it may ignore the
influence of bending. For tests where the ratio of the concave die’s diameter to
the specimen thickness is less than 100, it is recommended to check whether
the bending strain is sufficiently small compared to the actual thickness strain
ε3. The formula for determining the bending strain is as shown in formula (9):
Note: The biaxial stress-strain curve was obtained without using any
assumptions based on the yield criterion. The biaxial stress-strain curve can be
used to determine the yield locus of equiaxed stress, and to estimate the work
hardening curve of the material before it reaches uniform extension.
9.4 Appendix C gives the recommended method for obtaining the yield criterion
of equal biaxial stress points and the method for extrapolating the equivalent
stress-strain curve based on uniaxial tensile specimens from the biaxial stress-
strain curve obtained by hydraulic bulge test.
10 Test report
The test report shall contain at least the following information:
a) Number of this standard;
b) Operator;
c) Material designation;
d) The initial thickness of specimen;
e) Grid, camera and software used;
f) Location of protective glass;
g) Tonnage of test equipment;
h) Bulging or piston speed;
i) The calculation method of bulge test, especially the parameters for
calculating curvature and average strain;
j) The number of repeated tests;
k) For each bulge test, a table is required to record the absolute values of
Appendix C
(Informative)
Determination of equiaxial stress points of yield and work hardening
curves
C.1 Overview
In the hydraulic bulge test to obtain an equal biaxial stress-strain curve, the
average value of the primary stress and secondary stress corresponds to the
absolute value of the plastic true thickness strain. Generally, the true stress-
strain curve obtained from the uniaxial tensile test in the rolling direction can be
used as a reference curve for calculating the material hardening and yield point.
By comparing the stress-strain data of the iso-biaxial stress state and the
uniaxial reference curve, it can calculate the iso-biaxial stress point, meanwhile
the iso-biaxial strain curve can be transformed into an equivalent stress-strain
curve, which provides the work hardening data when the strain is higher than
the uniform strain of tensile test. C.2 describes how to determine the equal
biaxial stress ratio and how to extend the results of the bulge test on the uniaxial
stress-strain curve to more than uniform elongation.
C.2 Procedure
C.2.1 The procedure described below is one of the methods to deal with stress-
strain data in the bulge test. It is the user's responsibility to check whether the
basic assumptions are fully satisfied before using this method, so that this
method is consistent with the actual material behavior. If in doubt, it is strongly
recommended to consult experts in this field. The assumptions made by this
procedure are as follows:
- Isotropic hardening;
- The yield locus shape does not change with strain;
- Work hardening has nothing to do with the strain path (stress path);
- The loading path and strain path of the test are constant;
- The strain rate and temperature of the bulge test are close to those of the
tensile test. If this condition is not met, the effect of strain rate and
temperature on the material strength shall be known, to determine whether
correction is required.
C.2.2 The true strain ε1-UE of uniform elongation as obtained by the tensile test
in the rolling direction is taken as the reference point of the equivalent strain εE-
...