GB/T 38684-2020 English PDF (GBT38684-2020)

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GB/T 38684-2020: Metallic materials -- Sheet and strip -- Biaxial stress-strain curve by means of bulge test -- Optical measuring systems
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 n...