GB/T 38719-2020 English PDF (GBT38719-2020)

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GB/T 38719-2020: Metallic materials -- Tube -- Determination of biaxial stress-strain curve of tube by hydro-bulging test
GB/T 38719-2020
GB
NATIONAL STANDARD OF THE
PEOPLE’S REPUBLIC OF CHINA
ICS 77.040.10
H 22
Metallic Materials - Tube - Determination of Biaxial
Stress-strain Curve of Tube by Hydro-bulging Test
ISSUED ON: MARCH 31, 2020
IMPLEMENTED ON: OCTOBER 1, 2020
Issued by: State Administration for Market Regulation;
Standardization Administration of the People’s Republic of
China.
Table of Contents
Foreword ... 3 
Introduction ... 4 
1 Scope ... 5 
2 Normative References ... 5 
3 Terms and Definitions ... 5 
4 Symbols and Descriptions ... 6 
5 Principle ... 7 
6 Equipment ... 8 
7 Specimen ... 8 
8 Test Procedures ... 9 
9 Method for Determining Radius of Curvature, Strain and Stress ... 10 
10 Result Processing ... 12 
11 Test Report ... 12 
Appendix A (normative) Test Equipment ... 14 
Appendix B (normative) Speckle Spraying Method ... 17 
Appendix C (informative) Determination Method for Equivalent Stress-strain
Curve under Bidirectional Stress State ... 19 
Bibliography ... 21 
Metallic Materials - Tube - Determination of Biaxial
Stress-strain Curve of Tube by Hydro-bulging Test
1 Scope
This Standard specifies the terms and definitions, symbols and descriptions, test
principles, equipment, specimens, test procedures, calculation of biaxial stress-strain
curves and test reports of the biaxial stress-strain curve hydro-bulging test for metallic
material tubes.
This Standard is applicable to thin-walled metal tubes with a circular section (including
seamless tubes and welded tubes) whose wall thickness is not less than 0.5 mm and
diameter-to-thickness ratio is greater than 20.
2 Normative References
The following documents are indispensable to the application of this document. In
terms of references with a specified date, only versions with a specified date are
applicable to this document. In terms of references without a specified date, the latest
version (including all the modifications) is applicable to this document.
GB/T 228.1 Metallic Materials - Tensile Testing - Part 1: Method of Test at Room
Temperature
GB/T 15825.2 Sheet Metal Formability and Test Methods - Part 2: General Test Rules
3 Terms and Definitions
The following terms and definitions are applicable to this document.
3.1 Digital Image Correlation (DIC) Measurement System
Digital image correlation (DIC) measurement system refers to a measurement system
which adopts the digital image correlation method to track the speckle image of the
deformation of an object, and calculate the three-dimensional coordinates,
displacement and strain of the whole field of the object surface.
3.2 Speckle
Speckle refers to a randomly distributed spot on the surface of a test specimen.
Where,
L0 = (1.0 ~ 3.0) D0; L0 = 1.5 D0 is recommended.
7.3 The end of the specimen shall be deburred and smoothly polished. There shall be
no initial cracks, or relatively deep and sharp scratches. In addition, during the
preparation of the specimen, it shall be ensured that the surface of the specimen is not
damaged (defects like scratches and cracks, etc.).
7.4 The outer surface of the specimen shall be cleaned, so as to spray speckles. The
specific spraying method and precautions are shown in Appendix B.
7.5 During the preparation of the specimen, the specimen shall be prevented from
being deformed.
8 Test Procedures
8.1 During the test, the test temperature shall be recorded. The test is generally
conducted at room temperature 10 °C ~ 35 °C.
8.2 Measure the initial wall thickness and initial outer diameter of the specimen. Along
the circumferential direction, measure 8 points at equal intervals. The resolution of the
measuring tool shall be not lower than 0.01 mm.
8.3 Carry out necessary cleaning and examination of the molds and test devices used
in the test. In addition, check whether the pressure control system can normally
operate and whether the pressure line has any leakage.
8.4 Before the test, under no-load conditions, check whether the equipment can
normally operate; check whether the plug head and the sealing section of the mold fit
well.
8.5 Place the prepared specimen into the mold. After mold clamping, use the plug for
flaring and sealing.
8.6 In accordance with the wall thickness distribution measured in 8.2, determine the
area of the thinnest point and mark it, so that the intermediate point of the bulging zone
can be found when the DIC measurement system is used to analyze the data.
8.7 Pre-fill liquid (emulsified liquid or hydraulic oil) into the specimen to remove the gas
inside the specimen. Then, increase the plug thrust to achieve sealing of the tube end.
8.8 Use a pressure sensor to measure the liquid pressure during the test.
8.9 Use the DIC measurement system to synchronously measure the speckle image
of the surface of the specimen during the test with 8.8.
8.10 In accordance with the same time scale, record and save the pressure data in the
specimen and the deformation data measured by the DIC measurement system. It is
recommended to record at least 60 sets of data every minute. In order to ensure a
sufficient data size, at least 100 images shall be taken during the bulging test.
8.11 In accordance with a certain pressure increase rate (0.05 MPa/s ~ 0.1 MPa/s is
recommended), load until the specimen ruptures. Then, end the test; record the burst
pressure value; save the test data.
8.12 In order to ensure that at least 3 valid test results are obtained, a sufficient number
of specimens shall be prepared.
9 Method for Determining Radius of Curvature, Strain
and Stress
9.1 Radius of Curvature (zp, p)
The required radius of curvature is the axial and circumferential radius of curvature of
the intermediate point of the bulging zone. When the specimen is bulged, the axial
profile of the outer surface of the area near the intermediate point is elliptical, and the
circumferential profile is circular. Take the intermediate point of the bulging zone as the
center, within the DIC measurement range, select a local rectangular area for analysis,
as it is shown in Figure 2. The length l1 of the selected area is recommended to be l1 =
(0.2 ~ 0.5) D0; the width l2 is recommended to be l2 = (0.2 ~ 0.4) l0, or it may be adjusted
in accordance with the actual window size. In the axial direction of the selected area,
uniformly select several points (it is recommended to select at least 5 points; the
distance among the points shall be greater than 2 mm); extract the coordinate
information of the selected points. In accordance with the elliptic equation, use the
least squares fitting method to determine the axial radius of curvature of the
intermediate point of the bulging zone. In the circumferential direction of the selected
area, uniformly select several points (it is recommended to select at least 5 points; the
distance among the points shall be greater than 2 mm); extract the coordinate
information of the selected points. In accordance with the equation of circle, use the
least squares fitting method to determine the circumferential radius of curvature of the
intermediate point of the bulging zone. If necessary, it may also be negotiated to adopt
other fitting methods or calculation methods to determine the radius of curvature.
A.4 The test equipment shall provide sufficient mold clamping force. The provided
clamping force Ft shall satisfy Formula (A.3):
A.5 The test equipment shall be equipped with an interna...