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GB/T 20671.5-2020 English PDF (GBT20671.5-2020)

GB/T 20671.5-2020 English PDF (GBT20671.5-2020)

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GB/T 20671.5-2020: Classification system and test methods for nonmetallic gasket materials -- Part 5: Standard test method for creep relaxation of gasket materials

This Part of GB/T 20671 specifies the principle, instruments and apparatuses, sample, test procedure, precision and test report for the test of creep relaxation of nonmetallic gasket materials. This Part applies to the determination of the creep relaxation of Model 1, Model 5 and Model 7 materials which are specified in GB/T 20671.1. It can also be used for other models of materials as a reference.
GB/T 20671.5-2020
GB
NATIONAL STANDARD OF THE
PEOPLE REPUBLIC OF CHINA
ICS 23.100.60
Q 69
Replacing GB/T 20671.5-2006
Classification system and test methods for
nonmetallic gasket materials - Part 5: Standard test
method for creep relaxation of gasket materials
ISSUED ON: JUNE 02, 2020
IMPLEMENTED ON: APRIL 01, 2021
Issued by: State Administration for Market Regulation;
Standardization Administration of the PEOPLE Republic of
China.
Table of Contents
Foreword ... 3
1 Scope ... 6
2 Normative references ... 6
3 Principle ... 6
4 Instruments and apparatuses ... 7
5 Sample ... 8
6 Test procedure ... 9
7 Precision ... 12
8 Test report ... 13
Appendix A (Normative) Test method B - Creep relaxation tester bolt calibrating procedure ... 14
Appendix B (Informative) Examples of material technical indicator for relaxation rate tester of test method B ... 17
References ... 19
Foreword
GB/T 20671, Classification system and test methods for nonmetallic gasket materials, is divided into 11 parts:
-- Part 1: Standard classification system for nonmetallic gasket materials; -- Part 2: Standard test method for compressibility and recovery of gasket materials;
-- Part 3: Standard test method for fluid resistance of gasket materials; -- Part 4: Standard test method for sealability of gasket materials;
-- Part 5: Standard test method for creep relaxation of gasket materials; -- Part 6: Standard test method for adhesion of gasket materials to metal surfaces;
-- Part 7: Standard test methods for tension testing of nonmetallic gasket materials;
-- Part 8: Standard test method for flexibility of non-metallic gasket materials; -- Part 9: Standard test method for binder durability of cork composition gasket materials;
-- Part 10: Standard practice for evaluating thermal conductivity of gasket materials;
-- Part 11: Standard practice for determining resistance of synthetic
polymeric materials to fungi.
This Part is Part 5 of GB/T 20671.
This Part was drafted in accordance with the rules given in GB/T 1.1-2009. This Standard replaces GB/T 20671.5-2006, Classification system and test methods for nonmetallic gasket materials - Part 5: Standard test method for creep relaxation of gasket materials. Compared with GB/T 20671.5-2006, the main technical changes, except editorial changes, are as follows:
-- Add the scope of application of this Part (see Chapter 1);
-- In the scope, delete ?€?Use the value that is expressed in International System of Units (SI) as the standard?€? and ?€?This Part does not involve
safety issues that are related to its use. Users of this Part are responsible Classification system and test methods for
nonmetallic gasket materials - Part 5: Standard test
method for creep relaxation of gasket materials
1 Scope
This Part of GB/T 20671 specifies the principle, instruments and apparatuses, sample, test procedure, precision and test report for the test of creep relaxation of nonmetallic gasket materials.
This Part applies to the determination of the creep relaxation of Model 1, Model 5 and Model 7 materials which are specified in GB/T 20671.1. It can also be used for other models of materials as a reference.
2 Normative references
The following documents are indispensable for the application of this document. For dated references, only the dated version applies to this document. For undated references, the latest edition (including all amendments) applies to this document.
GB/T 20671.1, Classification system and test methods for nonmetallic gasket materials - Part 1: Standard classification system for nonmetallic gasket materials
3 Principle
3.1 Test method A
It is generally carried out at room temperature. Use a strain gauge that is calibrated on the bolt to measure the strain; place the sample between two flat circular plates to be compressed. The pressure is provided by the bolts and nuts that fasten the circular plates. In the test, from the beginning of the load to the end of the test, read the reading on the strain display at regular intervals. Convert the reading on the strain display into a percentage of the initial stress; then, draw a relation curve of the logarithm of the percentage and time (in hours). Within the range of the entire test time, the percentage of initial stress loss or relaxation at any given time can be found from the curve.
3.2 Test method B
3 -- upper flat circular plate;
4 -- test sample;
5 -- lower flat circular plate;
6 -- bolt;
7 -- fixed station;
8 -- connect the strain indicator;
9 -- wire.
Figure 1 -- Relaxation rate tester for test method A
4.2 Test method B
4.2.1 Relaxation rate tester: The relaxation rate tester shall meet the requirements of the test temperature calibrating procedure (see Appendix A). The relaxation rate tester consists of two flat circular plates with special holes, calibrated bolts, washers and nuts. The material is chromium-molybdenum ferritic steel or nickel-based high-temperature alloy steel or other alloys (see Appendix B), as shown in Figure 2.
4.2.2 Dial indicator assembly: It is composed of connecting sleeve, thimble, fastening screw and dial indicator, as shown in Figure 2. The precision of the dial indicator is not more than 0.0025 mm.
4.2.3 Socket wrench: 14.3 mm (9/16 in).
5 Sample
5.1 Test method A
The sample shall be a ring whose inner diameter is 33.02 mm ?? 0.05 mm and outer diameter is 52.32 mm ?? 0.05 mm.
5.2 Test method B
5.2.1 Rectangular sample: width of 10.16 mm ?? 0.38 mm, length of 31.75 mm ?? 0.38 mm. Four such samples are required for each test.
5.2.2 Annular sample: the area is 1 290 mm2; it is recommended to use the sample whose inner diameter is 15.62 mm and outer diameter is 43.56 mm. 5.2.3 For Model 4 Class 2 materials, the sample should be a long strip of 152.40 mm.
6.1.1 Clean the surface of the flat circular plate and the sample, to make it be free of wax, release agent and oil (use isooctane or other suitable solvent to wipe). Lubricate the threads of the bolts.
6.1.2 Use an initial stress of 13.8 MPa ?? 0.3 MPa, unless otherwise specified. 6.1.3 Perform the test at 21 ??C ~ 30 ??C, unless otherwise specified.
6.1.4 Take apart the relaxation rate tester and put the sample between two flat circular plates. The inner diameter of the sample and the inner diameter of the flat circular plate shall be as concentric as possible. Put the bolt head into the fixed station; screw on the thrust bearing and nut; tighten it lightly by hand or apply a force just enough to fix the sample.
6.1.5 Connect and balance the strain display. Record the reading. Then, adjust the reading of the strain display to the specified initial stress.
6.1.6 Apply pressure to the gasket by using a wrench to tighten the nut, until the strain display reaches equilibrium. For a pressure of 17.8 kN, a torque of about 30 N??m is required. The pressure shall be applied at a uniform rate and reach the specified pressure within 10 s ?? 2 s. This pressure is the ?€?initial stress?€?. 6.1.7 Start timing when reaching the specified ?€?initial stress?€?. Read the readings on the strain display for 10 s, 1 min, 6 min, 30 min, 1 h, 5 h, 24 h, until the end of the test (check the zero point of the strain display after the test). 6.1.8 Convert the reading of the strain display that is obtained in 6.1.7 into a percentage of ?€?initial stress?€?. On the semilogarithmic coordinate paper, draw the relation curve between percentage and time (in hours) logarithm.
6.2 Test method B
6.2.1 Clean the surface of the flat circular plate and the sample. Use lubricants such as graphite and molybdenum disulfide to lightly lubricate bolt threads and washers.
6.2.2 As shown in Figure 2, put four rectangular samples between two flat circular plates, ensuring that the distance between the samples and the edge of the flat circular plate is not less than 2 mm. If annular samples are used, the inner diameter of the sample and the inner diameter of the flat circular plate shall be as concentric as possible. If it is a Model 4 Class 2 sample, the sample between the flat circular plates shall be centered on the bolt hole, to ensure that the two ends overlap at least 6.35 mm.
6.2.3 Put the washer and tighten the nut by hand.
Appendix A
(Normative)
Test method B - Creep relaxation tester bolt calibrating procedure
A.1 Purpose
A.1.1 Before the bolts are calibrated, they shall undergo high temperature pretreatment to eliminate stress.
A.1.2 The bolts after calibration pretreatment shall be calibrated regularly after use. If the operating temperature exceeds 205 ??C, the calibration period shall be shorter than that at low temperatures.
A.1.3 Ensure that the bolts can be used normally.
A.2 Equipment
A.2.1 Relaxation rate tester: According to 4.2.1, make a slight improvement, so that each flat circular plate can be inserted into a pair of steel positioning pins. A.2.2 Dial indicator assembly: see 4.2.2;
A.2.3 Flat washer: the size is 177.8 mm ~ 406.4 mm; the material is carbon steel; the hardness is 38 HRC ~ 45 HRC.
A.2.4 Tensile testing machine: capable of providing and recording a load of 26.7 kN. The maximum allowable system error is 0.5% of the applied load.
A.2.5 Bolt calibration device mechanism: the mechanism that connects the relaxation rate tester and the tensile testing machine. See Figure A.1. A.3 Procedure
A.3.1 If the calibrated bolt is new, perform pretreatment according to the following procedures. Without the gasket, install the relaxation rate tester; apply a load to the bolt, to obtain a displacement value of 0.13 mm ?? 0.001 3 mm; record this value and use it as the original displacement value.
A.3.2 Remove the dial indicator assembly, and do not loosen the bolts and nuts; then, put the tester into a circulating hot air oven; adjust the temperature of the oven to 10 ??C above the maximum test temperature in advance. After keeping for 22 h, take out the tester from the oven and cool to room temperature. A.3.3 Install the dial indicator assembly on the relaxation rate tester; remove the load of the bolts; measure the displacement value, to an accuracy of 0.001 3 mm. Record this value and use it as the end point displacement value. A.3.4 Repeat the above operating cycle until the difference between the original displacement value and the end point displacement value of one cycle and the next cycle is 3% or less and shows no tendency to further decrease.
Note: For the pretreatment of general bolts, it is sufficient to have seven or less above operating cycles.
A.3.5 Disassemble the relaxation rate tester; replace the gasket with a flat washer; tighten the nut by hand.
A.3.6 Install the relaxation rate tester assembly, dial indicator assembly and bolt calibration device, as shown in Figure A.1. Install the whole on the tensile testing machine. Adjust the spacing, so that tensile load shall not be applied to the bolts of the tester. Adjust the dial indicator to zero scale.
A.3.7 Apply tensile force until the load reaches 4.45 kN. Keep this load; record the bolt elongation that is indicated by the dial indicator; estimate it to an accuracy of 0.001 3 mm. Continue to load pressure to the bolt, from 4.45 kN to 26.7 kN; for each increase of 4.45 kN, record the elongation of the bolt. A.3.8 Remove the tensile load on the bolt of the tester; observe the reading of the dial indicator. If the dial indicator does not return to the zero scale (within ??0.002 5 mm), this set of data shall be eliminated. Repeat this cycle. If the dial indicator reading does not return to zero after 3 cycles, invalidate the bolt. A.3.9 Reset the dial indicator to zero and repeat A.3.7 and A.3.8 three times.

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