GB/T 1685.1-2026Rubber, vulcanized or thermoplastic—Determination of stress relaxation in compression—Part 1:Testing at constant temperature (English PDF)

3.1 Compression stress relaxation

It refers to the phenomenon where the compressive force decreases over time following the application of a constant compressive deformation, expressed as a percentage of the initial force.

5.1 Compression device

It consists of two parallel, highly polished flat plates. The plates must possess high surface smoothness, parallelism, and rigidity. The compression plates are manufactured from chrome-plated material, stainless steel, or other corrosion-resistant materials, and the specimen is compressed between the two plates.

After the compression device is disassembled, the flatness of the compression plates shall be within 0.01 mm. The surface roughness Ra shall be no coarser than 0.4 µm (see ISO 4287). When the compression device is assembled without a test specimen, the gap between the two compression plates shall not exceed 0.01 mm.

After the specimen is installed and the load is applied, the deflection of either compression plate must not exceed 0.01 mm.

The compression plates shall be of sufficient dimensions to ensure that the entire compressed specimen remains within the compression area of the plates and is able to expand freely in the lateral direction.

For ring-shaped specimens, the compression plates shall have a through-hole with a diameter of at least 2 mm at their center to allow for pressure equalization and the circulation of liquid within the ring-shaped specimen.

Connect the compression device to the appropriate equipment. Compress the test specimen to the specified amount of compression at the specified speed. Measure the compression force with an accuracy of 1%.

Throughout the test, the device is capable of adjusting and maintaining a specific amount of compression, and it can be placed inside a constant-temperature chamber set to the specified test temperature.

5.2 Force-measuring device

The force-measuring device shall be capable of measuring the applied force within the required range with an accuracy of 1%. Preference shall be given to a device capable of monitoring the compressive force throughout the entire test. Such a device allows for the continuous measurement of compressive stress variations over time. Throughout the test, the deformation of the specimen shall be maintained within ±0.01 mm.

Another method involves using a compression testing machine to measure the applied force at specified time intervals. In this case, the force required to cause a slight increase in the compression of the test specimen is measured. This additional compression shall be kept to a minimum. In any event, for lever-type stress relaxation instruments, the applied force must not exceed 1 N. For stress-strain type stress relaxation instruments, the applied force must not exceed the stress corresponding to a strain of 0.05 mm. The entire force generated by the additional compression on the specimen must be exerted on the force-measuring device. The measurement accuracy of the compression device must be maintained within ±0.01 mm for each measurement.

5.3 Test chamber

The requirements of GB/T 3512 shall be met. For tests conducted in air, an air oven providing uniform heating and equipped with an appropriate temperature control device shall be used. The temperature control accuracy shall fall within the tolerance range specified in 7.2.For tests conducted in liquid, the compression device shall be fully immersed in the specified test liquid; if the liquid is volatile or toxic, the compression device shall be fully immersed within a sealed container. This ensures that the liquid can circulate freely through the central hole of the compression plate. The temperature of the test liquid shall be maintained within the specified range using a suitable controlled heater. Alternatively, the liquid and the compression device may be placed in a specified oven.

5.4 Temperature measurement device

Use sensor elements that meet the requirements. The temperature sensor shall be installed within the compression plate at a distance of no more than 2 mm from the surface of the test specimen.

6.1 Specimen type and preparation

6.1.1 Requirements

Test specimens are prepared by compression molding or cut from molded sheets or products in accordance with GB/T 2941.

NOTE. Data obtained using specimens of different sizes cannot be compared.

6.1.2 Cylindrical specimen

Cylindrical specimens. Type I. diameter phi is 10.0 mm ± 0.2 mm, height h is 10.0 mm ± 0.2 mm. Type II. diameter phi is 13.0 mm ± 0.5 mm, height h is 6.3 mm ± 0.3 mm.

NOTE. The results obtained from the two types of specimens cannot be compared.

6.1.3 Ring-shaped specimen

Ring-shaped test specimens can be cut from flat test material using a rotary cutter. The cross-section is square. Equipment suitable for preparing small ring-shaped test specimens is described in Annex B of GB/T 528.

Specimen dimensions.

Thickness. 2.0 mm ± 0.2 mm Inner diameter. 15.0 mm ± 0.2 mm Radial width. 2.0 mm ± 0.2 mm Test specimens can be produced by compression molding or prepared from finished products through cutting and grinding.

In addition, an O-ring (cross-section diameter. 2.65 mm; inner diameter. 14.0 mm) with the code BO140G, conforming to GB/T 3452.1, may be used as a standard test specimen.

Provided they are suitable, O-rings of other sizes, as well as seals or gaskets of other shapes, may also be used as non-standard test specimens.

NOTE. Most testing equipment is equipped with a clamping device. The specimen is compressed by tightening screws to drive a steel plate downward until it stops, thereby deforming the specimen's thickness. When a specimen falling within the specified tolerance range is tested in such a clamping device, the required compressive strain can be achieved. Therefore, careful matching of the clamping device and the specimen is essential to ensure that their proper fit yields the compressive strain specified in 8.3.4 and 8.4.3.

6.2 Measurement of specimen dimensions

The dimensions of the test specimens shall be measured in accordance with GB/T 2941.

6.3 Number of test specimens

The number of test specimens is three. However, one or two specimens may be used for routine tests and screening tests.

6.4 Time interval between vulcanization and testing

The time interval between vulcanization and testing shall comply with the requirements of GB/T 2941.

6.5 Conditioning of test specimens

6.5.1 Before testing, the test specimen is first subjected to thermal conditioning, followed by mechanical conditioning. See 6.5.2 and 6.5.3 for details.

6.5.2 Thermal conditioning consists of preheating the specimen at 70°C for 3 h. After

thermal conditioning, the specimen is allowed to stand at standard laboratory temperature for a period of not less than 16 h and not more than 48 h, before undergoing mechanical conditioning or testing.

NOTE. Some test specimens - particularly those made of thermoplastic elastomers -may contain molding stresses. Thermal conditioning is used to eliminate these stresses and improve the reproducibility of test results.

6.5.3 Mechanical conditioning is performed at the standard laboratory temperature

specified in GB/T 2941.The procedure is as follows. the test specimen is subjected to ......