Thread Content
Standard of the People’s Republic of China for the determination of flex crack in vulcanized rubber GB/T 13934—92 Rubber, vulcanized—Determination of flex cracking. This standard is based on the international standard ISO 132—1983 \"Determination of flex crack in vulcanized rubber\". 1 Subject matter and scope of application: This standard specifies the method for determining the flex crack resistance of vulcanized rubber on a dedicated fatigue testing machine. This standard is applicable to the determination of the resistance to flex crack formation and crack growth in vulcanized rubber after repeated flexing. 2 The reference standard is GB/T 2941, which specifies the standard temperature, humidity, and time for environmental conditioning and testing of rubber specimens. 3 Testing equipment: The instrument used is a dedicated fatigue testing machine. 3.1 The testing machine shall have fixed components, on which are clamps that allow one end of the specimen to be held in a fixed position, as well as reciprocating clamps for holding the other end of the specimen ； Its stroke is 57+0.50 mm, and the maximum distance between the two clamps is 75+10 mm (Figure 1). Figure 1 Schematic diagram of the clamps **Approved by the Technical Supervision Bureau on 12-12-1992; implemented on 01-10-1993. GB/T 13934—92 3.2 Each pair of clamps has a common center line; the movement path of the reciprocating clamps coincides with this center line, and the movement paths of all pairs of clamps lie in the same plane. The clamping planes of any pair of clamps must remain parallel throughout their movement. 3.3 The eccentric wheel that drives the reciprocating components is powered by a motor with a constant speed, resulting in a movement frequency of the gripper of 5.00±0.17 Hz (300±10 r/min) or 8.3±0.28 Hz (500±20 r/min). 3.4 The gripper should be able to hold at least 6 specimens per use, preferably 12 specimens, and it must clamp the specimens firmly. Note: An intermediate clamp that moves back and forth is used to divide the specimens into two groups; when one group of specimens bends, the other group remains straight, which helps to reduce instrument vibration. 3.5 If the test is to be conducted at high temperatures, the instrument can be sealed inside a device with high-temperature control. The temperature near the center of the sample should be controlled within ±2°C of the test temperature (an air blower can be used). 4 Specimens 4.1 Shape and Dimensions 4.1.1 The specimen is a strip with a semi-circular cross-section and molded grooves (Figure 2). Figure 2: Strip specimen with a semi-circular cross-section. 4.1.2 The specimen can also be a strip with a rectangular cross-section and molded grooves (Figure 3). Figure 3: Slab specimens with rectangular cross-sections – GB/T 13934—92 4.1.3 Test results cannot be compared for specimens of different shapes. 4.2 The specimen can be vulcanized using a multi-cavity mold, and during vulcanization, the molding grooves should be perpendicular to the rolling direction. 4.3 The grooves of the specimen shall have a smooth surface, without any irregularities that could lead to premature cracking ； The sample thickness must be strictly controlled at 6.3±0.15 mm, and the thickness measurement must be taken close to the groove in the sample. 4.4 The specimen shall be adjusted in accordance with the provisions of GB/T 2941 ； The specimen and the testing process should be protected from direct sunlight. 4.5 Number of specimens: There should be no less than 3 specimens for each test material. 5 Test temperature: The test shall be conducted at standard room temperature; high-temperature testing may also be carried out under the conditions specified in GB 2941. 6 Test Steps 6.1 Adjust the distance and stroke between the clamps in accordance with the provisions of 3.1. 6.2 Separate the two clamps to the maximum distance, insert the specimen so that it is flat, vertical, and free from tension, with the groove of each specimen located at the center of the two clamps. When bent, the groove should be on the outside of the formed corner. 6.3 Start the testing machine, bend it 5000 times and then stop; separate the specimen clamps to 65 mm, observe and record any initial cracks, and restart the testing machine. 6.4 To increase the interval or geometric progression of the deflection counts, a suitable geometric progression ratio is 1.5. Grade the degree of cracking in the specimen in accordance with the provisions of 7.1. 6.5 Tests should not be conducted in rooms containing any equipment that generates ozone. The drive motor of the testing machine should be a type that does not produce ozone during operation. 6.6 Record the results as follows: a. The crack grade of each specimen at each shutdown. b. Number of deflections. 7 Test Results 7.1 The degree of cracking is graded according to the following criteria: Grade 1 – The number of cracks visible to the naked eye, in the form of “pinprick-like” spots, is 10 or less. Level 2: It can be classified as Level 2 if one of the following conditions applies. a. The number of crack points exceeds 10. b. The number of crack sites is less than 10, but one or more of these crack sites have expanded; the depth of the cracks is very shallow, and they have a noticeable length, although this length does not exceed 5 mm. Level 3: One or more crack points expand into visible cracks; these cracks have a noticeable length and a relatively small depth, with a length greater than 0.5 mm but not exceeding 1 mm. Grade 4: The length of the maximum crack is greater than 1 mm, but not more than 1.5 mm. Grade 5: The length of the maximum crack is greater than 1.5 mm, but not greater than 3.0 mm. Grade 6: The length of the maximum crack is greater than 3.0 mm. Note: When the specimen has a rectangular cross-section, cracks appearing at the edges of the specimen should be ignored. 7.2 Draw a curve graph showing the relationship between levels 1 to 6 and the number of deflections, and use this graph to determine the number of deflections for any given level. 8 Test Report The test report shall include the following contents: a. Test name and number. The number of flexures required for levels 1 to 6 as specified in clause 7.1 of GB/T 13934—92, and report the number of flexures at which no cracking occurs. c. Sample shape and quantity. d. Test frequency. e. Test temperature and time. Additional note: This standard was proposed by the Ministry of Chemical Industry of the People’s Republic of China. This standard is under the jurisdiction of the Beijing Rubber Industry Research and Design Institute of the Ministry of Chemical Industry. This standard was drafted by Qingdao No. 2 Rubber Factory. The main drafters of this standard are Zhao Xiuqin and Luan Xiuming.