GB/T 4337-2025Metallic materials - Fatigue testing - Rotating bar bending method (English PDF)

6.1 Shape of the specimen

6.1.1 The specimen may be of one of the following shapes.

a) cylindrical (see Figures 1, 4 and 5);

b) conical (see Figure 2);

c) funnel-shaped (see Figures 3, 6 and 7).

Note. Under two-point and four-point loading conditions, the test material in the gauge length portion of the uniform-section cylindrical specimen shall experience the same maximum stress. For all other loading conditions, only one cross-section of the uniform-section cylindrical specimen and the funnel- shaped specimen shall experience the maximum stress.

corresponding stress depend on the specimen geometry and the length of the horizontal arm. For this type of machine, a cylindrical funnel-shaped specimen is recommended because the higher stresses are close to the stress calculated for the smallest diameter section.

6.1.6 The recommended ratio of the cross-sectional area of the test part to the cross- sectional area of the clamping part should not be less than 1.2.It is recommended to use clamps that will not cause significant stress concentration.

6.1.7 For certain materials, high stress and high-speed rotation may cause the specimen to heat up. If this occurs, the volume of material in the specimen subjected to high stress shall be reduced, or the test frequency shall be decreased (see 10.3). If the specimen is cooled during testing, the test medium should be stated in the test report.

6.2 Specimen dimensions

6.2.1 The specimens used in the same batch of fatigue tests shall have the same diameter, the same shape and dimensional tolerances.

6.2.2 To accurately calculate the applied force, the actual minimum diameter of each

specimen shall be measured to an accuracy of 0.01 mm. Before the test, it shall be ensured that the specimen surface is not damaged when measuring the specimen dimensions.

6.2.3 For cylindrical specimens subjected to constant bending (see Figures 4 and 5), the parallelism of the test part shall not exceed 0.025 mm. For cylindrical specimens of other shapes (see Figure 1), the parallelism of the test part shall not exceed 0.05 mm.

The radius of the transition arc between the specimen clamping part and the test section should preferably not be less than 3 d. For funnel-shaped specimens, the radius of the arc of the test section should preferably not be less than 5 d.

6.2.4 The shape and dimensions of the cylindrical specimen are shown in Figure 8.

Recommended diameters d are 6 mm, 7.5 mm, and 9.5 mm. The deviation of diameter d should preferably not exceed 0.005d. Figure 9 shows a typical funnel-shaped smooth specimen for high-temperature fatigue testing.

7.1 General

When determining the rotating bar bending fatigue properties of the material, attention shall be paid to the requirements for specimen preparation. If the test procedure is designed to measure the effect of a factor (such as surface treatment, oxidation, etc.), that is inconsistent with the specimen preparation requirements, deviations from the requirements may occur. Any deviations shall be stated in the report.

7.2 Sampling and marking

7.2.1 The sampling location, sampling direction, and specimen type shall be in accordance with the relevant product standards or the agreement between the parties.

Sampling from semi-finished products or parts may affect the test results; therefore, it is essential to sample under conditions where the situation is fully understood.

7.2.2 If necessary, the sampling diagram shall clearly indicate.

-- the position of each specimen;

-- characteristic directions (rolling direction, extrusion direction, etc.) of semi- finished product processing;

-- designation of each specimen.

7.2.3 The specimens shall be marked at each stage of processing. Reliable methods shall be adopted to ensure that the designation does not disappear or affects the test results during processing. When each specimen is finally machined, a unique identifier shall be engraved on both ends of the specimen to ensure that each half of the specimen can be clearly identified after the specimen fractures in the fatigue test.

7.3 Processing procedure

7.3.1 Heat treatment of test materials

If heat treatment is performed after rough machining, final polishing should be carried out after heat treatment. Otherwise, heat treatment should be performed under vacuum or inert gas to prevent oxidation of the specimen. Heat treatment shall not alter the microstructural properties of the material under study. Details of the heat treatment and machining process shall be reported in the test results.

7.3.2 Machining requirements

7.3.2.1 Machining may generate residual stresses on the specimen surface. These residual stresses may be caused by thermal gradients, material deformation, or changes in microstructure during the machining process. The effects of residual stresses are not considered in high-temperature fatigue testing, since such stresses are fully or partially relieved during the specimen soaking period at the test temperature. Appropriate machining methods should be adopted to reduce residual stresses, especially in the final polishing stage. For harder materials, grinding processes should preferably be used rather than turning or milling operations.

-- Grinding. The machining allowance before grinding the specimen shall be

+0.1 mm, and grinding shall be performed at a grinding speed of no more than

0.005 mm/revolution.

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