GB/T 6569-2006Fine ceramics (advanced ceramics, advanced technical ceramics) - Test method for flexural strength of monolithic ceramics at room temperature (English PDF)

5.1 Testing machine

A material testing machine with uniform crossbeam displacement velocity. It shall conform to ISO 7500-1.1999 Class 1, showing a load error of less than 1% at fracture.

5.2 Test fixture

5.2.1 Overview

The three-point or four-point flexure test shall use the structure shown in Figure 1.A four-point-1/4 point flexure structure is recommended. If the parallelism of the specimen meets the requirements of 6.1, a semi-adjustable fixture shall be used.

Otherwise, a fully adjustable fixture shall be used. A fully adjustable fixture shall be used for machined specimens.

NOTE 1.Sintered, heat-treated, and oxidized specimens typically lack flat and parallel surfaces.

Specimen distortion can severely impact strength evaluation; therefore, fully adjustable fixtures shall be used. The purpose of using adjustable fixtures is to ensure good contact between the fixture and the specimen surface.

NOTE 2.The fully adjustable fixture with bearings allows for free rolling to eliminate friction. Each roller maintains close contact with the specimen. (See Figure B.1 and Figure B.2.)

NOTE 3.A pair of rollers in the semi-adjustable fixture can slide freely and maintain close contact with the specimen. (See Figure B.1 and Figure B.2.)

5.2.2 Rollers

The specimen is supported and loaded by rollers. The rollers can be cylindrical bearings or round bars. When metal rollers are used, for specimens with a strength of up to 1400

MPa, the Rockwell hardness of the rollers shall not be lower than HRC40; for specimens with a strength of up to 2000 MPa, the Rockwell hardness shall not be lower than HRC46.For ceramic rollers, the elastic modulus shall be between 200 GPa and

500 GPa, and the flexural strength shall be greater than 275 MPa. The roller length shall be greater than or equal to 12 mm. The roller diameter shall be approximately 1.5 times the specimen thickness. The roller surface shall be smooth, with a diameter uniformity error within ±0.015 mm. The rollers shall be able to roll freely to eliminate friction.

NOTE 1.Friction will affect strength calculations. Roller rolling can be achieved in various ways. The method shown in Figure 2, where the roller rolls on the fixture surface, is a simple and feasible one.

and placed perpendicular to the specimen.

5.2.4 Four-point flexure structure. fully adjustable fixture

Figure B.1(b) in Appendix B illustrates the movement of the rollers in this structure.

All four rollers can roll freely. One roller does not require adjustment. The other three rollers shall be independently adjustable to ensure close contact with the specimen. The rollers shall be placed perpendicular to the specimen.

5.2.5 Three-point flexure structure. semi-adjustable fixture

Figure B.2(a) in Appendix B illustrates the movement of the rollers in this structure.

The loading roller in the middle shall be fixed and not roll, while the two supporting rollers shall be able to roll freely outwards. The parallelism error between the rollers shall not exceed 0.015 mm. All rollers shall be placed perpendicular to the specimen to ensure close contact with it.

5.2.6 Three-point flexure structure. fully adjustable fixture

Figure B.2(b) in Appendix B illustrates the movement of the rollers in this structure.

The middle roller shall be fixed and not roll. The two support (outer) rollers shall be able to roll freely outwards. Either of the two supporting rollers shall be independently adjustable to ensure close contact with the specimen. All rollers shall be placed perpendicular to the specimen.

5.2.7 Positioning of the rollers

The roller positioning shall be accurate to ±0.1 mm. In a three-point flexure structure, the middle roller shall be positioned midway between the two support rollers. In a four- point flexure device, the two loading rollers shall be placed between the two supporting rollers.

NOTE. The position of the rollers can be determined using positioning devices. The span shall be measured with calipers or other instruments to an accuracy of 0.1 mm. Alternatively, the span can be determined by measuring the distance between the positioning devices and then adding (for the outer rollers) or subtracting (for the inner rollers) the radius of the rollers.

5.2.8 Fixture material

The fixture shall have sufficient rigidity to prevent permanent deformation.

NOTE. Line contact loads may cause deformation of the fixture. The hardness requirement of the fixture is related to its dimensions. If the roller is at least 12 mm long and the fixture width is 12 mm or wider, then the fixture shall be made of a metal with a Rockwell hardness of at least 25.

5.3 Micrometer

The dimensions of the specimen shall be measured using a micrometer with an accuracy of 0.002 mm as specified in ISO 3611.Other measuring instruments with an accuracy of 0.002 mm or higher may also be used.

6.1 Specimen dimensions

6.1.1 Machining of specimens

The dimensions of the specimen are shown in Figure 3.The length and width tolerances of the beam specimen's cross-section are ±0.2 mm. The longitudinal surface parallelism tolerance is 0.015 mm.

6.1.2 Naturally sintered or heat-treated specimens

The dimensions of the specimens may differ from those specified, and any deviation from the specifications in 6.1.1 and Figure 3 shall be noted in the report.

6.2 Specimen processing

6.2.1 Overview

Various options are available for the surface finishing of the specimen. At least the two long edges of the tension face shall be chamfered as shown in Figure 3.It is recommended that all four long sides be polished. In various cases, no special treatment is required for the end surfaces of the specimen. Although surface treatment is not a major part of this standard, it is recommended that surface roughness be measured and reported.

Figure 3 -- Schematic diagram of specimen dimensions

6.2.2 Naturally sintered specimens (without machining)

The sintered specimen has not undergone any mechanical processing. It can be directly tested at this stage. Surface grinding shall be performed before sintering.

NOTE. Sintered specimens are particularly prone to twisting and warping. They may not meet the parallelism requirements specified in 6.1.1.In such cases, a fully adjustable fixture shall be used.

6.2.3 Conventional processing

When conventional processing methods are used, efforts shall be made to minimize damage to the sample (to minimize surface damage and residual stress caused by the processing). The long edges of the tension face of the specimen shall be chamfered as shown in Figure 3.

6.2.4 Component matching

The surface of the specimen shall have the same machining process as the surface of the component to be tested. The test report shall include detailed specimen machining steps, especially the abrasive (resin, metal, glass, or others) and the amount of grinding per cycle. The long edges of the specimen shall be chamfered as shown in Figure 3.

6.2.5 Basic processing methods

If the machining procedures in 6.2.2~6.2.4 are difficult to implement, the following procedures can be used.

NOTE. The processing steps mentioned below are for reference only. The purpose of this method is to minimize processing damage and residual stress in ceramics. For certain materials, faster and more aggressive removal rates may be more suitable. Conversely, some particularly brittle materials require more conservative removal rates.

6.2.5.1 The specimen shall be placed longitudinally as shown in Figure 4.

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