ASTM C1684-08 Test Method for Flexural Strength of Advanced Ceramics at Ambient Temperature—Cylindrical Rod Strength

Scope:

ASTM C1684-08 test method determines the flexural strength of ceramics using flexural tests. Four-point is commonly used, but three-point loading is also applicable. This test method is used for material development, quality control, characterization, and design data generation purposes. This method is used to test monolithic or particulate- or whisker-reinforced ceramics. It does not apply to continuous fiber-reinforced ceramic composites. It can also be used with glass test specimens, although Test Methods C158 is specifically designed to be used for glasses. The three-point test configuration exposes only a very small portion of the specimen to the maximum stress. Therefore, three-point flexural strengths are likely to be greater than four-point flexural strengths. Three-point flexure has some advantages. It uses simpler test fixtures, it is easier to adapt to high temperature and fracture toughness testing, and it is sometimes helpful in Weibull statistical studies. It also uses smaller forces to break a specimen. It is also convenient for very short, stubby specimens, which would be difficult to test in four-point loading. Nevertheless, four-point flexure is preferred and recommended for most characterization purposes.

Test Procedure:

The two most common types of flexure tests are three-point and four-point flexural bending tests. The three-point bend test consists of the sample placed horizontally upon two points and the force applied to the top of the sample through a single point so that the sample is bent in the shape of a “V.” A four-point bend test is roughly the same except that instead of the force applied through a single point on top it is applied through two points so that the sample experiences contact at four different points and is bent more in the shape of a “U.” The three-point flexure test is ideal for the testing of a specific location of the sample, whereas, the four-point flexure test is more suited towards the testing of a large section of the sample, which highlights the defects of the sample better than a 3-point bending test.

Specimen size:

This test method is applicable for ceramics with a strength of 50 MPa or greater. This test method is used to test machined, drawn, extruded, and as-fired round specimens and specimens that have elliptical cross-section geometries. 

Rod diameters between 1.5 and 8 mm and lengths from 25 to 85 mm are recommended, but other sizes are permitted. The fixture span length to specimen diameter ratio should be greater than 3.

A minimum of 10 test specimens is required for estimating the mean. A minimum of 30 test specimens is required if estimates regarding the form of the strength distribution are to be reported or if the data are to be used for design. A minimum of 30 test specimens is required if there are multiple-flaw populations.

Data:

The flexural strength of the test specimen is determined using three-point or four-point flexure tests.