Load capacity and strength testing of technical ceramics

Determination

Determination of strength to evaluate the load capacity of ceramics

Ceramics are brittle materials. When subjected to mechanical stress, they initially deform elastically, but with increasing load, there is no plastic deformation, but rather fatal rupture.
The tensile strength is the critical property of ceramic materials. Due to the enormous amount of preparation required, testing of this parameter is only carried out in an academic context. In practice, the testing of the flexural strength of a ceramic is primarily used. Here, tensile stresses on the underside of a specimen loaded in bending lead to rupture.

Strength determination
Figure 1: Diagram of time vs. applied force

Methods

What methods are available for determining strength by bending?

The 4-point bending test is used to determine the bending strength of high-performance ceramics. Small prismatic specimens measuring 3 x 4 x 50 mm³ are sufficient for this test. The sample is placed on a support consisting of two steel rollers (see Figure 2) and then loaded from above with two further steel rollers until it breaks. The breaking load allows the determination of the flexural strength of the material at room temperature (MOR).

Figure 2: Testing of flexural strength (4-point bending test)

The 3-point bending test can be used to determine the flexural strength of both small and large specimens, as there are variable support spans. The determination of the MOR in the 3-point bending test of refractory ceramics is carried out at WZR ceramic solutions on specimens measuring 150 x 25 x 25 mm³.

Festigkeitsprüfung
Figure 3: Testing of flexural strength (3-point bending test)

The compressive load from above is applied by a steel roller which loads the sample centrally until it breaks (see Figure 3). An advantage of this test is the possibility to determine the Young’s modulus (MOE). This provides information about the elastic deformation of a material before it breaks. The measurement is carried out by a displacement transducer, which is mounted centrally under the sample and records the deflection of the sample (the elastic deformation). Young’s modulus is calculated from the relationship between stress and strain.

Since the effective test volume is considerably smaller in the 3-point bending test than in the 4-point bending test, significantly higher flexural strength values are obtained.
Are you interested in determining the flexural strength at high temperatures? Then read on here: Hot flexural strength (HMOR).

Procedure

How is the probability of failure of your ceramic determined?

In order to obtain a first statistical confirmation of the strength of a ceramic material, at least 5 samples of a batch should be tested.
To calculate the failure probability of a ceramic material, 30 samples must be tested according to the standard. This is explained by the fact that the strength values of a ceramic material do not follow a classical Gaussian distribution, but the so-called Weibull distribution.

Limitations

What are the limits of strength determinations for ceramic materials?

We test the strength of ceramic materials up to a maximum force of 50 kN. Therefore, a specimen geometry that allows the testing of your material is necessary.
Working according to standards allows a reproducibility and a good comparability of the test results. For this reason, the exchange with you is very important to us in order to be able to perform the appropriate test for your material.
We would be pleased to discuss with you the options for testing the strength of your ceramic.

Further test methods

Further test methods for the strength of ceramics

In addition to the classical test methods for flexural strength, we at WZR ceramic solutions GmbH offer further tests to precisely characterize the strength of your material. Further test methods are:

For more information, click here.

From concrete testing, an indirect test of tensile strength is known, the so-called Brazilian test. Since this test is also very suitable for ceramic materials, WZR ceramic solutions also offers this test (see Figure 4).

Spaltzugfestigkeit
Figure 4: Test setup for Brazilian test.

In this test, the round specimen is loaded diametrically, causing tensile stresses to build up inside the specimen. This leads to tensile forces in the center of the specimen perpendicular to the direction of loading, which cause the specimen to crack. This method is therefore used to indirectly determine the tensile strength of ceramic samples. The geometry of the specimens is usually d=30mm and h=15mm. This method is not only used to test the strength of ceramic specimens in new condition, but is especially useful in cases of damage when only small specimens, drill cores or fragments are available. On a loaded material the change in strength can be determined, for example from the hot to the cold side (see figure 6).

In practice, biaxial stresses occur frequently. Therefore, in many cases, a biaxial test is more realistic than the conventional bending test, which uses uniaxial stresses. The B3B test is a biaxial strength test especially for ceramic or brittle materials. The specimens are usually thin discs with a diameter of a few millimeters. By selecting the sphere size, different specimen diameters can be tested (see figure 5).

B3B-Test
Figure 5: Test setup for the B3B test.

The advantage of the test is the good reproducibility and a low effort in sample preparation. Since the crack always originates in the middle of the specimen, edge effects have no influence on the test result - in contrast to the flexural strength test.

The test was developed at the Montan University in Leoben in the working group of Prof. Danzer. (A. Börger, P. Supancic, R. Danzer: The Ball on three Balls Test for Strength Testing of Brittle Discs - Stress Distribution in the Disc. Journal of the European Ceramic Society 22 (2002) 1425-1436, doi:10.1016/S0955-2219(01)00458-7).

One way to determine the strength of glass is the double ring flexure test (see Figure 6). This test involves a biaxial bending load. For this purpose, the glass pane is covered on the upper side with a transparent foil and placed on a cylindrical lower support. The load is applied via a cylindrical upper support with a diameter smaller than that of the lower support. As the load increases, the strength of the glass is exceeded, causing it to crack in a star-shaped manner (see Figure 6, right). This test allows thin glass sheets to be tested without time-consuming sample preparation.

Figure 6: Testing of glass in a double ring flexure test.

In addition to strength testing, WZR ceramic solutions offers the examination of bulk density and open porosity by means of water absorption and the hardness of your material. Furthermore, we determine physical parameters at temperatures above room temperature.


For more information please click here.

 

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Anika Braun
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