Load capacity and strength testing of technical ceramics
Determination of strength to evaluate the load capacity of ceramics
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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 fracture.
The tensile strength is the critical property of ceramic materials. Due to the enormous preparation effort involved, the testing of this parameter is only carried out in an academic context. In practice, the testing of the bending strength of a ceramic is primarily used. Tensile stresses on the underside of a specimen loaded in bending lead to breakage.
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 bending strength at room temperature (cold bending strength) of the material to be determined.
The 3-point bending test can be used to determine the strength of both small and large specimens, as there are variable distances between supports. The determination of the cold bending strength 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.
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 static modulus of elasticity (Young’s modulus). 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). The modulus of elasticity is calculated from the relationship between applied force and deflection.
Since the effective test volume is considerably smaller in the 3-point bending test than in the 4-point bending test, the result is significantly higher strength values.
Are you interested in determining the bending strength at high temperatures? Then read on here: Hot bending strength.
How is the failure probability 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 always 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.
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 contact you to discuss the possibilities of testing the strength of your ceramic.
Further test methods for the strength of ceramics
In addition to the classical test methods for bending 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 6).
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 stress states occur very frequently. Therefore the biaxial test is in many cases more realistic than the conventional bend test, which uses a uniaxial stress state. The B3B test is a biaxial strength test especially for ceramic or brittle materials. The specimens are usually thin round discs with a diameter of a few millimeters. By selecting the sphere size, different specimen diameters can be tested (see figure 4).
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. Figure 4: Test setup for the B3B test.
Figure 4: Test setup for the B3B test. (https://www.isfk.at/de/960/).
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 test (see Figure 5). This test also involves a biaxial bending load. For this purpose, the glass pane to be tested 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 5, right). This test allows thin glass sheets to be tested for strength without time-consuming sample preparation.
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.
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