Ceramic tensile test

Most ceramics have enormous yield stresses. In a tensile test, at room temperature, ceramics almost all fracture long before they yield this is because their fracture toughness, which we will discuss later, is very low. Because of this, you cannot measure the yield strength of a ceramic by using a tensile test. Instead, you have to use a test which somehow suppresses fracture a compression test, for instance. The best and easiest is the hardness test the data shown here are obtained from hardness tests, which we shall discuss in a moment. 

Fig. 17.2. Tests which measure the fracture strengths of ceramics, (a) The tensile test measures the tensile strength, CTj. (b) The bend test measures the modulus of rupture, o , typically 1.7 x CTj. (<) The compression test measures the crushing strength, a, typically 15 x...

The common tests are shown in Fig. 17.2. The obvious one is the simple tensile test (Fig. 17.2a). It measures the stress required to make the longest crack in the sample propagate unstably in the way shown in Fig. 17.3(a). But it is hard to do tensile tests on ceramics - they tend to break in the grips. It is much easier to measure the force required to break a beam in bending (Fig. 17.2b). The maximum tensile stress in the surface of the beam when it breaks is called the modulus of rupture, o for an elastic beam it is related to the maximum moment in the beam, M by... 

Fig. 9.30 The strength of a ceramic material as a function of the volume percentage of pores during a tensile test.

In practice, tensile tests on ceramic objects are hardly ever done because it is difficult to clasp them between the jaws of the testing equipment. The object is damaged by the jaw and a crack is very likely the result and this could be the place where the sample will break. The price of the test dumbbells is also a drawback of this type... 

Fig. 8.8 Tensile stress-strain curves for three specimens of [0,(0,90)2]SiCVLAS-IH glass-ceramic composite tested at 1000°C in air and in argon.25...

H. S. Starrett, A Test Method for Tensile Testing Coated Carbon-Carbon and Ceramic Matrix Composites at Elevated Temperatures in Air, Cer. Eng. Sci. Proc., 11[9—10], 1281-1294 (1990). 

Tensile testing of ceramics is time-consuming and expensive because of the difficulty in machining test specimens. Instead, the simpler transverse bending or flexure test is used, where the specimen is loaded to failure in either... 

Finally, another important ramification of the stochastic nature of brittle fracture is the effect of the stress distribution during testing on the results. When a batch of ceramics is tested in tension, the entire volume and surface are subjected to the stress. Thus a critical flaw anywhere in the sample will propagate with equal probability. In three- or four-point flexure tests, however, only one-half the sample is in tension, and the other one-half is in compression. In other words, the effective volume tested is, in essence, reduced. It can be shown that the ratio of the tensile to flexural strength for an equal probability of survival is... 

Fracture strength, Op, is the stress at a fracture. Because ceramics are often tested in bending we do not see any reduction in cross-sectional area during the test as we often see in a tensile test with a metal. As a result we would not expect to see a maximum in the a-8 curve corresponding to the tensile strength or ultimate tensile strength. 

The creep behavior of ceramic fibers has been reviewed extensively by Tressler and DiCarlo (1993, 1995). For example, DiCarlo and coworkers have investigated the time dependent mechanical properties of ceramic fibers (Yun et al., 1995a, 1995b, 1994). Tensile tests on single fibers have been used to study creep, and the BSR technique has been used as a screening test to compare the creep behavior of various fibers (Sabol and Tressler, 1990). 

Hermonssen L, Alderborn J, Burstrom M, Tensile testing of ceramic materials. High Technology Ceramics, Elsevier, 1161, 1981. 

Specimen size, in general, has an effect on mechanical tests, and ceramics or glasses are no exception. The shapes of ceramic and glass test specimens are often a matter of convenience, but, in most cases, the specimen s form dictates the test conditions. Various other techniques have also been suggested to improve the reliability of the strength result. One such technique, used for the tensile testing of ceramic fibers, is video extensometry [25]. Figure 1.8 illustrates a schematic video set up for the evaluation of the results of SiC monofilaments. 

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