Strength measurement for brittle materials

10.6.1 Strength measurement for brittle materials

Ceramics, glasses and carbons are well known to be brittle and there are two fundamental reasons for this. First, the residual porosity in sintered ceramics acts as potential nuclei for cracks. Secondly, dislocation motion is intrinsically difficult in crystalline ceramics and carbons (e.g. diamonds) because of their strong interatomic bonds, and glasses simply do not have dislocation plasticity as a possible mode of deformation because of their amorphous structures. Crack-tip blunting is therefore very limited in ceramics and glasses, and hence the fracture toughness of these materials is intrinsically low.

As discussed in Section 6.1, tensile and hardness tests are common methods for measuring the static strength of a material. For brittle materials, other methods include compression and bend tests. Amongst these, hardness and compression tests suppress fracture, which is the easier mode of damage in brittle ceramics and glasses, and unless the component is designed to carry only compressive loading (e.g. concrete columns in buildings), these two tests give unrealistically high values of strength. Tensile testing of ceramics and glasses is not generally favored since it is difficult to avoid damage of the specimen surface when it is gripped by the tensile machine. Flaws produced on the specimen surface will then seriously affect the reliability of the data. Bend or flexure tests are relatively easy to carry out, since very small pieces and simple shapes of specimens are required. Bend tests can be carried out in either the three-point support mode or the four-point support mode shown in Figure 10.21. The stress state in a bend test is non-uniform within the sample (Figure 10.22) and the strength indicator, known as the flexural strength or modulus of rupture (MoR), is the highest elastic stress in the sample when fracture just begins. The formulae to calculate the MoR are given in Figure 10.21.

A strength-related concern for using ceramics, especially in thermal applications, is their ability to withstand changes in temperature without fracture, called ‘thermal shock resistance’. Thermal stresses in a component arise from the constraints on the tendency for the material to expand when heated or contract when cooled, due to any external stoppage or the geometry of the component itself (e.g.

Non-metallics I – Ceramics, glass, glass-ceramics 541

Three-point bend

Four-point bend

Figure 10.21 Three-point and four-point bend test configurations. F = applied force, L = outer span, L i = inner span, b = breadth of specimen, d = depth of specimen.

Maximum tensile stress position

Figure 10.22 Stress distribution in a three-point bend test.

Table 10.7 Thermal shock resistance of some ceramics ( from Richerson, 1992). Al 2 O 3 SiC

Si 3 N 4 (reaction sintered)

Si 3 N 4 (hot pressed) β -Spodumene

96 ◦ C 230 ◦ C 570 ◦ C 650 ◦ C 4860 ◦ C

breakage of a cold teapot as hot water is poured into it). For shapes such as a bar constrained at both where E is the Young’s modulus, ν the Poisson’s ratio and α the linear expansion coefficient. The Clearly, materials with lower values of α are more thermal-shock resistant. Table 10.7 gives the

thermal-shock resistance of some ceramics. β-Spodumene has a very high thermal-shock resistance because of its extremely low thermal expansion coefficient ( −0.3 × 10 −6 K −1 ). Hot-pressed Si 3 N 4 has higher thermal-shock resistance than reaction-sintered Si 3 N 4 because of the higher fracture strength resulting from the hot-pressing condition, which reduces porosity.