Ceramics bend test

Figure 12.7 Fracture origins, (a) In a silicon nitride bending test specimen (b) In a barium titanate (PTC ceramic) bending test specimen (c) In an alumina bending test specimen. The origins are an agglomeration of coarse.

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...

Fig. 9.34 Four- and three-point bending test of a ceramic object.

Most ceramic materials exhibit elastic deformations with slight elongations, followed by fracture. In chapter 9 we discussed E-modulus measurements on ceramic materials. This can e.g. be done by means of three or four point bending tests or by measuring the speed at which a sound wave passes through a material. 

Through disk-bend testing on a series of ZrOj/Ni composite specimens fabricated by powder processing, we have examined the fracture behavior of ceramic/metal composites under an equibiaxial plane-stress loading, and derived, by making a micromechanical analysis of elastoplastic stress states, a brittle phase-controlled fracture criterion of the form, ( )max const., in terms of the equivalent normal stress a. This criterion is conceptually simple and quite useful particularly for our micromechanics-based approach to the FGM architecture. 

In the following these ideas are applied to describe bending test results of a commercial silicon nitride ceramic. 

The crystal phases in the glass-ceramics were determined by XRD analysis. All instruments were precisely and identically set to ensure a high precision to obtain the integral peak area. The microstructure of the fresh fractured cross section of the glass-ceramics was observed by SEM. The thermal expansion coefficient (TEC) was calculated from room temperature to 500 °C at a heating rate of 5°C/min in the dilatometry analyser (NETZSCH, DIL402PC). The flexural strength was determined in a 3-point bend test at a constant strain ratio of 0.5mm/min. 

Figure 10.3 The three-point bend test for ceramic samples (a) sideways and (b) end-on view of the test...

Experimental creep data for ceramics have been obtained using mainly flexural or uniaxial compression loading modes. Both approaches can present some important difficulties in the interpretation of the data. For example, in uniaxial compression it is very difficult to perform a test without the presence of friction between the sample and the loading rams. This effect causes specimens to barrel and leads to the presence of a non-uniform stress field. As mentioned in Section 4.3, the bend test is statically indeterminate. Thus, the actual stress distribution depends on the (unknown) deformation behavior of the material. Some experimental approaches have been suggested for dealing with this problem. Unfortunately, the situation can become even more intractable if asymmetric creep occurs. This effect will lead to a shift in the neutral axis during deformation. It is now recommended that creep data be obtained in uniaxial tension and more workers are taking this approach. 

W is the height of the beam and B is its thickness. For the case of bend testing a ceramic this equation is applicable only when the distance between the inner rollers is much greater than the specimen height. 

There are several techniques to determine for a ceramic. The two main approaches are to use indentation or bending. In the bend test a notch is introduced, usually using a diamond-tipped copper cutting wheel, into the tensile side of the specimen as shown in Figure 16.13. In Figure I6.I3a the... 

Flaws dominate the mechanical properties of ceramics. They determine how we test them and how we design components from them. Flaws are also the reason why ceramics are stronger in compression than tension. In this chapter we described the methods used to measure mechanical properties of ceramics. The important ones are bend testing, compression testing, and indentation. To determine the mechanical properties of small volumes we use nanoindentation. This technique is especially important for thin hlms, surfaces, and nanomaterials. An understanding of statistics is particularly important when using ceramics in load-bearing applications. The Weibull approach is the one most widely used for ceramics. 

High temperature mechanical characterization was performed on the PAIC compositions with Al/Si = 0.05 and 0.1. The elastic modulus has been measured in air at various temperatures between 800 and 1400°C by four point bend tests (40 X 20 mm) with a 0.2 mm min deformation rate. E has been calculated, through the standard equation valid for rectangular bars, by measuring the displacement with a LVDT. All the ceramic samples obtained by pyrolysis at 1000°C have been pre-annealed at 1400°C for 1 h in argon atmosphere, before the high temperature tests. This treatment lead to the crystallization of microcrystalline pSiC with a minor amount of aSiC. Aluminum atoms are present both as a solid solution of AI2OC in aSiC and in the residual amorphous phase. 

To compare the laminated sample with the monolithic control, three-point bend tests were carried out as shown in Fig. 16.16. The elastic properties of both samples were similar, with a flex modulus of 450 GPa. The bend strengths were also comparable, with 500 MPa for the monolithic and 633 MPa for the laminated sample. However, the resistance to cracking of the samples was entirely different. When a notched sample of the monolithic material was bent, the load/displace-ment curve was typical of a brittle ceramic material, with sudden cracking failure after an initial elastic deformation (Fig. 16.16(a)). The fracture toughness of the sample was calculated to be 3.6 MPa with a low fracture energy of 62 J calculated from the area under the curve. 

Figure 12.16 Bending tests at room temperature on an alumina ceramic [99]. Plotted is the cumulative frequency (a) versus time to failure for constant loading (0 = 270 MPa) and (b) versus strength for tests...

Fracture surface in a bending test specimen made from the same barium titanate ceramic. Qualitative differences between both types of fracture surfaces are hardly recognizable. 

Often in ceramics, fracture tests are replaced by a bending test (a flexural test), discussed earlier in Sect. 1.9, rather than by a tension test, for the aforementioned reasons. Figure 1.80 is a Weibull plot. In Fig. 1.80, Weibull plots of measured bending strength, Cf, are shown for different m values. 

Fig. 7.16 Fatigue of Ce-TZP ceramics in static and cyclic (pulsating and reversed bending) tests a Ce-TZP-11, b Ce-TZP-IV, and c Ce-TZP-V. The applied stress values are given as statie stress Ss and maximum stress Smax for R = 0.2 and stress amplitude Sa for R = —1 Cf and

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