Impact Testing of Ceramics

One of the important features of impact testing is the evaluation of the ductile-to-brittle transition temperature. What, then, is the purpose of discussing the impact testing of ceramics, since most are brittle at ambient temperature (and clearly at low temperatures) Yet, impact tests are also performed on classic, brittle materials in order to evaluate the energy absorbed during the fracturing process. Furthermore, some brittle ceramics are ductile at sufficiently elevated temperatures, so the brittle-ductile transition may still be of interest. Ductile and superplastic ceramics will be discussed in depth in Chap. 2 (on ductile ceramics), while the present section deals with the actual process of performing impact tests. 

Because of the brittle nature of ceramics, special instrumented Charpy Impact Test (henceforth CIT) machines were developed, primarily to evaluate the 

Particle VelecXy Measureinent Bleck -(horizontal light paths are 0.75 mm In diam.) 

Rupture Diaphragm (typicaNy thin Mylar) (arMgher Velocity Tests 

Ej is usually associated with crack initiation and, on a load-deflection curve, spans the area up to the maximum load, while Ep is believed to represent the apparent crack-propagation energy, appearing after the maximum load. Considering the curves of these ceramic materials shown in Fig. 1.72, it is clear that the load drops vertically and there is no additional area to represent Ep. This observation indicates that PSZ and SiC show typically elastic brittle fracture without plastic deformation and that  

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L. R. Deobald and A. S. Kobayashi A Bar Impact Tester for Dynamic Fracture Testing of Ceramics and Ceramic Composites, Experimental Mechanics, 32, 109-116 (1992). 

Relatively few impact strength data are available in the literature on ceramics and there are even fewer recorded experimental reports. The major limitations of performing such impact tests are the brittleness and low impact strength of ceramics at low and ambient temperatures, especially when the focus is on their applications at elevated temperatures, in light of their high strength properties. 

Impact resistance is determined usiag flyer plate impact tests, long rod impact tests, Hopkinson bar tests (50), and the Hquid jet technique (51). Impact damage resistance is often quantified by measuring the postimpact strength of the ceramic. 

Figure 9.20 shows the setup for a symmetric plate impact test. The projectile here has a facing plate of ceramic and is backed with a low-density foam, for support of the ceramic during launch. The facing plate of the target is also made of ceramic. The lithium fluoride slab, which backs the target sample, serves as a window for the laser velocity interferometer (VISAR) that measures the time-resolved particle velocity at the sample/window interface. 

The impact of thermal shock on the properties of a ceramic or a CMC is assessed by means of both destructive and non-destructive testing methods. Flexural or tensile (mainly for CMCs) tests of suitably-sized thermally shocked specimens are usually employed to measure retained mechanical properties as a function of the temperature difference. The temperature differential for which a significant drop in property values is observed is the A A- For monolithic ceramics and particle- or whisker-reinforced CMCs the property under investigation is usually strength, whereas in fibre-reinforced CMCs a drop in Young s modulus is usually a better indication of the onset of damage. 

Many applications of ceramic composites will expose the material to impact damage. Here again, the amount of work which has been done in this arena is limited, and needs to be increased. Impact tests can be performed using drop towers or Charpy impact testing apparatus. Such measurements have been done at room temperature30 and the apparatus can be adapted for use at elevated temperatures. 

Polymers tend to have rather lower fracture strengths than materials such as metals or ceramics, but not concrete The theory of brittle fracture applies for polymers as for metals, but with greater emphasis on the development of a plastic zone around the tip of the growing crack (Fig. 4.5). The brittle mechanism is favoured in unmodified epoxies, and as a result of reducing the temperature, increasing the strain-rate or specimen thickness, and having sharp notches. Traditionally, susceptibility to brittle fracture has been assessed by some form of impact testing. 

Figure 1.73 illustrates the instrumented CIT system in a block diagram and the impact-response curve method, as applied to the above tests. In addition to the aforementioned partially stabilized zirconia (PSZ ZrOi-S mol%Y203), samples of S3N4 were also investigated by Kobayashi et al. [32], Typical hammer load-times and strain-gage signal-time curves of PSZ and Si3N4 are found in Fig. 1.74. The impact-response curves of PSZ and S3N4, at several impact velocities are shown in Fig. 1.75. The impact curves of these specimens are impact-velocity-dependent. This technique enables the determination of the dynamic fracture toughness of these ceramics by means of the impact-response curve method. Impact-response curves quantitatively relate the response of the specimen to the impact, which depends solely on the elastic reaction between the specimen and the actual impact.

The ceramic specimens mentioned above, tested by dynamic loading, namely impact testing, are notched as is usual during the performance of such tests. During conventional impact testing, the radius at the notch-root is significant, since it affects the outcomes. For metals, the dimensions of the specimens and the notches are standard. A standard CIT specimen consists of a bar of metal or other material (ceramics included), 55 x 10 x 10 mm having a notch machined across one of... 

The classic method for evaluating the transition temperature from a ductile to a brittle state is by impact testing. The basic reasons for using such a test are the high strain rate that can be achieved by impact and its simplicity. Though there are currently many other ways to vary strain rate, those who choose to perform impact tests can enjoy the use of modem, instmmented impact machines. For most ceramics which are brittle at room temperature (henceforth RT), ductility is a high-temperature feature thus, it is more meaningfiil to discuss BDT, rather than ductile-to-brittle transition (DBT), the more common nomenclature. 

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