Fracture toughness of ceramics

There are two main techniques used to measure the fracture toughness of ceramics fracture stress and hardness indentation. The former measures the load to fracture of a pre-cracked specimen using a single edge notched beam (SENB) or a chevron notched beam (CNB) sample. The main drawback of this technique is ensuring that the crack tip is atomically sharp. The second method uses the crack formed at the corners of the indentation produced during a Vickers indentation hardness test. This technique is rapid and relatively inexpensive. However, the toughness values measured are those of the surface, unlike the values obtained by fracture of the pre-cracked beams which are a measure of the bulk material properties. 

Kuebler, J., Fracture toughness of ceramics using the SEVNB method from a preliminary study to a standard test method, in Fracture Resistance Testing of Monolithic and Composite Brittle Materials, ASTM STP 1409, ed. J.A. Salem, M.G. Jenkins and G.D. Quinn, ASTM, West Conshohocken, PA, pp. 93-106, January 2002. 

S. Suresh, T. Nakamura, Y. Yeshurun, K.-H. Yang, and J. Duffy, Tensile Fracture Toughness of Ceramic Materials Effects of Dynamic Loading and Elevated Temperatures, Journal of the American Ceramic Society, 73[8], 2457-2466 (1990). 

It is well established that improvements in the room-temperature fracture toughness of ceramics can be achieved by whisker reinforcement, and various mechanisms have been proposed to explain the phenomenon. Recently, two studies have examined the fracture behavior of SiC whisker-reinforced alumina composites at high temperatures to determine the dependence of fracture toughness on temperature, the effect of test atmosphere on fracture toughness, and mechanisms of fracture.33,34 In both studies, single-edged-notched bars... 

Before one explores the various strategies to increase the fracture toughness of ceramics, it is important to appreciate how K, . is measured. 

The fracture toughness Kjc of ceramics (Kic= 3-8 MPaVm) is situated clearly lower than those of metals (Kic= 40-80 MPaVm) and hard metals (Kic= 15-20 MPaVm) /MEN91/. The low fracture toughness of ceramics is responsible for their high brittleness (Table 1). 

M Kubouchi, K Tsuda, T Nakagaki, K Arai and H Hojo, Evaluation of thermal shock fracture toughness of ceramic particulate-fiUed epoxy resin , in Proceedings, ICCM-10, Whistler, BC, Canada. Cambridge, UK, Woodhead, 1995. 

Alternative geometries and test methods to determine the fracture toughness of ceramics and hard materials have been developed over the last decades Herzian indentation [88], the double torsion test [89], and compact tension tests [90 92]. 

Ceramic materials have a large fraction of ionic or covalent bonds, and this results in some special behavior - and, consequently, some special problems - with their reliable use in engineering. Within the temperature range of technical interest, dislocations are relatively immobile, and dislocation-induced plasticity is almost completely absent in ceramics l-3]. This is the basis for their extreme hardness and inherent brittleness, with typical values for the fracture toughness of ceramics ranging from 1 to lOMPa m, and the total fracture strain being commonly less than a few parts per thousand [4]. 

Kiibler, J. (1999) Fracture toughness of ceramics using the SEVNB method Roimd Robin, ESIS Document D2-99, EMPA. 

Shetty D.K., Rosenfield A.R. Duckworth W.H. 1985. Fracture toughness of ceramics measured by a chevron-notched diametral compression specimen. Journal of the American Ceramic Society 68 325-327. 

A natural consequence of such progress will be the need to measure fracture toughness of ceramic materials more frequently in the routine characterization of products. Herein lies a major outlet for the technique of hardness indentation within the field of ceramic technology. The convenience of the microhardness indentation method has already been stressed, involving as it does the use of robust conventional equipment acting on only a small area of polished surface of a sample from which... 

Measurement of Kc by other methods gives the constant in equation (5.44) as equal to 0.016 0.004 which means that equation (5.44) can be used, when E/Hy and c are determined at a known P, to obtain values for the fracture toughness of ceramics. Equally a plot of (c ) against P has a slope a, E/and such a plot can be made if a series of indents at loads in excess of the critical load are made on the same surface. 

The research works on toughening mechanisms and fracture toughness of ceramic composites with piezoelectric phases have been limited, while those for monolithic piezoelectric/ferroelectric materials have been available, as mentioned above. Therefore, details of toughening behavior and mechanisms in composite materials with piezoelectric phases have been still not clear. 

MEASURING THE REAL FRACTURE TOUGHNESS OF CERAMICS ASTM C 1421... 

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