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Published 1993 | public
Book Section - Chapter

Fracture of Brittle and Quasi-Brittle Engineering Materials

Abstract

The study of fracture of engineering materials involves a number of science and engineering disciplines. Continuum fracture mechanics is deeply rooted in the problem of fracture because it treats the relationship between a crack or inhomogeneity and the stress state in a material. Physics and chemistry are important because they help to explain the reactions between the environment and the crack tip. Finally, materials science is essential in understanding the relations among bond rupture, structure, processing and performance of a material. Fracture of materials can be divided into two broad categories - ductile and brittle fracture. Ductile fracture is associated with appreciable plastic deformation. "Cup and cone" fracture demonstrated in metals due to tensile overload is a classic example of ductile fracture. The different stages of such fracture are shown in Figure l.l(a). At the maximum load, plastic deformation is concentrated in a small gage length of the specimen and necking begins. Once this necked region has formed, fracture begins at the center of the specimen and extends along the dashed lines, finally producing the familiar cup and cone fracture. In fcc metals, plastic deformation continues on the conjugate slip planes until the specimen has necked down to a sharp point. Polycrystalline metals with second phase particles fail due to initiation, growth and coalescence of micro-voids formed in the necked region. Similarly, semicrystalline polymers exhibit necking which leads to localized strengthening of the specimen. Then the specimen elongates due to the propagation of this neck along the gage length. This ductile fracture in polymers is different from that in metals, in which all subsequent deformation is confined to the neck region. Alternatively, brittle fracture is associated with little or no deformation. A brittle material behaves elastically up to the maximum load at which catastrophic failure occurs (Fig. 1.1(b )). Silicate glasses are the most common example of such fracture. Brittle fracture is controlled by microscopic inclusions, surface and interior flaws and defects and pores present in the material. An intermediate category of fracture, known as quasi-brittle fracture, has recently been defined. A quasi-brittle material, a title which encompasses many polycrystalline ceramics and cementitious materials, shows measurable deformation prior to failure. The deformation, however, is not associated with dislocation motion. At the onset of nonlinearity in the load-displacement relationship existing flaws in the material start growing and new flaws form (Fig. 1.1(c)). Such materials are characterized by a softening curve after the peak load. This softening branch of the load-displacement relationship is associated with stable crack growth in the material before the final fracture. The failed specimen, however, need not look any different from a classically-brittle failed specimen. In a review article such as this, it is useful to first build the necessary background, and various aspects of fracture mechanics are reviewed in the next section. This review is followed by discussions of the microstructural aspects of toughening mechanisms, short crack fracture in comparison to long crack fracture, the statistical nature of fracture and its implications for quasi-brittle materials, and slow crack growth and other environmental effects on various fracture processes are presented. The implications to cementitious materials are discussed throughout.

Additional Information

© 1993 American Ceramic Society.

Additional details

Created:
August 20, 2023
Modified:
October 17, 2023