Ceramic fracture mirror

A crack in a ceramic material upon initiation may accelerate and interact with microstructure, stress field and the generated acoustic vibrations. These interactions may lead to some fractographic features such as fracture mirror, hackle or river patterns and Wallner lines. 

This type of fracture can be found in both very large and very small systems, including fractured natural rocks, in tested specimens of advanced ceramic materials, and even in broken ceramic fibers. The fracture mirror size measurements can be used for post-mortem determination of the failure stress in components [21]. 

In this chapter, a dose examination has been made of the phenomenon of fracture in ceramics. The macroscopic appearance of fracture and typical failure modes in ceramic materials has been analyzed, fracture mirrors and fracture origins have been identified, and the way in which fracture is intrinsically connected to the microstructure of a ceramic has been outlined. In particular, by detailing stress distributions it has been shown that fracture always starts at a single microstructural flaw, the stability of which can be described with simple linear elastic fracture mechanics. Notably, these features are responsible for the inherently statistical nature of failure in ceramic materials, an understanding of which can provide knowledge of the close corrdation between defect populations and fracture statistics, and of how to devdop materials parameters such as the characteristic strength. 

In order to review the calculation, tests were performed with circular disks and squares made from borosilicate glass and various glass ceramic materials [3.34]. As shown in Fig. 3.24, the disks were heated in a concentric circular area and heated up to fracture. The fracture mirror showed the type of stress (radial, tangential) and the distance of the maximum tensile stress from the centre of the plates. The magnitude of the tensile stresses was determined... 

Powder morphology was investigated using a transmission electron microscope (TEM, Model JEM-IOOCXII). Crystallite size of the powders and grain size of Nd YAG ceramics calcined at different temperatures were calculaied by X-ray diffraction (XRD, model D/maxrA, using nickel-filtered Cu-Ka radiation) patterns from the Scherrer s equation. Microstructures of the fractured and the thermal etched mirror-polished surfaces of Nd YAG specimens were observed by scanning electron microscopy (SEM, Model S-4800). Densities of the samples were measured by the Archimedes draining method. 

Mirror. The region around the crack origin. The crack travels in a single plane accelerating as it goes. The fracture surface is smooth and highly reflective. This can be seen in polycrystalline ceramics, but reflectivity is lower. 

A typical fracture surface of fibers with a mirror and hackle feature in SiC/SiC ceramics is shown in Fig. 7.24. The cyclic softening observed in SiC/SiC occurs either under stress control or strain control. Since this observation is crack initiation- and propagation-related, it is possible that similar phenomena may be more frequent after softening during fatigue deformation. The Manson-Coffin... 

J. A. Salem and M. G. Jenkins, Estimating Bounds on Fracture Stresses Determined From Mirror Size Measurements, J. Am. Ceram. Soc., 85 706 -708 (2002). 

J. J. Mecholsky Jr, R. W. Rice, and S. W. Freiman, Prediction of Fracture Energy and Flaw Size in Glasses From Mirror Size Measurements, J. Amer. Ceramic Soc., 57 440-443 (1974). 

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