S. W. Freiman

Crack propagation studies in brittle materials

An analysis was made of cantilever beam specimens used for crack propagation studies, Included in this analysis were the effects of a plastic zone at the crack tip, beam rotation, and the viscoelastic response of the material. This analysis showed that application of a constant bending moment to the specimen rather than a constant load provides a test in which the strain energy release rate,G, is independent of crack length. Other advantages of this test configuration are that corrections for shear or beam rotation effects are not necessary. Results of this test on both glass and ceramics are reported.

Microstructural aspects of crack propagation in ceramics

X-ray microradiographic examination supported by optical and SEM observations was used to study crack propagation in various ceramics, including glasses and cubic and noncubic polycrystalline bodies of different grain sizes. The nature of crack propagation in ceramics was often extremely complex. While cracks in glassy materials were generally simple, as would be expected, in cubic and non-cubic polycrystalline specimens both wandering and branching of cracks was observed. In cubic materials, wandering and branching occurred on the scale of the grain size, while in fine grain, non-cubic materials these were on a multi-grain scale. Results are consistent with the grain size dependence of fracture energy. Elastic anisotropy and thermal expansion anisotropy were suggested as major factors in crack wandering and branching.

Fracture energy of Si3N4

The fracture energy of Si3N4 made by hot pressing, reaction sintering, and chemical vapour deposition (CVD) was studied. Extrapolation of fracture energies to zero additive or porosity levels, as well as analysis of CVD Si3N4 all indicate an intrinsic fracture energy of 20–30J m−2. Higher fracture energies in dense bodies with increasing additive content, or in some more porous bodies (relative to expected porosity dependence) are associated with crack branching. In dense bodies such branching may arise due to micro-cracking from combined effects of crack tip stresses and mismatch stresses due to differences in properties, especially thermal expansion, between Si3N4 and the additive or its reaction products. In porous bodies such branching appears to be due to spatial distribution of pores.

Oxidation weight gain and strength degradation of Si3 N4 with various additives

Oxidation behaviour and strength degradation, due to long-term high temperature exposure in air, of Si3N4 with MgO, ZrO2 or Y2O3, as densification aids have been studied. It was found that the weight change and strength degradation in the specimens depend largely on the kind of densification aid. Strength degradation, which generally occurred and was greatest at higher additive levels, was related to the generation of pits in the specimen surface. Possible mechanisms for pit formation are discussed.

Effect of temperature and environment on crack propagation in graphite

Crack propagation studies were carried out on POCO AXF-5Q and ATJ-S graphite as a function of temperature and environment (for example H2 O, and CO and He). Crack growth rates in both graphites was essentially insensitive to the external environments at all temperatures. A transition from stable to catastrophic crack growth occurred at a particular temperature for each material. A definite effect of crack plane orientation with respect to the ATJ-S graphite billet was observed.

Press forging behavior of KCl crystals and resultant strengthening

Use of alkali halides for optical components of laser systems has been limited by the fact that single crystals deform and fracture at low stresses. As dislocation processes are responsible for both the yield and fracture of these materials, one should be able to strengthen by alloying (1) and forming polycrystalline materials (2,3,4) or possibly a combination of both.

Enhanced failure of ceramics irradiated by combined pulse and c.w. lasers

Several ceramic materials were subjected to the combined irradiation of a 1.06μ pulse and a 10.6μ continuous wave (c.w.) laser. The duration of c.w. irradiation required to cause failure from the combined exposure is compared to that from c.w. irradiation alone. For example, thin Pyroceram and soda-lime glass plates burned through when exposed to the c.w. laser followed by the single pulse laser, in about one half the time required to cause fracture during exposure to the c.w. laser alone. Also, enhanced catastrophic fracture of thick soda-lime glass plates resulted when the c.w. irradiation followed the pulse within 0.3 sec. Finally, the effects of the combined pulse and c.w. exposure are compared with the effects of single or multiple pulses, and the apparent enhancement is discussed in terms of beam size and power.