Microstructure flaws

TADB-derived ceramic fibers, with the idealized composition SiBNsC, do not reach the E-modulus of the most advanced SiC fibers at room temperature. However, they are clearly superior to the latter in a crucial point, namely the drop of the E-modulus and of the creep resistance at high temperatures. SiC fibers already lose a large part of their mechanical strength below 1400 °C, as can be measured by creep resistance. These limitations are fundamental in nature, since they are related to grain boundary sliding and thus to the crystallinity of SiC. In contrast, amorphous SiBNsC fibers do not show any grain boundaries and, moreover, the concentration of microstructural flaws is extremely low. 

The property that needs improvement is extrinsic and is therefore manufacture-dependent dielectric strength depends largely on microstructure. Flaws such as microcracks lead to local voltage peaks that may trigger dielectric breakdown. Moreover, the insulator in the X-ray tubes operates in a plasma that contains electrons and metal atoms that both interact strongly with the surface of the insulator. The values of the material properties that are listed in tables have not been determined in such an exceptional atmosphere and those values are therefore not very reliable selection criteria. The situation is similar to mechanical yield strength, which also depends on surface flaws. 

Influence of Microstructure Flaw Populations on Fracture Statistics... 

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. 

Polycrystalline films in a multilayered structure are observed to be more prone to the development of microstructural flaws than glass films during cosintering (53), so it is particularly important to minimize the presence of processing flaws in the polycrystalline layer during forming. When it does not lead to a deterioration in the properties of the multilayered system, the incorporation of a glassy second phase improves the stress relaxation in the polycrystalline film and can serve to alleviate the development of flaws. 

The variability in fracture stress when small artificial flaws were controlling strength was particularly pronounced for H-451 graphite, as can be seen in Fig. 12. Here the crack dimensions and crack trip process zone dimensions are comparable to the microstructural dimensions. Consequently, local variations in microstmeture... 

It is inherently difficult to measure the strength of a material since this is strongly influenced by the microstructure of the material, i.e., the distribution of flaws which strongly influence the propagation of cracks. This concept is illustrated in Fig. 31, where the elastic stress distribution in an ideally elastic, brittle material is seen to become infinite as the crack tip is approached. The key properties which characterize the strength of a material are ... 

The ultrasonic C-scan technique is the most widely used nondestructive method of locating defects in the composite microstructure. The through transmission C-scan is easy to implement and a large composite panel can be scanned in a matter of minutes. The problem with this technique is that a C-scan cannot reveal the type of defect present. Hence, there is no way to determine if a flaw detected by the C-scan is due to incomplete contact of an interply interface or some other type of defect in the composite microstructure. 

This equilibrium expression links material properties to surface state. Strength will therefore reflect to some extent the microstructural impact on F (also likely on E), but to a much greater extent it is a reflection of the influence of a wide range of mechanical, thermal and chemical operations on cf. Most ceramic surfaces have various surface flaws running up to 10 [Am deep. Even in carefully prepared samples, the largest of them determines the material strength. 

The preceding reasons for strength reductions are a consequence of microstructural deficiencies, but even perfect microstructures, in which the critical flaw size is the same as for the unreinforced matrix material, can yield smaller strength increases from particle-induced toughening than would... 

Padture, N.P., Bennison, S.J. and Chan, H.M. Flaw-tolerance and crack-resistance properties of alumina-aluminum titanate composites with tailored microstructures ,... 

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