Microscopic flaws

If it is assumed that the material carried into tension is predisposed to spall through a sufficient microscopic flaw structure when the energy condition is satisfied, then the expressions, (8.27) through (8.29), as equalities, can be solved for the spall properties. These are, for the fragment size... 

The problems with the ceramics lie as much in reproducibility as in absolute strength. With brittle materials, microscopic flaws concentrate stress and cause failure at stresses, which vary according to the classical crack theory as <5 1/2, where 8 is the characteristic dimension of the flaw. For materials formed from powders, the flaws represent structural... 

Of these four types, (a)-(c) are observed at the atomic level, whereas bulk defects are easily observed by the naked eye, or using a light microscope. These bulk defects are produced through the propagation of the microscopic flaws in the lattice. For crystals with a planar defect such as polycrystalline solids, the grain boundary marks the interface between two misaligned portions of the bulk crystal (Figure 2.23). 

Microscopic flaws oxmicroporosity axQ voids that are created by poor coalescence of particles. 

It is clear that macroscopic and microscopic flaws affect the properties of PTFE parts. It is important to characterize their impact quantitatively. These flaws can be directly measured by a number of techniques. The measurement methods are complex and not suitable for routine quality control. Macroscopic flaws are frequently detectable visually or by a simple magnifying glass. Internal defects can be detected by x-ray techniques such as radiography. ASTM E94 can be applied as a guideline in establishing procedures. 

Ceramic materials display a stochastic strength response because of the combination of brittleness and the random nature of the size, orientation, and distribution of inherent microscopic flaws. In addition, the ability of ceramic structures to sustain loads degrades over time due to a variety of effects such as slow crack growth (SCG), cyclic fatigue, creep, and oxidation. Life prediction codes such as CARES/Li/c, CERAMIC/ERICA, and STAU are available and have been demonstrated to... 

It is very important to recognise that fracture is inseparable from deformation. Yet, deformation and fracture are distinctly different phenomena. In general, for characterising deformation, the test piece is deformed uniformly. Otherwise, the stress-strain relationship becomes too complex to describe. Even for the treatment of a complex non-imiform deformation, it is sub-divided into small elements, which are assumed to deform imiformly. On the other hand, fracture originates from microscopic flaw, which is an extreme case of non-uniformity. Under the driving force of the stored mechanical energy, the flaw grows to become the fracture surface. 

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