Surface flaws

Typically, ozone cracking initiates at sites of high stress (flaws) on the mbber surface. Thus, in general, mbber articles should be designed to rninirnize potential sites of high elongation such as raised lettering. Similarly, the use of clean molds helps to reduce the incidence of surface flaws. 

AI2O3 has a fracture toughness Kic of about 3 MPa m. A batch of AI2O3 samples is found to contain surface flaws about 30 jUm deep. Estimate (a) the tensile strength and (b) the compressive strength of the samples. 

Moulded plastics will also have crack initiation sites created by moulding defects such as weld lines, gates, etc and by filler particles such as pigments, stabilisers, etc. And, of course, stress concentrations caused by sharp geometrical discontinuities will be a major source of fatigue cracks. Fig. 2.72 shows a typical fatigue fracture in which the crack has propagated from a surface flaw. 

Apart from the above three major engineered interface concepts, the ductile coating material may also heal up the surface flaws that are often generated during the fiber manufacturing processes, and protect the brittle fiber surface during subsequent processing. 

The area exposed by 1 g of powder is called the specific surface area . t Porosity is defined here as surface flaws which are deeper than they are wide. 

In the subcritical state, P < Pc, and consequently just a small fall in strength or none at all is expected in a brittle surface. Just before the critical load is reached, a limited stable expansion of surface flaws is likely to occur, but it will be imperceptibly small and thus obscured by fluctuation in the size of the flaws found during a typical strength test. 

The equation, unlike that for a spherical indenter, contains no terms relating to the state of the sample surface (flaw population, etc.). Instead, it considers the cone angle as a variable through its influence on the penetration field. 

Mono- and polycrystalline natural and synthetic materials are not subject to plastic strain and have no independent slip system. Stress concentration occurs in them at.crack tips and at flaws in the material, affecting the maximum strength which originates from the chemical or physical cohesion forces present. Non-plastic materials (crystals, rocks, ceramics, glass) show brittle cracks—forming at very low plastic strain—usually originating from surface flaws. 

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 acoustic energies of the pulses are shown in Fig. 11 and indicate the stress levels at which fracture has occurred in fibers of the same diameter. It is interesting to note here that fibers treated by the polymeric silane produce failure pulses at a higher energy than the untreated fibers. The protective effect of the polymeric silane coating in healing some surface flaws can be deduced from this observation. This effect is not significant when the monomeric silane is used. 

Maguire, J.F. Busbee, J.D. 8c LeClair, S.R. Surface Flaw Detection Using Spatial Raman-Based Imaging US 6,453,264 Bl Assigned to Southwest Research Institute Filed in 1999. 

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