Edge flaws

Figure 4 Virtual source distribution for a sharp-edged flaw.

The flaw was close to the edge of a plate, and if the contractor that moved the tank had cut it up along the welds—the usual practice—some or all of the flaw might have been removed. However, the tank was cut up close to the welds but away from them. The flaw was obscured by rust and residue and could not be seen. 

There is a convenient mathematical idealization which asserts that a cube of edge length, / cm, possesses a surface area of 6 f cm and that a sphere of radius r cm exhibits 4nr cm of surface. In reality, however, mathematical, perfect or ideal geometric forms are unattainable since under microscopic examinations all real surfaces exhibit flaws. For example, if a super microscope were available one would observe surface roughness due not only to the atomic or molecular orbitals at the surface but also due to voids, steps, pores and other surface imperfections. These surface imperfections will always create real surface area greater than the corresponding geometric area. 

The areas where a test piece is attached to clamps and cut edges are preferential sites for cracking. It is generally good practice to coat clamped areas with an ozone resistant paint (which does not affect the rubber in any way) but cut edges are best left. For most purposes a Hypalon-based paint is satisfactory. Clamps, even when made of material such as aluminium, should be soaked in ozone prior to use. Any pattern or flaws on the test piece surface will also tend to act as stress raisers and show preferential cracking. 

Microspectroscopy applies the identification power of infrared spectroscopy to the microscopic realm. Contaminants on printed circuit boards, blemishes in coatings, and other production defects can be isolated in situ and analyzed (see Electronics, coatings). Analysis of flaws that develop during use illuminates the method of failure. Microscopic samples, such as particulates filtered from air, can be analyzed individually. The forensic applications are many paint chips, single fibers, explosive residues, and inks on currency can all be identified nondestmctively (see Forensic chemistry). The structures of layered materials, such as laminated polymer films, are studied via microspectroscopy by cross-sectioning the materials and examining the individual layers edge on (47). 

Every effort was made to remove redundancy and add homogeneity to the material in this multi-author volume. Indeed, the more authoritative level of discussion afforded by having specialists write each chapter will have hopefully overridden any rough edges that remain from chapter to chapter. Undoubtedly, many flaws remain for which we as editors are wholly responsible we would welcome feedback on these. 

Where a molybdenum disulphide film has been formed on a surface in air, the nature of the interface between coating and substrate is not fully established, but it is probable that the initial adhesion, either chemical or mechanical, is preferentially at edge sites. This will inevitably result in discontinuities, flaws and gaps in the film adjacent to the substrate surface. 

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