Some Developments and Applications of Brittle Lacquers: A Survey of Published Work and Some Original Contributions

The calibration of brittle lacquer is an important part of the technique. The photograph, FIG. 10, gives some idea of the essentials of a cam operated cantilever beam bending rig with a steel beam in position, partially deflected; the equipment shown is in certain respects similar to an American apparatus the subject of U.S. Patent No. 2,310,845. FIG. 11 shows the typical formation of a crack pattern in an early type of cantilever calibration beam the dimensions of which are given in FIG. 12. The cracks observed may be grouped into two classes, the first incomplete cracks at A, distance L1 from the root, and the first complete crack at C, distance L2 from the root of the beam. Consistent results have been obtained using the mean position at B, corresponding to ×=1/2(L1 + L2) in. to compute the strain sensitivities of lacquers. The distance A to C, equivalent to (L1−L2.) in. is invariably quite constant for a given type of lacquer, and thickness of coaling, a typical relationship between dimension (L1−L2) in. and the thickness of a lacquer is illustrated by FIG. 13.