From the Rolls-Royce experimental archive: a quarter of a million communications from Rolls-Royce, 1906 to 1960's. Documents from the Sir Henry Royce Memorial Foundation (SHRMF).
Technical paper discussing the history and properties of brittle coatings for strain analysis.
| Identifier | ExFiles\Box 142\2\ scan0469 | |
| Date | 14th March 1940 guessed | |
| 2 approximate this ideal in several ways. While they cannot approach the accuracy of other mechanical gages they can furnish a very useful preliminary survey which need only be checked at a few critical points with instruments of higher precision. History of Brittle Coatings It has long been recognized that a brittle coating on strained metal parts furnishes an excellent means of recognizing overloaded regions. Since the days of wrought-iron bridges the cracking of the brittle oxide scale present on hot-rolled or annealed steel has been used to indicate local yielding. Whitewash over the scale has been used for many years as a special method to increase the visibility of the cracks. Attempts to improve on scale by using a varnish, lacquer or hot-applied resin coat which would crack at values of strain within the elastic range were published in 1932¹ and 1934². In 1937, a more complete study of brittle lacquer behaviour was started by one of the authors of this paper, at the Massachusetts Institute of Technology, and several of the limitations previously encountered have been removed. Properties of Brittle Coatings A broad definition of a brittle coating is one which will fracture in response to strain in the structure beneath it. Brittle oxide scale cracks and flakes in an irregular pattern when the metal beneath it yields. It indicates areas of high local strains. To indicate the direction of strain, the brittle coating needs the additional property that it will adhere to the surface of the structure at the same time it fractures within itself. Hot resin coats will do this very nicely and provide qualitative pictures of areas of maximum tensile strains and their directions, the cracks running normal to the principal tension strain. Quantitative measurements require in addition that the coating be uniformly brittle over the entire surface of the structure and possibly also over some calibration model. For practical use the coating must be uniformly brittle over a range of thickness since the other alternative, uniformity of thickness, is difficult to attain. The brittle coatings developed at M.I.T. are in the form of air-drying lacquers which may be sprayed on, dried over night | ||