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).
The properties and application of brittle coatings for stress analysis.
| Identifier | ExFiles\Box 142\2\ scan0470 | |
| Date | 14th March 1940 guessed | |
| 3 and used the next day. The brittle coatings produced are to a practical degree uniformly brittle within the thickness range of three to eight-thousandths of an inch. Under normal conditions of use, the coatings commence to fracture with about .0008 in.strain and produce a recognizable variation of crack pattern up to about .0012 in. per in. strain. In terms of stress on steel these values are : starting fracture at about 25,000 lb. per sq. in. and change of pattern up to about 35,000 lb. per sq. in. When strains increase above the upper limit, the crack patterns increase in number and regularity so slowly that the recognition of the amount of strain they represent becomes difficult. The sensitivity of the coatings may be increased by temperature manipulation. For instance, a coating dried at 75° F.{Mr Friese} and tested at 68° D.{John DeLooze - Company Secretary} would start to fracture at about 15,000 lb. per sq. in. stress on steel. The spacing of the cracks in a well-filled pattern has been found to be a function of the thickness of the coating, with the distance between cracks averaging five times the thickness of the brittle film. We believe this phenomena indicates that fracture initiates at the free upper surface of the coating. This conclusion is further substantiated by the facts that surface finish is of negligible effect on the sensitivity of the brittle coatings and that thin sections in bending cause the coating to crack at lower values of structure surface strain than occurs with straight tension loading. Rate of loading appears to be of little effect provided maximum load is reached within ten minutes. Since considerable time is usually required for inspecting new crack formation during loading, a convenient schedule is to load until a general inspection is desired; return the load to zero during the inspection, and then reload. Several rough checks appear to indicate that the coatings react normally at high rates of loading, even into the impact range. However, more exact measurements will have to be made before it is certain that the strain sensitivity remains fixed. Atmospheric conditions of temperature and humidity have been found to affect the lacquers. In order to obtain satisfactory results under different conditions it has been necessary to formulate a graduated series of brittle lacquers; and by means of a chart, pick the correct one for the prevailing temperature and humidity. | ||