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).
Page explaining the methodology for using brittle coatings in strain analysis.
| Identifier | ExFiles\Box 142\2\ scan0471 | |
| Date | 14th March 1940 guessed | |
| 4 After being painted, the coating should not be subjected during drying to a sudden temperature variation of more than 5° F.{Mr Friese} nor a slowly acting change of more than 10° F.{Mr Friese} During testing, temperature variation should be less. Humidity ordinarily does not change enough to cause trouble. Use of the Brittle Coatings in Strain Analysis. The brittle coating may be thought of as providing a large number of principal tension strain indicators with a minimum gage length of the order of .01 inches and with a workable range of approximately .0008 to .0012 in. per in. strain. Quantitative evaluation of the amount of strain is secured by calibration. A calibration strip such as seen in Figure 3 is sprayed and dried along with the structure under test. When ready the strip is given a known loading as a cantilever beam in the calibrator, as shown in Figure 1 and then placed in a scale marked with the values of strain the calibration strip has received. Evaluation of the strains on the structure is made by matching the regularity of the crack patterns on the structure and on the calibration strip rather than counting the number of lines per inch. Since the range of strains which can be evaluated is smaller than the range present on most structures, another principle must be utilized in order to effect a complete strain analysis. If we can make the assumption that Hooke's Law holds over the entire structure then we can measure local strains at different values of loading on the structure and by simple proportion interpolate or extrapolate all local strains to correspond to any value of loading we may choose. In general Hooke's Law is a good approximation of actual performance and in practice structure which seriously deviate from it are fairly easy to recognize. While the brittle coating is unaffected by compression strains as such, nevertheless it is possible to evaluate them by causing the coating to fracture from the relaxation of a compression strain. The procedure is to place an elastic structure under maximum load and then form the brittle coating on its surface. At the same time a calibration strip is loaded, with its coated surface under compression, in the calibrator. The test is run by relaxing the loads from their maximums on both the calibration strip and the structure in a manner very similar to the evaluation of tensile strains by the additions of loads. On areas of moderate strain concentration, values of principal tension and compression strains may be estimated within an error of about 15%. The direction of the principal strains is indicated with exceptional accuracy. When checks with other | ||