Complete determination of the stress tensor in a field using brittle coatings

Abstract

Brittle coatings are used today, almost exclusively, to determine the directions of the principal stresses at a point located on the surface of a loaded body. This is a strange paradox of history, because no other method can give more complete information about the stress tensor in a whole field. This is so, event if it is also true that the precision of the determinations of some of the components of the tensor may not be as high as that generally obtained using other methods.

This paper has the following objectives: (1) to show the application of the brittle-coating method to the determination of very complicated stress distributions like those which develop in thin-walled pressure vessels manufactured using circumferential and longitudinal weldings, (2) to show that the use of brittle coatings is the most practical method to solve some of these problems when boundary conditions are not well known, (3) to show that the method can, in certain cases (when the two principal stresses have the same sign), give the complete determination of the tensor in large parts of the field of complex stress distributions, and that it is the only method that can obtain that amount of information in one experiment, (4) to show how in other cases (when the two principal stresses are of opposite sign) that information can be obtained in two experiments, (5) to show the possibility of using the method to study yielding problems, (6) to review some of the advantages and limiations of the method and (7) to recommend an organized effort to find new materials which may permit the method to become popular again, without exhibiting some of the serious limitations that have significantly restricted its use. Some of the deficiencies in the present state of knowledge are described.