Torsion Testing as an Aid to the Porcelain Enamel Industry

The torsion test is one of a series of some fourteen test methods that have been developed by the porcelain enamel industry through the Porcelain Enamel Institute (PEI), the Enameled Utensil Manufacturers Council, and ASTM Committee C-22 on Porcelain Enamel for the evaluation of finished porcelain enamel and ceramic coating surfaces. A complete discussion of the development of standard test methods in the industry and their application is obviously outside the scope of the present symposium even though such test methods are invaluable in the precise determination of the physical characteristics of the coatings and the evaluation thereof. It is felt, however, that the presentation of a case history showing the benefits derived through the application of one of these test methods would be valuable to the present program in calling attention to the existence of the standard test methods and demonstrating their industrial usefulness. The torsion test should serve as an admirable example. Not only can the advantages gained through the use of the test method be demonstrated but, at the same time, attention may be called to the fact that, although porcelain enamel and ceramic coatings are glassy in nature and therefore are normally considered by the casual observer to be extremely brittle, such coatings can be formulated and applied so that they will withstand a surprising amount of torsion. The torsion test was originally developed as a standard test method by the PEI as the result of a research program instituted in 1937 for the purpose of determining the cause of field failures due to the fracture of enameled surfaces. The program was designed (1) to determine the cause of the failures and (2) to develop standard test methods whereby the stresses giving rise to the failures could be duplicated and the strength of the coatings thereby determined. The research program was successful in first demonstrating that a large percentage of the failures in question were due to torsion rather than impact, as had been previously supposed, and in developing a test method that was tentatively adopted in 1941.