Development of Compressive Flow Strength Testing for Gasket Materials

It is highly desirable to have a compression test of gasket materials (which are generally composites of fibers, fillers, and binders) since (1) a gasket tension testing (ANSI/ASTM F 152) does not represent the loading direction the material is used in service (anisotropic properties) and (2) there are generally significant differences between the tensile and compressive flow stress of composite materials. It is recognized that a compression test for axis-symmetric shapes is dominated by friction when low aspect ratio samples are used. It is predicted theoretically that when the sample aspect ratio approaches the interface coefficient of friction it may be possible to observe material flow in compression. Empirical observation of this fact has been made here and elsewhere. Typically the gasket industry relies on empirical relations between the maximum permissible compressive stress and gasket width-to-thickness ratios because different flow stresses are observed for the same material tested with different footprints. The application of a plane-strain compression test is discussed as a means to overcome the large friction effect in gasket compression tests. The plane-strain test offers the advantages of (1) loading the gasket in the service direction, (2) measuring the compressive rather than the tensile flow stress, and (3) largely eliminating concerns about sample edge damage during preparation since the compressed area is bounded by undeformed shoulders of the gasket sheet.