Rubber Hardness Testing

Abstract

Abstract Dictionaries give a confusing array of explanations of “hard” and “hardness”. A hard material, according to the Oxford English Dictionary, is one “that does not yield to blows or pressure; not easily penetrated or separated into particles; firm and resisting to the touch …”. “Hardness” implies “difficulty of penetration, inflexibility, rigidity, stiffness …”. Even the earlier scientific workers on rubber were equally vague; thus, Dawson in 1927 regarded hardness as “an ill-defined little understood property” and considered that resistance to penetration—the practical criterion of hardness—“brings into play not merely forces associated with the surface cohesion, but also more deep-seated disturbances connected with elasticity, plastic deformation (or set), internal friction, resilience, and energy absorption”—truly an intractable mixture! The practical man, disregarding theoretical difficulties, has always judged hardness by some form of indentation test, if only with the thumb-nail or a pencil point, and out of such crude beginnings have grown the precise standardized tests of today.