A Study of the Firmness of Soft Materials Based on Nutting's Equation

The Nutting power-law relating shear stress, strain and time has been applied to data derived from the compression and extension of cylinders and strips of soft materials each at a series of constant stress, due compensation being made for changes in the cross-section of the test-pieces. The majority of curves so obtained are linear, plotting log strain against log time, comparatively few materials giving curves showing marked deviations from linearity. A comparison of the three constants (two `quality' exponents and one `intensity' factor) is made for a variety of materials, including bitumens, rubbers (chiefly unvulcanized), clay and similar pastes, a variety of synthetic polymers, and cheese. In spite of the dimensional difficulties inherent in the equation, the treatment appears to have considerable advantages over the more usual practice of calculating viscosities and shear moduli by dividing the strains into recoverable and non-recoverable components.