The Measuring Method about Young's Modulus of Plastics Using the Indenting Hardness Test by a Spherical Indenter

This study aims at investigating the performance of a hardness testing machine. First, the phenomenon of a contact between a rigid sperical indenter and a plastic material is considered. By using Hertz's elastic contact law, the Young's modulus of plastics Es can be calculated from Eq. (1) using the cordal diameter of an indentation d, the elastic recovery of an indentation δ_r and the indenting load L : E_s = 0.9(3/2) L/(d δ_r)…(1) From the geometrical relation at the contact part, the contacting diameter d_c and the surface level diameter d_l are derived as follows. (δ_t : the total depth) : d_c = 2[D(δ_t-δ_r/2){1-(δ_t-δ_r/2)/D}]^<1/2>…(2), d_l = 2[Dδ_t×(1-δ_t/D)}^<1/2>…(3) Furthermore, the mean strain rate under the elastic recovery by a spherical indenter is related to the strain rate under the uniaxial stress field. Finally, the indenting experiments are carried out, δ_r and δ_t are measured so that a cordal diameter d is formulated as follows : d = d_l{1.71-0.707(d_l/d_c)}…(4) When the Young's moduli calculated from δ_t and δ_r using Eqs. (1)-(4) are compared to the Young's moduli measured from a compression test, their Young's values consist within an accuracy of about ±5-10%. Therefore, the measuring method about Young's modulus of plastics using the indenting hardness test by a spherical indenter can be applied to the present hardness testing machines