Correlation between ultrasound velocity, density and strength in metal-ceramic composites with added hollow spheres

Ultrasound velocity is known as a non-destructive predictor of strength in some construction materials like concrete. In multi-phase materials, where the physical properties of components differ by orders, the strength prediction becomes more ambiguous due to more complicated interactions between the components. As an example of such material, sintered metal-ceramic composites with added hollow spheres were taken. The matrix was a mixture of powdered iron and clay, where hollow admixtures were cenospheres or microsphreres of fly ash. By varying the iron-to-clay ratio, sintering temperature and cenospheres content, 17.4% variation of the bulk density rho and 35.3% variation of the compression strength sigma were achieved. Ultrasound velocity C was measured in cylindrical specimens and showed 8.6% variation. All mentioned parameters positively correlated with each other. The relationship between C and s had expressed non-linear character. Normalization of the relationship by. helped improving the C-sigma correlation. The non-linear monotonous function C = rho(-2)center dot 6(-4) provided the closest correlation with the Spearman rank correlation coefficient 0.942 and the Pearson linear correlation coefficient 0.943. The hollow spheres content was the main determinant of density rho in this material.