Modeling and Flexural Strength Properties of Calcium Sulfate Whisker / Epoxy Resin Composites

Abstract

This work is devoted to a comprehensive theoretical and experimental study of the effect of discrete reinforcement with calcium sulfate (CaSO4) whiskers on the mechanical properties of a two-component epoxy resin under static flexural loading. Using optical microscopy and statistical log-normal analysis, the geometric parameters of the whiskers within the polymer bulk were determined: the average diameter was d = 1.86 мm, and the weight-average length was  (l)=44.8 мm, corresponding to an aspect ratio of (l/d)=24.1. Anon-monotonic dependence of the composites strength characteristics on the
filler content was established, exhibiting an extremum at 10 wt/%, which provided a threefold increase in the elastic modulus (up to 180 MPa) and a 2.8-fold enhancement in the flexural strength (up to 6.2 MPa). To quantitatively describe the elastic behavior, the micromechanical Halpin-Tsai model was applied, modified by an effective volume function Veff=Vf.exp (-k.Vf). The extracted agglomeration intensity coefficient k=15.6 strictly justifies the degradation of stiffness at 20 wt.% illing by the avalanche-like clustering of the whiskers into isolated conglomerates. Using the Kelly-Tyson model, the ratio of critical to geometric crystal length was calculated lc/l - 1.63, proving the subcritical nature of the system’s reinforcement. It was established that mechanical energy dissipation and matrix toughening during bending proceed via the viscous pull-out mechanism of the microcrystals from the polymer matrix. The results obtained open up clear prospects for using calcium sulfate whiskers in the targeted design of high-strength structural compounds.