The present work on the mechanical properties of 10 wt% fly ash additions in 2.5 wt% increments to recycled high-density polyethylene (RHDPE) synthesizes experimental data from three similar published reports. The present work shows, as a function of increasing fly ash addition level, maxima at the initial fly ash addition level of 2.5 wt% for the tensile elastic modulus (+25%) and tensile strength (+10%); a slight general increase in the yield stress (+6%); and significant general decreases in the yield strain (-61%), elongation at break (-92%), and Charpy impact strength (-55%). Combining these data with data for higher-level additions of fly ash (40 wt%/23 vol%) and cenospheres (39 vol%) to HDPE or RHDPE provides the basis for design parameters and a generalized model for the interpretation of failure of composites of hard brittle spherical dispersant additions in ductile polymeric matrices. The relevant load-extension plots are characterized by three behavioral regions: ductile deformation (dispersion strengthening and stress concentration), crazing (debonding and cavitation), and brittle failure (fibril failure). The locations of these regions and their transitions are a function of five dependent variables: dispersant volume, dispersant particle size, intrinsic flaw size (viz., dispersant size), generated flaw size (viz., void size), and interfacial bond strength and associated load transfer. POLYM. ENG. SCI., 56:1096-1108, 2016. (c) 2016 Society of Plastics Engineers