An article of manufacture comprising a sag-resistant nucleus-forming monolithic composite capable of being located within a hollow interior portion of a structural material and being expanded therein. Also, articles of manufacture comprising open-cellular structural material containing within the open-cell or cell thereof, at least one sag-resistant nucleus-forming monolithic composite. The composite is desirably in the shape of a plug that is similar or close to similar to the shape of the hollow interior. In addition, there is described a process that comprises forming a pre-shaped sag-resistant nucleus-forming monolithic composite for use in reinforcing and stiffening a normally open-cellular structural material. Also described is a process for reinforcing or stiffening a normally open-cellular structure, any tubular structure, or any channel structure, by putting at least one sag-resistant nucleus-forming monolithic composite within a hollow interior portion of the structure. The invention is particularly desirable for stiffening and/or reinforcing honeycomb structures.