Aggregate (composite)

Aggregate is the component of a composite material that resists compressive stress and provides bulk to the composite material. For efficient filling, aggregate should be much smaller than the finished item, but have a wide variety of sizes. For example, the particles of stone used to make concrete typically include both sand and gravel. Aggregate is the component of a composite material that resists compressive stress and provides bulk to the composite material. For efficient filling, aggregate should be much smaller than the finished item, but have a wide variety of sizes. For example, the particles of stone used to make concrete typically include both sand and gravel. Aggregate composites tend to be much easier to fabricate, and much more predictable in their finished properties, than fiber composites. Fiber orientation and continuity can have an overwhelming effect, but can be difficult to control and assess. Fabrication aside, aggregate materials themselves also tend to be less expensive; the most common aggregates mentioned above are found in nature and can often be used with only minimal processing. Not all composite materials include aggregate. Aggregate particles tend to have about the same dimensions in every direction (that is, an aspect ratio of about one), so that aggregate composites do not display the level of synergy that fiber composites often do. A strong aggregate held together by a weak matrix will be weak in tension, whereas fibers can be less sensitive to matrix properties, especially if they are properly oriented and run the entire length of the part (i.e., a continuous filament). Most composites are filled with particles whose aspect ratio lies somewhere between oriented filaments and spherical aggregates. A good compromise is chopped fiber, where the performance of filament or cloth is traded off in favor of more aggregate-like processing techniques. Ellipsoid and plate-shaped aggregates are also used.