A method of making a composite core can include configuring a plurality of mandrels to have a desired thermal expansion characteristic, placing a composite material around each mandrel, placing the plurality of mandrels in a tool, the tool being configured to constrain the plurality of mandrels as the mandrels experience a thermal expansion, heating the mandrels so that the mandrels thermally expand, thereby applying a pressure to the composite material during a cure cycle, and then cooling and separating the mandrels from the composite core. The mandrel is configured as an internal form for making a tube member of a composite core.