Axial compression of a hollow cylinder filled with foam: a study of porcupine quills.

Porcupines use their lightweight quills, which are strong enough to support significant compression and flexure loads, for defense. Hystrix, with long and thick quills, belongs to the family of Hystricidae (Old World porcupines), while Erethizon, with smaller quills, belongs to the Erethizontidae family (New World porcupines). The objective of this work is to compare the structure and compressive properties of quills from Hystrix and Erethizon. Both quills have a thin keratinous cortex filled with closed-cell foam that has cell diameters decreasing from the center to the cortex. Hystrix quills have stiffeners that extend from the cortex towards the center. The local buckling strength is larger for Hystrix, and very good agreement is found between the predicted values and the experimental ones for both quills. The foam shows extensive deformation, both tensile and compressive, around the buckled cortex.