www.elsevier.com/locate/jmbbm Available online at www.sciencedirect.com Research Paper Failure analysis of porcupine quills under axial compression reveals their mechanical response during buckling Fernando G. Torres n , Omar P. Troncoso, John Diaz, Diego Arce Pontificia Universidad Catolica del Peru, Dept. of Mechanical Engineering, Av. Universitaria 1801, Lima 32, Peru article info Article history: Received 15 May 2014 Received in revised form 27 June 2014 Accepted 17 July 2014 Available online 25 July 2014 Keywords: Porcupine quills Failure mechanism Buckling Delamination abstract Porcupine quills are natural structures formed by a thin walled conical shell and an inner foam core. Axial compression tests, differential scanning calorimetry (DSC), thermogravi- metric analysis (TGA) and Fourier transform infrared spectroscopy (FT-IR) were all used to compare the characteristics and mechanical properties of porcupine quills with and without core. The failure mechanisms that occur during buckling were analyzed by scanning electron microscopy (SEM), and it was found that delamination buckling is mostly responsible for the decrease in the measured buckling stress of the quills with regard to predicted theoretical values. Our analysis also confirmed that the foam core works as an energy dissipater improving the mechanical response of an empty cylindrical shell, retarding the onset of buckling as well as producing a step wise decrease in force after buckling, instead of an instantaneous decrease in force typical for specimens without core. Cell collapse and cell densification in the inner foam core were identified as the key mechanisms that allow for energy absorption during buckling. & 2014 Elsevier Ltd. All rights reserved. 1. Introduction Porcupines are found in two main families (about 29 species of porcupines) distributed throughout most areas in the world. They are classified as Old World porcupines and New World porcupines. Old World porcupines are found in Europe, Africa and Asia. Some examples include the North African crested porcupine Hystrix cristata and the African brush-tailed porcupine Atherurus africanus. New World porcupines are found in North, Central, and South America. These include the North American porcupine Erethizon dorsatum and the prehensile tailed porcupine Coendou prehensilis. Both families have muscles at the base of the quills enabling the quills to be raised, thereby making the animal look larger when threatened. The Erethizontidae quills can be as long as 10 cm, while the Hystricidae quills are longer, up to 35 cm, and also have a proportionally larger diameter. All porcupine quills consist of a stiff outer sheath (cortex) and compliant porous core (medulla). The Hystricidae quills have additional thin solid longitudinal “stiffeners” extending radially from the cortex towards the centre of the core (Vincent and Owers, 1986; Karam and Gibson, 1994; Gibson et al., 1995). http://dx.doi.org/10.1016/j.jmbbm.2014.07.017 1751-6161/& 2014 Elsevier Ltd. All rights reserved. n Corresponding author. E-mail address: fgtorres@pucp.edu.pe (F.G. Torres). journal of the mechanical behavior of biomedical materials 39(2014)111–118