Towards better understanding of the strain–stress curve of cork: A structure–mechanical properties approach

Abstract

If the compressive strain–stress curve of cork has already been largely described, its relationship with the cork structure still requires a better understanding. In this study, quasi-static and Progressive Repeated Loading (PRL) tests in compression have been performed on natural cork with a tensile machine. The elastic modulus was measured by a quasi-static compression test on cube or parallelepiped specimens considering different strain measurements methods: crosshead displacement, extensometer, and full field measurements method (Digital Image Correlation or DIC method). Regardless of the considered strain measurement method, the elastic modulus of cork remains in the same order of magnitude. The PRL test carried out at different nominal strains (3 %, 20 %, 46 %, 66 %, 80 %) highlighted the time dependant behaviour of cork (viscous properties). Moreover, a stiffening of the material was observed when increasing the strain at each loading/unloading cycle. Finally, the full field measurement following compression revealed the heterogeneity of the strain distribution on the cork external surface, resulting in localised regions of deformation. From the perspective of compression behaviour, these results lead to a new interpretation of cork which can be considered as a composite material.

Keywords

• Cork structure
• Quasi-static compression test
• Progressive repeated loading test
• Extensometer
• Full field measurement
• Composite material