Fracture behavior of hydroxyapatite/polymer interpenetrating network composites prepared by in situ polymerization process

Experimental characterizations of both macroscopic and microscopic fracture behaviors have been attempted in hydroxyapatite/polymer interpenetrating network composites, which were prepared by an in situ polymerization process conducted into a porous hydroxyapatite skeleton. The polymeric compound was either nylon-6 or poly(methylmetacrylate). Toughening mainly occurred by micron-scale crack-bridging mechanism operated by polymer ligaments stretched upon crack opening along the crack wake. This mechanism, which is the same as that responsible for toughening in natural materials, led to an increase by about two orders of magnitude of the hydroxyapatite work of fracture. A piezo-spectroscopic technique based on microprobe measurements of Raman lines in the hydroxyapatite-matrix phase has been employed to quantitatively assess the microscopic stress field, which raised along the crack path. Such a crack-shielding microscopic stress field was mapped in situ on the profiles of advancing cracks. The crack-clamping effect could be rationally related to the macroscopic fracture characteristics (e.g. the rising R-curve behavior) of the composites.