Fracture mechanism and mechanical properties of porous HA/PLCL and HA/PLLA sandwich composite beams

Previously, we have developed a novel porous hydroxyapatite/poly l-lactic-co-caprolactone (HA/PLCL) composite sandwich beam as a new scaffold material for bone regeneration. This work presents the study of bending fracture mechanisms and microdamage of porous hydroxyapatite/poly L-lactic-acid (HA/PLLA) sandwich beam, in comparison to the previous developed HA/PLCL sandwich beam. Both beams were fabricated using the sandwich method in which the single porous composite beams were layered in between two porous polymer layers of their kind. Three-point bending tests were performed to assess their mechanical properties, and their microstructures along with the damage mechanisms were examined using field emission-scanning electron microscope (FE-SEM). Their crystalline structures and thermal properties were analyzed using Fourier transform infrared spectroscopy (FT-IR), X-ray diffraction (XRD) analysis, and differential scanning calorimetry (DSC) measurements. The microdamage and fracture mechanisms were then characterized by stepwise crack formations. It was found that HA/PLLA composite sandwich beam possessed higher mechanical properties compared to HA/PLCL sandwich beam, resulting from higher stiffness of PLLA polymer. PLCL layer showed a vast ductile fracture, with elongated PLCL strips, while the fracture surfaces observed on the PLLA layer have a relatively brittle fracture pattern with some polymeric pores that were still visible. Three stages of damage were observed on both beams with different locations of crack initiation, indicating that different polymer materials do affect the fracture behavior of each beam.