Damage accumulation studied by acoustic emission in bone cement prepared with core–shell nanoparticles under fatigue

Analysis of acoustic emission signals generated during the fatigue testing of core–shell nanoparticles filled acrylic bone cements is reported. Core–shell nanoparticles composed of a poly(butyl acrylate) rubbery core and a poly(methyl methacrylate-co-styrene) (P(MMA-co-St)) outer glassy shell with varying compositions (20/80, 30/70, 40/60, and 50/50) were incorporated into the solid phase of bone cement at 10 wt%. Acoustic emission data revealed that the damage accumulation process was discontinuous for all bone cement formulations, being less pronounced in formulations with higher fatigue resistance (20/80 and 30/70) which emitted a higher number of events before the final fracture. The capability of both 20/80 and 30/70 core–shell nanoparticles to hinder the microcracks or crazes propagation was attributed to their poor interfacial adhesion with the matrix in comparison with formulations with 40/60 and 50/50 core–shell nanoparticles. Based on the acoustic emission data and SEM micrographs, a crack propagation scheme for bone cements containing core–shell nanoparticles was proposed. In this regard, core–shell nanoparticles had a double role on the bone cement as reported for other systems; firstly, the nanoparticles promoted nucleation sites for crazes or microcracks, and secondly, they could also hinder the propagation of these defects, acting as a barrier to their growth. Crack propagation mechanism consisted mainly in crazing ahead of the cracks tip as well as the coalescence of these cracks which were responsible of the acoustic emission detected.