2.1. Specimen preparation procedure

Fresh bones from 22–26 weeks old porcine animals were harvested 24 h after slaughter from the Department of Animal Sciences at the University of Illinois, Urbana-Champaign. The donor pigs had an average diet of corn and weighed about 275 lbs. To keep the bone fresh, femurs were stored at −20 °C before use. Cortical bone specimens were thawed and then sampled using first a table-top band saw and then a diamond precision saw (Isomet 5000^®, Buehler, LakeBluff, IL). Two set of specimens were prepared to observe the fracture behavior in two different orientations. On the one hand, longitudinal-transverse (LT) specimens were used to study fracture perpendicular to the long axis of bone and they were cut into about 5-mm thick sections as shown in Fig. 1. On the other hand, short longitudinal (SL) specimens, approximately 15-mm long and 10-mm wide, were used to study fracture parallel to the long axis of bone. After sectioning, these smaller pieces were defattened, defleshed and then cleansed in a solution of 1.5% Alconox + 5% bleach. For further ease of handling during polishing and testing, each individual section was embedded in polymethylmethacrylate, allowed to cure for 8–9 hours, cut into 5-mm thick discs using the linear precision diamond saw and mounted onto aluminum discs using cyanoacrylate adhesive. To assure a smooth and flat surface finish, a rigorous grinding and polishing operation was carried out. Samples were ground using consecutively a 400, 600, 800 and 1200 grit size of alumina oxide abrasive pads. Wet coarse and fine polishing was ensued using consecutively 3, 1 and 0.25 μm diamond suspension solutions along with TexMet P^® (Buehler, LakeBluff, IL) polishing cloths. Finally, a 0.05 μm Microcloth (Buehler, LakeBluff, IL) pad was used for ultra fine polishing. The specimens were wrapped in a gauge soaked in Hank's Balanced Salt Solution to keep them wet and then stored at 4 °C until tested.

Optical microscopy and surface features: (a) digital photo of a Longitudinal-Transverse (LT) specimen and a representative optical microscopy image of the specimen surface; visible surface features are circular osteons with central Haversian canal and evenly distributed lacunae. (b) Digital photo of a Short-Longitudinal (SL) specimen and a representative optical microscopy image of the specimen surface; surface features are relatively elongated since the Haversian canals run parallel to this orientation. The final polished surface is clean, smooth and free of abrasion marks.