Measuring fracture toughness of bone at the microscale

Techniques to probe strength of materials at small scales have been established for over a decade but fracture toughness experiments are exclusively standardized for the macroscale. Notched microcantilever based methods have recently been proposed for performing fracture experiments at the microscale. Asymmetric loading conditions around the crack tip limit their viability, particularly for heterogeneous materials such as bone, a composite primarily comprised of collagen fibrils and bioaptite at the nanoscale. We developed a methodology that enables conducting 3-point bending beam fracture experiments on micron-sized samples on a variety of materials fabricated using an in situ SEM/nanoindenter. We extracted beams of ~50x10x5 µm from single crystalline silicon using a focused ion beam and measure a K_1c of 0.98 MPa m^1/2, in agreement with the accepted 1 MPa m^1/2 . We perform site-specific fracture experiments on similarly-sized dry bone beams, fabricated with collagen fibrils oriented orthogonally to the initial crack. We report a J integral toughness of 45 J/m^2 over 2.5 µm of stable crack growth. We discuss these results in the context of the rising R curve of bone and compare observed toughening mechanisms to those reported in similar macroscale experiments in literature.