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 cantilever and nanoindentation based methods have recently been proposed for performing fracture experiments on microscale samples. Asymmetric loading conditions around the crack tip and assumptions about the sample microstructure in these experiments limit their viability, particularly for multi-scale 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 with free boundary conditions and notches or fatigued pre-cracks on a variety of different materials that were fabricated using traditional nanofabrication methods and an in situ SEM/nanoindenter. We first extracted beams of roughly 50x10x5 µm from single crystalline silicon using a focused ion beam (FIB) and measure a microscale K1c of 0.98 MPa m1/2, in agreement with the accepted value of 1 MPa m1/2 . We performed these experiments on fused silica and an acrylate reporting K1c values of 1.33 and 0.82 MPa m1/2 , respectively. We employ the technique to perform site-specific fracture experiments on similarly-sized dry bone beams, fabricated with collagen fibrils oriented along the length of the beam and orthogonally to the loading direction. We calculate the J integral and report a 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.