News & Events / BME Seminar Series / Past Seminar Speakers / 2017-18Ange-Therese Akono, PhD
BME Seminar Series Fall 2017
Thursday, November 16, 2017 at 4-5 pm
Tech L361
Host: Professor Eric Perreault
Assistant Professor and Louis Berger Junior Professor, Department of Civil and Environmental Engineering, Northwestern University
Fragility of Cortical Bone via Microscopic Scratch Testing
Cortical bone is a highly complex material consisting essentially of water, type I collagen and hydroxyl apatite. The intricate ultrastructure and composition confers to bone extraordinary toughness properties, which are not fully understood. Previous theoretical and experimental investigations of the fracture behavior of bone have focused on a single mode fracture such as pure mode I (tensile opening) or pure mode II (in-plane shear). In the majority of cases, bone fracture occurs as a result of various superimposed loadings. Therefore, it is essential to understand bone fracture under mixed mode. It is equally important to probe the behavior at the microscopic level, given the multi-scale nature of bone.
I will present a new method to measure the fracture toughness at the microscopic length-scale. Herein, we employ scratch testing, which consists in pulling an axisymmetric probe across the surface of a small specimen and is frequently employed for quality control of thin films and coatings. We carry out progressive-load scratch tests on cortical bone specimens from bovine and porcine femurs. The acoustic emission sensor shows peaks characteristic of energy release events and optical microscopy reveals the presence of a residual groove, pointing toward material removal processes. Using scanning electron microscopy, we observe a transition from a smooth profile to curved microscopic fracture surfaces along the residual groove. These observations hint toward a ductile-to-brittle transition activated by the depth of penetration.
To verify our hypothesis, we examine the scaling of the scratch force. In particular, in a strength-driven regime, we expect the nominal strength to be invariant with the penetration depth; meanwhile for a fracture-driven regime, the nominal strength is inversely proportional to the square root of the penetration depth. In the asymptotic regime of a fracture-dominated process, nonlinear fracture mechanics analysis enables us to assess the fracture toughness directly at the level of osteons. The fracture behavior is found to be anisotropic with the fracture toughness and observed fracture micro-mechanisms that vary with the scratch orientation. The methodology and results reported here are important to gain a fundamental insight into the fracture of biological tissues.
Learn more about Professor Ange-Therese Akono and their research here.