Measurement of the microstructural fracture toughness of cortical bone using indentation fracture

Abstract

The purpose of this work is to investigate the use of indentation fracture as a method of measuring toughness at the microscale in cortical bone. Indentation fracture employs sharp indenters to initiate cracks, whose length can be used to calculate the toughness of the material. Only a cube corner indenter tip is found to initiate cracks at a suitable size scale for microstructural measurement. Cracks from 7 to 56 μm in length are produced using loads from 0.05 to 3 N. Preliminary data predicts rising toughness with increasing crack length (rising R-curve behaviour) at the microscale. This technique provides a new insight into fracture in cortical bone since it allows the investigator to observe mechanisms and measure toughness at a size scale at which in vivo damage is known to exist.

Introduction

Measurements of bone quantity, while useful in predicting fracture risk, cannot fully explain incidence of fracture (Hernandez and Keaveny, 2006). Other architectural factors such as geometry and material properties must therefore be influential. These factors are categorised as measures of bone quality (Hernandez and Keaveny, 2006). In order to improve our ability to assess fracture risk, our understanding of the contribution of bone quality must first be improved (Rho et al., 1998; Zioupos and Currey, 1998; Hernandez and Keaveny, 2006). One measure of quality that is directly related to the fracture of materials is fracture toughness.

Microcracks due to in vivo loading are known to exist in bone (Schaffler et al., 1989). In an effort to understand microcrack growth and fracture in bone, various methods have been used to measure fracture toughness, a review of which can be found in Yan et al. (2006). While these studies have measured the toughness of bone at a macroscale and revealed its variation with crack extension (rising R-curve behaviour), the techniques employed have not allowed a microstructural toughness measurement. It has recently been suggested that “in situ” measurement of toughness of the bone matrix at the microscale is the next stage in the examination of microcracks and bone quality (Zioupos, 2005).

Indentation fracture was first proposed by Palmqvist (1962) and developed by Evans et al. (1980) for the measurement of the microstructural toughness of ceramics. It has since been used to measure toughness of a wide range of materials and to investigate R-curve behaviour (Ramachandran and Shetty, 1991; Bleise and Steinbrech, 1994; Smith and Scattergood, 1996; Xu et al., 2000). Biological materials, such as enamel and dentin, often considered analogous to bone, because of their structure and anisotropy have also been the subject of studies using indentation fracture (Hassan et al., 1981; White et al., 2005). The technique employs a sharp indenter to grow cracks in a material at the microstructural level. The length of these cracks can then be used to calculate the toughness of the material using an empirical relation. Indentation fracture has some major advantages over traditional toughness testing methods: it allows measurement at the microscale. There is no need for machining microspecimens. The technique is extremely efficient, allowing many measurements on a small quantity of material. Finally, in the case of bone, the cracks produced are of the same size scale as those observed in vivo, and may therefore provide a more relevant insight into the nature of damage in bone.

The purpose of this study was to investigate indentation fracture as a method of measuring the microstructural toughness of cortical bone.