Issue 50

M.R.M. Aliha et alii, Frattura ed Integrità Strutturale, 50 (2019) 602-612; DOI: 10.3221/IGF-ESIS.50.51 603 the mechanism of failure and damage in bone materials often involves micro and macro cracking or sometimes other failure modes like delamination and etc. Accordingly, the common and general mechanical properties like tensile strength, elasticity modulus, flexural strength and hardness may not be sufficient to describe the resistance of bone against crack growth and other characteristic parameters such as fracture toughness ( K c ) or fracture energy ( G f ) must also be known for more accurate measurement of bone performance in vitro conditions against fracture. A number of research works have been published in the past for obtaining and investigating the fracture toughness and fracture energy of bone materials. K c and G f are the most fundamental parameters for explaining the resistance of material (like bone or other biomaterials) against crack growth and catastrophic failure. Norman et al. [1] tested the longitudinal fracture toughness of both human cortical bone and bovine cortical bone using the compact tension specimen. They obtained both K Ic and G Ic values for these two bone materials and derived an empirical relation between the fracture parameters (i.e. K Ic and G f ). Using a novel ring shape specimen, Barrero and Adams [2] obtained experimentally the mode I fracture toughness of rat cortical bone. The average value for the K Ic of rat bone was obtained about 0.55 MPa√m in their experiments. In another research paper, Behiri and Bonfield [3] investigated the effect of anisotropy and orientation effects on the fracture toughness of bovine cortical bone using a modified compact tension specimen. Lucksanasom et al. [4], investigated experimentally the effect of orientation and storage media on the fracture toughness of bovine femur and tibia bone. They tested a number of single edge notched beam bone samples taken from transverse and longitudinal directions and showed that the storing media ( i.e. saline and alcohol) and also the direction of test sample have significant influence on the mode I fracture toughness of investigated bones. Similarly, Yan et al. [5] studied the effect of temperature on the fracture toughness of bovine femur and manatee rib using single edge V notched beam subjected to flexural four-point bend loading. They tested some bone samples at four different temperatures ranging from zero to 50 o C and observed that the fracture toughness decreases by increasing the temperature. In another research paper, Nalla and coworkers [6] studied tensile type fracture and R-curve behavior in human cortical bone. They showed that in vitro fracture toughness value rises linearly with crack extension. Mode I, mode II and mixed mode I/II fracture behavior of human cortical bone was also obtained experimentally by Zimmermann et al. [7] using edge cracked four-point bend loading. Based on their observation, both fracture onset and fracture initiation directions were differed in longitudinal and transverse directions mainly due the anisotropy and non-homogeneous nature of bone in comparison with most brittle and homogeneous materials. Using double cantilever beam (DCB) specimen, Morais et al. [8] determined the pure mode I fracture toughness of bone tissue. In other research works, the mode I, mode II and mixed mode I/II fracture toughness of human and bovine cortical bone have also been using different test specimens including DCB specimen for mode I testing [9], End Notched Flexure (ENF) specimen for mode II testing [10,11] and Single Leg Bending (SLB) test specimen for mixed mode I/II testing [12]. Several test specimens with different configurations have been employed and suggested in the past for conducting the fracture toughness experiments in different materials. Edge creaked rectangular beam subjected to three or four-point bend loading [13-23], compact test specimen subjected to tension loading [24-28], center cracked Brazilian disc specimen subjected to diametral compression[29-35], semi-circular specimen containing an edge crack and subjected to symmetric or asymmetric three-point bend loading [36-46], square plate containing center crack and subjected to far field pin loading [47]; edge cracked triangular specimen subjected to bend loading [48-50], center cracked ring specimen under compressive loading[51,52] and edge notch disc sample under three-point bend loading [53-61] are some of the frequently used test samples for obtaining mode I, mode II, mode III or mixed mode fracture toughness of brittle and quasi-brittle materials. Among the mentioned specimens, the rectangular edge cracked bend beam and edge cracked compact tension specimens are more favorite test specimens for manufacturing small size test samples which leads easy loading setup in biomaterials like bone or dentures. In this research, the fracture toughness and fracture energy of bovine femur bone is obtained experimentally using single edge notch bend (SENB) specimen. The effect of sample preparation location on the crack growth behavior of the tested bone is studied. It is also shown that a linear relation exists between the mode I fracture toughness and fracture energy of tested bone material. B ONE FRACTURE TOUGHNESS TESTING wo fresh femur bones from a same bovine of unknown age were used. They were cut by means of band saw machine to obtain two cylindrical shape samples from each bone as shown in Fig 1. The bones were marked along a reference direction before cutting. A number of single edge notch beam specimens with size of 60 mm * 10 mm * 4 mm were cut and prepared from each cylindrical shape bone along the longitudinal direction. A vertical edge crack of length 5 mm was also introduced in the specimens using a very narrow saw blade. The crack length ratio a/W in the manufactured sample was equal to 0.5. Before testing the specimens were soaked in normal saline and wrapped in normal saline soaked gauge and T