13th International Conference on Fracture June 16–21, 2013, Beijing, China -5- embedded in a two-component acrylic (see Figure 6a) to provide the samples with a support that can then be used to polish the surface using an automatic grinder (see Figure 6b). The grinder holds down the sample, using suction cups, onto rotating (150 rpm) polishing plates to smooth the surface. The machine uses water as a lubricant. The polishing process is gradual being the carbide papers of the following grain sizes: 500, 1200, 2400 and 4000. Finally, a velvet disc was used to provide a smooth finish to the samples before they were tested (see Figure 6c). a) b) c) Figure 6. a) Embedding bone extremities in resin. b) polishing machine. c) polished bones Once the samples were polished, they were stored in a saline solution at 5°C to avoid the exposed polished surface from deteriorating. Prior to test the samples were preheated to 37°C in a controlled saline solution bath to recreate in vivo conditions. The UMI tests were carried out using a Shimadzu DUH-211 with a pyramidal Berkovich indenter tip (see Figure 7a and 7b). The test consists of two main phases, loading up to 150 mN and then discharging. Each phase is followed by a holding period: 10 seconds for the loading and 5 seconds for the discharge. The loading and unloading rate was 2,6648 mN/sec. On each sample, both the trabecular and cortical tissues were analyzed. For each area 14 indentations were made in order to obtain the hardness and elastic modulus. The hardness calculated is defined as the ratio between the force applied by the indenter and the area left after the indentation (see Figure 7c). a) b) c) d1 d2 d3 Figure 7. UMI test description. a) Machine. b) indenter tip geometry. c) projected area A typical UMI graph is shown in Figure 8. The loading and discharge phases are clearly visible and hf, hr, hs, hc and hm all represent specific indentation depths that occur during the test. The dashed line, with a slope S, is used to calculate the indentation elastic modulus. It intersects the x-axis at the depth hr. Once the data was collected it was analyzed using variety of statistical tools.
RkJQdWJsaXNoZXIy MjM0NDE=