13th International Conference on Fracture June 16–21, 2013, Beijing, China -4- In biomechanics, 4PBTs are important as they allow the application of a bending load to a bone shaft sample, which is generally how mid-shaft femur fractures occur [7]. During the 4PBTs the loading force was exerted by a hydraulic piston, through two points namely two cylindrical steel rods. The sample received this load whilst being laid between two cylindrical rods at 15mm apart. The load rate was 10 mm/min until the two points of the hydraulic arm reached the centre of the sample surface. Figure 4b shows the hydraulic actuator before it has reached the sample. F represents the force exerted by the hydraulic arm. In a 4PBT bone samples act as beams, the upper part of the bone being in compression and the lower part in tension. During the tests, the force-displacement graphs were obtained by collecting the numerical data obtained from the machine. These graphs enabled the determination of three mechanical properties: maximum force (Fmax), energy absorbed up to the maximum force (Ea) and rigidity which are represented in the typical force-displacement curve obtained during a 4PBT shown in Figure 5. a) b) Figure 4. a) Four point bending test setup and b) Detail of bone sample about to be loaded for a 4PBT Figure 5. Typical force-displacement curve of a 4PBT 2.3 Ultra-Micro Indentation Test (UMI) The second experiment undertaken on the samples was the UMI test, enabling the extraction of hardness [8-9] and elastic modulus parameters from the samples. The cut bone extremities were Rigidity
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